[skyscraper]

I found some time today to get started on another restoration. I looked at the 70, but it's missing a coil and the speaker is ruined, so I wanted to get some more practice on something easier first.

I've started on the 87. The underside of the chassis is immaculate, I don't think it has ever been messed with, I want it to look that way when I'm done. So far the ww resistor, coils and chokes ohm correctly, and I've tested about half the power transformer's windings.

The audio phase inverting transformer measures 2.5k on one leg of the secondary, and 70k on the other. I'm sure that isn't right, what options do I have for fixing or replacing it?

Also, the power switch is broken. I think it uses the same sort of switch on the 70, looks like layered wafers. Does anyone have one they could sell?


Thanks!
Will

[TA Forbes]

This is a common problem in early Philco sets.

Below is an article from this web site, it describes how to bypass these transformers. I have done this in a model 87 and a model 95 and it works VERY well!!

http://www.philcoradio.com/tech/audio.htm

[skyscraper]

Thanks TA! That's very helpful, although I suppose my plans of originality aren't going to entirely work out. Oh well.

I've gone over all the coils carefully now. Power and chokes are good, as well as all the RFs. The field seems to sometimes be opened and sometimes closed, I think there's a bad connection on the lug, but that could end up being another big problem.

There are four bad windings... The secondary of the first af transformer part 22 is bad, as are the primary and one secondary of the phase inverter part 25 and one primary of the output part 36. The latter two open windings are on the same phase of the push-pull amp. I wonder if it's possible that a the grid wire of that 45 somehow fell and hit the plate. That would short B+ to ground through those two transformers and open them.

Using the methods on that page, I can (I think) eliminate parts 22 and 25, then I'll just need a new PP output. How is the part 24 coil dealt with in making those modifications?


Schematic: http://philcoradio.com/tech/images/87.jpg

[morzh]

Question to both Tom and Sky:

From the article it seems like if you have an open half of secondary in push-pull inverter you cannot 1) use the transformer (you need a good secondary to invert) and 2) you cannot simply throw out the transformer as the third mod by Ron (transformerless) is intended for SE arrangement.
There is a way to do inversion without transformer but it relied upon the tubes other than triodes in the output or using the driving tube from both ends - driving one tube from Cathode (follower - no inversion) and Anode (common cathode - phase inversion) and this becomes more extensive mod than I'd like.

So how did you use the arrangements in this article (if you did)?

[skyscraper]

I couldn't use them for that reason. It's simple to eliminate the first, single ended af transformer that's bad on mine. The PP inverter is useless, I think.

I've set the whole thing aside for now. Hadn't thought of using the 26 as a phase inverter, but I agree that would be a bit too major surgery. Like I said before, mine is completely untouched after almost 80 years, I don't think it's worth changing.

Are the specs for those transformers available? Are there any modern replacements?

[morzh]

the trnsformer is described in Chuck's website.
The ratio is 1:4.
The primary is about 1.4k, one of the output halves meaures at 3.2k.

[Radioroslyn]

Hey Will
The output transformer you are looking for will have a 7-8k with a ct. That's 3.5 to 4k per tube. Wattage 250v x 70ma= 17.5 watts. 20w unit is fine. I've been told that the voice coil is very low impedance like 1.5 ohms. Most universal jobs will start at 4 ohms.
GL on getting the Might 87 singing again!!
Terry

[TA Forbes]

I used the mod in a model 87 and a model 95, and it worked fine.