help with an air chief 7308-8 radio

3/18/2014 4:17:00 AMjoecyphers

Hi all.
I am new to the radio restoration processes, so bare with me. I just purchased an air chief 7308-8 radio. I went through it. And replaced the electronic caps with the same rated ones I then powered it up after checking out the cord and other resistors and caps. I had no volume then within a min. It blew the new electronic I put in. Polarity was correct. Don't know why it did that. Any one have any ideas on what to look for? Any help would be aprecieated.

3/18/2014 8:43:35 AMBrianC

Did you run the radio before the cap change, and no caps blew? Then, you may have touched or moved something, shorting it to the chassis.
If you didn't run the radio before the cap change,
double check your caps installation polarity against a schematic, to make sure the new caps were put in properly. You may have copied a previous mistake, or, there is still a short in the B+ feed components somewhere.

3/18/2014 8:46:01 AMNorm Leal

Several things will blow electrolytic caps. Being reversed is usual but you verified these were installed correctly.

Was voltage rating same or greater than originals?

Were caps made more than 20 years ago? New manufacture caps should be used. They dry out over time.

Were cap connected to proper places in the radio. AC will blow these caps.

Rectifier tube could be shorted. This is not common.

Norm

:Hi all.
:I am new to the radio restoration processes, so bare with me. I just purchased an air chief 7308-8 radio. I went through it. And replaced the electronic caps with the same rated ones I then powered it up after checking out the cord and other resistors and caps. I had no volume then within a min. It blew the new electronic I put in. Polarity was correct. Don't know why it did that. Any one have any ideas on what to look for? Any help would be aprecieated.
:

3/18/2014 9:30:20 AMCV

Installation with reversed polarity or DC overvoltage are about the only things that will quickly (few minutes) cause a new electrolytic cap to
self-destruct. A short on B+ won't do it, since that won't stress the caps.

Assuming that you installed the caps with the correct polarity (since you said that you did) that leaves overvoltage as the most likely suspect.

Tube radios operate with high DC voltages and require electrolytic caps that aren't generally available at Radio Shack (or similar store)any more. You need a cap with the same capacity rating (MFD) as the original part, AND a DC working voltage rating the same as, or greater than, that of the original part. Installing a 22 MFD 50 Volt cap in place of a 22 MFD 450 Volt cap, for example, will result in the failure that you described.

3/18/2014 10:31:12 AMjoecyphers

3/18/2014 10:39:18 AMjoecyphers

:
: Installation with reversed polarity or DC overvoltage are about the only things that will quickly (few minutes) cause a new electrolytic cap to
:self-destruct. A short on B+ won't do it, since that won't stress the caps.
:
: Assuming that you installed the caps with the correct polarity (since you said that you did) that leaves overvoltage as the most likely suspect.
:
: Tube radios operate with high DC voltages and require electrolytic caps that aren't generally available at Radio Shack (or similar store)any more. You need a cap with the same capacity rating (MFD) as the original part, AND a DC working voltage rating the same as, or greater than, that of the original part. Installing a 22 MFD 50 Volt cap in place of a 22 MFD 450 Volt cap, for example, will result in the failure that you described.
:
Thanks for all the info. I replaced the electrylic cap with new same mfd. and voltage rating the original 10/15 450v cap was bad, new ones were installed same polarity. The one thats blowing seems to be comming from the chasis ground - to the speaker plug recepticle which is tied in to the power transformer. a voltage reading from this point to chasis showed 189 vdc but also 460 volts ac?? this does not seem right to me. The rectifier tube tested ok on my tube tester. I dont have nor can i find a schematic for this radio, makes it harder to troubleshoot.

3/18/2014 11:21:01 AMCV

With the set unplugged, try ohming from the center tap of the power transformer's high voltage winding to ground and let us know what the reading is. Should be fairly low (less than 100 ohms). If it reads very high or open, check the resistor stack that it goes to. This stack may take the form of a metal-wrapped strip riveted to an inside wall of the chassis; or it may just be a couple of big "dogbone" resistors. One end of the stack will go to the transformer HV center tap and the other end will go to chassis ground. The middle node in the stack will go off to various tube grids. If one of the resistor sections opens up the rectifier tube will lose its chassis-ground reference and odd things can happen.

