So I replaced all the output transistors, and all the little electrolytic caps but, I have discovered a new problem I think my main filter caps are shorting out.
After everything was replaced, it played great but I had the right limit indicator on, and it would come on from start up, without anything connected, no speakers, no input. sometime the right limit would go off if you turned the speaker relay on and off but, as soon as you would turn the amp on and off the light would be back on. When I connected some speakers (cheap test speakers) I noticed a loud thump on start up but, it would play great. So I noticed about the third or forth time I turned it on, I was adjusting things, there was a loud mechanical noise coming from the back of the amp, I believe from the main caps, It almost sounded like a low pitched creaking of an old wooden door opening. I'm guessing that the start up surge caused an internal short and that was the sound I heard (no speakers were connected at the time).
So where do I go from here? should I just replace the main filter caps? Did they probably just fail from age? Should I check the rectifier? And where can I find filter caps? The closest caps I was able to find was on digi same capacitance and normal voltage rating but, the surge rating was 63 volts not 65 like the originals, they're priced at $17. the next closest specs cap is about $40 so there's a large price gap, if there's a retailer out there that sells these caps besides the normal part stores like digi and mouser I would appreciate the heads up, since $80+ is a little rough for my budget right now.
After everything was replaced, it played great but I had the right limit indicator on, and it would come on from start up, without anything connected, no speakers, no input. sometime the right limit would go off if you turned the speaker relay on and off but, as soon as you would turn the amp on and off the light would be back on. When I connected some speakers (cheap test speakers) I noticed a loud thump on start up but, it would play great. So I noticed about the third or forth time I turned it on, I was adjusting things, there was a loud mechanical noise coming from the back of the amp, I believe from the main caps, It almost sounded like a low pitched creaking of an old wooden door opening. I'm guessing that the start up surge caused an internal short and that was the sound I heard (no speakers were connected at the time).
So where do I go from here? should I just replace the main filter caps? Did they probably just fail from age? Should I check the rectifier? And where can I find filter caps? The closest caps I was able to find was on digi same capacitance and normal voltage rating but, the surge rating was 63 volts not 65 like the originals, they're priced at $17. the next closest specs cap is about $40 so there's a large price gap, if there's a retailer out there that sells these caps besides the normal part stores like digi and mouser I would appreciate the heads up, since $80+ is a little rough for my budget right now.
One other notable thing, the output relay doesn't seem to be working right, it seems to be turning on with the amp instead of having a delay and instead of turning off slightly after the amp it doesn't turn off till the main caps discharge.
I'm wondering if this is causing the start up surge since the relay bypasses the thermistor, when energized.
I'm wondering if this is causing the start up surge since the relay bypasses the thermistor, when energized.
I'm not the best but let's give it try here.
1. We know the relay isn't working right, do you have a sub for it?
Check the voltages against the other relay.
It's old maybe the other might go too.
2. Check your caps. uF value, ESR, D.
checking the schematic the two big caps C27, C28 are 9300uF 45V.
Nuts, wrong schematic...those were for the MC250.
If yours are in series, they might be fine as according
to my schematic it's +40V & -40V. What are the original values?
1. We know the relay isn't working right, do you have a sub for it?
Check the voltages against the other relay.
It's old maybe the other might go too.
2. Check your caps. uF value, ESR, D.
checking the schematic the two big caps C27, C28 are 9300uF 45V.
Nuts, wrong schematic...those were for the MC250.
If yours are in series, they might be fine as according
to my schematic it's +40V & -40V. What are the original values?
the relay is working it's just turning on early and releasing late, it's the circuit that controls the relay that isn't working right. Just to be sure I did replace it with a new relay and the new one did the same thing, turns on with the amp instead of having a delay, and stays on till the main filter caps discharge. All of my electrolytic caps are new Nichicons that I just put in, it was doing the same thing with the old caps too. the only original caps are the two main filter caps 39,000 uf 50v with 65 volt surge rating. They appear to be working correctly, my capacitance meter doesn't go that high. I haven't seen any tell tale signs like leaking or bulging.
