My beloved Marantz 1200 board was rebuilt, it blew an output and driver transistor, (one of the electrolytic caps was touching one of the pre driver transistors, grounding the circuit). After playing music through the amp for several hours, I noted that the rebuilt board heatsink is warmer than the other channel. Not hot, just warmer.
Output transistors were replaced with new MJ21193 and MJ21194 transistors. One driver transistor and two pre driver transistors were replaced, as well as an electrolytic cap. Reinstalled the board, bias and DC offset were set on both channels and both channels were stable after an hour. I'm going to clean and apply new heat transfer compound and mica insulators on the un-rebuilt channel tomorrow- or should I just let it be?
Output transistors were replaced with new MJ21193 and MJ21194 transistors. One driver transistor and two pre driver transistors were replaced, as well as an electrolytic cap. Reinstalled the board, bias and DC offset were set on both channels and both channels were stable after an hour. I'm going to clean and apply new heat transfer compound and mica insulators on the un-rebuilt channel tomorrow- or should I just let it be?
Thanks Anatech.
I found that the bias reading was too high because I adjusted bias by clipping the leads onto the resistors on the attached photo that look like springs near the toroid inductor.
When I checked the bias by putting one lead on the PNP output transistor collector and the other lead on one of the NPN transistors collector, the bias reading was higher. So, I adjusted both channels bias with leads on the output transistors collectors. Both channels now run cool.
I found that the bias reading was too high because I adjusted bias by clipping the leads onto the resistors on the attached photo that look like springs near the toroid inductor.
When I checked the bias by putting one lead on the PNP output transistor collector and the other lead on one of the NPN transistors collector, the bias reading was higher. So, I adjusted both channels bias with leads on the output transistors collectors. Both channels now run cool.
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Too many. Don't replace transistors unless they are actually bad. I spent weeks replacing transistors only to replace them with the old transistors. I bought a 9V operated transistor tester online which showed the transistors were good, then borrowed a curve tracer from a colleague in the EE department and learned how to use it. That is when I found two transistors that tested as good with the 9V tester, but were bad when tested with the curve tracer.
What I found was the differential pair (Q501 and Q502) needs to be matched for Hfe range. Same with the output transistors (Q802-Q805), drivers (Q510 and Q511) and pre-drivers (Q507 and Q508). Today, the manufacturing process and equipment allows these components to be fairly closely match when they're manufactured, especially if they are from the same process lot.
If you can't bias the power amp board, it was usually Q801. DC balance problems were usually caused by Q501 and Q502. Blown output transistors were because of shorted or open resistors, and when the outputs went bad, it usually caused one or both of the driver transistors to go bad.
Tinkering with this amplifier caused me to learn a lot about these Marantz amplifiers, the cost was my time. I've also found UREI amplifiers to be good, and also like the Hafler amplifiers of the same era. Also, the Leach Amp is a good sounding amplifier that you can build yourself.
Because these amplifiers are old, the more you unsolder and solder the boards, the more likely it is that you will have lifted traces and other problems. Buy or borrow a desoldering tool, use a good soldering iron. Don't shotgun replace the transistors unless you definitely know it is bad.
What I found was the differential pair (Q501 and Q502) needs to be matched for Hfe range. Same with the output transistors (Q802-Q805), drivers (Q510 and Q511) and pre-drivers (Q507 and Q508). Today, the manufacturing process and equipment allows these components to be fairly closely match when they're manufactured, especially if they are from the same process lot.
If you can't bias the power amp board, it was usually Q801. DC balance problems were usually caused by Q501 and Q502. Blown output transistors were because of shorted or open resistors, and when the outputs went bad, it usually caused one or both of the driver transistors to go bad.
Tinkering with this amplifier caused me to learn a lot about these Marantz amplifiers, the cost was my time. I've also found UREI amplifiers to be good, and also like the Hafler amplifiers of the same era. Also, the Leach Amp is a good sounding amplifier that you can build yourself.
Because these amplifiers are old, the more you unsolder and solder the boards, the more likely it is that you will have lifted traces and other problems. Buy or borrow a desoldering tool, use a good soldering iron. Don't shotgun replace the transistors unless you definitely know it is bad.
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