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

[Philippe]

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