3/18/2014 12:13:41 PMjoecyphers

:With the set unplugged, try ohming from the center tap of the power transformer's high voltage winding to ground and let us know what the reading is. Should be fairly low (less than 100 ohms). If it reads very high or open, check the resistor stack that it goes to. This stack may take the form of a metal-wrapped strip riveted to an inside wall of the chassis; or it may just be a couple of big "dogbone" resistors. One end of the stack will go to the transformer HV center tap and the other end will go to chassis ground. The middle node in the stack will go off to various tube grids. If one of the resistor sections opens up the rectifier tube will lose its chassis-ground reference and odd things can happen.
:
Ok Il look at that with my meter and let you all know, I did find the scematic for this radio here in the resurces, im a little confused on the 10/15 mfd electrylit the schematic looks as if they do not share a common ground. if anyone can get a chance its under firestone radio model 7403-8 i messed up on model number originaly. thanks again for the help.

3/18/2014 5:48:04 PMCV

The ANODES of the two electrolytics are connected together, and go to the B+ bus. The CATHODES of the two electrolytics go to different places- one goes to the center tap of the HT winding of the power transformer; the other goes to chassis ground.

3/19/2014 3:33:31 AMjoecyphers

:The ANODES of the two electrolytics are connected together, and go to the B+ bus. The CATHODES of the two electrolytics go to different places- one goes to the center tap of the HT winding of the power transformer; the other goes to chassis ground.
:
Thanks again for the info. The cap to my center tap was polarity wrong. Corrected this then oh med out center tap to ground. U had open. Cold not find a bad resistor though. Powered up and gas a faint hum. But no stations. My speaker coil was very hot. May have a blown coil. I let you know what I find.

3/19/2014 9:20:41 AMCV

The "resistor stack" that I was referring to earlier are resistors 19 and 31 on the schematic. This radio uses a an odd pair of values: 180 ohms and 400K ohms so if you ohm the CT to ground, you will get approximately 400K ohms, not the "100 ohms" that I earlier mentioned.

Note that the speaker must be plugged in for the above ohm check, since the CT ground path goes through the field coil.

This arrangement is also a bit unusual (generally the field coil is driven via B+, not B-).

3/19/2014 9:30:29 AMjoecyphers

:The "resistor stack" that I was referring to earlier are resistors 19 and 31 on the schematic. This radio uses a an odd pair of values: 180 ohms and 400K ohms so if you ohm the CT to ground, you will get approximately 400K ohms, not the "100 ohms" that I earlier mentioned.
:
:Note that the speaker must be plugged in for the above ohm check, since the CT ground path goes through the field coil.

okay I will try that. On my speaker coil I should ohm a low resistance is that a correct assumption? The coil oh med out at 1.4k ohms.
:
:This arrangement is also a bit unusual (generally the field coil is driven via B+, not B-).
:

3/19/2014 10:39:36 AMCV

1400 ohms sounds about right for a speaker field. My guess is that there is some kind of short on the grid bias side of the speaker field (i.e., the side that connects to R31).

Try taking chassis voltage measurements per the troubleshooting table in the Riders service info and see if anything looks wonky.

3/19/2014 9:41:42 PMjoecyphers

:1400 ohms sounds about right for a speaker field. My guess is that there is some kind of short on the grid bias side of the speaker field (i.e., the side that connects to R31).
:
:Try taking chassis voltage measurements per the troubleshooting table in the Riders service info and see if anything looks wonky.
:
Well this radio sure has me stumped. I re-checked the two resistors R31 and R13, R-31 ohmed out at 185 ohms. but R13 which should be 400k ohms checked out at 56 ohms. I tryed installing a 400 k ohm resistor, then when i powered it up to check voltages the #27 10mfd 35 volt cap. blew, replaced it and the new cap immedieatly got hot, so i powered the radio down. this cap is tied into R6 470 k ohm resistor and going to the 6f6g output tube. there must be a short but i am at a loss on where to look next? I have aprecieted all your help in this, any other ideas would help. thanks.

3/19/2014 9:50:06 PMEdd w (Schema)

Sir Joe Cyphers . . . . .

In order to facilitate your troubleshooting and further enhance your knowledge of the involved circuitry paths, submitted herewith is my clarified custom schematic of your sets relevant circuitry.

Your power supply filters polarities now have been inserted, along with one bias filter (Item 27).

I also removed the garish ambiguity of the speaker plug and jack.

On this design of set, the center tap of the high voltage winding is NOT going to ground, but instead being held above ground with the series insertion of the field coil resistance of your speaker and then resistor items 31 and 19 and THEN it finally reaches ground potential by the left end of that final ree-sistor item 19.

