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Philco 90 restoration - Printable Version

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RE: Philco 90 restoration - Paul Philco322 - 10-26-2021

Hang in there kid, you will run it down. Frustration when resolved equals Victory!

Paul

RE: Philco 90 restoration - Ron Ramirez - 10-26-2021

Bob

If you have not replaced any resistor, or any paper capacitor...replace it. The original components are 90 years old and well past their prime.

RE: Philco 90 restoration - Bob Andersen - 10-26-2021

Every paper cap, electrolytic, and two micas have been replaced. About half of the resistors have been replaced. They rest tested within spec.

Restuffed both metal box caps and the tone control.
[Image: https://live.staticflickr.com/65535/51631889648_fe78ae5630_c.jpg]

Also replaced the tuning cap shock mounts.
[Image: https://live.staticflickr.com/65535/51632517810_06b9bc7433_c.jpg]

RE: Philco 90 restoration - Ron Ramirez - 10-26-2021

Replace all remaining resistors.

If you have spare tubes known to be good, try replacing tubes one by one and see if that helps.

But I don't have to tell you this, you know your stuff. Icon_smile Icon_thumbup I hope it works out.

RE: Philco 90 restoration - radiorich - 10-26-2021

Hello Bob,
what a major project that you took on and while you are working on this set remember one thing it is a one of a kind right ?
Anyway I have just started reading this thread on your set and you are a braver man then me to take this on !

Sincerely Richard

RE: Philco 90 restoration - Steve Davis - 10-26-2021

Bob, does the volume drop all at once or gradually? Does the AVC voltage change with the volume drop?

Steve

RE: Philco 90 restoration - radiorich - 10-26-2021

Hello Bob,
I know transformers really have started getting pricey and Yes I also seen that Speaker and far as the audio output transformer here is another company you may look at https://edcorusa.com/

Sincerely Richard

RE: Philco 90 restoration - Bob Andersen - 10-26-2021

Steve - the volume drops all at once. I have not checked the AVC voltage yet. I will later tonight and report back.

RE: Philco 90 restoration - Bob Andersen - 10-26-2021

I just made some measurements.

With good volume

RF AVC grid voltage = -1.1
IF AVC grid voltage = -0.56

When volume drops, the voltages get less negative which should boost gain.

RF AVC grid voltage = -0.7
IF AVC grid voltage = -0.23

So it seems gain is being lost somewhere in the RF/IF front end and the detector/AVC circuitry is working properly.

RE: Philco 90 restoration - BrendaAnnD - 10-26-2021

Those are both very low values. Should be around -10 or so for a strong signal.

RE: Philco 90 restoration - radiorich - 10-26-2021

Hello Bob,
At least you are narrowing down the area !

Sincerely Richard

RE: Philco 90 restoration - Steve Davis - 10-26-2021

This sort of thing can be very frustrating! A radio (or anything else for that mater) that will not work at all, is easier to troubleshoot than one that works, but just not quite right. 

Is the time from cold turn on until signal drop about the same each time or more random? If it is the same, I would guess a failing component. If random, it could be mechanical, possibly a riveted ground connection. You might use a clip lead connected to the chassis and touch the other end to every point under the chassis that should have a ground connection. Old style Philco tuning capacitors some times develop intermittent connections on the fixed plates, tighten all the fixed plate mounting screws. Recheck the alignment, does everything still peak in the same place or does the peak move one way or the other? I hade a 70 one time with a similar intermittent, it turned out to be a trimmer on an IF. There is a crimped brass eyelet that holds the thing together, the steel plate had rusted/corroded under the crimp, causing an intermittent connection.

Steve

RE: Philco 90 restoration - radiorich - 10-26-2021

hello Steve,
Well Said and yes those are the hardest to troubleshoot !
I remember some years ago I recapped one my little RCA radios and I ended up being a Mica capacitor after the set was warming up that would go bad.
Back in the 90s when my radio and TV shop was still going strong I had a Jcpenny tv built by NEC that after warming up a Zener would chance value that was a real pain .

Sincerely Richard

RE: Philco 90 restoration - Bob Andersen - 10-27-2021

It fails consistently about 5-10 minutes from a cold start.

-2 volts is the lowest voltage I've measured and that's with respect to ground. The cathodes are about 20 volts positive with respect to ground.

So that makes the grids around -20 to -22 volts with respect to the cathodes.

Thank you for all the suggestions. I'll replace the rest of the resistors and go over the mechanical connections.

Meanwhile I had fun digging through my stash for 24 tubes to try.
[Image: https://live.staticflickr.com/65535/51632151246_3fc7f025a6_c.jpg]

Seems to take longer to warm up and get good reception using 24s versus 24As. Here's a set of NOS RCA tubes I have in the set currently.
[Image: https://live.staticflickr.com/65535/51632151056_913721b1e5_c.jpg]

RE: Philco 90 restoration - Bob Andersen - 10-27-2021

I believe I found the problem - or at least one significant problem. No plate voltage on the RF tube!
Looks like we have an open primary on the transformer between the RF and 1st detector stages.
I'll dig it out for a closer examination. I may just swap it with the one from my parts chassis.


Also, it seems the voltage chart in the upper right cannot be right.

One issue is that is shows different voltage for the RF and IF cathodes even though they are connected together.
A much bigger issue is that it shows 20 volts on the 1st AF cathode even though it's grounded!
[Image: https://live.staticflickr.com/65535/51634548059_04e97498c2_b.jpg]