RS Stereo Amplifier PCB
 

Hi folks,
I found this at the side of a box of assorted parts that I'd acquired some time ago.
I don't have any circuit to go with it, so I'm not sure what IC it was designed for.
I initially thought it might be for a couple of Plessey SL403A but that wasn't rated at 5W output.
Was there a later higher power version of this IC that RS used to sell ?

Andy

Re: RS Stereo Amplifier PCB
 

Odd, the box says 5W but the pcb is silk screened 3W.
Perhaps another case of rms vs peak vs music power.

Peter

Re: RS Stereo Amplifier PCB
 

Or the other old trick of adding each channels power output, so instead of 2 x2.5W its 5W?

A.

Re: RS Stereo Amplifier PCB
 

It seems you were right:
https://www.vintage-radio.net/forum/...d.php?p=922763

Re: RS Stereo Amplifier PCB
 

I remember those and built one to provide an extra 2 channels when Quadrophonic discs first appeared. They worked well and I’ve probably got it somewhere in my piles of stuff. Don’t ask as it’s a large double garage that is almost impossible to get into.

Re: RS Stereo Amplifier PCB
 

Thanks for all the replies - having read the thread in the link posted above, it would be useful to see the RS datasheet R/185 :
Does anyone have a copy or scan ?
I still think there must have been an update to the IC used and that's why there's a yellow sticker on the envelope, which mentions "... each 5W AUDIO AMP"

Andy

Re: RS Stereo Amplifier PCB
 

I think that is just the way it was worded and slightly misleading. It was about the time when manufacturers started to make more extravagant claims about power outputs. 2.5 watts per channel at low distortion equals a 5 watt (RMS) stereo amp. The maximum output, incl distortion, was probably 3.5 watts. I believe these I.C's were made by Plessey who initially rated them at 1.5 watts and sold by the infamous Clive Sinclair as 10 watt amplifiers. They also need an additional heatsink or they will fail quickly.

Re: RS Stereo Amplifier PCB
 

The Sinclair amp was the IC10, late 60's, but it also had a pair of output transistors driven by the IC.

A whopping 59/6 in 1968.

Re: RS Stereo Amplifier PCB
 

The Sinclar chip package itself was branded IC10. I still have one that works and another that's died.

Alan

Re: RS Stereo Amplifier PCB
 

I vaguely remember something about those ICs originally being designed by Plessey for use as servo-drivers or Radar scanning or something where the output current slewed relatively linearly and slowly rather than being distinctly-peaky like an audio signal: in such applications they worked just fine. BUT the little wires linking the semiconductor 'slab' to the leadout-pins were rather thin, and when they were subjected to instantaneous high-power transients on a regular basis the effect of repeated thermal-cycling resulted in them embrittling and going open-circuit.

[I have a vision of a test-suite full of amplifiers all playing Ravel's Bolero into dummy-loads....]

Re: RS Stereo Amplifier PCB
 

Thanks everyone - yes, I've read that they weren't very reliable..
Maybe RS were trying to compete with the adverts for Sinclair IC10's !
Anyhow, I don't have any of the IC's needed and even if I did, I don't think I'd bother using them now.

Cheers,
Andy

Re: RS Stereo Amplifier PCB
 

Thread reopened by request.

Re: RS Stereo Amplifier PCB
 

I asked for this thread to be reopened as I picked up an assembled amplifier using the pcb in post No 1 at my local car boot sale this weekend for just £2
Photos attached.
I will run it up and see if it actually works and how it performs.

Peter

Re: RS Stereo Amplifier PCB
 

Thanks Peter, it will be interesting to see if they still work.
Might be best to use 16ohm 'speakers if you've got some, but certainly not less than 8ohm.
Good luck !

Andy

Re: RS Stereo Amplifier PCB
 

Thanks Andy, I don't have any 16 ohm speakers but will initially test on a resistive load.

The ICs are marked "RS AUDIO AMP 3W P7202"
Another variation on the Plessey SL403 / Sinclair IC10

Peter

Re: RS Stereo Amplifier PCB
 

I was going through some old magazines and spotted that in the July 1971 issue of 'Radio Constructor' there was an amplifier project that includes a circuit diagram and connection details of this RS PCB (no layout though) and an internal circuit of the RS amplifier IC.

Starts on page 34 of the pdf of said issue on the World Radio Histort site.

May be of interest...

Re: RS Stereo Amplifier PCB
 

Thanks Tony,
I have just finished tracing the schematic from the pcb!
I superimposed the top layout from post #1 with the track side photo from my unit and traced it out.
I attach the layout and both versions of the schematic, mine and RC.

I have started testing, dc only at present. The A channel may work, I can set the bias to the correct level.
Channel B output is at 0.8V however tapping the chip results in the output momentarily going to the correct level.

I understand that a common failure mode of these chips was a break internally connecting to a pin, looks like that is what I have.

There is a NOS chip available at £10, I am tempted but what would I do with the amplifier when I have finished testing?
I will probably not bother repairing it.

Peter

Re: RS Stereo Amplifier PCB
 

I have now finished testing, the A channel does indeed work. I have only tested using a scope and sig gen into a resistive load but it works.
As suspected the B channel does not work. The pre amp section works ok but the output section has no output.

As I noted in the previous post I could buy a NOS chip for £10 but I have decided to stop here, I don't know what I would do with the repaired amp.
I only bought it because I as curious and it was only £2.

If anyone wants the amp they can have it for the cost of postage.

Peter

Re: RS Stereo Amplifier PCB
 

Hello Peter,

I’d give it home (don’t tell the wife though :)) to go along with my curio amplifiers like the Sinclair 30, Mullard Unilex and other workshop-built gadgets.

Terry

Re: RS Stereo Amplifier PCB
 

It's yours Terry.
Peter