I borrowed a signal generator. Attached the o-scope and signal generator and the right channel is oscillating.
Now going to replace the components on the power amp board as recommend in the service manual (mostly pF capacitors), and retry. I should have done this long ago...
Too bad my coworker doesn't have a distortion analyzer.
Now going to replace the components on the power amp board as recommend in the service manual (mostly pF capacitors), and retry. I should have done this long ago...
Too bad my coworker doesn't have a distortion analyzer.
I also read on another audio forum that replacing these small value caps solved oscillation problems on a Marantz 250. These power amp boards are similar between the model 1200, 250 and 500.
Following the service manual troubleshooting section pertaining to hum and oscillation, I replaced all the parts recommended and reflowed several suspect solder joints on the power amp boards, especially around the DC Offset and bias pots. Powered the amp on and set DC Offset and Bias. Watched them both for 30 minutes. Both remained stable.
Hooked up the signal generator, o-scope and dummy loads. Squirted a 20Hz, 2kHz and 20kHz signal into the amp and they all looked clean. Both channels looked like the attached o-scope photo from 20Hz - 20 kHz.
I'm going to play music through the amp for a few days while I'm "working" from home.
While I can't say which component(s) exactly caused the humming and oscillation, I'll be sure to follow the service manual first. It says something about Marantz gear- the amps are well documented and schematics/manuals/manual addendums are available, individual components can still be bought relatively inexpensively, and good advice and help is available from experienced people on this forum.
Hooked up the signal generator, o-scope and dummy loads. Squirted a 20Hz, 2kHz and 20kHz signal into the amp and they all looked clean. Both channels looked like the attached o-scope photo from 20Hz - 20 kHz.
I'm going to play music through the amp for a few days while I'm "working" from home.
While I can't say which component(s) exactly caused the humming and oscillation, I'll be sure to follow the service manual first. It says something about Marantz gear- the amps are well documented and schematics/manuals/manual addendums are available, individual components can still be bought relatively inexpensively, and good advice and help is available from experienced people on this forum.
Attachments
If? the amplifier is a type that is not stable at unity gain and there is an issue with the nfb RC network to earth, there would be a problem since an open circuit there would drop the gain to unity. There are other possible causes of course but electrolytic capacitors are apt to dry out over the length of time elapsed since this amplifier was new.
You can relax now and enjoy the rewards of your efforts
Thanks for all your advice and sticking with me through this!
The amp sounds good as it ever did. I bought it from the original owner. He bought it while stationed in Germany in 1972. When I bought it in 2009, it had been sitting in need of repair for a couple years.
First thing was to rebuild the relay board and replace every electrolytic capacitor in the amp. The power supply board alone has at least 12 electrolytic caps. I hesitated to touch the power amp board since I don't like to fix what isn't broken, and I was intimidated by the complexity of the power amp boards.
The Marantz 1200 was my everyday driver at work for about 5 years. Then one Friday as I walked over to turn the integrated amp off for the weekend, it made a terrible noise and smoke came out of one of the speaker drivers.
When I came back on Monday, I started checking out the amp and found that the relay was fried, along with one of the TO-66 predriver transistors (Q510) and a few other TO-92 transistors.
I learned the hard way that you can't pull transistors and replace with modern replacements because some of these transistors have to be matched for Hfe, such as the differential pairs. And, you can't always use the little transistor testers available for sale online because they may test good under low voltage and current. They need to be tested on a curve tracer, and I had to learn how to use a curve tracer. Fortunately, I work for a university (I think Aussies like to call it "uni") and have access to a Tektronix 575 and 576, but nobody in the lab they are located in knew how to use either of them.
Lots of downloading and reading, so I wrote a procedure for both and tested each transistor on the two power amp boards. There were two flaky TO-92 transistors that tested good on the Chinese transistor tester, but were bad when tested on the curve tracer.
That took me about three months to get all sorted out. The amp went back into service, but failed again a few months later. I couldn't get the DC Balance to adjust and learned that on this amp, it was beacause one or both of the differential amp pair went bad. Matched replacements on the curve tracer and then couldn't adjust bias. That turned out to be a bad bias splitter transistor, (Q801).
Most likely because of all the lifted traces and removing/reinstalling (soldering) the power amp boards so many times, and the usual expansion/contraction of the soldered joints, one of the grounds on the power amp board came loose, had a cold solder joint, hanging wires, or gremlins.
For this humming/oscillating problem, it could have been resolved sooner if I hadn't been convinced that it was a ground loop problem. After all I did trying to fix a nonexistent problem, I should have put it on the oscilloscope at the first sign of the problems, but I'm "working" from home and didn't have an o-scope and signal generator at home. Didn't is now past tense.
Also, I stayed up until 1:30 AM last night, reading all I could about Marantz amps with humming and/or oscillation problems. The Marantz 1200 shares a similar power amp board with the 250 and 500 model. One person had the same problem on his Marantz 250 and said that he replaced certain caps on the power amp board and the problem was resolved.
That's what I did this morning. I also reflowed several solder joints as you pointed out.
I can move the signal wires and not induce any humming or oscillation. The amp sounds as good as it ever did, and I'm going to keep playing music on it until I have to go back to work.
