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Just bought a '83 HK PM660
- Thread starter almico
- Start date
ahh rightO... I see it now.Amplifier. It has no tuner.
That makes it even more worthy of a proper rebuild.
restorer-john
Grand Contributor
It uses twin secondary windings and the four 5A fuses are for each tapping.
As it is a high current amplifier, HK eschewed relays or even circuit breakers on the output of the amplifier. It relies on the 5A fuses to fail, which means any issues in the power stage can kill speakers. Back in 1983, my best friend bought a PM-660 and it failed after a few years, but luckily I had built him a sophisticated pair of speaker protectors, which saved his Infinity RS5bs when it went DC. He still to this day uses the speaker protectors as does my father also use a pair of protectors I built him for his NS-1000Ms.
The Harman Kardon PM-660 is one of the finest amplifiers out of the 1980s. It was commissioned to be the ultimate integrated by HK and subsequent models (PM-665/Vx/Vxii) were not as good on the bench, or to my ears.
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Hmmz... Let's see the layout for the amp section @restorer-john
I found the service manual for it @restorer-john
restorer-john
Grand Contributor
- Thread Starter
- #28
Looks like @almico is about to become intimately knowledgeable with his HK receiver.
Ha, so true. I like vintage stuff. I also like things to work right. That is a dangerous combination. I have a whole woodshop filled with machinery, nothing built after 1946. It just goes with the territory that if you want old things to work like new, or better then new, you will need to do a deep dive into your right brain and learn more than you ever wanted to know about a piece of equipment. There is no point in bringing it home if you are not willing to do that. The $15 it cost is irrelevant compared to the time and brainpower it will take to make it right. I grumble, but enjoy the process.
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- Thread Starter
- #29
I was thinking about testing the filter caps first. It sounds nice at low volumes but craps out when it gets pushed. It could be a power supply issue? I can't test them in the circuit, but I can easily pull one and test it.
Unfortunately, I don't really have the equipment, or knowledge, to "test the amplifier". I'll pull the driver board and check for connections.
The amp I'm comparing it to is a lot simpler. I built it almost 10 years ago and it has been rock steady, going 4-5 hours a day ever since. The sound is smooth and creamy, but clear and accurate. It's a cathode follower so the output tube is amplifying current rather than voltage. So even at only 3W it has very good bass drive. FWIW, there is very little to color the sound, unlike the much more complex PM660 circuit. They are very different animals.
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What are the chances that power supply filter caps are dried and so lack of bass and loud passages and so not sounding good. ?
sorry just read this. My newbie suggestion would be to run it for some more time so other caps in the circuit reform and then replace filter capacitors. If still not sounding good replace again with original.
Regards.
- Thread Starter
- #32
Will do.
I also noticed an issue with the -7dB "Main Direct" switch. You nailed it John. For example: with the switch "out" I get 85dB from the R ch and 93 from the L. When I press it in I get about 85dB from both. Just casually measured with an iPhone app, but you get the idea. The switch will sometimes crap out the R side completely. Funny, I used to have an ultrasonic cleaner when I was building a record cleaner, but I passed it on. I'll try and take the switch out and see what damage I can do...
MRC01
Major Contributor
I would follow @restorer-john advice first. But since you asked, I'll explain this procedure anyway.
Use REW to record a frequency sweep to a file. Play this file through your PM660 (burn it to CD or whatever). Record the PM660 outputs as it is playing. You can do this with a digital recorder, or by connecting the PM660 outputs to the inputs of a PC sound card. Make sure the volume level doesn't overload the recording device. Then in REW, import this sweep recording. REW compares the original to the recorded sweep and shows you the FR and distortion, among other things.
- Thread Starter
- #34
I got the board out that supports the Main Direct switch. There are two contacts that appear to not be soldered, but it seems on the schematic that they are not used.
I can see the contacts inside that need attention:
I can see the contacts inside that need attention:
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- Thread Starter
- #38
Oh man, it faked me out. I was so excited that it was sounding so good that I went ahead and made a bespoke set of speaker cables with Sewell banana plugs. But the speaker outs on the back are fake banana plug receptacles. They look like they'll take a banana plug, but the plastic screw connectors are solid plastic. There is no contact sleeve inside, just a flat plate at the "bottom". I would have to hold the cables in there while listening. I think I have some Sewell spring-loaded connectors that I can swap in and just screw them in like a piece of lamp cord. {sigh}.
But man, it does sound nice. I listened to a high-res copy of Simon and Garfunkel's greatest hits while I was making the cables, and it hasn't sounded that good in years. I guess it needs to be played on period-correct gear.
But man, it does sound nice. I listened to a high-res copy of Simon and Garfunkel's greatest hits while I was making the cables, and it hasn't sounded that good in years. I guess it needs to be played on period-correct gear.
restorer-john
Grand Contributor
Also note, the PM-660 runs very warm, even just idling. That's normal. She's a big, powerful amplifier and likes plenty of ventilation. Don't go stacking stuff on top.
It's phenomenally powerful into low impedances and also has a large transient capability. Consider its rating is a conservative 80wpc. It easily surpasses 120wpc continuous into 8R and nearly double that into 4R both channels driven. You will blow the fuses before you blow the amplifier.
It's phenomenally powerful into low impedances and also has a large transient capability. Consider its rating is a conservative 80wpc. It easily surpasses 120wpc continuous into 8R and nearly double that into 4R both channels driven. You will blow the fuses before you blow the amplifier.
dougi
Addicted to Fun and Learning
I remember having the successor PM665 at uni I bought second hand. Not sure why I ever traded it in on an Arcam Delta 290 that I had to fit fans to so it wouldn't shut down at parties! Would love to have it back now.
