[sdiy] SH101 Power circuitry transistor getting -very- hot

[Maarten]

Hi Philipe,

the input current is to high, for normal operation the SH-101 only 
consumes 85 to 100mA. The switch is ok, I also use the 6 pins with good 
results. Try to run the power supply without all the synth circuitry and 
see what kind of input current you have. If this seems all good, try 
disconnecting part of circuits to determine where the fault is. The only 
way to do this is by cutting traces. But before you start do a very 
careful visual check of both sides of the pcbs.
Cheers,
Maarten




Philippe schreef op 13-12-2014 20:05:
> Hi guys,
>
>
> first of all, thanks for your feedback to my « test message », I did not realize HTML was not accepted.
>
>
> so here it goes :
>
>
> I am currently trying to fix my trusty SH101 (bought new in the early 80's when I was still a young man !). I replaced several opamps (by TL062 and TL022), the 4050/4052 of the D/A, and replaced the two transistors (TR1 and 2) of the DC/DC converter with identical references bought from Dalbani.co.uk.. I also bypassed the power switch that was causing instability, until I replace it by the new one I just received - 6 pins only, vs 8 for the original.... so not quite the same).
>
> Things look better now : the tune is now OK - I had to change a resistor going to the VCO chip though I think to compensate for slightly different characteristics due to the new amps - except the range which is still 1/2 tone off.
>
> The issue is: TR1 - the 2SD571 that drives the primary of the transformer in the power circuitry - gets very hot after one minute. I do not think that comes from a short circuit somewhere as all voltages are good ( I feed the SH with 9V, with around 230mA current and it works fine). I tried replacing Tr2 by a 2 N2222A as it was advised in other forums but then TR2 becomes very warm as well. The 2SC1815GR used standard for TR2 has a HFE of around 350 so far superior to the 2N2222.
>
> Has someone got some advice to sort this ? Maybe TR1 HFE is still too low (I measured it at around 235) ? Or is the resistor R8 limiting the current at the entry of the power circuit too low (1 Ohm) and I should try testing 5 or 10 Ohms ? FWIW, I noticed that the oscillation at TR1's collector is not regular, ie the pulses have irregular widths... I should be a constant square wave I think ?)
>
> Thanks in advance for your feedback, I would really appreciate your inputs!
>
> Kind regards,
>
> Philippe
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