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Harman Kardon PM645 Questions

C

craigtd

Member
I just picked up a used H/K PM645 integrated amp.
It was offered at a great price, so I expected it would need work. It does.
I'm a newbie, so please bear with me.

This is what happened when I turned it on:
Lights work, no popping or humming.
But virtually no signal out of the headphone jack.
Turned all the way up, it is audible, but just barely.

Other observations:
the transformer got very warm in a very short period of time (normal?)
Some discoloration on the inside of the cover (looks like heat)
over what I understand are the output capacitors.

From what I have read, this is a nice piece of gear, so I'd like to resuscitate it. Should I start with the output capacitors or check the transformer.
If so, how do I go about doing that? Or should I start elsewhere?
Any help on the proper approach would be greatly appreciated. Thanks.
 
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First off, make sure your tape monitor switch(es) are in the OFF position. Next do a search on this forum for "DeOxit", it's an electronics contact cleaner. Over time oxidation builds up in your switches and pots (knobs) that causes symptoms ranging from scratchy noises when the controls are operated to channels dropping to no sound at all. Think of this oxidation as an insulating layer that's keeping your signal from getting through. Give all of the controls a good bath, I won't go into it here as it's been writtten up a thousand times.

The PM645 has no output caps, there aren't even any input caps on the power amp stage. The preamp section has a pair of input caps, 47uF. Highly unlikely they are causing a no-sound condition.

This thread:

http://www.audiokarma.org/forums/showthread.php?t=169829

details the hot running of a PM640 and a simple inexpensive fix. The resistors to change in your unit would be R483 and R484. Currently they are 68 ohms, replace with 220-330 ohms and the drivers run much cooler.
 
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The tape monitor switch is a dial.
I tried a tuner in the tuner inputs and listened through source,
and tried the tuner through a tape monitor input with the switch
set to monitor that input. No difference in out put.

I've become acquainted with de-oxit over the last couple months.
It has cleared up some noisy switces, etc.
I didn't realize near silence could be caused by oxidation.
I'll give that a go first.

I'll also track down the thread about the resistor causing the transformer
to heat up. I'd love it to be that simple a fix.

I'm still confused about the capacitors, though.
There are two 8000u 45v capacitors (amongst others, of course).
The schematic has them at C5 and C6 just downstream from the transformer. Given their location, I see that they can't be output related.
So what are they?
The heat mark on the inside of the cover was just above them.
That's why I was concerned. Thoughts?

Thanks.
 
They're filtering caps. If they are getting hot (feel them) they should be replaced. They filter out the 60/120Hz hum from the rectified AC.

Just set the tape monitor switch to SOURCE and the FUNCTION dial to "TUNER". I assume you did this already.

Is the transformer just warm, or is it actually hot to the touch? Hot = bad. Warm = normal. If there's a fault condition that's conducting too much current, that'd make the transformer hot, but the fuse(s) should blow. Do you own a multimeter?
 
I read the thread for the PM640.

Although a good deal of it went over my head,
the layout of the PM640 and PM645 look quite different.
Also, there are no obviously cooked transistors in my unit,
just some discoloration on the cover over the C5 and C6 capacitors
and on the underside of the board near but not under those caps.
In point of fact, I don't know if anything other than the transformer
gets hot on my unit because I get no sound so I don't run it very long.

Will changing the resistors as recommended cause the transformer
to run cooler?

Would pictures help?
 
After a thorough de-oxit cleaning of input jacks, switches, buttons and dials,
the PM-645 is generating output. Nicely. Moreover, the transformer is just warm,
not hot like it was originally. And the filter caps are barely even warm.
However, now some of the transistors near the filter caps are pretty hot.
Is this normal?
 
I have been driving the unit with a load for over an hour.
Transformer is pretty warm, but I can still touch it.
Heat sink is also pretty warm (doing its job?)
None of my other stuff (Scott 342b, Onkyo tx-2500, Nak TA-2A) gets this warm.
Is this heat level normal for this topology?
Is the resistor swap a worthwhile undertaking to cool things off (extend life)?
 
I did the resistor replacement in the thread linked above.

It may be helpful to have pics to see and confirm what you are talking about. You have been jumping around refering to capacitors, transformers, transistors.

But in my case it was the transistors and attached heat sinks that were painful hot to the touch and plus the main heat sinks.