Consult the new schema for the arrowed black bar/and/foo-shee-uh circuitry color coding.

Initially, the created NEGATIVE voltage gets filtered by C29 A (it gets its AC to ground loop via the series C29B) and then its passage on down thru the field coil inductance of the electrodynamic speaker.

The current being passed thru the provides for the magnetic field of the speaker in lieu of later day utilization of permanent magnets.

No extra charge . . . there is the filtering effect of the inductance of the field coil winding, while it also acts as a power dropping resistor by virtue of its inherent resistance.

The next series elements are the resistor items 31 and 19, and the combination of the filed coil and those two resistors that are developing two negative bias supply voltages at my marked in Red A and B nodes.

Reading my inserted YELLOW excerpt from the schematic, we see that a NEGATIVE 17 V volts should be flowing upwards from node B, in order to provide for the grid bias for the 6F6 audio output.

Over at marked up A node we see that resistor item 19 is serving as your 6SQ7's cathode resistor, but by virtue of being connected into the additional item 31 and field coil loop, a NEGATIVE 3 volts is introduced into the cathode circuitry .

By being at the bottom of the detector circuitry, that counterpoise voltage gives a wider dynamic swing to your sets overall AVC action.

And now . . . .AND NOW ! . . . . in looking at the partial components value charts listing that I have inserted, PLUS your feed back of a measured ~1400 ohm resistance of the series inserted field coil.

That 180 ohms for resistance item 31 REALLY seems plausible . . . . BUT . . . .in the specifying of 470 k for resistance of item 19 seems HIGHLY . . . to the power of 10 . . . . . IM-plausible, as your sets total current consumption has to pass thru this final resistor, for completing the DC ground loop of 3 series components.
Now, in getting ones brain fully in gear . . . .one would realize that having the specified 470 K as R19's value utilized, would not get enough power flow thru to power up a fleas nite lite .

The WHOLE sets Bt return must be made within this marked up series resistance train, with the 6F6 pulling the lions share of the power consumption . . . . if the volume level is being cranked up.

Soooooooooooo carefully check out your components in this series circuitry . . . part by part . . . and "war by war" and confirm for us that R31 is 180 ohms and that after lifting the wires from the dynamic field coil, that it is not ohmming out any resistance to the speaker frame IF the speaker frame is eventually getting grounded to chassis by its mounting.(Accounting for your mentioning of the warmed up field coil.)

And then the final info would be to measure the present resistance of R19, along with its color coding or marked on value if still being viewable.

With my off the head computations of the series current flow presented in this circuitry; yet not being able to eyeball the power transformer sizing to confirm if its being a 40 ma or 90 ma transformer secondary.

I figure on a proper resistance of R19 as really being ~50 ohms on the low side of a value or ~280 ohms on the very high side.

If you don't come back with that approximate value presently being installed there, we may just have to consult with a STEW-HART
WARMER . . . . equivalent sets schematic, since they are being this sets REAL manufacturing Sugar Daddy, with Firestone company just merely tacking on its name . . . . . since they really just make vee-hicle "Tars" and other 'sociated stuffs.

Thasssit . . . .

73's de Edd

The sole purpose of a child’s middle name . . . . . . . is so that they can tell when they're REALLY in some deeeeeeeeep doo-doo with their parents.

Firestone Mark Up SCHEMATIC:

::1400 ohms sounds about right for a speaker field. My guess is that there is some kind of short on the grid bias side of the speaker field (i.e., the side that connects to R31).
::
::Try taking chassis voltage measurements per the troubleshooting table in the Riders service info and see if anything looks wonky.
::
:Well this radio sure has me stumped. I re-checked the two resistors R31 and R13, R-31 ohmed out at 185 ohms. but R13 which should be 400k ohms checked out at 56 ohms. I tryed installing a 400 k ohm resistor, then when i powered it up to check voltages the #27 10mfd 35 volt cap. blew, replaced it and the new cap immedieatly got hot, so i powered the radio down. this cap is tied into R6 470 k ohm resistor and going to the 6f6g output tube. there must be a short but i am at a loss on where to look next? I have aprecieted all your help in this, any other ideas would help. thanks.
:

3/19/2014 11:09:20 PMCV

The 400k ohm value for the lower resistor of the bias divider appears to be a schematic error. If one can trust the accuracy of Notes A and B, the voltage at the top of the divider should be -14 V, and the voltage at the center of the divider should be -3 V. This means that 11 volts are being dropped (elevated?) across R19; if it is in fact a 180 ohm resistor, that would give a divider current of ~60 mA.
Since most of the current is going to flow through the lower resistor (the grid branches drawing a miniscule amount of current, that would mean that the lower resistor would have to be 50 ohms- pretty close to what you actually measured.

