Sharp STM-31 Spage Age, only hum..

[Rob DeFries]

Got this Sharp Space Age Amp to see if one could get it to work again.

A quick look revealed 3 blown power transistors, all 2SC1060, and a C-E shorted 2SC984. The last one could give some trouble to find.
EDIT: I had some in one of the drawers.

The amp is far from mint, knobs are totally ruined, so keeping things original is not an option or needed.


The 2SC1061 is the same as the 60, so I'm going to use the 61's. I've got a drawer full of them.

 

[Rob DeFries]

Here are all the transistors back in position. The little amp seems to be working again, but it almost died on me. It might be bad pots or push switches (Alps....)
I don't have a service manual or schematic. That is a bit of a problem. There are a lot of wires going to and from switches. What is what...... hard to find out this way.

 

[Rob DeFries]

It turned out to be the pushknobs, that made the noise and killed the sound. This amp is running ok for the moment. It had some cleaner spray and it seems to be stable.
I do have 20.3 V over the resistors,;on each side and a healty 40.6 V power supply. That is good enough. The amp might be running a bit lean at the moment, 5 to 8 mV idle isn't much. But it's working.
Sound is a bit dulll, lacking highs. And there's a lot of noise. Some from the main amp, with no input, a bit of hiss, but more when you open the volume.
So the tone control needs attention too.
There's only one 33uF/50V electrolit on the board, but two transistors, the 2SC458.....?
I replaced the 458's with "our" universal transistor, BC547. I changed the output caps and used a muse on the input.


I had to take the front off to get reach for the toneboard. So it looks a bit sad at the moment.

Since most of the caps have been done, it's time to listen if there's any improvement.
And there is improvement. Amount of hiss went down enormous. All that's left is from the main board. There's another two 2SC458 waiting to be swapped.
Highs improved a lot too. It's very transparant and there is detail to the max. Wow, it sure is a nice little amp....

 

[Rob DeFries]

Silence is golden.....

 

[Moving Ahead]

Good job.
Looks cap coupled. Adjusted center voltage? bias will be low, monitor heat sink temp (thermal gun is great here) idle around 30 degree. driven should go 40-50 degree Celsius.

 

[Rob DeFries]

It's cap coupled, like most amps with the SC1060 transistors. Don't know what year it is. I think very early 70's.
Center voltages is 20.3.(+/-0.1V) 40.6 for the supply.
It runs fairly cool, I can feel it's a little more than body temperature. Gets slightly warmer when it is playing loud. Not hot!
Guess everything seems to be in good working order now. Been playing for 3 or 4 hours now.
The push knobs started to get noisy again.
It's a cute amp, and that's what it is.

 

[Goldie99]

Nice job & write up - the following link has some more info + schematic.

http://www.radiomuseum.org/r/sharp_stm_31stm3.html

 

[Rob DeFries]

Yeah, I know, but joining them is a real pain in the butt. And the only way to get the schematic is...... to join.
I tried a couple of times but never succeeded. It'll cost too......

 

[Goldie99]

Actually you can download (request) page copies free of charge, as a guest - but it's limited to 3 pages/day, 10 in a month. I'll pm you tomorrow (I have pgs 1, 2, & 4 now).

 

[Rob DeFries]

Ah. Now I've got the schematics I found out that they used 4 tantalums disguised as normal caps, on the tone board. Maybe I'll take it apart again to change them into something Wima.

 

[Muppet06]

Got one myself to repair. replaced all the caps and 458's. Can anyone tell me the adjusment procedure with pinouts, values or something else (kan ook in het nederlands)? I never adjusted an amp before. There are only 4 pots idunno . Here to learn.

 

[LoneToteBoy]

Hello,
I'm brand new here on AK, delighted to see enthusiasm for this lovely little amp.
It is so nice to see something at least partly hand-built.
I have one and it seemed original and undisturbed.
Mine has an output stage comprising 4x 2N5298, one of which was open-circuit base-emitter, the others tested fine.
So I thought I would replace the whole set with bunch of TIP31C available cheaply in quantity from China.
On fitting, they blew both 1.5A fuses immediately with each going s/c.
I reckon the cheap source may have supplied defective examples as fitting a second set in only one channel ran ok with an external variable supply working gradually up to about 32 volts. When removing external supply and providing the amp's own internal supply of about 41v (45v measured open load) and the otherwise working TIP31C (as labelled) both blew instantly again.
Spec sheet pdf info would suggest the TIP31C set up ought to have worked and I struggle to find 2N5298 spares so I will find another source for the TIPs.
A cct diagram would be very helpful and once I get the output stage sorted I think I will replace the tone board tants.
Thanks very much for the pictures; eye candy for the afflicted!

 

[Scott75s1974]

TIP31C's are easy to source and cheap from reputable dealers: Mouser, Digikey, Arrow. No sense in taking chances with unreliable providers on those.

 

[LoneToteBoy]

I found I had a bunch of 2SD1138 pulled from computer monitors years ago and tried a couple on one channel.
They were in the frame output stage I think, vertical rate low frequency application.
Replaced the other channel with 2 of the original 2N5298 and couldn't tell the difference between channels!
Ran cool on a pair of used and abused bench test speakers.
Will try out on a nice pair of AR's later.
Time to bin the cheap Chinese TIP31C's, labelled Fairchild, must have been reject production wandering their way into circulation via the back door.