Film-Tech Forum ARCHIVE: Really old theatre amp.....

This is topic Really old theatre amp..... in forum Film-Yak at Film-Tech Forum ARCHIVE.

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Posted by Jim Koehler (Member # 10546) on 08-11-2018, 09:44 AM:

Hello!....
My first post on this eclectic forum and I'm doing some research in order to obtain a schematic and any info on the following....
This unit is a PhotoPhone Type PA-36. The Model number is 4-PA-36-B. Serial number is 1019. It's a five tube affair utilizing the following: VT-24 (1st audio) VT-53 (duo-triode) Two VT-47's (output) and a VT-80 (rectifier). The correct speaker comes with the amp in it's own carrying case. It looks to be an early RCA 10" model. It is electrodynamic.
Also in the mix, which I haven't seen yet, is a PhotoPhone Pre-Grooved Disc Recorder. Type PR-14 Model 4-PR-14-B1. Two speed at 78 and 33 RPM. Serial Number 1023.

OK...With tube compliment I would say this unit is from the late 20' early 30's. I'm familiar with RCA radio chassis's from this era (I restore vintage radios as a business) and this chassis is very typical of RCA's early construction. Going thru my early Riders and RCA Redbooks, no light was shed on where I could find the schematic needed. Googling came close but no cigar.

Now, the question: Would anyone know where I could find this particular schematic? Or, are there other circuits that come close to what I'm looking for?

Appreciate the help!
Tnx and a Hat Tip...
Jim Koehler
Antique Radio Repair Service

Posted by Leo Enticknap (Member # 534) on 08-11-2018, 11:09 AM:

I'm not sure where you would find the schematic, but if the disc cutter can be restored to working condition, it would be highly sought after by a growing sub-cult of geeks who seem to be making coarsegroove records cool again. Initiatives such as The 78 Project are restoring and using these machines, and regular lacquer blanks will work with them and effectively function as dubplates for a few playings afterwards. I'm seen consumer and radio reporter field disc cutters from the 1930s and '40s, claimed to be in good working condition, go for $5k plus on Ebay, and rusty old ones that are probably only good for a few parts go for hundreds.

Posted by Jim Koehler (Member # 10546) on 08-11-2018, 12:22 PM:

Hello Leo.....

The Phonograph is the second thing on the list that will be restored also.
It seems, on the amp, a few "hands" have already been inside the chassis. The E-Caps, of the ones that were in view, were changed sometime in the past and left outboard of what I'll call a capacitor can network. What's inside the can without a schematic would be anyone's qualified guess. Even with the old E-caps changed out, we're still experiencing 60 hz. hum. The volume control can't control the level so there must be one or two more E-caps in there that must be reached. Actually, using, of all things, a crystal set and it's audio connected to the grid cap of the VT-24, I will get controllable audio out of the speaker.
...Jim

Posted by Kenneth Wuepper (Member # 1174) on 08-11-2018, 04:51 PM:

Jim,

This sounds like a great project for an EE student. It is known as "reverse engineering" and requires the student to use tube base charts and the Multi-Meter to reconstruct the diagram of what is present.

The fact that the piece of gear is in questionable operating condition will up the challenge a notch or two.

Once you have the basic schematic put together you will be able to see what the engineer had in mind. Just show resistors and capacitors without their value. This will result in a schematic without any component specifications. (Values)

Often the high value resistors become open or their resistance changes radically. Often the color coding is of little use in determining the original value.

Take the explorer approach to this one and chart your way through the design. Remember that it did work at one time.

Posted by John Eickhof (Member # 323) on 08-11-2018, 10:32 PM:

that is an old rca public address system i dont think i have a schematic but i will check rca theatre systems used the PG series model identification and components would have a MI number ie: MI-1040 sound reproducer. the pa equipment used ordinary rca component numbers, there might be a riders manual or a sams photofact around for it. good luck

Posted by Jim Koehler (Member # 10546) on 08-12-2018, 07:52 AM:

Tnx fer the responses....

Most of the circuits I've already figured out but the cap cans with a dozen or so wires running in and out of it is where I hit the proverbial brick wall. It "seems" the amplifier section is in somewhat good shape. It's the input side of the amp that's giving me the vapors.

Apart from my own library (Riders, RCA Red Books etc.) I have very little on commercial amplifiers and such. So when an amplifier like this comes across the bench, the learning curve is somewhat heightened. However, with that said, if you can find a crossover number or such, or maybe even have the print, I'll gladly pay someone for their time in obtaining it.

...Jim

Posted by Jim Koehler (Member # 10546) on 08-15-2018, 08:22 PM:

On another Forum, Ken Layton had mentioned the late Dick Prather who had a sizeable stock of RCA Amplifier schematics and such. Does anyone know what happened to his collection?

...Jim

Posted by Sam D. Chavez (Member # 1841) on 08-16-2018, 01:52 AM:

Gary Hoselton on this very site would be a good choice.
He know Prather.

Posted by Jim Koehler (Member # 10546) on 11-03-2018, 05:09 PM:

Sam....
Any way I can get in touch with Gary Hoselton?

...Jim

Posted by Mark Gulbrandsen (Member # 72) on 11-03-2018, 05:55 PM:

Dick's wife sold everything that was left to a cinematographer down in Los Angeles about a year ago. He went to Portland and loaded up everything in a truck and trailer... there was a lot! And now it's all down in Los Angeles. I tried to look for his posts with the pictures of all the stuff on Motion Picture Technology on Facebook but could not find them. When I run across the guy again I will post his name here. I don't think he intends to keep all of it, but Dick's wife wanted it all out of the house and garage.

Mark