Hello Forum,
I have been e-mailing various people with
specific questions but now that I have a common problem
I figured I'll post this for others to use.

I have a 1941 Zenith Universal Portable 5G500/5A01 that
I finished recapping. I added individual electrolytics
under the chassis for filter cap replacments.

After getting the radio to work fine I made the mistake
of showing it off to a friend who started poking at it.
Upon turning it back on I experienced humming. There
is no humming with low volume but when I increase it
I get the hum and with high volume I get motorboating.

After observation I noticed a shorted lead from a
.002 mfd cap to the center pin (chassis ground)of the
1LH4 Det-Amp tube.

The cap is connected to, I believe the grid (- - - -) ,
of the same tube so basically the grid was shorted to ground.
The other end of the cap connects to the volume control
pot. The pot also connects to the second IF transformer. The
schematic shows a trimmer base for the IF tower can.

After removing the short I still get the hum in the same way
but not as loud.
I checked 3 caps in the general area from the schematic
and they all measured OK with a cap meter. I found one
of a pair of resistors connected in series to pins 1 and
8 of the same tube to be reading high (4.7k reads 6.4k).
I don't remember it reading that high when I initially
checked it. Unless I have another short somewhere obviously
something blew.

Questions:

1. Is the out of tolerance resistor enough to cause this condition ?
2. Can a cap still be bad even though it measures OK ? Can it still leak ?
3. Is the tube bad ?
4. Is the IF can bad ?
5. If none of the above, what else can I check ?

Thanks

Dana

Hi,
The answer is "yes". All of those can contribute to the problem.

First, check the condition of the solder joints in the vicinity of the 1LH4, the filter caps you replaced, and then the 1LB4.

Test the tubes, and gently tap it while testing for shorts, and values. Loose grid wires can be a source for hum.

One place to check also, is the cap across the output, "C3". If that has dried out, it may test okay, but may have too small an amount of capacity to do what it needs to do.

The resistor is borderline, but is not likely the source of the hum, I would be inclined to replace it. It is there to "decouple" the cathode of the tube from the rest of the RF stages.

I would then check to make sure there are no open windings in the IF, and even the output transformer.

The smaller values of capacitors can be checked for leakage with a VTVM, or a high impedance DMM. The capacitor will need some time to charge; as it does so, the resistance increases. You should be able to see it charge in just a few seconds for a .1Mfd. You should also attain a resistance that is greater than 1.5 Meg. Anything less than 1 Meg should be replaced.

: Hello Forum,
: I have been e-mailing various people with
: specific questions but now that I have a common problem
: I figured I'll post this for others to use.

: I have a 1941 Zenith Universal Portable 5G500/5A01 that
: I finished recapping. I added individual electrolytics
: under the chassis for filter cap replacments.

: After getting the radio to work fine I made the mistake
: of showing it off to a friend who started poking at it.
: Upon turning it back on I experienced humming. There
: is no humming with low volume but when I increase it
: I get the hum and with high volume I get motorboating.

: After observation I noticed a shorted lead from a
: .002 mfd cap to the center pin (chassis ground)of the
: 1LH4 Det-Amp tube.

: The cap is connected to, I believe the grid (- - - -) ,
: of the same tube so basically the grid was shorted to ground.
: The other end of the cap connects to the volume control
: pot. The pot also connects to the second IF transformer. The
: schematic shows a trimmer base for the IF tower can.

: After removing the short I still get the hum in the same way
: but not as loud.
: I checked 3 caps in the general area from the schematic
: and they all measured OK with a cap meter. I found one
: of a pair of resistors connected in series to pins 1 and
: 8 of the same tube to be reading high (4.7k reads 6.4k).
: I don't remember it reading that high when I initially
: checked it. Unless I have another short somewhere obviously
: something blew.

: Questions:

: 1. Is the out of tolerance resistor enough to cause this condition ?
: 2. Can a cap still be bad even though it measures OK ? Can it still leak ?
: 3. Is the tube bad ?
: 4. Is the IF can bad ?
: 5. If none of the above, what else can I check ?

: Thanks

: Dana

I don't have a VTVM. I have 2 digital multimeters (Fluke 75 and Triplett 9015) and one little analog meter. The Triplett measures caps. I have found the smaller value caps don't produce any reaction on a meter but rather measure wide open.

The C3 cap was one of the 14 paper caps that I replaced. Can it dry out as a result of the short ? I'm thinking that something destructive happened because of this.

