MV-125 restoration & repair help

[ListenerM]

Hey everyone,

I recently picked up an MV-125 which needed a little love.

When I had first received it the amp would continuously blow fuses and would not set bias. Following the owners manual I went ahead and replaced the original GE 6550's tubes along with the 5751 and 6FQ7 tubes for good measure. This fixed the fuse issue.

The amp ran wonderfully for a short while. After that a hum which sounded like it was coming from the front right (over the power button) capacitor started and is now audible in the right channel. Is there a schematic & parts list for the MV-125 that might help point me into the right direction when it comes down to replacing the PSU and other caps?

The MV-125 has wonderful synergy with our current gear and I'd hate to give up on it due to the caps.

[admin]

First, welcome to CJO, great to have you with us.

Congrats on getting the MV-125, very nice little unit. Unfortunately, gear this age is prone to some failures. We don't have the MV-125 schematics in our database unfortunately so that will not be of help.

As for trouble shooting. If you actually hear a hum from the unit (ie not the speakers but an actual vibration), you may be able to track that down by applying gentle pressure on individual components with a non-conductive probe. If the hum stops with pressure on a specific component you could identify a potential component failure. I would look specifically at any transformer as those are prone to vibration/hum. Caps can hum as well.

[ListenerM]

Thanks for the input and warm welcome!

I agree that its a nice unit, not sure about little though haha. I will follow your troubleshooting steps this weekend and hopefully I can narrow down whats going on.

[microstrip]

I owned a MV-125 and loved it - in fact it was my first "serious" (more than four output tubes ... ) amplifier. Although we do not have the schematic of the MV-125, the smaller MV 55 is similar enough in basic design to be of help diagnosing it.

Although these are only hints - it is hard to diagnose remotely - if light hum happens only in one channel it suggests it is not a power supply issue. Perhaps it can be due to the an unbalanced pushpul output. Do the tubes bias well in similar position of the bias potentiometers? Have you access to an IR temperature meter to measure individual tube temperature? These measurements with reflective surfaces and glass are not very reliable, but a power tube that shows a very different behavior should be looked as suspicious.

[SolderSlinger]

Hello ListenerM,
Use the Premier 4 and MV100 schematics from this site for references. If you look on the web for pictures, one thing that you will notice is the Premier 4 and the MV125 have two more large electrolytic capacitors than the MV100. More than likely there are additional differences, but parts are labeled so you should be able to make alterations to a Premier 4 schematic to match your MV125. The MV125 LED reference voltage needs to be set at 1.2V for 6550 tubes.
I'm not one to unnecessarily replace parts, however, consider replacing the large electrolytic bulk caps. I like to buy from Digikey or Mouser, and preferred brand is TDK/EPCOS.
My first CJ amp was a Premier 4 with EL34 tubes. Excellent amplifier.
Good luck,

[Robbie]

For what do I know, the MV125 doesn't feature any electrolytic cap in the audio circuits and neither in the power supply

[SolderSlinger]

Hello Robbie,
Very interesting. You are correct. CJ website MV125 information states polypropylene capacitors for the main power supply reservoir, and trans-spectral output transformers. I would like to hear one.
Thank you for correcting.