>From my
limited knowledge I react on the voltage 0.6V because it sounds like a
PN-junction voltage drop.
>So I would guess that something, coming from 0V/ground, via a reverse PN-junction, reaches the emitter of Tr20.
>For example, its own base-emitter junction, if the base of Tr20 is 0V. Is it? *no, base is > 0V*
>So I would guess that something, coming from 0V/ground, via a reverse PN-junction, reaches the emitter of Tr20.
>For example, its own base-emitter junction, if the base of Tr20 is 0V. Is it? *no, base is > 0V*
I would expect it to be around -15.7V or so when
the circuit is operating properly. Exactly how much is it?
How much is base voltage of Tr21, Tr22 and
Tr23?
Check if D7 is good.
BR,
Peter
----- Original Message -----From: Adrian CorstonSent: Saturday, July 20, 2013 5:31 PMSubject: Re: [vintagesynthrepair] CS-70m power supply issue, -15V rail is dead
On 20/07/13 5:50 PM, Adrian Corston wrote:I'll be having another look at the unit tonight and will try out the excellent suggestions received.
Here we go... my responses are in red in the quoted text below.
Change ANY electrolytic capacitors in that -15 circuit. *done*
Diode "check" any diodes ( make sure they conduct one way and NOT the other) *I don't see any in the -15V circuit*
If there are any tantalum capacitors ( usually teardrop shape) replace them too *not relevant - there are none*
The -15' and +15' values are probably right as they're derived from a zener diode + a PN drop, so 15-0.7 and -15+0.7, roughly.
Sounds about right. *OK*
From my limited knowledge I react on the voltage 0.6V because it sounds like a PN-junction voltage drop.
So I would guess that something, coming from 0V/ground, via a reverse PN-junction, reaches the emitter of Tr20.
For example, its own base-emitter junction, if the base of Tr20 is 0V. Is it? *no, base is > 0V*
Also switch it off and check for a short to ground from Tr20 base. *no short found*
If IC3 is socketed you could try replacing that. Who knows what will happen if that's fried... *IC3 replaced (and socketed for next time)*
Also check for shorts between Collector-Emitter on Tr22 and Tr23. *no shorts found*
I'd also check the electrolytic on the emitter of TR20 isn't pulling anything to ground *electrolytic replaced*
Also check for any shorted tantalum caps throughout the synth... *not relevant, rest of synth is disconnected*
5. No voltage is put out for -15V. Other voltage positions operate correctly.
Probably cause: Tr21 - Tr20 - 0.22ohms.1P *tr20 replaced, tr21 replacement pending, 0.22ohm resistor measures OK*
None of the above things that I tried had any effect; the -15V rail is still sitting at around 0.6V.
I have no 2SB560 (or well-documented substitute) handy for tr21, but I did find a 2SA1534A (in another old power supply) which has a similar spec. The 2SA1534A is only 60V rather than 100V, but I assume that won't matter in a -15V circuit (I only measured 26V for Vce in the circuit with the 2SB560 in place). Pinouts are the same for the two parts. However according to my wonderfully-useful transistor comparison book it is not "lo-sat" (low collector-emitter saturation voltage) whereas the 2SB560 is. According to data sheets:
2SB560 Vce(sat) = -0.3V typ, -0.8V max
2SA1534A Vce(sat) = -0.2V typ, -0.4V max
Datasheets:
2SB560: http://www.futurlec.com/Transistors/2SB560.shtml
2SA1534A: http://www.alldatasheet.com/datasheet-pdf/pdf/12706/PANASONIC/2SA1534.html
This looked near enough to me, so I tried substituting it. Unfortunately the 1K resistor between tr20/tr21 collectors and tr22/23 collectors immediately started getting too hot to touch, so I removed power straight away. I will try again next week after I find a real 2SB560.
Thanks again,
A.