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Just bought a '83 HK PM660

almico

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I couldn't resist. It was my dream "receiver" when I was too poor to buy it. It was a lot cheaper now than it was then.

Looks clean. I downloaded the UM and SM from hifiengine. I may have to blow the dust off my AR XA. I do have a pair of Frazier Manhattans to pair with it, but I never did get those Shahinian Obelisks. Anyone have an 8-track player I can borrow?

Hopefully it won't need more than a bulb replacement and a 1/4 can of Dioxit.
 

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Doodski

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Nice. 4 Ohm capable with MM, MC and subsonic too.
 
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almico

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And it has a tone control bypass switch that takes the signal straight from the input selector to the amp.

Let’s see. In 1983 I was driving a ‘70 Beetle with a pioneer kp-500 under the glovebox and a pair of Altec Lansing 6x9 4a’s in the back. And no heat of course.
 

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That brings back good memories. Back in the 80s when I was a starving college student audiophile I saved up my pennies from various jobs and got a PM645. It served me well for about 15 years, when I upgraded and gave it to a friend who enjoyed it for many more years. It always ran warm/hot, blew an output transistor once, which I replaced. Other than that it was reliable. Volume knob never got scratchy.
 
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almico

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My first set of gear was a Kenwood KR-4600, BIC turntable and a pair or Acoustic-phase speakers from Crazy Eddies. I had $600 in the bank. My parents asked me if I wanted to buy a car or a stereo. Even back then I had my priorities in order.

But a few years later, my best friends parents got a PM660 and I always coveted it.
Should I just turn it on or put it on a variac? Probably needs a full recap.
 

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MRC01

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One test you can make is turn it on, let it warm up for 15 mins, then measure the DC offset across the speaker terminals.
 
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almico

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Will do. I know how to use a soldering and I even have a small scope, but I'm no electrical engineer or tech. I found this video of a guy working on a PM 640:


Right now I have a pair of HiVi 3.1 on the bench waiting for veneer.
 

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almico

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One test you can make is turn it on, let it warm up for 15 mins, then measure the DC offset across the speaker terminals.

PM660 arrived today. I'm pretty thrilled. Everything seems to work. Even the Tape2 light the seller said was out is working. UPS must have smacked it just right.

DC offset 0.10 one channel, -39mV the other. I'll open it tomorrow and hunt for the trim pots and see what else I can see. It sounds great!
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I have the service manual for correct values:

Screen Shot 2020-11-14 at 10.30.30 PM.png
 
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almico

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I couldn't wait till morning. Popped the top and set the bias and DC offset on the speakers. Dead on nuts now. Might be the best $120 I've spent in a long time.

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almico

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So I hooked up the PM660 into my main system and poop, it's sounds like crap. Now I know what grainy means. Compared to the 3W/ch single-ended cathode follower tube amp I've been using, it sounded horrible. It gets loud, but the sound is unbearable. I ordered most of the caps for the driver and power stage and have some homework to do.
 

MRC01

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Can you measure it?
Like connect its output to a sound card, play a REW sweep into its inputs, then REW will plot FR, noise & distortion.
 

Doodski

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So I hooked up the PM660 into my main system and poop, it's sounds like crap. Now I know what grainy means. Compared to the 3W/ch single-ended cathode follower tube amp I've been using, it sounded horrible. It gets loud, but the sound is unbearable. I ordered most of the caps for the driver and power stage and have some homework to do.
That's not what is should sound like. It should sound neutral and good. :D
 
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almico

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That's not what is should sound like. It should sound neutral and good. :D

To be fair, the amp I'm comparing it to is one of the best sounding amps I have ever heard. I've been a tube guy for years. But I just built a couple of Amp Camps and I'll try one or both in the system tomorrow for a SS/tube comparison. The Amp Camp sounds fluid and musical on my 604Cs. Let's see what it sounds like on my A5s.

Can you measure it?
Like connect its output to a sound card, play a REW sweep into its inputs, then REW will plot FR, noise & distortion.

I just downloaded REW again. I even still have the MiniDSP mike that I bought years ago when this was an obsession. I used to to sweep speakers and my room, but never an amp. Not sure how that works. I have a little Lab-2 scope that I can blow the dust off of.
 
