[bobclausen]

http://pacifictv.ca/schematics/philco37-3630manual.pdf

I have a Canadian Philco 3630. I changed all the caps and all the out of range resistors and it was working. This is a three section chassis and I took it apart to access two of the caps and I ordered new chassis supports. I thought it went back together very well but now it is motorboating at a frantic pace when I turn up the volume. I am not receiving anything just dull hum from the speaker. 

The voltages for the tubes are: 6F6 273 pin 3, 286 pin 4, 6K7 RF 267 pin 3, 104 pin 4, 6A8 268 pin 3, 104 pin 4, 193 pin 6, 6K7 IF 285 pin 3, 104 pin 4 and 6Q7 238 pin 3.

All the voltages are higher than on the schematic especially those going to pin 4. Where would I begin to check or replace components to reduce the voltages or are they ok as is.

The motorboating is my main concern. I am new to this hobby and have not encountered it before. I’m a little lost as to how to proceed from here and would appreciate any suggestions from the learned members of the Phorum.

[RodB]

Motorboating sources are many, so you can start with an IF alignment, make sure the AGC is properly connected, find out if the motorboating occurs at all the tone switch settings. Usually occurs somewhere before the first AF circuit. It's a low frequency feedback. Might be because the tube shields are off or a dirty tube pin connection or grid cap. The grid cap wire for the detector tube should be inside the tube shield against the tube.

[bobclausen]

Rod

The tube shields are firmly on, grid caps are good and the 6K7 wire is inside the shield. Sorry but I don’t know what the AGC is.

I’ll check the IF alignment tomorrow.

[Arran]

It could be a grid cap lead where the wire is breaking down inside, usually at the clip end that goes over the grid cap, though this can happen at either end with the leads attached to the tuning condenser. One thing that could also happen is that two of the grid cap leads could be swapped, that can easily happen with two tubes situated next to each other. Wrong tube in wrong socket, tube will still light, but radio will not work. It sounds like the set was working until you tried changing the last few caps, so you should retrace what you did, it's very easy to make a mistake when there are two terminals next to each other.
Regards
Arran

[radiorich]

hello guys,
I agree with you Arran I have been there and it was a 1950s Philco !
Lead dressing too is another thing .
Sincerely Richard

[RodB]

Bob, AGC is Automatic Gain Control. The rf amplifier tubes are specifically designed as variable gain tubes. At the detector tube, the duo diodes generate a negative voltage of a couple volts depending on the strength of the rf signal being received. This signal is also the audio being passed on to the audio amplifier. The negative voltage is fed back to the grids of the rf amplifiers where it adds and subtracts to the gain of a tube depending on the strength of the radio signal. Strong stations reduce the gain and weak stations increase the gain. If the AGC isn't working properly then this feedback circuit could cause the motorboating. In your radio parts 44, 15 and 6 are providing the AGC. Be sure part 15, the 0.05 mfd cap, is the correct value and both resistors are within 10 percent tolerance. No cold solder joints, anywhere.

[bobclausen]

Thanks for all the help and suggestions.

I can faintly pick up the two am stations that are near me but if I turn up the volume the motorboating starts.

I will change the wires going from the tuning condenser to the grid caps. The tubes are in the right sockets and the grid caps do go to the proper tubes. Since I do not have a signal generator I tried to do a half assed alignment by ear to see if it made any difference but it didn’t. The station signals did not get any better only worse.

I will check the three parts with the AGC and change them if needed.

Thanks again.

[Arran]

I think that it should be pointed out that motorboating is basically low frequency feedback, and most of the time when I run into it it's because of an open bypass capacitor, or in the last instance a missing one because the guy who "restored" it left one out. In these old schematics it isn't called AGC, it's called AVC "automatic volume control" different terminology for the same thing. Another thing that can cause motorboating is having the primary and secondary leads of an IF transformer crossed.
Regards
Arran

[462ron]

Hi Bob, I noticed in an earlier post you mentioned the grid wire for the 6K7 was run inside the tube shield. The grid wire for the 6Q7, 1st audio amp, should be inside the tube shield. I’ve run into the same problem on a 630 chassis but the 1st audio/2nd detector was a 75 tube!

Ron

[RodB]

You're right Arron, it is called AVC in the AM sets. I just finished working on an FM set and used some residual memory where it's called AGC. Another cause of motorboating is a dead oscillator. Check the components and connections in the 6A8 oscillator circuit, see if the motorboating occurs on all bands, is the oscillator running at all, try a different tube. One of the toughest problems to have.

[radiorich]

Hello Rob,
I agree with you "One of the toughest problems to have"like the old saying goes about chasing a red herring !!

Sincerely Richard

[bobclausen]

I checked the parts Rod had suggested and they are all good. I changed the leads to the grid caps. I did notice that on the parts list #35 was a .25 cap but the schematic showed it as .05. I originally had put in a new .25 but I changed it to the .05, still no difference.

I’m not sure how to check whether the leads of the IF transformers have been crossed. I didn’t do anything to the IF cans other than took the top off to replace that grid cap wire.

I have noticed that if I put a finger on top of the 6K7 RF tube the faint stations come in better but I dont know if thats just me acting as an antennae. The motorboating is still there. I do
appreciate all the advice/ suggestions.

Ron, I typed in 6K7 but it is the 6Q7 tube where I have the wire running inside the tube shield.

Rod, I will check around the 6A8 tube and report back. Is there specific checks I should do? I’m very new to radio restoration.

[RodB]

So, if you are receiving stations then the oscillator is running. Now concentrate on finding out where the rf signal is getting blocked. Try touching the grid cap of the 6A8, simulating an antenna signal to the converter.

Have you cleaned the band switch contacts?

[bobclausen]

Rod

No but I just ordered from Deoxit D5 and F5 from Amazon. Will arrive tomorrow and I’ll clean those switches.
Nothing changes when I touch the top of the 6A8.
Thanks again. I know it like finding a needle in a haystack especially with an inexperienced pupil!

[radiorich]

Hello bob,
Good to hear that you ordered some craig products and cleaning those switches will help the set and I know what you mean about "like finding a needle in a haystack "

I remember when my Repairshop was going full steam back in the 90s a customer brought in a Jcpenny tv wow I did finally find the needle using heat and freeze spray it was a zener diode .

Good luck and we are here to aid you in your adventures of repairing old radios.

Sincerely Richard