Posts: 31
Threads: 12
Joined: Mar 2008
Hello.
I'm restoring a 37-620 Chairside.
It belongs to the father of a friend of mine. Supposedly it worked within the last 20 years or so.
I've replaced all the capacitors (a total PITA due to the sub-chassis) and it will now power up and produce a small hum. But that is all...
Adjusting the tone control appears to filter the hum. If I touch (or even come close to touching) the grid cap of detector 2 (the 6Q7G) I get a very very loud 60hz hum in the speaker. If I tune my little grundig M400 am radio to 1070khz and tune the philco to 625ish khz I can hear the oscillator in the grundig speaker. The tuning capacitor moves the oscillator around as I would expect (you can hear it on the grundig and also see it on the scope if I look at the grid cap on that tube). The frequency changes as i would expect when moving between the three bands.
Apart from that I get nothing. The volume control doesn't do anything that I can tell. There is no fuzz anywhere on any of the bands.
The "socket voltages" listed in the rider pages mostly line up. However the voltages labeled 70V show up more around 90V. I'm unsure what those actually are, though?
All of the tubes check out on my (admittedly crappy Radio City Products 802N) tube tester.
So now I'm stumped.
This is the second radio I have attempted restoration on. My Philco 112 Lowboy pretty much worked great from the moment I powered it up after the re-capping. Apparently I got lucky on that one.
Does anyone have any hints on what I should look at next?
Posts: 7,214
Threads: 266
Joined: Dec 2009
City: Roslyn Pa
Well think your not passing any signal though your IF stage. The easiest way to check the IF is to use a signal generator. If you don't have one the other things to check would be the resistance across the IF transformers windings to make sure you don't have an open one, check the plate, cathode, and screen grid voltage at the 6K7 IF tube,and check the plate voltage on the 6A8 mixer. I have the same set here. Haven't gotten round to working on it. It needs some major cabinet work , top veneer is shot. Chassis is good and clean but someone cleaned the dial so got to order a new one for 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: 1,394
Threads: 69
Joined: Oct 2007
City: Linn Creek, MO
Audin,
You might try bypassing the RF stage by conecting the antenna to the grid of the 6A8.
Steve
M R Radios C M Tubes
Posts: 31
Threads: 12
Joined: Mar 2008
I've checked both IF transformers with an ohm meter and they seem fine.
Connecting the antenna (both by way of a random long piece of wire and by way of a jumper from the grid cap wire from the 6k7) to the 6a8 doesn't result in any change. But i'm not sure what should happen?
In terms of voltages:
I'm not entirely sure how to go about checking them. Going from chassis ground to the points labeled in the "socket voltage" section of the rider results in the following for the 6a8:
"P": 265ish ... should be 245
"S": 63ish ... should be 70
"OSC R": 120ish ... should be 150
and for the 6k7:
"S": 62ish ... should be 70
"can't read it": 265ish ... should be 245
the filament voltages for both are normal.
I do have a signal generator if that would help. It is a modern new fangled digital thing (telulex sg-100/a). I'm not sure if it is useful in this case.
Posts: 7,214
Threads: 266
Joined: Dec 2009
City: Roslyn Pa
With the signal generator adjust it so you have a modulated signal at full output at 470KC or for you modern guys 470KHZ. Take the output from the generator though a small cap, size doesn't matter here anything from 100pf to .1@ 600v is fine and connect it to the plate of the 6K7. But careful not to let the test lead touch anything as it will have HV on it when the set is on. If that stage (detector) is working you will hear the modulated tone from the generator in the speaker of the set with the volume set up a bit.
What is the cathode voltage on the 6K7? and is the filament lit?
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: 7,214
Threads: 266
Joined: Dec 2009
City: Roslyn Pa
One other quick question do you have a signal tracer? If you feel a bit intimidated by the generator we could use a tracer to do some trouble shootin'
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: 31
Threads: 12
Joined: Mar 2008
Victory!
The suggestion to look at the cathode voltage on the 6k7 made me notice a resistor+capacitor combination that I had replaced. The schematic read 400 ohms. the resistor read 400k ohms. Clearly I grabbed the wrong one. Swapping that resistor out for a 400 ohm got me very very quiet audio! (and took the cathode voltage from ~15v to ~1.5v).
However the audio was indeed quite quiet and the volume control didn't work. A couple hours of head scratching and wire tracing led me to the discovery that the wiper on the volume was not connected to the grid on the 6q7 like the schematic indicated! It was connected to one of the diode plates in that tube instead. Moving it to the right place resulted in a working volume control and good loud audio!
I've listened to it for a couple hours now and it seems to be working reasonably well. The dial scale is not accurate. I guess i'll have to figure out how to align it. All three bands work, though sometimes when switching between them nothing will come in (not even static) until power cycling the whole radio. No idea what that means.
One worry I have is that the output tube (6f6) runs very hot. It is a metal tube so I can't tell anything visually. The voltages match the rider. So maybe that one just runs hot?
Thanks for everyone's help! This forum is a great resource!
Posts: 1,394
Threads: 69
Joined: Oct 2007
City: Linn Creek, MO
Great, glad you found the problems. It would be normal for a 6F6 to run very hot. You wouldn't want to touch one after it has been on for a while.
Steve
M R Radios C M Tubes
Posts: 7,214
Threads: 266
Joined: Dec 2009
City: Roslyn Pa
Congrats!!
I was thinking that the plate voltage on the IF amp tube was a bit high which could indicate an open cathode circuit. With the cathode open or too high or a resistance the 6K7 won't draw much plate current as it should and won't amplify. It will just sit there and look pretty and fool you.
As for your 6F6 Steve in correct I does run pretty hot. For laughs take your voltmeter and connect the - lead to pin 8 and the + lead to pin 5 of the 6F6. What you should see is about -10 or -15vdc (grid bias) If not you've got a problem in the bias side of the power supply or a bad coupling condenser.
The SW problem maybe a dirty bandswitch. Try a bit of DeOxit to clean it up. Apply sparingly. Caution!! After cleaning be sure the switch is totally dry before applying power to the set. If not your bandswitch wafer could burst in to flames. Don't ask me how I know. It's very unpleasant replacing a bandswitch!
Thank for the update
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
