Marantz 2500: Left Channel Outputs Running Hot

[Mag84ilfully]

The left channel outputs of my Marantz 2500 are acting like little toasters!
The left side reaches ~160-185F within minutes and fan kicks onto high speed (hmmm this isn't right...).

Right channel outputs are chillin' at around ~90F.

The odd thing is that my Fluke multimeter was measuring the DC-Offset voltage to be -4mv and -1mv in the left and right, respectively. Yes, they were negative DC-offsets (and yes I did match red-red, black-black). Does this mean a careless monkey swapped the speaker terminal wires? I'm the ___ owner of this 2500 so I'm assuming its been handled and prodded more times than someone who calls their local red light district home. :yes:

I've included a thermal image of the outputs and photos of the DC-offset.

I'm an amateur at this so I'm looking to learn as much as I can from everyone here. :thmbsp: If the problem is too big to handle at home, I'll take the big bastard to a good technician. Hate to haul it somewhere though. I have a service manual in hand to reference as well.

Thanks everyone for your guidance on this! :thmbsp:

 

[SoCal Sam]

Bias current adjustment needed. Nice thermal image.

 

[Mag84ilfully]

Bias current adjustment needed. Nice thermal image.

Thank-you! :thmbsp:

 

[Mag84ilfully]

Okay I'm reading in section 5 (Audio Adjustment) of the 2500 service manual as follows:

1. Voltage adjustment
Connect a DC voltmeter across the pins 4 and 3 on J805. Adjust the trimming resistor R819 until the DC voltmeter reads 32 V.

2. Main Amplifier DC off-st alignment
Connect a DC voltmeter...blah blah...to the speaker terminals and adjust trimming resistor R760 for "zero" DC output on the meter. Repeat the same procedure for the other channel.

3. Idle current adjustment
Connect a DC voltmeter (in 50 mV range) across TP1 and TP2 on JX01. Adjust R716 on the P.W. board on the left side of the tunnel heatsink until the voltmeter reads 25 mV (32 mA). In turn, connect the voltmeter across TP3 and TP4 on JX02 and similarly, adjust R716 on the P.W. board on the right side of the tunnel heat-sink until the voltmeter reads 25 mV (32 mA).

4. Check DC off-set voltage aligned in the procedure 2 and if an DC output is observed on the DC voltmeter, adjust the R760 again for "zero" output.


Okay, great. My question now is it acceptable to skip to procedure no.3 since my DC-offset is good? Or is it suggested to go back and start at no. 1 and check DC voltage across pins 4 and 3 on J805?

 

[SoCal Sam]

Reducing the idle current is what will cool down the amp. I would go with the service manual and check everything.

 

[Mag84ilfully]

Great, will do this tomorrow(maybe) and share the outcome. Thanks for the help on this!

 

[bryans12v]

May I ask how you've achieved that thermal image? Very very cool and quite helpfull.

 

[bktheking]

If you're bias was like mine, around the 250mv measurement read below.


There are a pair of to220 transistors IIRC that go leaky on the inside of that heat sink, this is going from memory from when I fixed mine. You have to remove the heat sink and split it in half, if you have a transistor tester you can check them.

 

[Mag84ilfully]

Brief detour...

May I ask how you've achieved that thermal image? Very very cool and quite helpfull.

FLIR E4 thermal imaging camera. Upgraded from E4 IR resolution of 80X60 to E8 resolution 320X240. I extracted (Hacked I guess you could call it...) the E4 software and inserted the E8 model software. Seeing that E4 model was ~$900 new versus an E8's $4000 price tag, definitely worth it. I'd like to think I'm better at software coding than hardware aspects of electronics

PM me for details on this if you would like!

 

[Mag84ilfully]

Okay slight problem I have run into. Service manual says to adjust R716 on the P700 board. This one is undoubtedly a fixed resistor. I see a pot at R761. Could this be a typo in the the service manual? or am I :stupid:?

 

[KutzlerTrans]

760 Is your DC Offset. 761 is probably the right one. Mark the starting point and see if it changes on the volt meter. Looking for 25mv. If a 1/5 turn does not produce any results,"Stop!" and check your meter connection location again.

What is your starting voltage??

 

[Mag84ilfully]

760 Is your DC Offset. 761 is probably the right one. Mark the starting point and see if it changes on the volt meter. Looking for 25mv. If a 1/5 turn does not produce any results,"Stop!" and check your meter connection location again.

What is your starting voltage??