By installing the bogus 400K resistor in lieu of the 50 ohm one, the center junction of the divider rose to a voltage close to that of an open circuit- maybe 60 V or so. This is what popped the #27 cap.

If you replace the popped cap and re-install the original 50 ohm (56 ohm?) resistor, you should be able to measure the current through the speaker field by breaking the connection and inserting an ammeter in series. Since we know that the B- design current (from Notes A and B) should be around 60 mA, if the speaker field grossly exceeds that figure, there must be a short on the "downstream" side of the field (i.e. the side that feeds the resistor divider).

3/20/2014 9:00:43 AMjoecyphers

:The 400k ohm value for the lower resistor of the bias divider appears to be a schematic error. If one can trust the accuracy of Notes A and B, the voltage at the top of the divider should be -14 V, and the voltage at the center of the divider should be -3 V. This means that 11 volts are being dropped (elevated?) across R19; if it is in fact a 180 ohm resistor, that would give a divider current of ~60 mA.
:Since most of the current is going to flow through the lower resistor (the grid branches drawing a miniscule amount of current, that would mean that the lower resistor would have to be 50 ohms- pretty close to what you actually measured.
:
:By installing the bogus 400K resistor in lieu of the 50 ohm one, the center junction of the divider rose to a voltage close to that of an open circuit- maybe 60 V or so. This is what popped the #27 cap.
:
:If you replace the popped cap and re-install the original 50 ohm (56 ohm?) resistor, you should be able to measure the current through the speaker field by breaking the connection and inserting an ammeter in series. Since we know that the B- design current (from Notes A and B) should be around 60 mA, if the speaker field grossly exceeds that figure, there must be a short on the "downstream" side of the field (i.e. the side that feeds the resistor divider).
:
:

Hi again. Well I followed your directions and this is what i got.

voltage across R19 was -7 volts. across R31 and R19 was 35 volts. I then seriesed my ampmeter inline with my field coil and was getting arround 134 ma. sounds like double what I should have for voltages and ma. I will start trying to track down possible shorts in the feed to to R19 and R31.

3/20/2014 9:35:28 AMCV

A couple of last tests: turn off the set, unplug the speaker, and measure the resistance from the speaker-plug end of R31 to chassis ground. You should get in the neighborhood of 50+180 ohms = 230 ohms.

Now, plug the speaker back in, turn on the set, and take a DC voltage reading of both B+ (anode end of the two filter electrolytics) and B- (speaker end of R31); both with respect to chassis ground.

My thinking is that you could have two possible problems- a partial short of the speaker field which is causing it to draw excess current, OR a heavy current drain on B+ which is drawing down the B+ voltage while tending to further lower (drive more negative) the B- voltage. Measuring the B+ and B- voltages will give a clue as to what is going on.

Don't leave the set on too long as you take these measurements or the speaker field coil can burn out.

3/20/2014 10:20:39 AMjoecyphers

:A couple of last tests: turn off the set, unplug the speaker, and measure the resistance from the speaker-plug end of R31 to chassis ground. You should get in the neighborhood of 50+180 ohms = 230 ohms.
:
:Now, plug the speaker back in, turn on the set, and take a DC voltage reading of both B+ (anode end of the two filter electrolytics) and B- (speaker end of R31); both with respect to chassis ground.
:
:My thinking is that you could have two possible problems- a partial short of the speaker field which is causing it to draw excess current, OR a heavy current drain on B+ which is drawing down the B+ voltage while tending to further lower (drive more negative) the B- voltage. Measuring the B+ and B- voltages will give a clue as to what is going on.
:
:Don't leave the set on too long as you take these measurements or the speaker field coil can burn out.
:
Okay I ohmed the speaker plug with speaker unpluged to the the end of R31 reading was 242.6 ohms.
Then i plugged in the speaker and powered up and voltage checked the anodes, of the elec. caps to ground i only had .27 mv. Then I checked B- end to R31 and had 37.7 vdc. B+ was almost non existent.

