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Old 1st August 2009, 07:45 PM   #1
PB2 is offline PB2  United States
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Question Marantz 250M Repair

I have a Marantz 250M here from the original owner who says it worked fine until he put it in a new system and the speaker leads were shorted blowing one channel.

He had it worked on, but it is still not working. I think it was checked out but not completed.

I have the schematic. Does anyone know the story behind these Motorola SJ2519/sj2520 output devices?
Are they simply beta selected?
The beta range is listed for many of the transistors.

This is a nice looking piece, however it is a rather unusual design. Seems it does not tolerate a shorted output even though it has protection.

Pete B.
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Old 4th August 2009, 02:19 AM   #2
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Default Driver Substitutes

Looking at a Motorola Product Selector Guide and trying to determine what the TO-66 drivers might be,
I see that there are the:
2N3584/2N6421 which are 2A, 35W, 250V, hfe 25 min at 1A, ft = 10 MHz
And the:
MJ3248/MJ3238 which are 8A, 75W, 150V, hfe 40 min at 3A, ft = 20 MHz

The drivers drive the load through a 5.6 ohm resistor but are also boosted by the output transistors.
The outputs offer no help when the output is shorted to ground and the drivers would see
very high currents until the current limiter turns on. This makes me lean toward the 8A part, especially
considering thermal derating and SOA.
MJE15034/35 should be a good substitute.

Pete B.
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Old 4th August 2009, 02:34 AM   #3
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KSA1220A/KSC2690A would be my choice for predrivers:
160V, 1.2A, 20W, 155 MHz ft
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Old 4th August 2009, 03:10 AM   #4
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Hi,
Quote:
Does anyone know the story behind these Motorola SJ2519/sj2520 output devices?
Could they be based on the Toshiba 2SD424/2SB554?
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Old 4th August 2009, 05:22 AM   #5
PB2 is offline PB2  United States
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Here is the schematic:
http://www.audio-circuit.dk/Schemati...0M-pwr-sch.pdf

I don't think they could have been based on the Toshibas, since this amp was sold in 1976 - for just one year.

Pete B.
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Old 4th August 2009, 06:09 AM   #6
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Pete,
pretty interesting design for being from 1976. No recommendation, but I would omit a number of stages.
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Old 4th August 2009, 06:09 AM   #7
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With only a single pair of output device per channel beta selecting (matching) isn't a big deal. If the beta is low it will likely just increase THD a bit at high power levels.

What might be an issue is the stability (compensation) if you put modern faster output devices in. I've worked on some old Marantz amps that hated newer transistors but I don't have experience with the 250M. If you're not familiar with repairing power amps, you might want to either let someone else do it, or read up on how to try to avoid lots of collateral damage if things don't go well when you power up a repaired amp.

If the amp oscillates on power up, for example, it will probably self destruct before you realize it--even with such tricks as a lightbulb in series with the AC power. And a blown output transistor can take other parts with it. So it can be both expensive and frustrating. There are lots of tricks to help avoid problems, but they vary in their effectiveness, applicability, and depend on what test equipment you have access to, etc. If you don't have the right equipment, it can be very hit or miss.

Especially if someone else has attempted a repair, and failed for some reason, I'd be cautious. If it was a simple repair, it probably would be working now. If it left the shop still dead, it was likely either an expensive repair, a failed repair, and/or the shop knew from experience substitute parts wouldn't likely work, etc.
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Old 4th August 2009, 06:40 AM   #8
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I had the non metered version of this amp - the 240 - for about 18 months in the late 1970's before it blew the output stage (I think RF ingress caused it to oscillate, fryng the output stage).

Bcause of the CFP output, the output devices are probably quite critial in that if you stuck a low ft device in there, it migh t not behave very well. For th e small signal devices, I don't think anything special is needed. This amp could do with a bit of degeneration in th e long tailed pair, but that's not a topic for here.

However, left with little info and not much else, I'd just go for the highest FT devices you can find consistent with the package type (T03). This will probably b e On types, and back in th e 70's, Ft's in thes e big device s were not much bette r than 4 of 5 MHz.

I still have the caps (2 x Sprague 22mfd) from this amp!

If this amp had lasted and blown on me now, I would probably know enought to ge t it going again - but not back in the late 70's - I pretty green then.
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Old 4th August 2009, 03:02 PM   #9
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Thanks for all the tips; I've had plenty of experience repairing, debugging, and scratch building amps, usually ones others were unable to fix.

I am curious as to if these were in any way stable.

Perhaps I should first look at the working channel to see if I can measure the open loop transfer function and how well the output protection works.

There are a lot of compensation caps around the output stage and it looks like they had trouble stabilizing this amp.

Pete B.
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Old 4th August 2009, 06:34 PM   #10
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PB2,
the CFP is a great compound when implemented "right", which can be very fastidious to do. In order to avoid headaches, oscillation and bothersome crossover distortion I would convert it to to common EF. Paralleled slaves is a problem as well.
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