Hi All,
I hope you can help me with this one. I have been fixing old radios since I was a kid and never got one this bad before.
Zenith H723Z2 is an AM FM form the 50's. A schematic is on this site. When I got it I found that the filament dropping resistor was open and that one or two of the tubes had heater to cathode shorts. Easy fix. I recapped it, too, plugged it in and WOW! It played great. Sendsitiviy, selectivity, and tone quality were so good, it made me want to recall all the other Zenith 723 and 724's I have done.
For about a week I played it on the work bench to make sure the repairs were good. I even heard Fibber McGee and Molley on it. After the week it still played fantiastically. Then the socket under the 35C5 exploded. It took off pin 4 and cracked the bulb. I replaced the socket and tube and the radio again played well. Just after I put it in its case I played it, it played for 15 minutes and then went silent.
I turns out that the socket under the second IF tube had a short in it, too. I then came to the conclusion that I had some how gotten hold of bad solder, perhaps 'No Clean' solder. From then on I did repairs using solder from Radio Shack and got cleaner for No clean solder. I sprayed this cleaner on this radio and another that I was doing at the same time. That fixed the other radio, not this one.
I also found a problem in the dual frequency IF, the 10.7 coil measure 4.7 ohm, but the schematic says it should be 1.5 ohm. I replaced that, too.
Presently the radio is dead on both bands, except that I get a tiny bit of audio on strong AM signals. When I change R15 to 47K I can get reasonably good AM sometimes.
The voltages seem OK, except that on the IFs the plate and screens which should be 90 volts are around 97. Pin 1 of the new 35C5 can be as high as 6 volts (but R29 checks), and the rest of the voltages on the tube are OK. I rechecked this socket for shorts and it is OK. Also Pin 7 of the 12AT7 should be around -5 volts on FM but is around -0.5 volts.
Any ideas? I am ready th throw this one in the trash tubes speaker and all.

Best Regards,

Bill Grimm

Hi Bill

Is the audio section operating? Touch center terminal on your volume control, there should be hum.

http://www.nostalgiaair.org/PagesByModel/568/M0025568.pdf

Voltage on pin #1 of the 35C5 is dependent on tube emission. Anything around 6 volts is ok.

Your FM oscillator isn't operating. This can be a bad 12AT7 even if it tests ok.

Don't think solder had anything to do with the problem unless it was acid core? Acid core is conductive & corrosive.

Difference in plate reading is due to line voltage and tube emission, not a problem. Be sure screen voltages are also positive.

Norm

:Hi All,
: I hope you can help me with this one. I have been fixing old radios since I was a kid and never got one this bad before.
: Zenith H723Z2 is an AM FM form the 50's. A schematic is on this site. When I got it I found that the filament dropping resistor was open and that one or two of the tubes had heater to cathode shorts. Easy fix. I recapped it, too, plugged it in and WOW! It played great. Sendsitiviy, selectivity, and tone quality were so good, it made me want to recall all the other Zenith 723 and 724's I have done.
: For about a week I played it on the work bench to make sure the repairs were good. I even heard Fibber McGee and Molley on it. After the week it still played fantiastically. Then the socket under the 35C5 exploded. It took off pin 4 and cracked the bulb. I replaced the socket and tube and the radio again played well. Just after I put it in its case I played it, it played for 15 minutes and then went silent.
: I turns out that the socket under the second IF tube had a short in it, too. I then came to the conclusion that I had some how gotten hold of bad solder, perhaps 'No Clean' solder. From then on I did repairs using solder from Radio Shack and got cleaner for No clean solder. I sprayed this cleaner on this radio and another that I was doing at the same time. That fixed the other radio, not this one.
: I also found a problem in the dual frequency IF, the 10.7 coil measure 4.7 ohm, but the schematic says it should be 1.5 ohm. I replaced that, too.
: Presently the radio is dead on both bands, except that I get a tiny bit of audio on strong AM signals. When I change R15 to 47K I can get reasonably good AM sometimes.
: The voltages seem OK, except that on the IFs the plate and screens which should be 90 volts are around 97. Pin 1 of the new 35C5 can be as high as 6 volts (but R29 checks), and the rest of the voltages on the tube are OK. I rechecked this socket for shorts and it is OK. Also Pin 7 of the 12AT7 should be around -5 volts on FM but is around -0.5 volts.
: Any ideas? I am ready th throw this one in the trash tubes speaker and all.
:
:Best Regards,
:
:Bill Grimm

