Marantz PM230 - DC on right channel output - UK Vintage Radio Repair and Restoration Discussion Forum

bluecheese · 4th Jul 2017, 10:20 pm

I'm trying to get a Marantz PM230 amplifier working. The left channel is working well but the right channel output has DC (up to about 10-15V) overlaid on the expected signal.

The signal is the expected AC for a second or so, then there is a pulse of DC before it settles back to normal. This seems to repeat fairly periodically.

Does anyone have any ideas as to what might be causing this problem? I'd be very grateful to hear any suggestions.

Repairs I've made so far:

- Reassembled tone pots (damaged by a previous owner dropping it on its front).
- Replaced and heatsinking the STK3042 IC.
- Replaced a couple of capacitors on the tone amp board.
- Replaced two resistors in the power supply section.

Thee specs and manual are available here: http://www.hilberink.nl/codehans/luxman59.htm

kevinaston1 · 5th Jul 2017, 10:53 am

d.c. offset is usually caused by an imbalance in the differential amplifier, which is the STK3042.

Measure the voltage on pins 10 an 11 to see if they copy the d.c. offset. Alternatively, a hair dryer and freezer spray will provoke it quickly if it is intermittent.

Kevin

paulsherwin · 5th Jul 2017, 12:20 pm

I would also suspect a bad STK module, but you said you'd changed it?

Nickthedentist · 5th Jul 2017, 12:52 pm

Don't forget that fake STK modules do exist.

Have you seen this thread? https://www.vintage-radio.net/forum/...ad.php?t=83682

I have replaced STK-modules in amps of this era before, only to find that the symptoms still remained. The actual problem in every case was a dried-up electrolytic capacitor nearby. Do you have an ESR tester?

bluecheese · 5th Jul 2017, 2:29 pm

Thanks for the replies all! I will check the outputs on pins 10 & 11 when I get home and report back.

The replacement STK module I ordered did have different markings than the original - I suppose it could be fake. I believe both were the STK3042II (mk2) type. The symptoms seem the same with both the original and the replacement module which led me to believe the problem probably lies elsewhere. The left channel seems okay with the replacement STK too (testing with dummy loads and a generated sine wave at the moment).

I don't have an ESR tester but I have got one on its way in the post. None of the capacitors have obvious signs of bulging. I could start replacing capacitors in that section of the board in the meantime.

As an aside I'd love to gain a better understanding of the circuit design, but I'm not sure what sort of reading I should be looking for. I've got a good electronics textbook and know some basic theory but beyond that I am not sure what to do next to further my knowledge. Perhaps building very simple amps as DIY projects? How do I figure out where this amplifier fits in the classes A/B/AB etc.?

vidjoman · 5th Jul 2017, 3:06 pm

As you said it has been dropped you might have a slight crack on the PCB or a break in a copper track as it's the same with both IC's. Might even be bad joint where the drop has shaken the part loose.

bluecheese · 12th Jul 2017, 12:53 am

Sorry for the delay - just got back to this.

I've checked most of the electrolytic caps on the main board in circuit and all the ones I've successfully tested have low ESR (<2Ω). I get no readings for C709-C712, C807 and C810 so I may need to lift a leg of those and re-test.

Here are traces from those STK output pins (apologies for the cheap scope but it seems to be doing the job):

Pin 6 (Lch+ out) - sine looks good to me: https://amoss.me/pm230/stk-pin6-reading.jpg

Pin 10 (Rch+ out) - no visible sine, you can see the big DC spike and then a return to a normal-looking DC bias: https://www.youtube.com/watch?v=2tfr0v66u1g

Pins 5 and 11 are mirrors of these respectively.

Some other measurements I made earlier in the stage (leading to the inputs to the STK):

RK05 - good signal with no DC bias.
RK06 - gradual drop to -4VDC or so.

Seems dubious?

Any ideas much appreciated, thanks!

It seems odd that there isn't a good signal going into the STK on the right channel. Perhaps there is an issue with the output of the tone amp board. Thinking of swapping the channel outputs of the tone amp board next to see if the power section is indeed at any fault.

PJL · 12th Jul 2017, 12:26 pm

Not sure what RK05 and RK06 refer to.

Pin 1 of the STK is the +ve input and this should be near zero and is used for the input.
Pin 2 is -ve input used for feedback and should also be near zero.

Leakage in C703 could cause an offset voltage
Leakage in C705 could cause on offset voltage

bluecheese · 17th Jul 2017, 10:59 pm

Thanks again for the advice - I've got it working!

It turned out to be mostly frayed ribbon cables (probably from a combination of me pulling the tone amp board away the chassis, and the original damage) in combination with a bad slide pot for the bass adjustment (the wiper was actually damaged and I had to adjust the contacts back into shape).

