Bought a Marantz PM7200 - dead channel

With black probe connected th chassis, spkr selector set to "1", measure
dc voltage at diode 6280, banded end/see pic.

Set selector to spkr "2".
Measure voltage at diode 6279, banded end/pic

With headphones plugged in, measure voltage at diode 6281, banded end/pic

Expect +24Vdc in each case
phones.JPG
 
Thanks for the info MBZ. False alarm. The speakers I connected to do not work. These were the speakers that were attached to my Onyko amp when it blew. That must have damaged these speakers. I'm checking the fuse in them now.

Other speakers work fine.

Thanks again my friend :D
 
My listening chair position is pretty much nearfield, the amp is normally close enough for me to reach out with my toe to turn the volume knob.....that's my remote for all amps :D:D

I had a Marantz cd-72 without a remote, I got one for cheap on e-bay from china, it worked well.
 
You should checdk the bias, page 34 of the SM. I'm somewhat hesitant in posting this, since what you've been through, however it's part of normal maintenance.
Maybe take the readings and post without attempt to adjust.
 
You should checdk the bias, page 34 of the SM. I'm somewhat hesitant in posting this, since what you've been through, however it's part of normal maintenance.
Maybe take the readings and post without attempt to adjust.

Yea I did consider that but after checking the offset I thought it was probably ok. The amp is in its new home all wired and connected up, however apparently there is a phono mod which makes it sound better so I'll check bias when I do that.

These are all the little buggers that gave me grief :whip:

C04jsb.jpg
 
My listening chair position is pretty much nearfield, the amp is normally close enough for me to reach out with my toe to turn the volume knob.....that's my remote for all amps :D:D

I had a Marantz cd-72 without a remote, I got one for cheap on e-bay from china, it worked well.

I can use my LG G3 mobile temporarily but it's not the best solution.....What Ebay seller did u use?
 
You should checdk the bias, page 34 of the SM. I'm somewhat hesitant in posting this, since what you've been through, however it's part of normal maintenance.
Maybe take the readings and post without attempt to adjust.

I've attempted to adjust the bias now. For the bias for AB, set from cold, I can set both variable resistors to measure 0.4mV < 1 min. When I leave the amp on for 30 mins I am only reading 10mV when the manual says it should be 18mV+-3mV. Odd.

When setting bias for A, after the 30 mins warming, I can get the right channel into spec (65mV - 70mV after 30 secs) however measuring from 3335 I get 68mV and 3337 72mV (out of spec). If I lower the resistor to get 3337 into spec 3335 goes out (less than 65mV). I had compromised on the 68/72 spec as it's about as good as it gets.

EDIT: Woops! Missed the bit in the manual where it says to use the higher value to set :crazy:
 
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Question....when making the Class A measurement do I turn the amp off, then back on again in class A then measure? The service manual does not say to turn the amp off, just "Next, switch to class A operation." Now when you switch between class A and AB, going by the manual, you are supposed to turn the amp off first.

Now the funny thing is the voltage actually drops so that it's within the 65mV - 70mV range after 30 secs however it continues to drop to around 50mV but apparantly it's supposed to "stablize" to 90mV but it just keeps falling to around 43mV?

What on earth is going on?

I can only assume I'm doing something wrong.

For the class AB measurement I'm testing from cold across the two outer legs of 3335. I'm adjusting to get 0.3mV - 0.4mV under 1 min. Then I turn off and wait for 5 mins (to make sure I'm starting from cold again) and then swap to 3336, repeat. Same process for 3337 and 3338.

I'm going off my multimeter for the readings.
 
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Now when you switch between class A and AB, going by the manual, you are supposed to turn the amp off first.
Seen problems with other amps when switching between AB and A. I'd be inclined to power off before switching, including the bias procedure.
The main focus should be on the steady state bias values. Class A falling to 43mV does not sound right, maybe thermal tracking is not ok, maybe a diode
on the heat sink?? Will check schematic.
 
Thanks.

It all depends on if I switch from AB to A by powering off and then IMMEDIATELY powering back up for the A measurement (within 2 secs) or whether I wait a bit.

If the amp has been running in class A for 30 mins and I then set the bias the volltage stabilizes to about 80mV - 82mV, still shy of the specs.
 
The bias transistors 7267&7268 are heat sink mounted so they should
track with the outputs as they heat up.

If both channels are behaving the same then I'd just adjust the bias and
leave it be. The bias should be adjusted when cold according to the SM.
Don't pay any attention to readings made 0-10sec after power ON. I would
decide what mode I would mostly listen to (Class A, AB) then then adjust
the bias for that mode, if the other is a little out of spec then ok.

If you are getting 80mV then maybe ok, but I would be a bit concerned about
40mV.
 
Marantz has you start with a cold amp and a low idle current because they know the idle current will rise as the heatsinks warm up. I think they assume all of this will be done after a repair and want the bias at a safe level to start with. You can do exactly what the manual says and you'll be on the safe side as the idle current doesn't seem to increase as much as it should, line voltage and/or room temp. dependent or you can shoot for the final idle current no matter how long it takes to get there.
 
Shows you know what you are doing and have some confidence. Some would not have tinkered after all the pain you've been through but
you are doing the right thing. The bias should be checked. When you lost sound (faulty spkrs) I thought that the bias might have been wound way up
and eventually destroyed some transistors. Check bias, set as best you can, follow llwhtt suggestions.
 
These pots are RUBBISH! I breath near them with a blade screwdriver I made out of a timber fork (didn't want to slip while adjusting and short it out all over again!) and I get a jump of dozens of mV with barely a touch!
 
These pots are RUBBISH! I breath near them with a blade screwdriver I made out of a timber fork (didn't want to slip while adjusting and short it out all over again!) and I get a jump of dozens of mV with barely a touch!
Many of them are like that. But it's not the pot's fault.
You need to be very patient. What I find works is this : with the tool, give a small turning force in the direction you want (without actually turning it but nearly) and then wriggle the tool a few mm back and forth, the pot often will move a very small amount.
That's why we sometimes put in "multi-turn" pots when replacing them :biggrin:
 
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