Conrad Johnson MF-2250 horribly distorted

[Steely Joe]

I've purchased a used Conrad Johnson MF-2250. The output is horribly distorted.
Is a service manual available?
Can the problem be bias adjustment?
PLMK if there is a recommended repair technician in the Pennsylvania/ New Jersey area.
Thanks

MF-2250 b.jpg (51.19 KiB) Viewed 201 times

[AnotherJohnson]

CJ will not cooperate with schematics or parts. There are no authorized service centers. The old guys who retired having worked at CJ are dying off.

It is unlikely to be a simple bias adjustment.

You can call CJ. Solid State … might be more useful to chase down a McCormick technician. The 2250 was one of the McCormick (sp?) projects iirc.

These are worth on the order of a grand in working order (not likely more) so remember that as you’re thinking about bench fees and estimates.

Best of luck.

[admin]

Unfortunately we do not have the MF-2250 schematics in our database. I think horrible distortion would be unlikely from a biasing issue, especially in a SS unit. I would do a good visual inspection of the inside to look for obvious signs of faults such as burn marks, bulging caps, bad solder joints, etc.

[Steely Joe]

Unfortunately we do not have the MF-2250 schematics in our database. I think horrible distortion would be unlikely from a biasing issue, especially in a SS unit. I would do a good visual inspection of the inside to look for obvious signs of faults such as burn marks, bulging caps, bad solder joints, etc.

I opened the bottom plate and found a few "shady tree mechanic" soldering attempts.

[admin]

I would use the continuity tester on a multi-meter to make sure those solders are solid and not touching other circuits. But otherwise it doesn't look too bad to me.

[AnotherJohnson]

I would use the continuity tester on a multi-meter to make sure those solders are solid and not touching other circuits. But otherwise it doesn't look too bad to me.

[AnotherJohnson]

Try Steve here if you can’t get help from CJ.

https://smcaudio.net/

[Big Dog RJ]

Bad distortion... either from one or both channels could also mean a blown fuse. Check all fuses: b+ fuses, ac mains inlet fuse, and whatever other fuses are located inside the unit.

I can't quite remember how many of these fuses are in there but similar situation happened on an earlier amplifier, MF2100. It was one of the fuses on one channel. If it's the ac mains tray fuse, which is usually located right next to the power cord inlet, then the amplifier wouldn't work at all. In this case, you're getting power but very distorted, as you described

Hope you manage to find out the problem.
Best, RJ

[Steely Joe]

both fuses test good.

[admin]

both fuses test good.

I thought there are a total of 5 fuses in the MF2000 series. Are you sure you didn't miss any?

[AnotherJohnson]

From manual

IMG_1793.jpeg (295.31 KiB) Viewed 151 times

I was wrong. I think this predates the McCormick collaboration.

[Steely Joe]

both fuses test good.

I thought there are a total of 5 fuses in the MF2000 series. Are you sure you didn't miss any?

EUREKA- 2 of the rear fuses were blown.
THANKS.

[Big Dog RJ]

mmm... that's what I thought. Thanks.
Woof! RJ

[Steely Joe]

both fuses test good.

I thought there are a total of 5 fuses in the MF2000 series. Are you sure you didn't miss any?

EUREKA- 2 of the rear fuses were blown.
THANKS.

I replaced the 2 rear fuses. IT WORKS. Sounds like Nina Simone is in the room.
Thanks all in CJO.

[paulCJ]

Congrats!! Glad the old 2250 sounds great and was only fuses!!

[microstrip]

Happy to know you are now listening again to the MF-2250 with the help of our administrator and members experience!

Although we do not have the schematics of the MF series we have the schematic of the premier 350 solid state amplifier. As SS cj designs share the same simple building blocks with a few variations, using it a qualified technician can easily solve any problem in these amplifiers. These amplifiers are not rocket science - in Europe we have plenty of technicians that are able to solve such problems without sending units across the Altantic Ocean, I can imagine the US also have excellent independent service people. By mistake I once shorted the output terminals of one channel of a Premier 350 paying loud, burning several output transistors and resistors and it was easily solved locally with a bill around 300 euros.

If we have a similar problem the first thing to look is if the distortion exists in just one of both the channels. If only one channel is faulty most technicians can solve it just by comparison between the channels. Experienced technicians know a lot more than we sometimes imagine - my usual technician once solved a problem in an amplifier just looking at the aspect of the circuit. He looked at a few burnt resistors, got a few transistors and resistors, replaced them and just adjusted bias after powering it on ...

Please note I am particularly addressing conrad johnson gear - I would not say the same about some other very complex brands.