Best not to replace IF transformers. If resistances go higher, you could have faulty connections within. Check and repair. Don't adjust the slugs at all...the radio played properly, and should be in adjustment. You just have a bad connection or a bad tube. Often in these radios, the AM and FM IF coils are in series. A fault in the FM coils can ruin AM as well. Often the AM coils are shorted out for FM. This is the case with the 1st IF in your radio. Notice how the 2nd IF has more of a wave trap configuration. Both signals pass through this one transformer. A short or open in either condenser in this transformer can cause both bands to go dead. Check all condensers including those within the IF transformers for shorts. Check for opens if you are able to. Also, regarding condensers and connections within IF transformers, wiggling of the cans while assembling the radio can knock things loose, and can destroy those nasty mica wafers that are inserted in the bases of some transformers.

T.

:Best not to replace IF transformers. If resistances go higher, you could have faulty connections within. Check and repair. Don't adjust the slugs at all...the radio played properly, and should be in adjustment. You just have a bad connection or a bad tube. Often in these radios, the AM and FM IF coils are in series. A fault in the FM coils can ruin AM as well. Often the AM coils are shorted out for FM. This is the case with the 1st IF in your radio. Notice how the 2nd IF has more of a wave trap configuration. Both signals pass through this one transformer. A short or open in either condenser in this transformer can cause both bands to go dead. Check all condensers including those within the IF transformers for shorts. Check for opens if you are able to. Also, regarding condensers and connections within IF transformers, wiggling of the cans while assembling the radio can knock things loose, and can destroy those nasty mica wafers that are inserted in the bases of some transformers.
:
:T.

Hi Thomas,
Your comments are good ones. Those IF screws are seductive, "turn me! turn me!" they seem to say. Happily I did not turn them. The problem was a bad connection between tube and socket.
The high impedance on that IF transformer I suspected was a bad solder joint. It is hard to immagine what in the coil would cause impedance to climb and not open. That IF had been rebuilt because there was a leak across the wafer, so it did need readjustment.
I only adjust an IF if I rebuilt it or if it looks like someone has been messing with the IFs, which can sometimes be a hard judgement.

Thank you for helping and reading my long tale of woe.

Bill Grimm

:Hi Bill
:
: Is the audio section operating? Touch center terminal on your volume control, there should be hum.
:
:http://www.nostalgiaair.org/PagesByModel/568/M0025568.pdf
:
: Voltage on pin #1 of the 35C5 is dependent on tube emission. Anything around 6 volts is ok.
:
: Your FM oscillator isn't operating. This can be a bad 12AT7 even if it tests ok.
:
: Don't think solder had anything to do with the problem unless it was acid core? Acid core is conductive & corrosive.
:
: Difference in plate reading is due to line voltage and tube emission, not a problem. Be sure screen voltages are also positive.
:
:Norm
:
::Hi All,
:: I hope you can help me with this one. I have been fixing old radios since I was a kid and never got one this bad before.
:: Zenith H723Z2 is an AM FM form the 50's. A schematic is on this site. When I got it I found that the filament dropping resistor was open and that one or two of the tubes had heater to cathode shorts. Easy fix. I recapped it, too, plugged it in and WOW! It played great. Sendsitiviy, selectivity, and tone quality were so good, it made me want to recall all the other Zenith 723 and 724's I have done.
:: For about a week I played it on the work bench to make sure the repairs were good. I even heard Fibber McGee and Molley on it. After the week it still played fantiastically. Then the socket under the 35C5 exploded. It took off pin 4 and cracked the bulb. I replaced the socket and tube and the radio again played well. Just after I put it in its case I played it, it played for 15 minutes and then went silent.
:: I turns out that the socket under the second IF tube had a short in it, too. I then came to the conclusion that I had some how gotten hold of bad solder, perhaps 'No Clean' solder. From then on I did repairs using solder from Radio Shack and got cleaner for No clean solder. I sprayed this cleaner on this radio and another that I was doing at the same time. That fixed the other radio, not this one.
:: I also found a problem in the dual frequency IF, the 10.7 coil measure 4.7 ohm, but the schematic says it should be 1.5 ohm. I replaced that, too.
:: Presently the radio is dead on both bands, except that I get a tiny bit of audio on strong AM signals. When I change R15 to 47K I can get reasonably good AM sometimes.
:: The voltages seem OK, except that on the IFs the plate and screens which should be 90 volts are around 97. Pin 1 of the new 35C5 can be as high as 6 volts (but R29 checks), and the rest of the voltages on the tube are OK. I rechecked this socket for shorts and it is OK. Also Pin 7 of the 12AT7 should be around -5 volts on FM but is around -0.5 volts.
:: Any ideas? I am ready th throw this one in the trash tubes speaker and all.
::
::Best Regards,
::
::Bill Grimm