I got everything ironed out, it was a long road but, I'm finally finished. Here's the recap, the amp got all the output transistors replaced, all connections and pots cleaned with deoxit, all the electrolytic caps (except for the main filter caps) replaced with Nichicon's audio caps, the ribbon cable was replaced since it was starting to go bad, rust on the chrome was removed and then waxed, the glass bubbles were removed and repainted, and the thermal compound was removed and replaced since the original compound was dried out. The problem with the relay and limit light was caused by a spec of something conductive making a path to ground, once it was removed everything functioned as it should. I biased each side to 5.8 mV. She runs pretty cool the only thing that gets warm is the power transformer after being on for a while. Here's some pics.
Thinker,
Which large caps did you end up using?
I'm in the same boat with an old MC250.
I found what I think is the bad driver
transistor the PNP transistor on the
left channel heat sink.
Whoop tee do.
Me thinks these were all matched.
I wonder if I could just find one.
Some on I forget whom suggested part of why
these amps fry is that they start coming off the
heat sinks.
They are all mounted with sheet metal screws...instead of
small nuts, bolts, washers and lock washers.
What a PITA to work on. Too bad they didn't consult with
Leo Fender about layout for servicing an amp. The more
stuff I see, the more I like his stuff.
Cheers,
Which large caps did you end up using?
I'm in the same boat with an old MC250.
I found what I think is the bad driver
transistor the PNP transistor on the
left channel heat sink.
Whoop tee do.
Me thinks these were all matched.
I wonder if I could just find one.
Some on I forget whom suggested part of why
these amps fry is that they start coming off the
heat sinks.
They are all mounted with sheet metal screws...instead of
small nuts, bolts, washers and lock washers.
What a PITA to work on. Too bad they didn't consult with
Leo Fender about layout for servicing an amp. The more
stuff I see, the more I like his stuff.
Cheers,
I ended up not replacing my large filter caps, since it was a short else where that was causing my problems, I probably will end up changing them once I save up some money. What makes you think your driver transistors bad? The bad one in mine got really hot really fast and the meter showed full power use, that's what made me suspicious and when I removed the transistors the one transistor tested bad, it wasn't completely dead but was beginning to conduct slightly the wrong way, so I think it crapped out from age, I do see what you mean about the screws I've heard one guy tightens them every so many years in his, I've heard other people put a little bit or solder or locktite on them, mine managed to go 36 years without trouble I think that's pretty good. I've been told that you can replace just one driver if you can find the same type, I had to change all mine out since the ones in mine are no longer made, they show up on eBay sometimes when people upgrade or replace because one went bad maybe you'll luck out and find the one you need, I saved my four drivers since I think they're still good just incase I have trouble in the future
I followed the McIntosh service bulletin for the MC2505 for my amp.
Since I couldn't read the copy of the power output PCB, I monitored
it according to the schematic values on the MC250 and wrote those
on the L/R channel Power Output PCB and Input section PCBs.
Once all the voltages were written on it, I then started monitoring it
as I brought up the voltages from 0 - 30 VDC , then 30 - 60 VDC.
Started taking measurements. The input board voltages tracked
with the variac. The output boards didn't track.
So I then detailed the L/R channel power output board and found
problems with the current on wires 9 and 6 of each board.
Right 9 = -14 mV, 6 = -21.4 mV
Left 9 = -0.9 mV, 6 = -5.4 mV
Then continuing on with the Service Bulletin recommendations, I measured all the output transistors and their drivers and found the bad one.
Will replace then go through it again, then make changes/mods when it working.
Since I couldn't read the copy of the power output PCB, I monitored
it according to the schematic values on the MC250 and wrote those
on the L/R channel Power Output PCB and Input section PCBs.
Once all the voltages were written on it, I then started monitoring it
as I brought up the voltages from 0 - 30 VDC , then 30 - 60 VDC.
Started taking measurements. The input board voltages tracked
with the variac. The output boards didn't track.
So I then detailed the L/R channel power output board and found
problems with the current on wires 9 and 6 of each board.
Right 9 = -14 mV, 6 = -21.4 mV
Left 9 = -0.9 mV, 6 = -5.4 mV
Then continuing on with the Service Bulletin recommendations, I measured all the output transistors and their drivers and found the bad one.
Will replace then go through it again, then make changes/mods when it working.
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