Hi,
The Fluke should be a high impedance. Some DMM's do not respond well, and you can see if yours is one that has too much delay in the response, switch the test prods around, and you should see some resistance, then a quick climb to what would appear as an open circuit.

None would dry out from anything but age, however, depending on the type of cap you used, there is a small chance that the voltage rating was exceeded on one or two caps, possibly even the power supply output cap.

I would look a little bit closer at the cap that is essentially across the filament of the 1LA6 converter (small possibility). Look to see if there is any bulging in any of the filter caps, I would focus on the possibility that one of those five around the RC filter are cooked, and may not show it, especially if the working voltage rating is close to 160 volts (better possibility).

I would also check R11, R12, and R9, R13, and the resistors in the RC filter circuit. These are the ones that have a value listed rather than a "R-" number. If these were overheated, their values may shift with no outward appearances of defect (better possibility).

: I don't have a VTVM. I have 2 digital multimeters (Fluke 75 and Triplett 9015) and one little analog meter. The Triplett measures caps. I have found the smaller value caps don't produce any reaction on a meter but rather measure wide open.

: The C3 cap was one of the 14 paper caps that I replaced. Can it dry out as a result of the short ? I'm thinking that something destructive happened because of this.

All the resistors measured ok and all the caps reacted appropriately to my ohm meter check. There were 2 caps (C6 and C7)that didn't move the meter at all but neither did new replacements. I measured pins 1 and 8 of the 1LH4 tube and I get around 7.5 ohms. The 2nd IF can measured ok as well.

I guess I'll start measuring voltages from the tube sockets to AC line at changeover switch according to the 2nd page of the schematic. Which terminal on the changeover switch should I use ?

Dana

Hi,
All of the votage measurements are referenced from the socket terminal, but if you have the chassis out of the radio, you can use the switch terminals on the volume control.

All of those tubes have an internal shield, and should not be overlooked as a possible source of trouble. If you have replacements, and you have tried the plug as mentioned next, try swapping the tubes out one at a time.

One thing to try first, if you have not tried it already, is to flip the plug over in the wall. Many of the old transformerless radios would hum when plugged in one way, and would not hum when plugged in the other way. I should have thought this as one possibility a bit sooner.

Also, have any flourescent lights been turned on (that were not on when you re-capped), or put into the area since the time it was re-capped (like a grow light)?

The other possibility is still lead dress, there might be some componenet, or wire that is routed incorrectly, perhaps too near to the tube heater wires, or even a loose rivit that is used to tie a terminal to ground. There is a small chance that a bypass cap was tied to the wrong terminal too. Cold solder joints are a possibility, especially when the onset is about the same as what hapened with yours.

All of this, covering up to what has been looked at already, and what is mentioned above, would basically cover over 99% of the cases of an unwanted hum.

At this point it has to be something simple. Any large repair runs the risk of something minor causing major problems, I have had my share of them. In a couple of instances I took them down to the bare chassis and started over from square one because of the time that it had taken to get numerous things ruled out. Sometimes simple errors will be missed, or just remain because they seemed to look right somehow.

: All the resistors measured ok and all the caps reacted appropriately to my ohm meter check. There were 2 caps (C6 and C7)that didn't move the meter at all but neither did new replacements. I measured pins 1 and 8 of the 1LH4 tube and I get around 7.5 ohms. The 2nd IF can measured ok as well.

: I guess I'll start measuring voltages from the tube sockets to AC line at changeover switch according to the 2nd page of the schematic. Which terminal on the changeover switch should I use ?

: Dana

The radio was plugged in the same way in the socket. On the schematic it lists various tube voltages. It says to measure the voltages from socket terminals to A.C. line at changeover switch. I understand where the positive lead on the meter goes but where does the negative lead go ?

Hi,

Since you are going to be looking positive voltages, the reference point for the "-" lead will be the location on the switch where the yellow wire from the battery plug will attach to; through the resistor. This should be pin 7 of the 117Z6. All voltages listed on the page will be referenced with this point as the "-" (negative lead of the meter). If you are using one of the Dmm's, your voltages will likely be higher than expected due to the fact the DMM's have a higher impedance than what was used to establish the reference voltages originally, and will represent less of a load..

: The radio was plugged in the same way in the socket. On the schematic it lists various tube voltages. It says to measure the voltages from socket terminals to A.C. line at changeover switch. I understand where the positive lead on the meter goes but where does the negative lead go ?