Hi Norm,
Your line, "Your FM oscillator isn't operating. This can be a bad 12AT7 even if it tests ok," was like a dope slap. The local oscillator is kind of important!
You came just slightly high of a bulls eye. I had tried another 12AT7 there with no luck. I didn't mention it because my long tale of woe was already long enough I had to edit somenting. I wondered if I may have had two bad 12AT7's. I took one from an operating radio, the one I wrote about the bad speaker earlier this month. The radio started working. I then tried the old 12AT7 and it again did not work. I found however that if I pulled that 12AT7 half way out of its socket, then it, too, worked! That Is why I said your were a little high of a bulls eye, but still scored big!
The radio chassis plays well. Red Skelton tonight at 7!
I will be happy to return the favor sometime, but you are good. I may not have a chance.

Thank you,

Bill Grimm

Bill, I would save that weak 12AT7 - it'll probably work in another radio.

Sometimes an oscillator will start running if it's kick-stated, e.g., turn the radio off then on again. Maybe pulling the tube out a little from the socket gave it a kick? But once a weak tube is kick-started, it will usually poop out after a while. Anyway, that's been my experience.

Just for grins, check the various components in the oscillator ckt - see if a resistor or cap has drifted in value such that only a primo tube will run. Also look for any suspicious-looking solder joints.

: You came just slightly high of a bulls eye. I had tried another 12AT7 there with no luck. I didn't mention it because my long tale of woe was already long enough I had to edit somenting. I wondered if I may have had two bad 12AT7's. I took one from an operating radio, the one I wrote about the bad speaker earlier this month. The radio started working. I then tried the old 12AT7 and it again did not work. I found however that if I pulled that 12AT7 half way out of its socket, then it, too, worked! That Is why I said your were a little high of a bulls eye, but still scored big!
: The radio chassis plays well. Red Skelton tonight at 7!
: I will be happy to return the favor sometime, but you are good. I may not have a chance.
:
:Thank you,
:
:Bill Grimm

:Bill, I would save that weak 12AT7 - it'll probably work in another radio.
:
:Sometimes an oscillator will start running if it's kick-stated, e.g., turn the radio off then on again. Maybe pulling the tube out a little from the socket gave it a kick? But once a weak tube is kick-started, it will usually poop out after a while. Anyway, that's been my experience.
:
:Just for grins, check the various components in the oscillator ckt - see if a resistor or cap has drifted in value such that only a primo tube will run. Also look for any suspicious-looking solder joints.
:
:
:
:: You came just slightly high of a bulls eye. I had tried another 12AT7 there with no luck. I didn't mention it because my long tale of woe was already long enough I had to edit somenting. I wondered if I may have had two bad 12AT7's. I took one from an operating radio, the one I wrote about the bad speaker earlier this month. The radio started working. I then tried the old 12AT7 and it again did not work. I found however that if I pulled that 12AT7 half way out of its socket, then it, too, worked! That Is why I said your were a little high of a bulls eye, but still scored big!
:: The radio chassis plays well. Red Skelton tonight at 7!
:: I will be happy to return the favor sometime, but you are good. I may not have a chance.
::
::Thank you,
::
::Bill Grimm

Hi Doug,
The grid resistor should be 10K, but reads 9K. I considered that close enough. Now I wonder.

Bill

Bill, I wouldn't worry about that grid stopper's value too much, not within 10%.

Old carbon-composition resistors seem to most often increase in value. But, whatever.