The amp sounds good as it ever did. I bought it from the original owner. He bought it while stationed in Germany in 1972. When I bought it in 2009, it had been sitting in need of repair for a couple years.
First thing was to rebuild the relay board and replace every electrolytic capacitor in the amp. The power supply board alone has at least 12 electrolytic caps. I hesitated to touch the power amp board since I don't like to fix what isn't broken, and I was intimidated by the complexity of the power amp boards.
The Marantz 1200 was my everyday driver at work for about 5 years. Then one Friday as I walked over to turn the integrated amp off for the weekend, it made a terrible noise and smoke came out of one of the speaker drivers.
When I came back on Monday, I started checking out the amp and found that the relay was fried, along with one of the TO-66 predriver transistors (Q510) and a few other TO-92 transistors.
I learned the hard way that you can't pull transistors and replace with modern replacements because some of these transistors have to be matched for Hfe, such as the differential pairs. And, you can't always use the little transistor testers available for sale online because they may test good under low voltage and current. They need to be tested on a curve tracer, and I had to learn how to use a curve tracer. Fortunately, I work for a university (I think Aussies like to call it "uni") and have access to a Tektronix 575 and 576, but nobody in the lab they are located in knew how to use either of them.
Lots of downloading and reading, so I wrote a procedure for both and tested each transistor on the two power amp boards. There were two flaky TO-92 transistors that tested good on the Chinese transistor tester, but were bad when tested on the curve tracer.
That took me about three months to get all sorted out. The amp went back into service, but failed again a few months later. I couldn't get the DC Balance to adjust and learned that on this amp, it was beacause one or both of the differential amp pair went bad. Matched replacements on the curve tracer and then couldn't adjust bias. That turned out to be a bad bias splitter transistor, (Q801).
Most likely because of all the lifted traces and removing/reinstalling (soldering) the power amp boards so many times, and the usual expansion/contraction of the soldered joints, one of the grounds on the power amp board came loose, had a cold solder joint, hanging wires, or gremlins.
For this humming/oscillating problem, it could have been resolved sooner if I hadn't been convinced that it was a ground loop problem. After all I did trying to fix a nonexistent problem, I should have put it on the oscilloscope at the first sign of the problems, but I'm "working" from home and didn't have an o-scope and signal generator at home. Didn't is now past tense.
Also, I stayed up until 1:30 AM last night, reading all I could about Marantz amps with humming and/or oscillation problems. The Marantz 1200 shares a similar power amp board with the 250 and 500 model. One person had the same problem on his Marantz 250 and said that he replaced certain caps on the power amp board and the problem was resolved.
That's what I did this morning. I also reflowed several solder joints as you pointed out.
I can move the signal wires and not induce any humming or oscillation. The amp sounds as good as it ever did, and I'm going to keep playing music on it until I have to go back to work.
Interesting point about the amplifier being originally purchased in Germany.
The online service manual shows how to change the transformer primary connections from 220 Volts a.c. to to suit 100 Volts a.c.
If the mains supply in USA is 120 Volts a.c. +/- 6% - dependent on the load on the grid - that poses some questions.
It would pay to check the level of your supply rails if these are significantly higher than 52 Volts - over time that could account for the demise of the original 60 Volt rated supply capacitors and some of your semi-conductor complement.
It was wired for 100Vac when I bought it. Changed it to 120Vac connections right after I bought it. My house voltage is pretty consistently ~120Vac.
Not for now. The Marantz 1200 has been playing for the past two days, minimum 8 hours per day. Heatsinks are barely warm, bias and DC offset are stable. Best of all, it sounds good.
@mjona-- the amp played for almost a week, then I had to go and check the DC offset. They were both about 300 mV and jumping around like kids on a trampoline.
So, I decided that I'm going to put it aside and make a Leach Amp. Next decision is whether to convert the Marantz 1200 into a Leach Amp. I've got all the parts.
Since this Marantz 1200 has lifted traces/pads on the power amp boards, I'm thinking why not take out most of the guts and replace with Leach Amp boards and transformer?
I thought about making some new boards for the Marantz, but the design is too unstable and I'm not motivated to reinvent the wheel for this thing.
We have an old circuit board machine I can use to make the Leach Amp boards and I have a transformer, all the bits and pieces to make the Leach Amp.
So, I decided that I'm going to put it aside and make a Leach Amp. Next decision is whether to convert the Marantz 1200 into a Leach Amp. I've got all the parts.
Since this Marantz 1200 has lifted traces/pads on the power amp boards, I'm thinking why not take out most of the guts and replace with Leach Amp boards and transformer?
I thought about making some new boards for the Marantz, but the design is too unstable and I'm not motivated to reinvent the wheel for this thing.
We have an old circuit board machine I can use to make the Leach Amp boards and I have a transformer, all the bits and pieces to make the Leach Amp.
The Leach amplifier is a proven design probably built in the thousands since it first appeared in Audio Magazine in the mid 70's.
I am one in that number - mine was one of the early versions. The performance of this was improved in later versions published on the w.w.w.
I am one in that number - mine was one of the early versions. The performance of this was improved in later versions published on the w.w.w.