RCA room amps

[shooter]

 wound up with 10, what appear to be room amps, (stereo?), intercom?
just got started, surfed n faceplanted on info.  the only "Info" paper sticker on chassis RVB 2747119  BS/252K
there are stickers saying bedroom, study, ..... so far only obvious date looks like a tech used marker to write '71

1st quick pass schematic below
my Xref book only has RCA 129, then jumps to 142, the power transistors on this say 132 4, in an arc, then below, RCA.  all 4 say the same thing, on the schematic N is just a "placeholder" for now

the one in the pic must be like Beta test, the other 9 have a potted pre, this one I can reverse Engineer 

[PRR]

You would assume intercom, but the layout suggests stereo power amplifiers. So this was for some rich person to play music in every room of a large house. (Bathroom?)

I don't see much interest in the circuit. I guess I would connect the "120V" to say 12V AC with a 10 Ohm series resistor, see what the rectifiers put on the first cap (that dubious looking thing in the strap). That will give an estimate of the power output per channel. It's not small, look at that PT! In fact compare that PT size to Hammond's plain power transformers to get a ballpark VA value.

EDIT -- "12V" s/b "120V"

[shooter]

(Bathroom?)

yup, one labeled Mbath
It does appear to be a stereo amp with intercom/remote control. AC power is also "controlled", or series daisy-chained (I didn't draw that part).  I'm gonna try and get a close proximity schematic, then do a power-up n see.  For some reason, without logic, my brain "sees" a cute little phone docking/playing station, bright colors n all, 2 3 -5" "full range" cabs 

[shooter]

updated schematic in post 1, got inputs now

PT is PT# 1461174-24      3947115
found a couple ppl asking about variations of the main # -19, -9.  both were fixing console phonographs

best I could do on the PA transistors,  maybe, kinda, possibly NTE 152

[shooter]

another schematic update.

2minute test, no audio out, i'll try another, but I suspect there's a "mute" or on/off on a remote circuit, probably find it when I get the last section sorted out and drawn up.  I do get good loudish hum and it sounds like it wants to audio.
busting out meter n scope next

[shooter]

success!  brkfstrm was bad, bedrm1 works, although sounds like it has a gain of 2 

proper #'s soon, then it's back to tubes for a minute

[shooter]

made some quick measurements;
B+ ~ 33.4vdc
PA 1 ohm sense resistor ~ 40mA idle (R414)
with ~ 20mVacrms in, get ~ 280mVacrms out, anything bigger @ input seizes up the amp.  similar to the QX747, so next time I order parts probably add ecaps for one of these and see.  low power audio is good n clean, sounds nice, but nothing to write home about 

[shooter]

so I changed audio sources and walla, LOUD!  my matchbook sized MP3 works fine at max vol into powered speakers, but this amp anything above 1/2 vol and it cuts out, even spiking B+ ?

I hooked up the notepad/sig gen and annoyed myself with 1khz, then switched to modern phonograph with phono out/line out and played an album, both sides, just fine on line out, again annoyingly loud for a bedroom amp and just on the edge of distortion since there is no volume on input or amp.

time to find a new chassis that is NOT steel, then i'll re-work into.....something

[shooter]

It's still firkin snowing here!, was able to find something in the barn, 6 kittens I didn't know about 
and an old face-plate cover for a Phillips display that went on the magnet face for table positioning

should have just enough room to eck in a pair of 3" speakers.

[PRR]

For reference, from RCA SC-12 1966, example 10 Watt audio amp. I believe it is of the era. I do not believe RCA commercial products came out of the same room as the application examples, though they may have peeked.

Congrats on your growing family.

[PRR]

Well, I just realized your beast is all plastic, so can't be (from RCA) 1966.

There's a WIDE variety of designs from the SC-12 era to mid-1970s when "all" small amps went to complementary outputs. The plastic puts this in the thick of the 1970s.

I would assume the amp is what it is, with rotted electros and probably some blown *sistors, which may be cross-checked kitchen to bedroom like a young handsome plumber. Will never be great, wasn't bad. If it is crap to your ears (or unfixable within your schedule) you can find a chip to run on that supply and those heatsinks.

[shooter]

thanks for the datasheet!  The "plastic" is my modern chassis concept, the originals are all steel chassis, earliest date so far '71.

The sound is good when driven, but needs V control, beta test will be a dual gang 250k across the 33k's at the input.

been spec'in speakers;
https://www.parts-express.com/peerless-by-tymphany-pls-series-75f25al04-3-aluminum-full-range-speaker--264-1185


https://www.parts-express.com/kenwood-kfc-835c-3-1-2-dual-cone-speaker-pair--267-4300#lblProductDetails

forgot to copy link, but leaning 296-717-fountek-fe85-specifications

[shooter]

 
I took out the TS,(SSBuild thread), wired a pair of these to my bose and
I gotta give it to the design engineer(s) on these!
It's like when you get the tube amp right, you just can't stop playin.  I listened for 1.5hrs, these little things don't need EQ.
need to track down "the other side" which didn't work on 3 of 3, same side, I'm thinking there's a mute I haven't found, so I ran 2 as mono blocks. nice room volume, I drove them with a pocket MP3/radio ~ 1Vacrms.  I think anything hotter will start to distort
'70's class A meets '90s (?) speakers and they rock 2018!!!

best track, Afroman's "I drive drunk", It was like sitting in the middle seat !!!

[shooter]

While not elegant, pretty confident the schematic in 1st post is now really close-ish
In the upper right is the only difference, (R418,419, c428) 
Hopefully Monday I take them out and play stereo

Not sure why it kills the channel, there is nothing on H, guessing the cap is iffy 

I had to move J Geil's harmonic to #2, North Mississippi Allstars "Jr" move to #1 

[shooter]

Murphy Wins!
If you look at the pic of the amp, the 2 output caps stick out, and get/got banged enough to blow out the foil runs  , once I resolved that I made up an input cable, and yup, if flexed one side drops out 
the truly odd thing, all 3 I tried had the same cap issue on the L ch, the input flex, yup, same, L ch!  I need to go put a big bet down on RED!!! 

[shooter]

I took this snippet from the full article in reply #9

It talks about the "limiting factor" of the diode, is the limiting factor V-drop   

also got updated schematic, there is an RC circuit in || with the speaker, filtering?, impedance balancing?....