:Hi Doug,
: The grid resistor should be 10K, but reads 9K. I considered that close enough. Now I wonder.
:
:Bill

:Hi Bill
:
: Is the audio section operating? Touch center terminal on your volume control, there should be hum.
:
:http://www.nostalgiaair.org/PagesByModel/568/M0025568.pdf
:
: Voltage on pin #1 of the 35C5 is dependent on tube emission. Anything around 6 volts is ok.
:
: Your FM oscillator isn't operating. This can be a bad 12AT7 even if it tests ok.
:
: Don't think solder had anything to do with the problem unless it was acid core? Acid core is conductive & corrosive.
:
: Difference in plate reading is due to line voltage and tube emission, not a problem. Be sure screen voltages are also positive.
:
:Norm
:
::Hi All,
:: I hope you can help me with this one. I have been fixing old radios since I was a kid and never got one this bad before.
:: Zenith H723Z2 is an AM FM form the 50's. A schematic is on this site. When I got it I found that the filament dropping resistor was open and that one or two of the tubes had heater to cathode shorts. Easy fix. I recapped it, too, plugged it in and WOW! It played great. Sendsitiviy, selectivity, and tone quality were so good, it made me want to recall all the other Zenith 723 and 724's I have done.
:: For about a week I played it on the work bench to make sure the repairs were good. I even heard Fibber McGee and Molley on it. After the week it still played fantiastically. Then the socket under the 35C5 exploded. It took off pin 4 and cracked the bulb. I replaced the socket and tube and the radio again played well. Just after I put it in its case I played it, it played for 15 minutes and then went silent.
:: I turns out that the socket under the second IF tube had a short in it, too. I then came to the conclusion that I had some how gotten hold of bad solder, perhaps 'No Clean' solder. From then on I did repairs using solder from Radio Shack and got cleaner for No clean solder. I sprayed this cleaner on this radio and another that I was doing at the same time. That fixed the other radio, not this one.
:: I also found a problem in the dual frequency IF, the 10.7 coil measure 4.7 ohm, but the schematic says it should be 1.5 ohm. I replaced that, too.
:: Presently the radio is dead on both bands, except that I get a tiny bit of audio on strong AM signals. When I change R15 to 47K I can get reasonably good AM sometimes.
:: The voltages seem OK, except that on the IFs the plate and screens which should be 90 volts are around 97. Pin 1 of the new 35C5 can be as high as 6 volts (but R29 checks), and the rest of the voltages on the tube are OK. I rechecked this socket for shorts and it is OK. Also Pin 7 of the 12AT7 should be around -5 volts on FM but is around -0.5 volts.
:: Any ideas? I am ready th throw this one in the trash tubes speaker and all.
::
::Best Regards,
::
::Bill Grimm

Hi All,
I still have a problem. It seems that now the local oscillator is intermittant. Even with the 12AT7 tube that always works in the other radio and with the original 12AT7 half in the socket, the oscillator seems to start only sometimes.
Red Skelton was a no show last night on it. The oscillator only started after he was off.
I plan on replacing the low resistance 10K ohm resistor, but also plan on looking elsewhere in the oscillator circuit, as it is only down 1K to 9K. I have tested the 50 pF capacitor and it is good.

Best Regards,

Bill Grimm

::Hi Bill
::
:: Is the audio section operating? Touch center terminal on your volume control, there should be hum.
::
::http://www.nostalgiaair.org/PagesByModel/568/M0025568.pdf
::
:: Voltage on pin #1 of the 35C5 is dependent on tube emission. Anything around 6 volts is ok.
::
:: Your FM oscillator isn't operating. This can be a bad 12AT7 even if it tests ok.
::
:: Don't think solder had anything to do with the problem unless it was acid core? Acid core is conductive & corrosive.
::
:: Difference in plate reading is due to line voltage and tube emission, not a problem. Be sure screen voltages are also positive.
::
::Norm
::
:::Hi All,
::: I hope you can help me with this one. I have been fixing old radios since I was a kid and never got one this bad before.
::: Zenith H723Z2 is an AM FM form the 50's. A schematic is on this site. When I got it I found that the filament dropping resistor was open and that one or two of the tubes had heater to cathode shorts. Easy fix. I recapped it, too, plugged it in and WOW! It played great. Sendsitiviy, selectivity, and tone quality were so good, it made me want to recall all the other Zenith 723 and 724's I have done.
::: For about a week I played it on the work bench to make sure the repairs were good. I even heard Fibber McGee and Molley on it. After the week it still played fantiastically. Then the socket under the 35C5 exploded. It took off pin 4 and cracked the bulb. I replaced the socket and tube and the radio again played well. Just after I put it in its case I played it, it played for 15 minutes and then went silent.
::: I turns out that the socket under the second IF tube had a short in it, too. I then came to the conclusion that I had some how gotten hold of bad solder, perhaps 'No Clean' solder. From then on I did repairs using solder from Radio Shack and got cleaner for No clean solder. I sprayed this cleaner on this radio and another that I was doing at the same time. That fixed the other radio, not this one.
::: I also found a problem in the dual frequency IF, the 10.7 coil measure 4.7 ohm, but the schematic says it should be 1.5 ohm. I replaced that, too.
::: Presently the radio is dead on both bands, except that I get a tiny bit of audio on strong AM signals. When I change R15 to 47K I can get reasonably good AM sometimes.
::: The voltages seem OK, except that on the IFs the plate and screens which should be 90 volts are around 97. Pin 1 of the new 35C5 can be as high as 6 volts (but R29 checks), and the rest of the voltages on the tube are OK. I rechecked this socket for shorts and it is OK. Also Pin 7 of the 12AT7 should be around -5 volts on FM but is around -0.5 volts.
::: Any ideas? I am ready th throw this one in the trash tubes speaker and all.
:::
:::Best Regards,
:::
:::Bill Grimm
:
:Hi All,
: I still have a problem. It seems that now the local oscillator is intermittant. Even with the 12AT7 tube that always works in the other radio and with the original 12AT7 half in the socket, the oscillator seems to start only sometimes.
: Red Skelton was a no show last night on it. The oscillator only started after he was off.
: I plan on replacing the low resistance 10K ohm resistor, but also plan on looking elsewhere in the oscillator circuit, as it is only down 1K to 9K. I have tested the 50 pF capacitor and it is good.
:
:Best Regards,
:
:Bill Grimm