Yes, 761 worked! Thank-you! I'll explain the method I used below. Still a problem lurks in the left channel, however.

 

[Mag84ilfully]

Partially Fixed! Key word is Partially

Okay, first off R761 was in fact the correct adjustment pot for the idle current. Being the paranoid person that I am and not wanting to short anything, I didn't use voltage readings to make the adjustment as the probes on the multimeter were too large to squeeze in a tight space (also a case of "Where did those damn alligator clips go?").

I improvised. Instead using voltages, I used my infrared camera to check the temp of the left channel outputs while turning the R761 pot clockwise.

Presto!!! The temperature was dropping in the left outputs.

185F...175F...165F...155F...145F...140F... Fan kicks down to low speed. Now I'm thinking that if I can match the left and right channel temp, I'm golden (BTW, I was measuring DC offset in the left channel with the multimeter while turning R761 and it bumped up 3 mV from 4 mV to 7 mV after adjustment. Still okay! :thmbsp

Kept turning R761 pot until...hmmm...can't turn it anymore! And the left outputs are running 50F-60F hotter than the right outputs!

(Just as a FYI, I only adjusted the R761 pot BARELY 1/4 a turn clockwise until I hit the end)

So I still have a problem on my hands...I think and it may be much more involved. Tomorrow I will do it the right way and check the Right amplifier P700 board voltages coming off of the right side R761 pot.

If the right side reads 25 mV at R761, then there is current leakage somewhere past the pot in the Left side R761, correct?

I remember when I first got this 2500, it had been "serviced". I am making a big assumption that the service meant someone set the DC-offset because it was nearly perfect when I checked it the other day. I also remember the fan didn't kick into high speed as much as it started to over a period of months. So I am assuming the "service" included setting the idle current. Another thing I am assuming is this is one of those problems that will come back to haunt me!

Included is a schematic from the manual as well as more infrared photos of the progress.

 

[Mag84ilfully]

If you're bias was like mine, around the 250mv measurement read below.


There are a pair of to220 transistors IIRC that go leaky on the inside of that heat sink, this is going from memory from when I fixed mine. You have to remove the heat sink and split it in half, if you have a transistor tester you can check them.

Will check the voltages tomorrow. Thanks for the advice! This might be what is wrong with the left side of my amp!

 

[SoCal Sam]

If you're bias was like mine, around the 250mv measurement read below.


There are a pair of to220 transistors IIRC that go leaky on the inside of that heat sink, this is going from memory from when I fixed mine. You have to remove the heat sink and split it in half, if you have a transistor tester you can check them.

bktheking is the man! If those resistors are leaky, you might as well replace them on both sides as preventative maintenance.

 

[bktheking]

Yup do the pairs. Transistors , im sure that's what you meant. Also check the emitter resistors to make sure they are in spec.

 

[bktheking]

Found this pics I took when I ran my tests and found the issue.

I'm pretty sure I replaced with MJE15032/15033.

 

[blhagstrom]

I spent the day going through the amp.
Didn't find any real culprits.
All transistors test fine. I did swap the drivers since I was in there.
All the resistors are fine.
All the diodes.
I even went through the peak indicator and soft start board.

I pulled and tested every transistor, except the outputs, they can be tested on board once the driver board is pulled.

The problem side works. The good side lost all bias!?!?

Went back in and went through all of that side. Pulled transistors and tested after checking driver replacement.

Works!

Bad solder joints? The connectors are clean and tight.

Odd.

 

[anatech]

Hi Guys,
The insulators melt under tension and heat over the years. This releases the mounting force that keeps the transistors cool on the heat sink. You need to retension all the mounting screws for the outputs and drivers. Note that the torque is something like 8 in-oz or something like that. DO NOT over tighten mounting screws for transistors. BTW, the torque is 6 in-oz. Motorola / On-Semi has an application note that deals with mounting power transistors.

If you find bad output transistors, they all need to be matched and be the same type. If you are going to "hack one" together, please leave it for someone who cares to do a good job on these excellent receivers.

If the insulators (not the mica) appear to have been compressed, you need to find another solution. Those also act as standoffs, so they are critical.

-Chris

 

[Tanner02480]

Hi guys,
I'm new here, I' rebuilting my 2500, everything looking good, I just notified that the big resistors on the peak indicator control card, comes very hot, do you think it's normal ?
Also the ceramic 680homs resistor near the solf start relay.
Mag84ilfully,
Do you have an other thermal picture of your 2500 in normal running condition, showing this section of the receiver ?
Thanks,
Jeff.