Yes, there are several threads pointing out the heat problems with these series of HK components. Yes, heat can affect the life of a component but figure its lasted this long ... You definitely want to have good clearance above the amp.

Up to you if you want to try it. Not very difficult at all.

dspear99ca pointed out the resistors to replace. When I replaced my resistors, the transistors cooled down dramatically. It did not noticeably degrade sound quality and it can give you some peace of mind.

Here's the service manual for the PM645.
http://manuals.harman.com/hk/Service Manual/pm645 sm.pdf
You'll want to see the Alignment Procedures to test and perform Idling Current adjustment.
 
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Thanks for the feedback.
The reason for the jumping is that I'm a newbie, and lost.
I thought it was the filter capicitors that were the problem because of discoloration in the area. Turns out it is probably the transistors nearby.
The transformer gets warmer than what I am used to, but appears OK.
I'm going to pick up the recommended resistors, replace them and then
try to do the alignment. I'll probably need some guidance with that, too.
I will try to post photos in the interim.
Thanks again.
 
The driver stage in the PM645 is different than the one in the PM640. In the PM645 there is two NPN drivers mounted in parallel (Q424 and Q426 in one channel), and two PNP drivers mounted in parallel (Q428 and Q430). They share the load and the 68 Ohms resistor change I recommanded for the PM640 is not needed in the PM645.

It is possible that the heated area near the main filter capacitors comes from the zener diodes (D13 and D14), the series pass transistors (Q15 and Q16) or the resistors R47 and R48.

Adust the bias in your amp for 33mV between TP2 and TP1, with VR401, and between TP4 and TP3 with VR402.

The service manual for the PM645 is here:

http://manuals.harman.com/hk/Service Manual/pm645 sm.pdf
 
thanks ecluser for catching my mistake on the service manual, I corrected the link in my above post just now!
 
The temperature of the transformer will be dependent upon a number of factors including the design. I had an old tube console amp where the tranny got hot... not hot enough to burn you but pretty toasty, maybe 60 or 65C. Other amps, as you note, stay cool. I have a PM-650 that gets warm at idle, haven't actually felt the transformer, nor have I done any modifications. Also, if there's any DC riding on the supply voltage, it can jack up the temperature pretty quick. You can check this with a multimeter, just set it on DC voltage and stick the probes into your AC outlet.

The DC offset and bias adjustments are a piece of cake, however you may want to give the trimpots a blast of good ol' DeOxit before you start as they are pots just like the balance and tone pots. Be VERY CAREFUL with the pots, they're ancient plastic which will have become very brittle with heat and they're very easy to break. They also only cost about $0.15 apiece, I think that unit uses 100 ohm single-turn. Should you replace them, I always opt for multi-turn pots as it's far easier to adjust to a specific value.

If you are interested in learning electronics (or any other type of) repair, I have a bit of advice, feel free to disregard but I learned a lot of things the hard (and expensive!) way. First off, slow down. About the worst, most frustrating thing you can do is jump to conclusions (discoloration around filter caps = must be causing all my problems). Logic is your best friend when doing fault diagnosis... forget logic and you may as well just chuck it in the dumpster along with a bag of cash. There are many eager repair wannabes who are quick to replace parts without having a reasonable understanding of what they're doing. I'll tell you it's a far more satisfying experience to determine which part(s) is/are at fault and getting a broken piece of junk back to working order. I'll also tell you that the shotgun (or guess 'n check) method of parts replacement is frustrating, time consuming, expensive and, ultimately, very unrewarding. The folks on this forum are incredibly generous with their time and expertise and are more than willing to help you; however, the more specific and targeted questions you ask, the quicker and easier your repairs will go. There are tons of online tutorials on electronics in general and audio equipment in particular.

Just my 2 cents.

Dave
 
Also the test points for the idle current adjustments are usually vertical pegs. I couldn't find a nice diagram for them in the service manual, so I hope they are clearly labeled on the circuit board.

I recommend getting some hook on adapters for your multimeter so your multimeter probes hook on to them and don't accidentally slip (available at Radio Shack for a few bucks). It will also free up your hands for actual adjustments.
 
terra1 said:
I recommend getting some hook on adapters for your multimeter so your multimeter probes hook on to them and don't accidentally slip (available at Radio Shack for a few bucks). It will also free up your hands for actual adjustments.
Click to expand...