Bill,

Have you checked the pins in the socket for a broken one. Some years ago, working on tube communications equipment, on a few rare occasions I found a pin in the socket was broken. This was a plastic molded socket. The fix was to replace the broken pin. On some other occasions I have found a pin in the socket corroded and had to be replaced.

Radiodoc

:::Hi Bill
:::
::: Is the audio section operating? Touch center terminal on your volume control, there should be hum.
:::
:::http://www.nostalgiaair.org/PagesByModel/568/M0025568.pdf
:::
::: Voltage on pin #1 of the 35C5 is dependent on tube emission. Anything around 6 volts is ok.
:::
::: Your FM oscillator isn't operating. This can be a bad 12AT7 even if it tests ok.
:::
::: Don't think solder had anything to do with the problem unless it was acid core? Acid core is conductive & corrosive.
:::
::: Difference in plate reading is due to line voltage and tube emission, not a problem. Be sure screen voltages are also positive.
:::
:::Norm
:::
::::Hi All,
:::: I hope you can help me with this one. I have been fixing old radios since I was a kid and never got one this bad before.
:::: Zenith H723Z2 is an AM FM form the 50's. A schematic is on this site. When I got it I found that the filament dropping resistor was open and that one or two of the tubes had heater to cathode shorts. Easy fix. I recapped it, too, plugged it in and WOW! It played great. Sendsitiviy, selectivity, and tone quality were so good, it made me want to recall all the other Zenith 723 and 724's I have done.
:::: For about a week I played it on the work bench to make sure the repairs were good. I even heard Fibber McGee and Molley on it. After the week it still played fantiastically. Then the socket under the 35C5 exploded. It took off pin 4 and cracked the bulb. I replaced the socket and tube and the radio again played well. Just after I put it in its case I played it, it played for 15 minutes and then went silent.
:::: I turns out that the socket under the second IF tube had a short in it, too. I then came to the conclusion that I had some how gotten hold of bad solder, perhaps 'No Clean' solder. From then on I did repairs using solder from Radio Shack and got cleaner for No clean solder. I sprayed this cleaner on this radio and another that I was doing at the same time. That fixed the other radio, not this one.
:::: I also found a problem in the dual frequency IF, the 10.7 coil measure 4.7 ohm, but the schematic says it should be 1.5 ohm. I replaced that, too.
:::: Presently the radio is dead on both bands, except that I get a tiny bit of audio on strong AM signals. When I change R15 to 47K I can get reasonably good AM sometimes.
:::: The voltages seem OK, except that on the IFs the plate and screens which should be 90 volts are around 97. Pin 1 of the new 35C5 can be as high as 6 volts (but R29 checks), and the rest of the voltages on the tube are OK. I rechecked this socket for shorts and it is OK. Also Pin 7 of the 12AT7 should be around -5 volts on FM but is around -0.5 volts.
:::: Any ideas? I am ready th throw this one in the trash tubes speaker and all.
::::
::::Best Regards,
::::
::::Bill Grimm
::
::Hi All,
:: I still have a problem. It seems that now the local oscillator is intermittant. Even with the 12AT7 tube that always works in the other radio and with the original 12AT7 half in the socket, the oscillator seems to start only sometimes.
:: Red Skelton was a no show last night on it. The oscillator only started after he was off.
:: I plan on replacing the low resistance 10K ohm resistor, but also plan on looking elsewhere in the oscillator circuit, as it is only down 1K to 9K. I have tested the 50 pF capacitor and it is good.
::
::Best Regards,
::
::Bill Grimm
:
:Bill,
:
:Have you checked the pins in the socket for a broken one. Some years ago, working on tube communications equipment, on a few rare occasions I found a pin in the socket was broken. This was a plastic molded socket. The fix was to replace the broken pin. On some other occasions I have found a pin in the socket corroded and had to be replaced.
:
:Radiodoc
:
Hi Radiodoc,
With this radio's history of bad sockets, I will check again. This socket is molded as you described.
A while ago I pulled a 6T8A out of its socket and it left a pin behind. That had been causing a serious intermittant problem. There it was the tube. I understand you are talking about the socket.
I will have the resistors replaced this evening, hopefully.