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Doodski

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@almico have you cleaned and lub'd the controls and switches yet? Did you resolder any joints that may have cracks or crumbly solder from the heat over the years?
 
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almico

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@almico have you cleaned and lub'd the controls and switches yet? Did you resolder any joints that may have cracks or crumbly solder from the heat over the years?
I bought a fresh can of Dioxit for the task, but it doesn't need it. Nothing scratchy, all controls are smooth as new. I don't think it was used much and must have been nestled in a cabinet, because there is very little dust inside. I'm hoping the caps have just dried out. It sounds better at lower volumes, but when I hit the gas, it just whimpers. All the really good amps I have used are like great sports cars. You have no idea how fast your going until you look at the speedometer and notice you're doing 90 on an exit ramp when it felt like 30. This amp sounds like 90 when it's doing 50. While I'm in there I'll clean all contacts anyway.
 

Doodski

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I bought a fresh can of Dioxit for the task, but it doesn't need it. Nothing scratchy, all controls are smooth as new. I don't think it was used much and must have been nestled in a cabinet, because there is very little dust inside. I'm hoping the caps have just dried out. It sounds better at lower volumes, but when I hit the gas, it just whimpers. All the really good amps I have used are like great sports cars. You have no idea how fast your going until you look at the speedometer and notice you're doing 90 on an exit ramp when it felt like 30. This amp sounds like 90 when it's doing 50. While I'm in there I'll clean all contacts anyway.
Yes, I would clean the switches and pots anyway. They have been known to cause intermittents, scratchy sounds and even grainy sound. If there is a speaker relay that has poor contacts that could cause grainy sound too. How does the amp PCBs look? Any signs of heat?
 

restorer-john

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So I hooked up the PM660 into my main system and poop, it's sounds like crap. Now I know what grainy means. Compared to the 3W/ch single-ended cathode follower tube amp I've been using, it sounded horrible. It gets loud, but the sound is unbearable. I ordered most of the caps for the driver and power stage and have some homework to do.

The PM-660 is an excellent amplifier, when it is working. Unfortunately, the driver board is a known problem area. It uses a vertically mounted sub board and dry joints are a common problem. As it uses no protection (no relays, just muting), you need to check all the connections are good. Trust me, you don't want it to give you trouble.

Do not start poking around with capacitors- you are wasting your time and will likely cause problems you don't have right now. There are absolutely no capacitors in the signal line from input to output when main direct mode (-7dB) is engaged and the amplifier is ruler flat from 0.5Hz-well over 120KHz. Before you do anything. At all. Test the amplifier. What you are describing is either a) your imagination or b) the other amplifier is non-linear to the point it is audible.

The biggest issue with all of the HKs from that era are the silver plated pushbutton switches for everything. They seriously oxidise inside and there is only one way to fix them. Do not use any contact cleaner. They must be removed from the PCBs and dismantled and cleaned internally with silver polish, then ultrasonically cleaned, dried and re-assembled. Not a job for a beginner. The issues will be most evident in the MM/MC headamp switches.

Ask me anything about the PM-660, I have 3 of them, 2 PM-650s and a few PM-640s. I love them, with the PM-650 (the model below the 660) being my favourite. In fact there's two in my loungeroom right now to play with as they have been in storage for about 15 years.
 

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EJ3

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I couldn't resist. It was my dream "receiver" when I was too poor to buy it. It was a lot cheaper now than it was then.

Looks clean. I downloaded the UM and SM from hifiengine. I may have to blow the dust off my AR XA. I do have a pair of Frazier Manhattans to pair with it, but I never did get those Shahinian Obelisks. Anyone have an 8-track player I can borrow?

Hopefully it won't need more than a bulb replacement and a 1/4 can of Dioxit.
I have an AKAI RTR with a built in 8 track. Have no idea if the 8 track works though. I have Frazier Super Monte Carlo's that I bought (77-78) to go with the ADVENT 300 AMIRM recently tested. The ADVENT sounds better since it went to Peter at QuirkAudio after it's not so good test. The Frazier's efficiency goes well with the ADVENTS low power of it's on board amp.
 
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