Army Signal Corps RP-104/UIH-2 Schematic Request

[Martinihenry]

I recently came into one of these olive-drab "Public Address and Sound Reproducer" amplifiers, but it needs service. Unfortunately, I cannot find a schematic for the blasted thing anywhere.

One of the metal-can capacitors blew, and took out a couple of carbon comp resistors, and baked a couple more to the point where their color-code value is just not legible.

There are copies of service manual online, yes, but during the scanning process, they neglected to fold out the schematics, and wiring diagram foldouts, so you only see a small slice of them...not enough to glean the information I need to repair the amplifier. They didn't even scan the parts list for crying out loud.

OR....if anybody has one of these, could they please, please, PLEASE take a picture of the small circuit board containing 4 large carbon comp resistors so that I may determine their values? I've attached a photo of the small circuit board in question. Believe me, the values are not legible, despite how it looks in this photo.

The cap that blew is the 50µF/50V cathode bypass cap bolted to the chassis, just above and to the right of the resistor circuit board. I gutted it and restuffed it with a modern cap (after scraping and cleaning up the potted tar that blasted all of the chassis). Oddly, this didn't appear to be an oil filled cap, it had a gummy black tar inside, like old Philco bakelite block caps.

Thanks!

 

[Tinkerbelle]

That looks like a well built unit.

I can't help with the schematic but it looks like the 4 resistors are two pairs, so perhaps only two values need to be determined. Where in the circuit are these - part of the bypass? When you say that the values are not legible, I assume this rules out that you have lifted one side and looked at the underside to see if that is equally legibly destroyed. Also, you say two are blown & two baked, so for the ones that are baked - are they just baked but still resistive & can be measured or are they also open circuit?

 

[larryderouin]

Expanding the photo the values appear to be 100ohm 10% 0.5W(?) for the smaller pair and 47K 10% 1w(?) for the larger pair. +1 on Tinkerbelle suggestion of pulling the out of circuit and verifying values either with VOM/DMM or finding a NON-Burned section of the resistor.

 

[Martinihenry]

Hello, thanks for your response. Yes, a very well built unit, and well protected by the military MFP coating (mold and fungus protection).

Tracing where these resistors go in the circuit is a commitment I'm not ready to submit to. The connections from this daughter board go off into a wiring loom and off to another section of the amp.
I just don't have the strength or desire to chase it down. Especially with the MPF on the wiring. What a pain. Am attaching close-ups of the burned-up resistors...ironically, the open ones are quite legible...they didn't get hot at all.

Here's a shot of the two that got toasty. You can see the rear resistor actually has a crack running down the side. It measures 436Ω out of circuit (you'll notice I clipped them both). It's amazing how the heat distorts your perspective of the colors. The tolerance band looks gold, but it must be silver, since there are no other resistors in the set with a gold tolerance band. The first significant digit in the cracked resistor looks like brown in this shot, but in person, it looks either gray or orange. The multiplier band looks to be brown as well, but it could also be orange. I think the MFP coating is responsible for helping to distort this.

Here's a close-up shot of the cracked resistor.

And here's a close-up of the other resistor (the one closest the camera in the first shot). It clearly looks to be a 100Ω in this shot, but again, in person, the colors look different.
Out of circuit, its value measures 965KΩ!!!!

 

[Martinihenry]

I just went in an rolled the cracked 436Ω resistor on its back, and lo and behold! It's a 1KΩ. I'd have bet real money that it was a 100Ω. Just goes to show how a hot resistor's color bands can deceive!

And yes! The tolerance band IS silver on it! CRAZY!

The other resistor looks worse on the underside, so no joy there.

 

[Tinkerbelle]

It seems likely that the two sets are two identical sets, so knowing the value of one burned out resistor in the set should get you to where you need to be. On the larger pair, one photo makes the 3rd (multiplier) band look yellow, the one pic makes it look orange on my screen, so depending on whether that is from the significant light reflection effect (making more yellow for example) or difference in screen color setting vs. your photo capture setting might make that difference.

 

[Amorphous]

I've just found this.. so the 4 resistors are 1K (R26 and R27) and 47K (R28, and R29)
I have the amps and the circuit diagram. Hope this helps.

 

[TriodeLuvr]

The OP posted this query 4-1/2 years ago as a new member and only has three messages to his name since joining back then. You might consider a private conversation to let him know you have this info.

Jack