All the Best,

Bill

You probably already know this, but if the pins break out of the tubes, don't try to repair these. You can attempt to repair broken contact clips in the sockets, though. Press on them with a very small screw driver to compress them if they're loose. Some just won't tighten up.

T.

:Hi Bill
:
: Is the audio section operating? Touch center terminal on your volume control, there should be hum.
:
:http://www.nostalgiaair.org/PagesByModel/568/M0025568.pdf
:
: Voltage on pin #1 of the 35C5 is dependent on tube emission. Anything around 6 volts is ok.
:
: Your FM oscillator isn't operating. This can be a bad 12AT7 even if it tests ok.
:
: Don't think solder had anything to do with the problem unless it was acid core? Acid core is conductive & corrosive.
:
: Difference in plate reading is due to line voltage and tube emission, not a problem. Be sure screen voltages are also positive.
:
:Norm
:
::Hi All,
:: I hope you can help me with this one. I have been fixing old radios since I was a kid and never got one this bad before.
:: Zenith H723Z2 is an AM FM form the 50's. A schematic is on this site. When I got it I found that the filament dropping resistor was open and that one or two of the tubes had heater to cathode shorts. Easy fix. I recapped it, too, plugged it in and WOW! It played great. Sendsitiviy, selectivity, and tone quality were so good, it made me want to recall all the other Zenith 723 and 724's I have done.
:: For about a week I played it on the work bench to make sure the repairs were good. I even heard Fibber McGee and Molley on it. After the week it still played fantiastically. Then the socket under the 35C5 exploded. It took off pin 4 and cracked the bulb. I replaced the socket and tube and the radio again played well. Just after I put it in its case I played it, it played for 15 minutes and then went silent.
:: I turns out that the socket under the second IF tube had a short in it, too. I then came to the conclusion that I had some how gotten hold of bad solder, perhaps 'No Clean' solder. From then on I did repairs using solder from Radio Shack and got cleaner for No clean solder. I sprayed this cleaner on this radio and another that I was doing at the same time. That fixed the other radio, not this one.
:: I also found a problem in the dual frequency IF, the 10.7 coil measure 4.7 ohm, but the schematic says it should be 1.5 ohm. I replaced that, too.
:: Presently the radio is dead on both bands, except that I get a tiny bit of audio on strong AM signals. When I change R15 to 47K I can get reasonably good AM sometimes.
:: The voltages seem OK, except that on the IFs the plate and screens which should be 90 volts are around 97. Pin 1 of the new 35C5 can be as high as 6 volts (but R29 checks), and the rest of the voltages on the tube are OK. I rechecked this socket for shorts and it is OK. Also Pin 7 of the 12AT7 should be around -5 volts on FM but is around -0.5 volts.
:: Any ideas? I am ready th throw this one in the trash tubes speaker and all.
::
::Best Regards,
::
::Bill Grimm

Hi All,
I replaced the 10K bias resistor on the oscillator, it was 9K and should have been 10K. I also replaced the cathode resistor on the mixer side of the 12AT7. It is supposed to be 2.2k but read 3K, well out of spec. I also checked the capacitors and they seemed fine, except for C10 which doens't make sense. I will get back to that.
After the replacement, no oscillation, dead. It doesn't seem to even be intermittant anymore.
Voltages on the 12AT7 are:
pin 1 97.5 volts
pin 2 0
pin 3 1.73 volts
pin 6 101.3 volts
pin 7 -0.6 volts (switch is in FM position)
pin 8 0

I can get a little audio on AM on some strong frequencies, but nothing that can be listened to.