4th Jul 2017, 10:20 pm	#1
bluecheese Retired Dormant Member Join Date: Jul 2017 Location: London, UK. Posts: 4	Marantz PM230 - DC on right channel output I'm trying to get a Marantz PM230 amplifier working. The left channel is working well but the right channel output has DC (up to about 10-15V) overlaid on the expected signal. The signal is the expected AC for a second or so, then there is a pulse of DC before it settles back to normal. This seems to repeat fairly periodically. Does anyone have any ideas as to what might be causing this problem? I'd be very grateful to hear any suggestions. Repairs I've made so far: - Reassembled tone pots (damaged by a previous owner dropping it on its front). - Replaced and heatsinking the STK3042 IC. - Replaced a couple of capacitors on the tone amp board. - Replaced two resistors in the power supply section. Thee specs and manual are available here: http://www.hilberink.nl/codehans/luxman59.htm

5th Jul 2017, 10:53 am	#2
kevinaston1 Hexode Join Date: Aug 2014 Location: Featherstone, West Yorkshire, UK. Posts: 386	Re: Marantz PM230 - DC on right channel output d.c. offset is usually caused by an imbalance in the differential amplifier, which is the STK3042. Measure the voltage on pins 10 an 11 to see if they copy the d.c. offset. Alternatively, a hair dryer and freezer spray will provoke it quickly if it is intermittent. Kevin

5th Jul 2017, 12:20 pm	#3
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,947	Re: Marantz PM230 - DC on right channel output I would also suspect a bad STK module, but you said you'd changed it?

5th Jul 2017, 12:52 pm	#4
Nickthedentist Dekatron Join Date: Jan 2005 Location: Oxford, UK. Posts: 17,846	Re: Marantz PM230 - DC on right channel output Don't forget that fake STK modules do exist. Have you seen this thread? https://www.vintage-radio.net/forum/...ad.php?t=83682 I have replaced STK-modules in amps of this era before, only to find that the symptoms still remained. The actual problem in every case was a dried-up electrolytic capacitor nearby. Do you have an ESR tester?

5th Jul 2017, 2:29 pm	#5
bluecheese Retired Dormant Member Join Date: Jul 2017 Location: London, UK. Posts: 4	Re: Marantz PM230 - DC on right channel output Thanks for the replies all! I will check the outputs on pins 10 & 11 when I get home and report back. The replacement STK module I ordered did have different markings than the original - I suppose it could be fake. I believe both were the STK3042II (mk2) type. The symptoms seem the same with both the original and the replacement module which led me to believe the problem probably lies elsewhere. The left channel seems okay with the replacement STK too (testing with dummy loads and a generated sine wave at the moment). I don't have an ESR tester but I have got one on its way in the post. None of the capacitors have obvious signs of bulging. I could start replacing capacitors in that section of the board in the meantime. As an aside I'd love to gain a better understanding of the circuit design, but I'm not sure what sort of reading I should be looking for. I've got a good electronics textbook and know some basic theory but beyond that I am not sure what to do next to further my knowledge. Perhaps building very simple amps as DIY projects? How do I figure out where this amplifier fits in the classes A/B/AB etc.?

5th Jul 2017, 3:06 pm	#6
vidjoman Dekatron Join Date: Jun 2013 Location: East Sussex, UK. Posts: 3,326	Re: Marantz PM230 - DC on right channel output As you said it has been dropped you might have a slight crack on the PCB or a break in a copper track as it's the same with both IC's. Might even be bad joint where the drop has shaken the part loose.

12th Jul 2017, 12:53 am	#7
bluecheese Retired Dormant Member Join Date: Jul 2017 Location: London, UK. Posts: 4	Sorry for the delay - just got back to this. I've checked most of the electrolytic caps on the main board in circuit and all the ones I've successfully tested have low ESR (<2Ω). I get no readings for C709-C712, C807 and C810 so I may need to lift a leg of those and re-test. Here are traces from those STK output pins (apologies for the cheap scope but it seems to be doing the job): Pin 6 (Lch+ out) - sine looks good to me: https://amoss.me/pm230/stk-pin6-reading.jpg Pin 10 (Rch+ out) - no visible sine, you can see the big DC spike and then a return to a normal-looking DC bias: https://www.youtube.com/watch?v=2tfr0v66u1g Pins 5 and 11 are mirrors of these respectively. Some other measurements I made earlier in the stage (leading to the inputs to the STK): RK05 - good signal with no DC bias. RK06 - gradual drop to -4VDC or so. Seems dubious? Any ideas much appreciated, thanks! It seems odd that there isn't a good signal going into the STK on the right channel. Perhaps there is an issue with the output of the tone amp board. Thinking of swapping the channel outputs of the tone amp board next to see if the power section is indeed at any fault.

12th Jul 2017, 12:26 pm	#8
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Marantz PM230 - DC on right channel output Not sure what RK05 and RK06 refer to. Pin 1 of the STK is the +ve input and this should be near zero and is used for the input. Pin 2 is -ve input used for feedback and should also be near zero. Leakage in C703 could cause an offset voltage Leakage in C705 could cause on offset voltage

17th Jul 2017, 10:59 pm	#9
bluecheese Retired Dormant Member Join Date: Jul 2017 Location: London, UK. Posts: 4	Re: Marantz PM230 - DC on right channel output Thanks again for the advice - I've got it working! It turned out to be mostly frayed ribbon cables (probably from a combination of me pulling the tone amp board away the chassis, and the original damage) in combination with a bad slide pot for the bass adjustment (the wiper was actually damaged and I had to adjust the contacts back into shape).