Conrad Johnson MF-2250 horribly distorted
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Conrad Johnson MF-2250 horribly distorted
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
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
- AnotherJohnson
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Re: Conrad Johnson MF-2250 horribly distorted
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.
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.
Last edited by AnotherJohnson on Thu Nov 07, 2024 9:51 am, edited 2 times in total.
It’s just stuff. I like mine. I hope you like yours. I probably like yours too.
Re: Conrad Johnson MF-2250 horribly distorted
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.
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Main stereo: ART Amplifier and ET7s2. 2nd stereo: PV-14L and MV-55. Previously Owned: PF2 preamp, Evolution 2000 Amp, PV-12AL preamp, D/A-2b Vacuum-Tube Digital Processor.
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Main stereo: ART Amplifier and ET7s2. 2nd stereo: PV-14L and MV-55. Previously Owned: PF2 preamp, Evolution 2000 Amp, PV-12AL preamp, D/A-2b Vacuum-Tube Digital Processor.
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Re: Conrad Johnson MF-2250 horribly distorted
I opened the bottom plate and found a few "shady tree mechanic" soldering attempts.admin wrote: Thu Nov 07, 2024 9:46 am 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.
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Re: Conrad Johnson MF-2250 horribly distorted
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.
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Home Theater in Member Gallery
Main stereo: ART Amplifier and ET7s2. 2nd stereo: PV-14L and MV-55. Previously Owned: PF2 preamp, Evolution 2000 Amp, PV-12AL preamp, D/A-2b Vacuum-Tube Digital Processor.
Home Theater in Member Gallery
Main stereo: ART Amplifier and ET7s2. 2nd stereo: PV-14L and MV-55. Previously Owned: PF2 preamp, Evolution 2000 Amp, PV-12AL preamp, D/A-2b Vacuum-Tube Digital Processor.
- AnotherJohnson
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Re: Conrad Johnson MF-2250 horribly distorted
admin wrote: Thu Nov 07, 2024 2:39 pm 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.
It’s just stuff. I like mine. I hope you like yours. I probably like yours too.
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Re: Conrad Johnson MF-2250 horribly distorted
It’s just stuff. I like mine. I hope you like yours. I probably like yours too.
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Re: Conrad Johnson MF-2250 horribly distorted
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
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
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Re: Conrad Johnson MF-2250 horribly distorted
both fuses test good.
Re: Conrad Johnson MF-2250 horribly distorted
I thought there are a total of 5 fuses in the MF2000 series. Are you sure you didn't miss any?
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Home Theater in Member Gallery
Main stereo: ART Amplifier and ET7s2. 2nd stereo: PV-14L and MV-55. Previously Owned: PF2 preamp, Evolution 2000 Amp, PV-12AL preamp, D/A-2b Vacuum-Tube Digital Processor.
Home Theater in Member Gallery
Main stereo: ART Amplifier and ET7s2. 2nd stereo: PV-14L and MV-55. Previously Owned: PF2 preamp, Evolution 2000 Amp, PV-12AL preamp, D/A-2b Vacuum-Tube Digital Processor.
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Re: Conrad Johnson MF-2250 horribly distorted
I was wrong. I think this predates the McCormick collaboration.
It’s just stuff. I like mine. I hope you like yours. I probably like yours too.
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Re: Conrad Johnson MF-2250 horribly distorted
EUREKA- 2 of the rear fuses were blown.admin wrote: Fri Nov 08, 2024 5:15 pmI thought there are a total of 5 fuses in the MF2000 series. Are you sure you didn't miss any?
THANKS.
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Re: Conrad Johnson MF-2250 horribly distorted
mmm... that's what I thought. Thanks.
Woof! RJ
Woof! RJ
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Re: Conrad Johnson MF-2250 horribly distorted
I replaced the 2 rear fuses. IT WORKS. Sounds like Nina Simone is in the room.Steely Joe wrote: Sat Nov 09, 2024 8:52 pmEUREKA- 2 of the rear fuses were blown.admin wrote: Fri Nov 08, 2024 5:15 pmI thought there are a total of 5 fuses in the MF2000 series. Are you sure you didn't miss any?
THANKS.
Thanks all in CJO.
Re: Conrad Johnson MF-2250 horribly distorted
Congrats!! Glad the old 2250 sounds great and was only fuses!!
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Re: Conrad Johnson MF-2250 horribly distorted
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.
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.
Currently listening mostly to dCS Vivaldi, cj GAT2 preamplifier, cj ART amplifiers and SoundLab A1Px's.