C10 on the schematic is an enigma. The schematic says it is 0.68mmf. That can't be 0.68pf. Two wires in close proximity will have more capacitance than that. Since it is in the middle of everything I tested it on the bridge. It seems to read around 18pf, with no leakage.

The last thing I did was pull the 12AT7 and place it in a working H724Z1. That radio played fine.

Any ideas? I am still at wits end.

All the Best,

Bill Grimm

Hi All,
It seems that when I changed the resistors in the oscillator it threw the front end out of alignment.
With great trepidaiton I retuned the FM ANT coil on the front end and got reception. Not great reception, but listenable. It looks like the same for AM.

Thank you for all the help.

Best Regards,

Bill G.

You should really use a generator for aligning the set, especially FM. Changing resistors in the FM oscillator should not affect AM reception. Perhaps your set needs a complete alignment.

T.

:You should really use a generator for aligning the set, especially FM. Changing resistors in the FM oscillator should not affect AM reception. Perhaps your set needs a complete alignment.
:
:T.
Hi Thomas,
I tried realignment. AM is fine now. FM, however, didn't align well. I was able to peak the three IFs OK, but the descriminator will not null. The place where it seems it ought to null is the place where I get the highest deflection. Sensitivity on FM is poor and the sound highly distorted. The 19T8 is known to be good.
Both AM and FM tuned as though the IFs were well out of alignment, however, it did play well on AM and FM before the 35C5 socket exploded. Any ideas on how that could throw the IFs out of alignment?

Any thoughts would be appreciated.

Best Regards,

Bill Grimm

OYE! Shouldn't peak FM IFs, especially the discriminator. Doing so causes distortion and poor audio quality. FM works by shifting the transmitted frequency up and down to the audio signal. The radio which receives this signal must be able to pass the widest expected frequency shift, not just the mean frequency. Not quite sure what the bandwidth should be, but I think I remember it being 10KC in either direction from center (could be wrong...maybe it's more like 19 or 20). At any rate, don't peak the transformers, especially the discriminator transformer. The discriminator transformer should pass the appropriate band width with a flat top--no dip or peak in the middle.

Thomas

:OYE! Shouldn't peak FM IFs, especially the discriminator. Doing so causes distortion and poor audio quality. FM works by shifting the transmitted frequency up and down to the audio signal. The radio which receives this signal must be able to pass the widest expected frequency shift, not just the mean frequency. Not quite sure what the bandwidth should be, but I think I remember it being 10KC in either direction from center (could be wrong...maybe it's more like 19 or 20). At any rate, don't peak the transformers, especially the discriminator transformer. The discriminator transformer should pass the appropriate band width with a flat top--no dip or peak in the middle.
:
:Thomas
Hi Thomas,
I am following the alignment proceedure and using a signal generator. I have used the same procedure with other H724 and H724 radios with good results.
The primary of the descriminator is supposed to be peaked according to the direciotns, the secondary is to be nulled. On the secondary I can only get a peak, no null. I suspect that a capacitor in the descriminator has disconnected inside.
Your point is well taken that these IFs need to pass a wide band width, though. THat is something that needs watching.

Best Regards,

Bill

Well, if you're following directions, then that's good. I've seen some sets where you peak the IFs, and some where you stagger them. The discriminator is always staggered, though. Make sure you don't have any corroded micas in the base of the IF transformers. Those will make proper tuning and resonance impossible. You'll be able to peak the transformers, but they won't resonate strongly, and won't give you a strong signal.

Thomas

:Well, if you're following directions, then that's good. I've seen some sets where you peak the IFs, and some where you stagger them. The discriminator is always staggered, though. Make sure you don't have any corroded micas in the base of the IF transformers. Those will make proper tuning and resonance impossible. You'll be able to peak the transformers, but they won't resonate strongly, and won't give you a strong signal.
:
:Thomas

Hi Thomas,
I tried a little experiment. I drilled a hole in the discriminator IF can and sprayed tuner tonic in it. I now get the null on the A9 tuning, but no peak on A8. Sound is still weak and distorted.
It looks like you assessment is correct. I seem to have lost connection to the capacitor on the primary of the discriminator. I tried an exterior 15pF in place of it, but there wasn't a change. I may try a 33 pF this weekend.
Even if that works I am reluctant to solder in a capacitor, tune it and call it done, since the old capacitor could reconnect itself at any time.
By the way the local oscillator not opertaing turned out to be a red herring. I made the same check on a working radio and got the same voltage. It seems my meter loads the circuit.

