Help Please! 38-116 Troubles
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Joined: Jun 2011
City: Lexington, N.C. 27292
I have 38-116 that I hae recapped, rebuilt all the bakelite caps, checked and replaced most of the resisitors on the B+ side, and checked all tubes. The radio humms faintly and I have approximately 30% less voltage across the board on all pins, of the higher voltage tubes such as 6l6's, 6j5 driver, 6r71st audio etc. If I pull one of the 6l6's I then get 80-85% of the volyage back, I get static theru the speaker which I can adjust tone and volume. I did not worry about an actual audio signal yet. Would it be safe to say I have a bad/weak transformer? If not any suggestions on where a short may be that I am missing? Plate ac voltage at pins 3and 5 of the 5x4 are 381 and 385v.
New input transformr and speaker coil/ output transformer check good along witht he 75 ohm and 400 ohm chokes.
Bob
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I think I'd take a look at the grid bias on the 6L6's Should be about-22v or better.
Terry
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
Terry
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City: Evanston, IL
That's a good start. With insufficient grid bias, those output tube are drawing way too much current from the B+ source and pulling it lower than the expected nominal values.
Chuck
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City: Lexington, N.C. 27292
If I did this correctly....I measured voltage at pin #5 of the 6l6's and got a voltage reading of -.115 on both tubes.
I am now wondering if I did the change back to the 6r7 from the 6sr7 correctly. If someone has a copy of that tube sub and what needs to be changed in the circuitry would you please scan and send to me? bmarcis@netzero.net.
If I have no grid voltage(pretty much 0 voltage) what would be some of the obvious causes? I've been thru all resistors except those on the sub chassis.
Any suggestions, instructions are welcome or if someone lives near Charlotte, or Asheville, N.C. and is willing to spend an afternoon to work with me, I'd love to learn more about what I'm working on. I am willing to pay for your time to teach and or fix.
Bob
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Ok, Next check dc voltage at the junction of resistors 134 and 135. With the set on, - lead to chassis and + to the junction of the resistors. What do you get?
Then measure the resistance with the set off from the junction of 134 and 135 to pin 5 of one of the 6L6 tubes and then the other 6L6. What do you get?
Terry
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
Terry
Posts: 53
Threads: 12
Joined: Jun 2011
City: Lexington, N.C. 27292
Thank you very much in advance for this help Terry.
I have the code 125 chassis so I compared the schematics and I need to do the check at resistors #115&116 which I will do this evening and post my findings
Bob
Posts: 53
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Joined: Jun 2011
City: Lexington, N.C. 27292
Good Evening,
I ran the tests and found out while looking for resistors #115 &116 that 115 was missing. After finding that issue I think resistor 111 is missing also but If it goes whre I think it should it makes no sense. Anyhow putting resisitor 115 in the radio fixed the issue of having to remove the one 6l6. Now I just have the light static but still no signal. The voltage reading you asked for is -25.4 and the resistance to the 6l6's are 11510 and 11860ohms.
I checked voltage readings on the 6j5 driver, pin 3 should have 260 and I have 283.
the 6r7g ist audio should have 70v at pin 3 and I have 46v. The 1st and 2nd IF 6K7's should have 250,90and -3.5 and actual readings are 285,80,and -3.15 (pins 3,4,5,)
I still question my 6r7 change back, if someone would please double check the schematic I have pin #3 wire going to the potentiometer, and one going to a 50,000ohm resistor by the 6j5 driver, 4 &5 are grounds, 7 heater and #8 is the tube cap(grid plate) going to the tube top at a junction point and also going thru a 1 meg resistor that goes to a junction terminal(not grounded) and goes nowhere from there. Then a ground wire and a.1 condenser going to ground. I think the schematic shows from the 1meg resistor it should continue back to resistor 110 but if I connect that I lose all static and the radio goes back to the low hum.
I also checked votages at the 6a8 oscillator tube and they were good within 10 volts.
I could not check the voltage on the 6a8 mixer and the 6k7 rf with my meter. I'll have to borrow one with long leads to check those.
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City: Roslyn Pa
What I was thinking was that the driver transformer was open. Seems the be OK. Do you have -25 on pin 5 of both 6L6's?
111 maybe your problem with the 6R7 circuit. 111 controls the grid bias for that tube. Without that resistor the bias volt is going to be too high.
If you touch the grid cap in should make a loud buzz in the speaker. Does it?
Terry
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
Terry
Posts: 53
Threads: 12
Joined: Jun 2011
City: Lexington, N.C. 27292
OK, I found #111, not where it should be and way out of spec, like 220ohm instead of 25,000 so I changed it out and put it back by the bakelite condenser.
I do have feedback when I touch the top of the 6r7 and I went one step further, the 1 meg resistor in the tube cap circuit that wnet nowhere, well it should go to resistor #111, I did that and my pin #3 voltage on the 6r7 went from 45 volts to 89 volts(spec is 70v) and it did not kill the static like before. Should I be concerned now that the voltage is 10 - 20 volts higher everywhere if the negative bias is pretty much at spec?
Grid voltage still holding at -25.5 by the 6l6's
Static can be controlled by the volume and treble/selectivity but bass compensation has little to no effect. We are making great progress!!
