McIntosh MC240- Help needed, new to tube amps.

[sanjith]

Me got a McIntosh MC240 tube amplifier which was sitting idle for last many years. The condition known to me was the power transformer is blown and need replacement.

A few months back I gave it to a technician to get it fixed. He replaced the power transformer with newly built one matching original specs. But unfortunately, now the amp is not singing on one channel. The explanation technician gave me is one of the output transformers which is sitting next to power transformer has got short inside. He says the fuse is getting blown when he connects the output transformer sitting in middle and there is also no sound on one channel. Tested all tubes and they are in good condition.

It seems like there is no one here in Australia does rebuilding unity coupled transformers. I tried contacting Audio Classics, US and they are able to fix it if I can send the bad transformer to them.

Yesterday I opened the unit and I can see the main four capacitors are very old, maybe the ones came from the manufacturer in 1960s or replaced very long back. They should be in very bad condition. One of the capacitors has bulged on top(left black colored big one in the second photo). I'm wondering if this/bad components inside could be the issue for the problem with one channel and fuse to get blown? How is the unit going to behave if it has got bad big capacitors?.

 

[Tinkerbelle]

Bulging caps are not a good sign and if these are the originals they are likely in need of replacement. If the large caps are PS filter caps, then one of them going awry is going to affect the supply to both channels, not just one. It could be a contributing cause for the fuse blowing.

However, it seems that it is passing signal in the one good channel, indicating that channel is getting some level of DC supply. If it blows when he connects the second OPT, then is he powering one channel at a time or just powering both and the one channel plays for a short time before the fuse blows? It is assumed he checked voltages & such, including the relevant cap & resistor performance relative to spec in each channel.

This is the Tube forum, which is frequented by many members who specialize in tube equipment so your thread is in a good place. There is also a dedicated McIntosh forum here and this thread can be moved there by a moderator is you feel that might have more focused response. They might be more familiar with this type of issue in an MC-240 and whether an OPT is likely to fail short or open and the causes/possible things to look for

 

[sanjith]

Bulging caps are not a good sign and if these are the originals they are likely in need of replacement. If the large caps are PS filter caps, then one of them going awry is going to affect the supply to both channels, not just one. It could be a contributing cause for the fuse blowing.

However, it seems that it is passing signal in the one good channel, indicating that channel is getting some level of DC supply. If it blows when he connects the second OPT, then is he powering one channel at a time or just powering both and the one channel plays for a short time before the fuse blows? It is assumed he checked voltages & such, including the relevant cap & resistor performance relative to spec in each channel.

This is the Tube forum, which is frequented by many members who specialize in tube equipment so your thread is in a good place. There is also a dedicated McIntosh forum here and this thread can be moved there by a moderator is you feel that might have more focused response. They might be more familiar with this type of issue in an MC-240 and whether an OPT is likely to fail short or open and the causes/possible things to look for

Thanks, Me already collected the unit back from the technician. Anyway, I will give him a call tomorrow and get more detailed information.
It will be good if this thread can be moved to McIntosh forum by a moderator here.

 

[s-petersen]

The Electrolytic can caps are not the originals, but I am not sure how old they are. The originals are silver, and one is paper covered. IIRC they were twist lock and not clamped. It is hard to see the bulge in the picture, sometimes the plastic insulator on top of some shrink wrapped caps will bulge a little, if you push it and it flattens without much effort, it is normal.