MF2500 throwing 68 DCvolt out on right channel

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jahatl513
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MF2500 throwing 68 DCvolt out on right channel

Post by jahatl513 » Thu Dec 21, 2017 12:43 pm

Is there anyone who can help identify if this is possibly a fixable issue; left channel plays fine. I don't mind sending it to Jeff to have it fixed, just wonder what it could be and the amp is not in my hands.

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Re: MF2500 throwing 68 DCvolt out on right channel

Post by admin » Thu Dec 21, 2017 3:57 pm

Unfortunately, we do not have the MF2500 schematic in our database. If you have access to the unit you can try to take measurements backwards along the signal path and try to identify at what component the signals deviate from each other.

However, you would have to be very careful and kind of know what you are doing as working on a live amp can be dangerous.
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Re: MF2500 throwing 68 DCvolt out on right channel

Post by ronenash » Fri Dec 22, 2017 3:55 pm

Most likely it's the output transistors failed. It's not unlikely they took out other parts as well. The amp needs to be professionally serviced.
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Re: MF2500 throwing 68 DCvolt out on right channel

Post by bambadoo » Wed Jan 03, 2018 4:53 am

Have one that did excactly the same. Many parts are fried including one pair of output transistors.
Look here
viewtopic.php?f=17&t=1118

and here

http://www.diyaudio.com/forums/solid-st ... f2500.html

CJ was kind enough providing schematics.

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Re: MF2500 throwing 68 DCvolt out on right channel

Post by roberto » Tue Jan 09, 2018 3:02 pm

Hola. Bambadoo is right! Follow his instructions.
68 DC Volts can fried your speaker coils. Do not plug any speakers to the output terminal. If you want to load the amp, use a 30 Watt resistor, from 4 to 8 ohms.

My advice is, you must check all the output transistors from emitter to collector leakage. Drives could suffer of leakage too. Do the same thing. Usually at the input, there is a diferential amplifier, you must check here for semiconductor leakage too.

My best advice is, contact Conrad Johnson for help and advice. Mr. Jeff Fisher is a very helpful man. He can guide you better with your problem.
Last edited by roberto on Mon Jan 15, 2018 12:41 am, edited 1 time in total.
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designing engineer for Evulation 2000?

Post by runner70 » Fri Jan 12, 2018 10:28 am

I have been enjoying the Youtube videos by Stan McGowen from PS Audio. A great deal of information concerning many audiophile topics, but a great interview with both Bascom King and Arnie Nudell. The discussion was great (sorry to see Arnie pass away not long ago). I owned and operated for 13 year a high end audio store in Oshkosh, Wisconsin (Audiophile Specialists). We handled a large number of great products (one of which was Infinity).
I currently have an Evolution 2000 power amp (had it for over 6 years and great sound). But wondered about all the talk about tube front ends and MOSFET outputs on the new PS Audio BHK (Bascom King) designed power amp. He (Bascom) stated that this was one of the first times this application was used. I thought this is the design of my CJ power amp. Bascom became technical when discussing circuit design of outputs, and I am not qualified to be able to understand that, but did Bacom King have any input on the design of the CJ 2000 power amp, and how much is different between the new PS Audio BHK design amp, and my CJ power amp? Stan did mention that Bascom did work with CJ, so just wondering how the two compare? I know my unit is a few years old, and this is a new product from PS Audio, but would like to know how they compare in design and maybe, sound? Love my CJ amp. Is bi-amped with the CJ driving my Acoustat Monitor 4 panels, and a ARC D150 driving my two large subs, with a modified Dahlquist electronic crossover, along with a tube preamp and tube CD player. These are products that I sold for many years, and still lover them after 30 years. Work great and still sound great. Do you recommend any reasonable priced mods for the amp? No mods to date on the unit.

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Re: MF2500 throwing 68 DCvolt out on right channel

Post by admin » Sat Jan 13, 2018 7:59 am

Hi runner70. I also have a 2000 Evolution amp and love it. I, unfortunately, do not know its design history in detail. I too would be interested in the sound comparison of this now "older" amp vs something designed more recently. As for mods, one simple cheap mod that can be done and which my unit has is the addition of an IEC plug so the power cord can be changed. Whether this makes a significant difference in sound,... I don't know?

-sorry about this post being a little off-topic.
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Re: MF2500 throwing 68 DCvolt out on right channel

Post by jahatl513 » Tue Jan 23, 2018 11:53 pm

Lol; off topic Admin. UPDATE: it went to CJ and I should know in a week or so about what is in need of repair and will try and follow through.

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Re: MF2500 throwing 68 DCvolt out on right channel

Post by jahatl513 » Fri Feb 16, 2018 12:11 am

Update: Sent the amp to CJ. Spoke with Keith and I had decided that it's worth saving so what I am ending up with is a just about a new amp. It has gotten the "A" upgrade. All new main caps; upgraded bridge rectifier w/ discrete slow diodes; Van den hul output wires, removed un-necessary filters, and the different resistors for the power caps for slow drain; makes it faster. All said and done I'm told it's 90% Premier 350 in sound; this from them. To finish it off, a new face plate from NOS. Worth it? What do you think?

