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Need Help With Marsh Amplifier
Old 29th April 2018
  #1
Gear Nut
 

Need Help With Marsh Amplifier

Hi Guys:
I have a Marsh Sound Design A200s power amplifier. Can't seem to find a schematic. It keeps blowing fuses, F901 and F902 which appear to be related only to one channel (some sort of protection circuit). The other channel functions OK. These fuses are not for the mains. The mains fuse does not blow.

Any suggestions would be appreciated.

best regards
Steve
Old 30th April 2018
  #2
Lives for gear
 

Speaking very generally for conventional solid-state amplifiers . . . what you describe sounds like the + and - rail fuses . . . and it's most likely that the output stage has failed, with the output transistors shorted.
Old 2nd May 2018
  #3
Gear Nut
 

Thanks for the reply kirkus, Do you think that the output transistors shorting is the cause of the failure?

best regards
Steve

Last edited by Northamusi; 2nd May 2018 at 04:40 PM..
Old 2nd May 2018
  #4
Lives for gear
 

Quote:
Originally Posted by Northamusi View Post
Thanks for the reply kirkus, Do you think that the output transistors shorting is the cause of the failure?
I think the shorted output transistors is the main failure . . . and the cause of which for a typical solid-state amplifier is usually excessive output current in combination with excessive operating temperature. Internal protection circuitry is supposed to safeguard against this and prevent it from happening, but nothing's every completely perfectly foolproof.
Old 2nd May 2018
  #5
Lives for gear
 

Haven't heard that name in awhile.

Usually when you have a hard time finding schematics its because they are imports or out of business, especially allot of amps that weren't very popular to begin with or pre dated the internet.

In this case its because they are built from kits. You should be able to get a schematic from the company that sells the kits pretty easily. http://www.marshamps.com/ They have an EMail address under contacts.

I suspect since all the kits they sell are clones of other manufacturers gear. If they were to sell a power head kit it would likely be a very simple design for people to build and troubleshoot. As Kirkus mentioned, Starting by testing the power transistors would be my first guess too. Also checking the power caps and voltage regulator. Make sure the transistors are good before connecting any speaker or you can risk passing 120VAc through them with a shorted transistor and blow them instantly.
Old 2nd May 2018
  #6
Lives for gear
 

Quote:
Originally Posted by wrgkmc View Post
In this case its because they are built from kits. You should be able to get a schematic from the company that sells the kits pretty easily. http://www.marshamps.com/ They have an EMail address under contacts
I think this is a different Marsh . . . IIRC Marsh Sound Design was back in the late 1990s or early 2000s, designed by Richard Marsh, after he left MIT cables.

I think he may have written some articles for Jan Didden's Linear Audio publication, and you might search/inquire around for his contact information, and give him an email.
Old 3rd May 2018
  #7
Gear Nut
 

Yes kirkus, Marsh Sound Design was Richard Marsh after he left MIT cables. I'll search using Linear Audio and see what comes up.
thank you
Old 14th June 2018
  #8
Gear Nut
 

Update: I removed the output transistors and tested them. Some of them were shorted and I replaced them. The amplifier is now working, passes audio and sounds fine. However I notice that two of the new transistors get very hot, very quickly (within a minute or two of turning on the amp). The corresponding transistors in the other channel do not heat up anywhere near as much. Is there a bias adjustment that needs to be made?

thank you again,
Steve
Old 14th June 2018
  #9
Lives for gear
 

Quote:
Originally Posted by Northamusi View Post
The corresponding transistors in the other channel do not heat up anywhere near as much. Is there a bias adjustment that needs to be made?
The bias adjustment on a solid-state amplifier is almost always made via a trimmer potentiometer on the circuit board, near the output transistors. Usually there's only one of these . . . if there are two, the other one usually affects the DC offset at the output.

To make the adjustment, first locate the "emitter resistors", which are low-value (i.e. 0.47 ohm or less) and relatively high-wattage (usually 5 watt) . . . and there's usually one per main output transistor. They're connected in series with the output transistors' emitters (for an emitter-follower output stage design) or with their collectors (for the Sziklai pair design, less common). Start with the amplifier off and completely cold, put your leads across one of the resistors in the good channel, flip the amplifier on, and note the reading after it stabilizes for about 10-15 seconds. Flip the amp off, move the leads to the same resistor on the overheating channel, turn it on, and adjust the bias trimmer for the same reading.

This will require a digital voltmeter that has a decent low-voltage range, as the measurement value will usually be in the range of 5-20 millivolts, depending on the emitter resistor value and the design of the amplifier. If you find that on the bad channel, the trimpot makes no difference, or it can't be adjusted anywhere near the proper value . . . then something else has gone bad in the circuit. If this is the case, then the first thing I'd look for is a defective "Vbe multiplier" bias transistor, which is usually a small-signal transistor that's stuck to the main heatsink (such that its temperature somewhat tracks the temperature of the output transistors). It could also be caused by a defective driver transistor, or something else . . . but in any case it's overwhelmingly likely that this would be caused by a component that's pretty close to the output transistors.
Old 18th June 2018
  #10
Gear Nut
 

Hi Kirkus:
Thanks again for your help. Your analysis appears to be spot-on.

I was able to get a schematic and I am posting it. I highlighted six transistors that were replaced. It looks to me like VR206 is the bias adjust pot, and VR204 is the DC offset adjustment. Does that look correct to you?

This AM, I made some preliminary measurements, and the voltage across the emitter resistors for the repaired channel are considerably higher than the working channel.

I appreciate your help,
Steve
Attached Files
File Type: pdf A200S schematic P2.pdf (243.3 KB, 10 views)
Old 18th June 2018
  #11
Lives for gear
 

Quote:
Originally Posted by Northamusi View Post
It looks to me like VR206 is the bias adjust pot, and VR204 is the DC offset adjustment. Does that look correct to you?
Yep, spot on. The offset is maintained in operation by the DC servo U200, so VR204 is more of a "coarse" adjustment . . . mainly to account for wide variations in Vgs tolerances for the input JFETs. If you're going to adjust this, it'd be easier to do if you first short across C248, so the servo isn't pulling the offset around while you're adjusting the trimpot. As with the bias, make this adjustment without a speaker load on the amplifier.

The design of this output stage is an emitter-follower, preceded by a Sziklai-pair driver stage. If you can't get the bias in the right range, I'd check Q230/Q232 and R252/R260 . . . it's possible that these could be defective in a manner that still allowed the amp to stumble along and "kinda" work, but not work correctly . . . and the out-of-control bias might simply be the most observable of several symptoms. Q224 is the aforementioned "Vbe multiplier" transistor, which is used for temperature-compensation of the output-stage bias.
Old 25th July 2018
  #12
Gear Nut
 

Hi Kirkus:
I think that we have solved the issues. I was able to get the bias of the repaired channel within a few millivolts of the working channel. I could not get them to match precisely. Do you think that is a problem?

The new transistors are running much less hot and the amp sounds great. I'm posting a couple of photos of a 'scope showing a 1 kHz sine wave input to each channel. I input the sine wave to the repaired channel at a variety of levels and it looks the same regardless of input level (no clipping).

I did not mess with the DC offset because I measured approximately 0.1 mV of DC at the outputs.

Let me know what you think, and thanks again for the help.
best regards
Steve
Attached Thumbnails
Need Help With Marsh Amplifier-scope-left.jpg   Need Help With Marsh Amplifier-scope-right-repaired.jpg  
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