Help Need To Repair Marantz Pm 94 Class A Amp - diyAudio
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Old 25th February 2009, 09:25 AM   #1
seafire is offline seafire  Mauritius
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Red face Help Need To Repair Marantz Pm 94 Class A Amp

Hi all my sister donated me her marantz pm 94 class a amp.It switches on goes through all the moves but no music from output posts.Being the fretler i am i opened it up and started scanning.Please remember my knowledge of electronics is extremely limited but im intrigued and willing to learn.The amp is basicaly 2 x mono design and on further inspection i found a burnt mosfet.My sister confirmed that one of her kids had dead shorted the speaker cables some years back.I then stripped it out and the mosfet in question reads K 405.oN I phoned a supplier in South Africa who said it wasnt made anymore the replacment is a K 1529 pared with J200.
QUESTIONS
1-Is the replacement part K1529/J200 CORRECT to replace k405/j115.
2-Do i replace only k405(burnt) with k1529 or do i need to do the pair ie. replace j115(not burnt) but its opposite as well with J200 ie. replace as a pair
3-What about the other 2 pairs of K405/J115 that are not burnt on the left side channel ie.should i replace all six.
4-what about the right side amp which shows no damage do i leave as is ie. left channel x3 pairs J1529/J200 right channel x3 pairs K405/J115(original)I am sure this will be heard in the sound???
5-My instinct says replace only burnt and oposite if it works then proceed to replace all on left and right channel as to balance amp with new components-is this correct
Only one mosfet is burnt(from what i can see??) but no sound at all from the amp right channel not burnt is it some protection circuit kicking in??
Will i have to reset bias current and how would i do this?
PLEASE PLEASE guys would love to do this as my first project.I have a very good fluke meter the wiring diagram of the amp(of the net) and a very good set of ears to listen and learn Your help for this newbie would be appreciated and i hope ive posted on the right place regards Seafire
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Old 26th February 2009, 01:56 PM   #2
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1-Is the replacement part K1529/J200 CORRECT to replace k405/j115.

I don't know this one off the top of my head but I'm sure that someone will provide some information and advice.

2-Do i replace only k405(burnt) with k1529 or do i need to do the pair ie. replace j115(not burnt) but its opposite as well with J200 ie. replace as a pair.

I always replace outputs as a matched pair. You may need to replace the driver transistor as well and any of the resistors and capacitors that are directly related to the burned out transistor. You can check to see if these components are still good with a resistance check - but you might need to disconnect one end of the device from the circuit (unsolder and pull away for the PWB) to get a valid measurement with your meter.

3-What about the other 2 pairs of K405/J115 that are not burnt on the left side channel ie. should i replace all six.

Because you are replacing the originals with updated parts this might be a good idea so that both channels have the same performance.

4-what about the right side amp which shows no damage do i leave as is ie. left channel x3 pairs J1529/J200 right channel x3 pairs K405/J115(original)I am sure this will be heard in the sound???

Same answer as 3

5-My instinct says replace only burnt and oposite if it works then proceed to replace all on left and right channel as to balance amp with new components-is this correct

This is a sometime yes and sometimes no answer - it depends on the design of the circuit. You could replace just the suspected bad components and leave the other channel alone. If you were using the same parts that the amplifier came with then this would most likely work out OK. HOWEVER - you are using updated parts - and this could be a little be trickier - especially with class A because of the power being dissipated. By using the updated devices you might need to change of bias resistors and this might or might not trip up any circuits that sense an amplifier overload condition. Basically your instincts are correct - but don't be suppressed if you need to go with plan B - replacing everything on both output channels. Even then the overload circuits might get tripped.

6 Only one mosfet is burnt (from what I can see??) but no sound at all from the amp right channel not burnt is it some protection circuit kicking in??

Most likely the case. See if removing the output xsistors on the bad channel will result in the good channel firing up. Depending on the output protection circuit it might let you get away with it.

7 Will i have to reset bias current and how would i do this?

Yes - information should be in the service manual.

Check the service manual - most of them will have a troubleshooting guide and recommended repairs. There should also be some warnings about things that you should not do.

There should also be a section about bias and alignment instructions for protection circuits.

