Morning Gents

Thanks for the replies. Will now apply this advice to the job. Yes Robbbert, R4 was tested to chassis ground. Fred, unsure which capacitor you are speaking of. We have replaced all the papers and all the electrolytics. There is what appears to be a capacitor near the coils on the cct diagram but it is not referred to in the list of components, and I can not locate it physically, presuming it to be inside the cans??

Anyway, I will take this advice and revisit the job today.

Your concern for the tubes under incorrect voltage is also noted Robbbert...thanks for that. We will test more sparingly.

Aaron

A AGC bypass is usually about 0.05 μF and near a coil end to ground.
I seem to agree with Marc just replace any capacitor if there is any doubt.
Once old caps are removed you can short them out with a clip lead, then remove the clip lead and hook a voltmeter across the leads.
Every now and then you will find a capacitor that generates voltage into the DMM!
That wont effect a capacitor in a low impedance circuit but in a megohm isolated AGC circuit it's a different world.
Fred.

Morning Fred Lever et al

I am unsure to which capacitor you are referring. We changed all of the listed ones (paper and electrolytic). I see on the diagram some (what look like) capacitor symbols off the grid of the 6A7 to the switch, but where are they physically? Inside the cans? I don't see any other caps. I'm loathe to open the cans for all the reasons Marc pointed out, but if unavoidable we will do it.

We did do some continuity checks yesterday, and there were some interesting results that might lend themselves to capacitor failure, if only we could find it.

Interestingly, we also decided to do the plate voltage tests recommended on the schematic with wave change switch in SW and off station tuning. This is what we found.

With primary supply voltage of 237V

80 - 380V nominal - 390V measured
42 - 225V nominal - 235V measured
75 - 75V nominal - 121V measured
6D6 - 230V nominal - 248V measured
6A7 - 215V nominal - 230V measured
6D6 - 23V nominal - 254V measured

The grid voltage remains +1V and drops to .05V with finger close to cap of the 6A7 tube, with the receiver coming to life with lots of audio. Are these deltas in nominal voltage concerning?

Regards
Aaron

6A7 is listed by RCA as a horror tube and 6D6 is not much better. You should not be able to get variance putting you hand etc. near it as it should have a tin can over it like mine. If there are no shields, as in the photo of mine, which has them; It will be about as stable as a bucket of weeping gelignite.

Beware: If its a Mica and not on the parts list and on a coil, leave well alone. The Gang and its coils are a matched set and calibrated in manufacture as one. That is why its a "coil set". Those caps are to get the coils & gang to "track" properly and messing with them and those in IF's without good cause, can have serious ramifications.

Marc

Re post 18.
voltages.

6D6 23 nominal 254?? measured.

Something is wrong there!

Fred.

Yes Fred 231 volts over what it should be ! Is there a typo?

There is a typo, by Aaron. The schematic correctly says 230.

Yep, sorry guys. 230v is nominal. It is reading 254V