Kikusui COS6100 power supply problems - Page 1
Forum section: electronics / repair

author: andersthuresson
author_rank: Contributor
timestamp: January 19, 2025, 01:56:44 pm
content: 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!

author: fenugrec
author_rank: Frequent Contributor
timestamp: January 20, 2025, 01:45:34 am
content: Quote from: andersthuresson on January 19, 2025, 01:56:44 pmThat includes the +55 and +150 rails ?QuoteCheck 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 ?

author: andersthuresson
author_rank: Contributor
timestamp: January 20, 2025, 03:27:55 pm
content: Thanks @fenugrec for some good input. I will checked the questions you have and came to the following conclusions,1. Yes. the +55V and +150V rails are okay. I checked them all and none is below or above what would be normal.2. The OP Amp is socketed so I lifted pin 567 out of the socket and run the scope. The temp of the IC was around +40C to +50C in that configuration.3. I checked the voltage drop over R1240 = 0,62V so the current would be around 2.8mA.4. All BJT's measure okay. I tested them all out of circuit.5. Voltage drop over R1235 = 7,8V so the current would be around 115mA (0,9W).6 Voltage drop over R1224 = 7,1V so the current would be around 104mA (0,7W). This is at the +12V rail.R1235 is the resistor that is at >+102C. I didn't keep the scope on once I reached +102C because I worry it will fry. The resistor is rated 2W so it should be plenty of margin but ...the temp is worrying. It could perhaps be normal but I have no other COS6100 to compare with. It was also the -12V part of U1212 that didn't work before I changed it. It should have been regulated to around -12V but before changing U1212 the -12V rail was measuring around -22V.When I said that the -12V was sticking out I actually meant temperature wize. If I compare temperatures of components in the +12V rail I can see that the -12V rail run much hotter. Perhaps something in the -12V rail draw abnormally high current but so far I have not been able to pin point anything. So the question is if this is normal or abnormalMaybe bad design?!

author: m k
author_rank: Super Contributor
timestamp: January 23, 2025, 02:26:17 pm
content: Back in the day some sections of CRT circuit were problematic.For example before suitable power semis there were small transformers.So it's normal that some resistors are taking the heat.2W is also 2W, that's why some resistors are lifted and away from other parts.It was also sort of a norm that constructions were failing around a certain spot.There joints and traces were giving up first and so component's environment changed.