Best Regards,

Bill

You should remove IF transformers in question and disassemble them. Do your transformers have tunable slugs or condensers? If slugs, then they definitely have fixed condensers in the base. If you can't see any obvious condensers, look at the base. If it's clear plastic, look for metal items inside, bridging between the primary or secondary posts (one condenser per set of posts). If a phenolic wafer base, look for sheets of mica sandwiched between metal clips and another sheet of phenolic. Look for corrosion on the silver of the micas. To replace these, you must remove all traces of the old ones. Take a razor blade and hack out all of the mica. Do not leave any traces that might short across the contacts. Solder in new condensers of the appropriate size. If you don't know what size this is, it is best to work on only one transformer at a time (that's a good idea anyway). Then, with all other transformers as original (or already repaired, as you work down the line), try to peak the transformer you are repairing. Keep changing capacitance (adding) until you can peak the transformer with the slugs a little out from center (top slug above the coil and bottom slug below the coil).

Thomas

:You should remove IF transformers in question and disassemble them. Do your transformers have tunable slugs or condensers? If slugs, then they definitely have fixed condensers in the base. If you can't see any obvious condensers, look at the base. If it's clear plastic, look for metal items inside, bridging between the primary or secondary posts (one condenser per set of posts). If a phenolic wafer base, look for sheets of mica sandwiched between metal clips and another sheet of phenolic. Look for corrosion on the silver of the micas. To replace these, you must remove all traces of the old ones. Take a razor blade and hack out all of the mica. Do not leave any traces that might short across the contacts. Solder in new condensers of the appropriate size. If you don't know what size this is, it is best to work on only one transformer at a time (that's a good idea anyway). Then, with all other transformers as original (or already repaired, as you work down the line), try to peak the transformer you are repairing. Keep changing capacitance (adding) until you can peak the transformer with the slugs a little out from center (top slug above the coil and bottom slug below the coil).
:
:Thomas

Hi Thomas,
I rebuilt the discriminator IF. As you know this is quite tedious. When I went to tune up the discriminator, it tuned like a dream. The primary has a good peak and the secondary has a distinctive null. Perfect!
The radio still doesn't work.

The discriminator was definitely bad of course, but there is seomthing else going on. Further troubleshooting shows that all the plate votages are low, 75 volts, should be 90 volts. Also, when trying to check the tuning of the other two IFs I found that one has a very nice peak even when the signal genertator is turned off!
The term 'parts radio' has never sounded so good, but I had this set playing well in June and it just faded out. Also, I have been telling people that I can get a tin can full of corroded wire to play as a radio. It ain't bragging if I can do it. This thing certainly rates as a tin can full of corroded wire.
Have you ever seen such a misbehaving IF, peaks with the signal generator off?

All the Best,

Bill

By the way, for rebuilding IF cans on this kind of Zenith radio I have found the ideal insulator. They are available from Digi-Key, part numbers BER173-ND or MER105-ND. Either will work.

Well, yes.

Thomas

Hi All,
When I am feeling masochistic I bring this one out and torture myself with it.
As you could read from this radio's sordid history, just about aeverything has happened to it, acid solder, actively advancing silver mica, subtly bad tubes, exploding tube sockets, on and on.
Schematic is http://www.nostalgiaair.org/Resources/568/M0025568.htm
While aligning the FM something very odd happens. I connect the meter to the resistor that comes from the secondary of the second IF, as described in the alignment instructions. I align for maximum, while cutting down the signal generator output to avoid AGC action. Alignment goes well until the final part. Suddenly the voltage at the point which should be around -2 or -3 volts jumps to -41. It will do this for any strong FM station, too. The resulting sound is extremeny distorted.
I have already rebuilt the second IF once, using 24pF capacitors. The primary has about 90 volts on it. The other end of the secondary, which connects to the 12AU6 grid has about -2 volts on it, when this is going on. Where could it be getting this high negative voltage? Any ideas what the fix is?

Best Regards,

Bill Grimm