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Re: MF2500 throwing 68 DCvolt out on right channel

Post by roberto » Fri Feb 16, 2018 12:50 am

Hola. A very clever decision! Congratulations! You can't go wrong.

Happy listening!
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Re: MF2500 throwing 68 DCvolt out on right channel

Post by admin » Fri Feb 16, 2018 8:19 am

Great choice. Enjoy.
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Re: MF2500 throwing 68 DCvolt out on right channel

Post by jahatl513 » Mon Feb 19, 2018 12:45 pm

I received the amp Friday, it is almost as big as my Premier 350, but you can actually pick it up and move it. I hooked it up to my Premier 16LS2 and started a weekend long listening session. Immediate thoughts are that I'm completely happy with the outcome. It drives my system output w/ less volume needed, has some very nice bass and mid-range. I''ll get back on the high's later; giving it a good listen before any further judgement.

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Re: MF2500 throwing 68 DCvolt out on right channel

Post by bambadoo » Sun Mar 04, 2018 4:11 pm

Did CJ say what was wrong with the amp? What was fixed?

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Re: MF2500 throwing 68 DCvolt out on right channel

Post by jahatl513 » Tue Mar 06, 2018 4:53 pm

Yes, Keith and I worked out a plan to restore it to a glory that its former could not obtain. You see I bought this at estate the same day I purchased my Premier 350. Here is what was done to it. No expense was spared (absurd or not) New: Main Caps, Bridge Rectifier w/ discrete slow diodes, different resistors for power caps to drain them down over time and give it even more life. Board was completely gone over, anything needing replaced was, and room on board for new Van Den Hul Output Wires, New power switch capacitor, Removed unnecessary output filters. New face plate, so cosmetically it looks amazing Tested and burned in over a week at CJ. Keith had said, this about what all he ended up doing, "Its now 90% Premier 350". My initial impressions are:
The highs are very well defined, the mid is big and sweet at times, and the low is very present and controlled. Is it as well defined as the P350? I'll have to wait and see. I have every confidence in this amp. It is extremely powerful and at times feels more powerful than the P350. I have it listed on AG for a pretty penny. I will most likely keep it as it sounds absolutely terrific on the Premier 16LS2; will try it on the ET-7 in a few weeks.

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Re: MF2500 throwing 68 DCvolt out on right channel

Post by bambadoo » Thu Mar 08, 2018 2:58 pm

Thanks alot for a good answer.
Did they respond on why it threw 68Vdc on right channel?

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Re: MF2500 throwing 68 DCvolt out on right channel

Post by jahatl513 » Sat Mar 10, 2018 10:46 pm

Bambadoo- No, but its hidden cryptically in the replacement parts I suppose. I did not have it when it had blown so I could'nt tell you more.

Update- I put it on my ET-7 yesterday (just got a new ET-7 and a new Classic 120SE) and it sounded outstanding. Keeping everything at fairly low volume as the ET burns in slowly, but you can hear the subtle essence of very good highs and superb mid, a very tight bass. The amp is a superb winner. The upgrades have made it quite a contender. As I said I have a Premier 350 as well, so the MF is still not a Premier 350 but very close call, and as I've had many amps and I could live with the MF forever w/ out looking back. The ET-7 combo will prove to be super nice.

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Re: MF2500 throwing 68 DCvolt out on right channel

Post by bambadoo » Mon Mar 12, 2018 5:20 am

Thanks alot for your reply :)

Started up my "fixed" MF2500 yesterday. No smoke, no hum in transformer. Adjusted dc offset and bias. Looks stable and fine. Will do some more testing perhaps today.
Also have done some small modifications. And as far as I understand - it seems as the biggest difference between an mf2500 and mf2500a is new discrete bridge rectifier which is easy to implement.
Removed unnecessary output filters
Are you able to tell more about that? :)

BTW: CJ provided schematics for MF2500 to me. I am not sure if it is ok to share on this site?

Sincerely

Kid doing repair :)
20180311_184637.jpg
(2.91 MiB) Not downloaded yet

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Re: MF2500 throwing 68 DCvolt out on right channel

Post by admin » Mon Mar 12, 2018 7:49 am

Cute picture,... I hope it's unplugged!

Glad the work went well.

As for the schematic you can email it to me at "admin@conradjohnsonowners.com" and I can add it to our database. Essentially all our schematics in the database were donated by our users so there should be no problem.
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Re: MF2500 throwing 68 DCvolt out on right channel

Post by bambadoo » Mon Mar 12, 2018 2:29 pm

Yes - unplugged for about a year :)

Now it sings..

Schematic coming soon to an email near you..

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Re: MF2500 throwing 68 DCvolt out on right channel

Post by bambadoo » Wed Mar 14, 2018 3:07 pm

It plays extremely well.
Had a ground loop problem with one of my preamps. That was fixed and now it is dead silent and absolutely no transformer humming also.

According to my knowledge the only "soundwise" difference between a 2500 and a 2500A is the new discrete rectifier diodes.

jahatl513 - is it possible to be so kind and take a picture from inside your amp?

Would be very helpful :)

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