Hope this helps. Are there any Marantz forums on the net?
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Old 26th February 2009, 05:30 PM   #3
seafire is offline seafire  Mauritius
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Thank you very much so far C2Cthomas you have been very helpfull.My biggest concern now is to find the right mosfets so i will be wating to see if someone on the forum has some info
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Old 26th February 2009, 05:43 PM   #4
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Try here: http://www.anatekcorp.com/faq/compcref.htm

or put this into your search engine of choice: "electronic component cross reference"
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Old 26th February 2009, 07:43 PM   #5
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Not that this will help your repair but why are you calling it "class a"?
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Old 26th February 2009, 07:59 PM   #6
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http://www.hi-fiworld.co.uk/hfw/olde...rantzpm94.html

"The '94 also boasted Marantz's unusual 'Quarter-A' system, in which lower level signals up to a quarter of the maximum power output were handled in pure Class A. When called upon to pump higher levels, the amp switched into Class B, resulting in a not inconsiderable 140W per channel. The system was surprisingly effective in practice, and not, as many assumed, just a Japanese-style sales gimmick. "
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Old 26th February 2009, 10:29 PM   #7
seafire is offline seafire  Mauritius
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Hi c2c just to keepyou updated i took your advice searching for cross reference and came up with a supplier in the states ICE COMPONENTS.
They have the original k405 and j115 in stock so thanx for your help there!.Just noticed tonight that the preamp (not sure i think so) on both left and right channels have to cerafine caps on each.One of the two caps on each side is slightly bulged does that mean they are popped or semi popped and where do i find hi quality replacement caps what make do you recommend
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Old 27th February 2009, 12:54 AM   #8
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Quote:
Originally posted by seafire
.Just noticed tonight that the preamp (not sure i think so) on both left and right channels have to cerafine caps on each.One of the two caps on each side is slightly bulged does that mean they are popped or semi popped and where do i find hi quality replacement caps what make do you recommend
Well 1st of all you really need to look at the service manual you have and see if this is the pre-amp, amp, power supply, etc. It could be the hidden compartments that was sometimes included on the older gear and had secret spare parts stored in it. That's where I used to store all the parts I had left over after I fixed I 'em.

IF you can disconnect one side of the cap put your ohm meter across it and see if it is shorted. Depending upon it's value you might see the resistance reading on the meter decrease as the capacitor charges up. Also - the cap's value might serve as a clue as to it's function. Small values - signal cap - large value (10,000 uFd) power supply cap. In between value - say 470 UFd - most likely signal cap - but may serve some other purpose such as in over protection circuit. That's where the service manual comes into play. Can you read electronic schematics? I don't intend to be rude by asking this question - but I have no idea how familiar you are with electronics and knowing this may help in assisting you.

With this unit you should be using the best caps that you can afford. Parts Express has some - but most often used in speaker cross-overs. You might get lucky depending on your value required.
Mouser and Digi-key are good sources for regular parts but I don't know if they carry high end caps. x-bay might be a good source - but you need to watch out for counterfit parts.

BTW - you are correct about a swollen cap being suspect. Bulges usually indicate an internal short and the heat causes expansion of the electrolytic material. Wear eye (and maybe ear) protection because when one of 'em let's loose it's like a fire cracker going off and for sure can cause you some physical damage.

When you fire this bad boy up for the 1st time be well away from it and near a switch that can shut off the electricity. A nice long extension cord that you can unplug from the wall end of things is one way to go.

Hope this helps!
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Old 27th February 2009, 09:06 AM   #9
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Quote:
Will i have to reset bias current and how would i do this?
You still have a working channel; try to measure the current through the fets, usually done by measuring the voltage across source resistors (large power resistors usually).

That gives you a starting point in case you don't find a service manual.

Have fun, Hannes
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Old 27th February 2009, 01:37 PM   #10
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Quote:
Originally posted by c2cthomas
http://www.hi-fiworld.co.uk/hfw/olde...rantzpm94.html

"The '94 also boasted Marantz's unusual 'Quarter-A' system, in which lower level signals up to a quarter of the maximum power output were handled in pure Class A. When called upon to pump higher levels, the amp switched into Class B, resulting in a not inconsiderable 140W per channel. The system was surprisingly effective in practice, and not, as many assumed, just a Japanese-style sales gimmick. "
It was the same mode as Yamaha M-85 ?
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