EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: andersthuresson on January 19, 2025, 01:56:44 pm
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Hi everyone! I bought a second hand Kikusui COS6100 a few years ago. After bringing it home I only powered it up for about 10 minutes and after that the power supply gave up, with a nice puff of magic smoke. I have finally started to repar this oscilloscope and it is now up and running again. Everything seems to be working and all the power rails seems to be correct. I have traces on the CRT and even the probe compensation is working. I do get the traditional square. The problem is that the power supply seems to be running a bit hot. The -12V rail seems to be sticking out compared to the other power rails. So far I changed some broken components and those are,
- CR1217 rectifier due to short.
- CR1218 rectifier as precaution. It looked a bit crusty.
- U1211 OP Amp.
- U1212 Regulator.
- Power supply re-capped with all new electrolytic capacitors.
The U1211 OP Amp is running at a temp around +65C to +70C and it feels a little bit high to me. It is not crazy hot of course but I don't see why it should run at that temperature. Highest temperature comes from R1235 (68R/2W). Once it surpassed +102C and then I decided to turn off the oscilloscope. Temperature was still climbing. I tried to look at the other boards and measure resistance between -12V rails and ground but there is no short found anywhere. R1224 (68R/2W) is the corresponding resistor for the 12V rail and the temperature for that resistor during operation is only around +60C or there about. I did find the complete service manual available on the web.
My question is simply if it could be normal for R1235 and U1211 to be running at such temperatures during normal operation?
Many thanks!
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all the power rails seems to be correct
That includes the +55 and +150 rails ?
The U1211 OP Amp is running at a temp around +65C to +70C and it feels a little bit high to me.
Check Q1212, Q1214, but don't forget the other half of that opamp, driving Q1218.
At a glance I don't understand the purpose of Q1217 (also, how are they getting +5V from bridge CR1218 producing a negative rail ??)
but you could check voltage drop accross R1240 for an idea of how much current U1211A is providing. It's driving a power bjt directly; depending on its operating Ic and hFE, it may be normal for the opamp to be pushing a fair amount of current.
Worth checking all those BJTs for obvious failures (although unlikely if the rails are good).
R1235: what's its V_drop and therefore current ? They did size it as 2W so they expected some heat in there; if the opamp is powered from -12V rail (unclear) and it's drawing more current than it should, maybe...
You said the -12V was "sticking out", is it still off from nominal ? If regulator U1212 or its pass transistor Q1216 aren't operating correctly, that could also explain some of this. AFAIK feedback network R1230 and R1231 should ensure fairly symmetric rails + and -12 ?