3/20/2014 10:53:54 AMCV

So, it appears that the set has a dead shorted B+ bus. Try disconnecting the feed wire that runs from the anode of the two filter electrolytic caps to the rest of the B+ bus, and power up the set with the wire left disconnected (leave the connection from the rectifier heater pin to the caps' anodes). Measure the voltage at the caps' common anode connection. It should now be over 300 VDC.

Next, check the B- voltage: it should have risen to around -14 volts at the speaker end of R31.

If B+ and B- now look OK, you will need to trace out the B+ bus to find where the short is.

A likely cause of a B+ short is a shorted RF bypass cap hanging between it and chassis ground. Part #20 is a prime suspect. You can just lift one end of this cap and see if the B+ voltage pops up- the cap is just there for stray RF suppression.

If the B+ voltage at the rectifier's heater pins (cap anodes) doesn't come up with the rest of the B+ disconnected, you may have a shorted electrolytic cap- the one that connects the B+ node to ground, not the other one.

3/20/2014 9:01:57 PMjoecyphers

:So, it appears that the set has a dead shorted B+ bus. Try disconnecting the feed wire that runs from the anode of the two filter electrolytic caps to the rest of the B+ bus, and power up the set with the wire left disconnected (leave the connection from the rectifier heater pin to the caps' anodes). Measure the voltage at the caps' common anode connection. It should now be over 300 VDC.
:
:Next, check the B- voltage: it should have risen to around -14 volts at the speaker end of R31.
:
:If B+ and B- now look OK, you will need to trace out the B+ bus to find where the short is.
:
:A likely cause of a B+ short is a shorted RF bypass cap hanging between it and chassis ground. Part #20 is a prime suspect. You can just lift one end of this cap and see if the B+ voltage pops up- the cap is just there for stray RF suppression.
:
:If the B+ voltage at the rectifier's heater pins (cap anodes) doesn't come up with the rest of the B+ disconnected, you may have a shorted electrolytic cap- the one that connects the B+ node to ground, not the other one.
:
Well, I just got finished. I decided to go with your recommendation of the suspect 20 .1 mfd cap on the B + buss. And low and be hold it tested shorted to chasis. I removed the cap and installed a new one, then verified my B+ voltage. Got 278 vdc. Hooked my antenna and had music to my ears! Playing great. Thanks for all the help and patience with me. I have learned a great deal from all of your info and support.

3/21/2014 11:20:14 AMCV

I can't begin to tell you how many tube radios I've repaired with shorted B+ bypass caps. When these happen in Euro radios it's not so bad, since most Euro sets are internally fused and the fuse will usually prevent major set damage. Not so for most older USA sets- if B+ shorts, you can count on major damage occurring when an overcurrented part finally gives up the ghost and burns out. It's not so much of a fire-safety issue as a where-the-heck-do-I-find-a-replacement-70-year-old-power-transformer? issue.

3/21/2014 12:21:04 PMClifton

CV,

In the 'old' days in radio school the teachers taught us to "Ohm" out the power supply before replacing the fuse and turning on the radio. I still do that in restoration of old radios.

Clifton

:I can't begin to tell you how many tube radios I've repaired with shorted B+ bypass caps. When these happen in Euro radios it's not so bad, since most Euro sets are internally fused and the fuse will usually prevent major set damage. Not so for most older USA sets- if B+ shorts, you can count on major damage occurring when an overcurrented part finally gives up the ghost and burns out. It's not so much of a fire-safety issue as a where-the-heck-do-I-find-a-replacement-70-year-old-power-transformer? issue.
:

3/20/2014 8:30:48 PMClifton

CV,

With -3 volts specified on the cathode of the 6SQ7 I can not see any way that R19 could be 400K Ohms. If it was there would be a fairly high voltage on the cathode instead of the -3 specified in the data.

Clifton

3/21/2014 9:30:54 AMCV

:"CV,
:
:With -3 volts specified on the cathode of the 6SQ7 I can not see any way that R19 could be 400K Ohms. If it was there would be a fairly high voltage on the cathode instead of the -3 specified in the data.
:
:Clifton"
:
Yep, the schematic was wrong. Correct value for the bottom resistor in the divider pair was around 50 ohms, not 400K. Not the only problem with the schematic- at least one resistor appears on the drawing but is not called out in the parts list.

My general operating rule is "trust the documentation"- since it is far more likely to be correct than not. Except when it isn't. ;<)