The 20 ohm resistor (your#133) has seen a lot of heat at one point, (all the paint is burned off still checks decent 24ohms), I have a 33ohm would it hurt to put that big of one in?
And yes -25.4 on both 6l6's, voltage on the plate of both tubes are now 300v and at pin 4 they are at 312volts.
Bob
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City: Roslyn Pa
I do have feedback when I touch the top of the 6r7 and I went one step further, the 1 meg resistor in the tube cap circuit that wnet nowhere, well it should go to resistor #111, I did that and my pin #3 voltage on the 6r7 went from 45 volts to 89 volts(spec is 70v) and it did not kill the static like before. Should I be concerned now that the voltage is 10 - 20 volts higher everywhere if the negative bias is pretty much at spec?
I'm not too concerned about that. Keep in mind that the line voltage is a bit higher than it was back in '38. If it bugs you just go down the string of resistors that feed the b+ to the 6R7 till you find the one that has drifted high in resistance and that is going to fix that problem.
If you want to give the audio stages a test you can connect an audio source to the grid cap of the 6R7and chassis ground. At that point there is no volume control so keep your source level down low lest it will blast you out of the room.
Something else to have a look at is there is a muting switch that is triggered when you push in the arm that is next to the tuning knob. Double check that it's isn't muting the audio from the detector.
On the 20 ohm resistor I'd put a new one in at some point mark it down for replacement and order a new one with a high wattage rating.
Terry
You might think I'm smart but not really, I've got a 37-116 that I fixed up back in the '80's. It need a good going over again.
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
Terry
Posts: 53
Threads: 12
Joined: Jun 2011
City: Lexington, N.C. 27292
terry, you make me look stupid!! please take that as a compliment.
Tonight I dug into the tube voltages on the sub chassis and found o voltage at the plate of the 6a8 mixer and the 6k7 rf tube. I traced the lack of voltage to resistor #47 a 1000ohm that was open. I replaced it and now I have voltage to those tubes, that fixed the higher b+ voltage readings, now I'm about 10 to 15 volts less on the plates. Unfortunately that killed all my static now I 'm back to the low humm. I disconnected the wire from the 1meg ohm resistor to the #111 resisitor (tube top wire circuit 6r7) for now because it did not do anything but raise the plate voltage from 46 to 89 volts.
Now the voltage reading at resistors 115 &116 (134&135) is -15.4 instead of -25.4. I s that better or too low?
I still have feedback when touching the 6r7 top.
Is my next step to trace the plate wire of the 6r7 and try to find out why I'm 25volts low? There was a ground wire running to pin 3(plate) and I moved it to pin 8, did I look at the schematic correctly or should I move back to pin 3?
If I tack on a phono jack off the center tap of the volume control and it works, does that tell me my 6r7 circuit is working properly?
Bob
Posts: 7,212
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Joined: Dec 2009
City: Roslyn Pa
90 the 1M resistor is feed for the grid bias voltage to the 6R7. I'm thinking that you should have about -3v at it and the junction of 110. Is 90 good? Did you replace the electrolytic caps around 110 and 111.
What's happening is without 90 in place there is no bias on the grid of the 6R7, that cause the tube to draw more plate current. That causes more of a voltage drop across tube and lowers the plate voltage. With 90 in place less plate current higher voltage.
Yes you can tack in a jack at the wiper of the volume control and it will control the volume.
It's better to have more - voltage cause it will cause the tube to draw less current. That's why your plate voltage went up when you got the proper bias voltage on the 6L6's. They can haul down a lot of current like 100ma each.
The plate of the 6R7 should NOT be grounded!
Terry
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
Terry
Posts: 53
Threads: 12
Joined: Jun 2011
City: Lexington, N.C. 27292
#90 checked good but I'll pop a new one in place. After I posted last night I went back to work on it and found that I accidently knocked off the grid wire of the 6k7avc tube. Reinstalled that and the radio started to play, not well but play. Amount of volume is low and I can only receive stations from 1500 to around 850-900 on the band, anything lower is just squeals and squallers(if thats a word).
I'll reattach the grid circiut(1meg to 111) of the 6r7 since it will not hurt anything. I'll double check the bias but if I remember correctly it was at -3.5volts on that tube and then I will check again at the 6l6's and at the resistors(115,116).
We are getting closer!!!
Bob
Posts: 7,212
Threads: 266
Joined: Dec 2009
City: Roslyn Pa
Great! we're gettin to the place where you are going to need to get a signal generator.
Terry
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
Terry
Posts: 53
Threads: 12
Joined: Jun 2011
City: Lexington, N.C. 27292
I replaced the 1 meg resistor and re-connected the cap wire circuit on the 6r7 1st audio tube. I have 89 volts at pin 3 on the 6r7. The 1st & 2nd IF tubes(6k7's) 247 volts pin 3, 84volts pin4, and -4.5 pin 5.
The voltage at resistors is -15.4
I cleaned the tuning condesor and blew the dust out of the IF coils but it did not help the squeal. I will have to see if I can get my hands on a signal tracer. Ebay maybe?
Bob
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