[Bob Andersen]

Hello.  I'm taking a break from home improvement and TV projects to dive into a classic Philco 90.
The basement workshop is undergoing renovations after a rat infestation and water damage was discovered.  So I'm working in the backup workshop / guest bedroom.

I acquired this heavily modified 90 about 8 years ago.  Apparently, the original owner was an engineering whiz and really sooped it up.
Many of the tubes were replaced with a more modern equivalent.  The original 27 detector with an octal 6H6 for example.  Even an eye tube was added!
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Yikes! That must have been so much work to modify. The front end looks mostly intact but from the detector down, it's been rebuilt.
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Rather than try to make sense of all that, I track down a nearly untouched chassis with single 47. It came with a full set of correct knobs, but no tubes.

I discovered three paper caps had been tacked in across some of the bakelite blocks. I believe everything else is original
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[Ron Ramirez]

Great to hear from you, Bob. How have you been? Looking forward to reading more about this resto.

[morzh]

Speaker is not original, which is obvious. Looks to be a PM one. Might sound better than the original.
If this is 1x47 output, it has a detector-rectifier that makes it sound way better than the 1st 2x45 version with the plate detector.

[Ron Ramirez]

It does not have the NORMAL - MAX. switch yet it has a four gang tuning condenser, so it was originally a "mid" chassis with 1-47 output.

Yes, not only is the speaker wrong, but the rear arch is also missing from the cabinet.

All things which I am sure Bob can take care of.

[klondike98]

Yes this will be fun to watch!

[Bob Andersen]

Thank you for the responses. I've been doing well, but maintaining our 106 year old house has been taking up a lot of my free time.

Yes, the speaker is not original and it does not have the prop 4-pin plug to fit into the unmodified chassis.

With that in mind, I have been on the hunt for an original. I thought I stuck gold with this one a few years ago and carefully stored it away.
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Well, I finally got around to doing some basic resistance checks and discovered that it has a center tapped primary. That would indicate it is for the PP 45 or PP 47 version I believe ?

Now I'm thinking it might work OK if I use the outer primary windings and ignore the center tap. However, I've read that if the core is not gapped properly for single ended use it could saturate and distort. We shall see.

Meanwhile, I tacked in a new power cord and clipped out the old AC line bypass caps. A power up with no 80 tube installed was successful All the other tubes lit up and I have around 700 VAC across the B+ secondary.

[Bob Andersen]

Next up, the old electrolytics came out.  One had leaked out all it's electrolyte and there was a large crusty mess underneath.
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Here's a look at my test setup.  I threw a 4K resistor in place of the speaker field coil.  I have a Hammond 125D for the output transformer feeding a modern 8 ohm PM speaker.  The 125D is design for PP output, but it's all I had handy.  I left the primary center tap disconnected.

I also managed to get an order in to Renovated Radios for some rubber tuner mounts before he closes shop.

It's getting a little cramped on my little Ikea table but I'll manage 
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I fired it up again while monitoring B+ and got a crackle out of the speaker.  B+ is only about 205.  I think it should be closer to 275?

A bit of signal injection testing confirmed that the audio portion is working.  I get a clear tone out of the speaker and it does respond to the volume control.  No reception yet though.

Not a bad start I'd say.

[Arran]

The replacement speaker, with the original chassis, is a Utah speaker of similar vintage to the original, early to mid 1930s, it is a field coil speaker, and ironically used in other makes and models of cathedrals, and tombstones. I remember mentioning to Bob on Y.T., after he got this set that not having the Philco pie pan style speaker wasn't a total loss as the replacement probably sounded better. I agree, model 90 chassis are common enough that it isn't worth dealing with a badly hacked up one, especially the mid production ones with the single ended #47, looks like the "Hamster" added a 6.3 volt filament transformer to power the octals he added. The pie pan replacement he has looks like it was paired with a model 20 chassis, those have that big stud, and nut, holding the pole piece in place, but I think the output transformer should still work with P-P #47s or #47s.
Regards
Arran

[Bob Andersen]

Yes, that is a badly hacked in 6.3 filament transformer.  At least I'm able to salvage some good parts from that chassis.  I also realize the original speaker may not sound that great.  Even so I wanted to try finding one to hear for myself.

After a bit of tube swapping and aligning by ear, I am getting some reception.  AM 820 in particular is booming.  Others are quite faint.  That has me thinking the AVC isn't working properly.

All in all, I'm quite impressed that just my replacing two caps, adding a line cord and clipping out the AC bypass got this radio working.  Oh also the old repairs where three paper caps were added.

I'm glad I took out the bypass.  The caps has swollen and only the tar/wax inside was holding the bakelite together.
I'll salvage one from the hacked up chassis if it's still present.
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[Steve Davis]

The line caps are probably both shorted, if only one is shorted, it will put line voltage on the chassis, if both are shorted, in becomes a short across the line and will blow the bakelite block apart. When you replace the block, those week stations will come in stronger, as those caps provide the ground path for the antenna circuit.

Steve

[Bob Andersen]

Thank you for the information. I was assuming they were just for line filtering. Makes perfect sense given that I haven't hooked up the antenna ground to anything.

I also appears that the volume control has been replaced on both chassis. I believe it should be 500K but am not sure about the wattage or taper. I just might have a good old type "J" pot that will fit. Those are excellent quality.

[Ron Ramirez]

Correct. On the 1-47 and 2-47 chassis, the volume control is 500K, audio taper.

[morzh]

A centertapped transformer might be ether for P-P 47 or 45. Or it can be for Philco 20.

In either case it will work for SE if only ends of the primary are used (and I heard people did exactly that) but at least in theory it is not an adequate thing to do as a P-P xfmr usually has smaller core as it is not afraid of saturation. A SE xfmr usually is a larger one, to deal with DC saturation. So the P-P one might get somewhat saturated which might distort the output and lose the power, plus your primary inductance and all will be screwed up.

[Steve Davis]

In the case of the Philco 90s, I'm pretty sure the cores of the P-P and the SE transformers are the same.

Bob, the 70/90 speakers used an improved spider and would sound much better than the 20 speaker you have.

Steve

[morzh]

Many people resorted to reconing or re-spidering (in case the paper cone is in good shape, which is rarely the case in K speakers, as even if the paper is not torn, it is kinda brittle) 20 K speakers.
45 RPM one is very good; in fact it is used by many early Philcos and in case of say 37-116 or 38-690 it provides for a great sound.