MC2105 blowing fuses

[DrRobocop]

Hello all,

I recently acquired a cosmetically very good condition MC2105. When I first attempted to turn the unit on nothing lit up, but the meters did a weak "bump" and then went back to resting. There was no function after that. I then proceeded to replace the filter caps (triple checked polarity) and the multicap can capacitor with the hamfest replacement. All the wiring looks good. I turned the unit on, the bulbs flickered then a fuse blew, I saw a small wisp of smoke and smell but could not locate it's source. There are no obviously burned or damaged parts after checking the unit. I then pulled the driver boards and attempted power on again, hoping the issue was isolated to them. Nope, another fuse blew. Now I'm not sure how to proceed. What would you recommend as the next step in troubleshooting?

Thanks for your help!

 

[Bob the Tech]

Keep in mind that pulling the drivers leaves the heatsinks powered, something may be smoked/shorted on the heatsinks.

I'd check the supply rectifiers then disconnect loads you can and use a variac and dim bulb to see if the voltages build correctly just to make sure the supply is ok. If they build w/ no issues w/ no load then I'd check heatsink and drivers with a meter looking for shorted devices / burnt parts to isolate any issues and resolve them. Then you can reconnect (you could even do one channel at a time) and check again with the variac and dim bulb to see that voltages build and the heatsinks bias correctly. I've seen bad caps on input, driver, and meter boards in these amps all the time, they don't generally cause smoke, but you may have other issues once you get it powered.

 

[DrRobocop]

Thanks so much Bob. I'll connect a dim bulb tester and start checking. My first stop will be the rectifiers (2 1N3492 and 2 1N3492R ) on the filter caps. I may even just replace them anyway. Would the original diodes be preferred (looks like they are still available) or would the 512-GBPC3504 bridge rectifier be preferred?

 

[Bob the Tech]

I wouldn't replace the rectifier unless it's shorted.

 

[SaturationPt]

Unless you can be sure that everything you do, and every replacement part you buy, is perfect, it is far better to diagnose as-is, leaving all of the original parts in unless they test bad.

Most likely there is one bad part causing your symptom, find that part through voltage checks and waveform outputs. Power supply is likely the first thing to isolate and test as Bob has mentioned, then other directly-connected high-current circuits like output transistors and the heatsink isolators. Heatsinks should be electrically isolated via their plastic mounts and there are some power transistors that are isolated, some that are not, very important.

 

[w3jn]

^^ Absolutely correct.

Get at it with an ohmmeter to find out where it's shorted. Isolate the short to the bad component, and replace that and only that. Shotgunning replacements is a recipe for failure and damage to the amp.

 

[twiiii]

remember you always replace all the emitter resistors and outputs if you have to replace 1. Other wise the repair life expectancy will be very short. Same goes for transistors on driver boards. Check all caps and resistors.

 

[quartersaw]

Thanks so much Bob. I'll connect a dim bulb tester and start checking. My first stop will be the rectifiers (2 1N3492 and 2 1N3492R ) on the filter caps. I may even just replace them anyway. Would the original diodes be preferred (looks like they are still available) or would the 512-GBPC3504 bridge rectifier be preferred?

One of my 1N3492R's was the culprit, for my '2100 blowing fuses. I goobered up the heat sink, as that press fit, is very tight. I was fortunate to find NOS replacements mounted in heat sinks, on the 'bay. (Looks like DigiKey, has them in stock)

 

[DrRobocop]

Thanks all for your recommendations and information! Your feedback is incredibly valuable.
While checking I found that one of the output transistors was shorted (probably more were, but I didn't check all of them). So I went ahead and replaced all 12 with MJ15003G transistors. After I plugged the amp into my dim bulb tester, the bulb was shining brightly and there was no activity from the amp (no light, no meter movement etc...). UGH!
I did not yet replace the emitter resistors. I intend to, but I'm not sure if this should be the next stop in finding the short?

With the driver boards pulled, the filter caps replaced, the can cap replaced, and all the output transistors replaced what else could it be? Could the meter board or input section board be the culprit? At this point I'm not sure what to check next so I can power on the unit. I think my next stop will be to replace the rectifiers. All I have for diagnosis is my DMM.

For the rectifiers, It looks like I need to buy the 1N3492 and 1N3492R in quantities of 10 on digikey. However, that may be my best bet as the price is comparable to buying just the needed quantity of vintage parts on ebay.

 

[w3jn]

Get at it with an ohmmeter to find out where it's shorted. Isolate the short to the bad component, and replace that and only that. Shotgunning replacements is a recipe for failure and damage to the amp.

I'll repeat my prior advice. You're still shotgunning components.

I don't think you can replace all the transistors with MJ15003s because there's a mix of transistors in each output module. Q115/116/117/118 are different than Q119/120/121/122/123/124/125/126. I do not know what the proper replacements might be for Q115/116/117/118, but the MJ15003 is probably reasonable for the rest of them.

First off, you need to pull all those MJ15003s and figure out which ones, if any, are shorted. Then test ALL the emitter resistors and replace those that are open, although you might as well do all of them if one is bad. Then install the proper transistors in the Q115/116/117/118 sockets and the proper transistors in the rest. Next, I'd check the driver transistors on the driver boards for shorts.

After all that is done, you need to check the +35V line to ground for shorts and then the -35V line to ground for shorts. If you find a short then disconnect a board or module one at a time until that short goes away. That module/board then is where your problem is. If you STILL have shorts then disconnect all the wiring from the rectifiers and check using the diode test function on your multimeter. If it doesn't have a diode test there are other ways to test it - ie low resistance with your probes on the rectifiers one way and very high resistance when the probes are reversed.

Until you clear up all shorts in this amp, and are CERTAIN that any parts you've replaced are installed correctly and are of the proper value, don't power up the amp.

We can guide you through this but please, STOP shotgunning components and focusing your attention on capacitors, rectifiers, or anything else. The short could be anywhere or anything and you need to keep an open mind.

 

[SaturationPt]

+1 on what @w3jn said.

Also, if you read my above post #6, did you verify that none of the heatsinks are shorted to ground, and that the transistors that are supposed to be isolated electrically from the heatsink are not shorted to the heatsink? You will need to consult and understand the schematic to do this.