Bought a Marantz PM7200 - dead channel

[Homer4beer]

Hi guys,

I bought a PM7200 amp today for $30 and the right channel doesn't work. I think the problem is in the power amp so I've pulled it apart.

Any direction on what to test and what to look for?

Cheers,
Homer

 

[Homer4beer]

Found this post about my issue. Linking here for others that may need help.

https://forum.lowyat.net/topic/2771010/all

Hmm but not much real detail about what fixed it apart from them recapping the boards?

 

[mbz]

Are you saying the left channel works fine/music out, protection relay clicks 4-10 seconds after power ON?
Same issue through headphones?
Try Source Direct? bypass tone circuitry
Didn't see any pre-out/main in links
Don't suppose you have an oscilloscope?

 

[Homer4beer]

double post woops

 

[Homer4beer]

Are you saying the left channel works fine/music out, protection relay clicks 4-10 seconds after power ON?
Same issue through headphones?
Try Source Direct? bypass tone circuitry
Didn't see any pre-out/main in links
Don't suppose you have an oscilloscope?

Yep.

Left channel works perfectly. Left "Processor" out going into left processor in has music on left side only. Reversing and going from (right out into left in) and (left out into right in) still has music on left side only. Same through headphones. Source direct makes no difference. Using the amp in Class A mode makes no difference.

I do have an oscilloscope. Need guidance on how to use it to fault find this amp

Thanks for your help!

 

[petehall347]

is that little board the protect circuit ?
i would change the caps on it first as others did .. i suggest high temp caps .

 

[Homer4beer]

is that little board the protect circuit ?
i would change the caps on it first as others did .. i suggest high temp caps .

Yep checking caps now

 

[Homer4beer]

All the 47uF caps are reading at 39uF

 

[mbz]

All the 47uF caps are reading at 39uF

Just within 20% tolerances. Sounds like there is some history, so maybe not a bad idea to replace the caps.

The unit has 2 separate (L&R) relays, maybe the supply/mechanism is ok for one channel but not the other.

Since you have a scope would be good to confirm relay is the issue by checking for audio signal at part numbers
3355(L) and 3356(R), both 330ohms resistors, test point highlighted below.

 

[Homer4beer]

Thanks very much MBZ!

 

[Homer4beer]

I'm changing all the caps on the main power amp board and have found a few with ESR 0.00.

Are these bad caps?

 

[Bert 1100]

a few with ESR 0.00.

The lower ESR, the better. you don't want the caps to act as resistors.....
So zero ohm ESR is good....

ESR = equivalent series resistance

 

[Homer4beer]

Just within 20% tolerances. Sounds like there is some history, so maybe not a bad idea to replace the caps.

The unit has 2 separate (L&R) relays, maybe the supply/mechanism is ok for one channel but not the other.

Since you have a scope would be good to confirm relay is the issue by checking for audio signal at part numbers
3355(L) and 3356(R), both 330ohms resistors, test point highlighted below.

View attachment 1020940

Ok silly question I suppose but do I test with the amp powered up? What do I earth the ground lead on the scope to? Is there a risk I may blow up my oscilloscope doing this? Lastly do I just run music into the amp using an input (CD say) or do I use a signal generator?

My guess is to run a signal generator from my phone into the CD input. I guess I'd ground the probe to the chassis when testing the resistor? I should see the signal generated by my mobile on the oscilloscope if it's good yes?

I am just very wary of blowing up my oscilloscope by connecting it incorrectly. I've seen guys on youtube remove the earth pin on the oscilloscope power cord to prevent inadvertently arcing it and blowing it up!

Thanks for answering some pretty dumb questions

 

[Homer4beer]

The lower ESR, the better. you don't want the caps to act as resistors.....
So zero ohm ESR is good....

ESR = equivalent series resistance

Thanks Bert,

I have read that the are supposed to have some ESR and if they showed zero it was shorted somewhere and no good?

I had to use low ESR caps for the main amp board due availability.

 

[Bert 1100]

Hmmm... I guess if there was a short there would be no capacitance ?
What is your meter saying in regards to capacitance of these caps ?

 

[Homer4beer]

My meter can't test capacitors.

 

[Homer4beer]

.
.
Boards done with the same fault remaining....no right channel.

 

[mbz]

Ok silly question I suppose but do I test with the amp powered up? What do I earth the ground lead on the scope to? Is there a risk I may blow up my oscilloscope doing this? Lastly do I just run music into the amp using an input (CD say) or do I use a signal generator?

My guess is to run a signal generator from my phone into the CD input. I guess I'd ground the probe to the chassis when testing the resistor? I should see the signal generated by my mobile on the oscilloscope if it's good yes?

I am just very wary of blowing up my oscilloscope by connecting it incorrectly. I've seen guys on youtube remove the earth pin on the oscilloscope power cord to prevent inadvertently arcing it and blowing it up!

Thanks for answering some pretty dumb questions

First up, no such thing as a dumb/silly question on AK. If in doubt it's dumb/silly not to ask.

Yes, amp will need to be powered up/playing music. If you have a signal generator or PC SW like Audacity or from phone, then a 1kHz sine wave is preferred, however
audio from a CD is also ok. Basically looking for a flat line=no music or bouncing around=music. Yes, the oscilloscope earth should be connected to the chassis.

Don't understand how "any" voltage measurement would blow up an oscilloscope, will search youtube. Below are extracts from the Tektronics 2220 user manual.

 

[Homer4beer]

Thanks I'll do this in a few hours.

Have a look at 5:15 on this YouTube clip to see about oscilloscope grounding.

And this tutorial on how not to blow up your oscilloscope.

 

[mbz]

The scenario that he is talking about is where the chassis of the item under test is not at 0Volts, chassis not grounded. Your amp will have the earth wire from the
power cord connected to the chassis, ie, grounded. For this problem to occur you need a live chassis, meaning 2 faults, 1) a clear break in the power cord earth wire.
2) An internal wiring fault where a hot wire makes contact with chassis. The amp actually has 2 earths, the earth wire (protective earth) and an operational earth zero
volts, normally derived from a mid point between transformer windings, this would cause fault 2) to simply blow the amps fuse.

The main point he makes is that the probe ground connector shouldn't be connected to a high voltage. Always connect it to the chassis of a grounded amp.