^^^^^^^^^^^^^^^^^^^^

Yah, what he said. Nuttin' worse than a probe slipping and frying a transistor (or 5), it's good cheap insurance. If you don't have access to micro-grabber hooks, tape up the probes on your meter so that only a few millimeters of metal is showing. Every tech alive has had probes slip.
 
Thank you all for your input, insight and advice.

Although I bought the resistors with a plan to swap them out,
the note from ecluser and Dave sobered me up. I'll stick with the
"if it ain't broke, don't fix it" approach.

A thorough de-oxit cleaning seems to have brought the unit back to life.
I adjusted the idling current to 33mV (one was 16 mV other was 21mV).
I buttoned it up and it is currently driving some AR2a's I'm rehabbing.

A "nudie" pic is attached for posterity.
(Note bottle of de-oxit next to screwdrivers!)

Thanks again!
 

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"Adjust the bias in your amp for 33mV between TP2 and TP1, with VR401, and between TP4 and TP3 with VR402."



Just acquired a PM645 myself and wanted to adjust the idle Current to 33mv. Cannot find TP1, TP2, TP3 or TP4 on the PCB. Not even on the schematics. Any help in locating these test points? Near what component? Doesn't look like its labeled. Thanks, going blind trying to find the test points.....
 
The test points are visible on the schematic (page 12 of the 25 pages PDF file), on the far right side. They are, in fact, the emitters of the power transistors.

If you look at the picture on post #15, you will see two white boxes close to the main filter capacitors. Each one of these boxes contains two resistors in series. The pins on the top, opposite to the coil, are the test points.
 
Where to begin on repair?

I have one of these in the garage that was working very hard when it died a couple of years ago.
Replacing the blown fuse - it blows it again instantly, as soon as the power button is depressed.
Any suggestion on where to begin looking for repair? I'm mainly looking at it for a soldering practice, diagnostic practice project. What is most likely to have died when listening at very high volume with low impedance load?
 
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If you still need help...

My first advice would be to check the Output transistors (the big ones under the big heatsinks) for shorts, if they are shorted they would cause the fuse to blow instantly, also if they are shorted you may or may not be able to find replacements (don't know the trannies in there), so you might want to stop right there so you don't waste valuable time and nerves.

Good luck!
-Bi_Nom
 
dspear99ca said:
The temperature of the transformer will be dependent upon a number of factors including the design. I had an old tube console amp where the tranny got hot... not hot enough to burn you but pretty toasty, maybe 60 or 65C. Other amps, as you note, stay cool. I have a PM-650 that gets warm at idle, haven't actually felt the transformer, nor have I done any modifications. Also, if there's any DC riding on the supply voltage, it can jack up the temperature pretty quick. You can check this with a multimeter, just set it on DC voltage and stick the probes into your AC outlet.

The DC offset and bias adjustments are a piece of cake, however you may want to give the trimpots a blast of good ol' DeOxit before you start as they are pots just like the balance and tone pots. Be VERY CAREFUL with the pots, they're ancient plastic which will have become very brittle with heat and they're very easy to break. They also only cost about $0.15 apiece, I think that unit uses 100 ohm single-turn. Should you replace them, I always opt for multi-turn pots as it's far easier to adjust to a specific value.

If you are interested in learning electronics (or any other type of) repair, I have a bit of advice, feel free to disregard but I learned a lot of things the hard (and expensive!) way. First off, slow down. About the worst, most frustrating thing you can do is jump to conclusions (discoloration around filter caps = must be causing all my problems). Logic is your best friend when doing fault diagnosis... forget logic and you may as well just chuck it in the dumpster along with a bag of cash. There are many eager repair wannabes who are quick to replace parts without having a reasonable understanding of what they're doing. I'll tell you it's a far more satisfying experience to determine which part(s) is/are at fault and getting a broken piece of junk back to working order. I'll also tell you that the shotgun (or guess 'n check) method of parts replacement is frustrating, time consuming, expensive and, ultimately, very unrewarding. The folks on this forum are incredibly generous with their time and expertise and are more than willing to help you; however, the more specific and targeted questions you ask, the quicker and easier your repairs will go. There are tons of online tutorials on electronics in general and audio equipment in particular.

Just my 2 cents.

Dave
Click to expand...
This was a good read, i took some notes. and accurate. I know more about cars than electronics, and with cars troubleshooting is usually a lot more complex than people think. and usually a careful process of elimination, rather than the first quick guess someone makes online.
 
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