pm78 schematic or service manual

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Hi anyone have this?
Can only find one place on google that sells cds for 25 dollars.
This thing has a blown channel and I havent recieved it yet. It also needs a new transformer, what are the rails on it?
It uses HDAMS so I am thinking of replacing drivers and outputs on the blown channel, but a manual would help for some tweaking:)
 
Sorry I can't help with the schematic, but what makes you think the power transformer is bad?


Hi Chris,

Well the story goes, its belongs to a guy who was working abroad and got the voltage wrong, zapped one channel.
This amp is 110V so I would change it 230V even if it were ok.
What I dont know is if the transistors are hard to get and if I should just change them to the latest Toshibas on both channels anyway.
one of my aims is to tweak the amp to see what performance I can get out of a low price amp. Currently I have aleph 2s, Krell ksa50, JLH couple of gainclones, so I am curious what I can get out of a midfi tweaked amp.

seeya arthur
 
Moderator
Joined 2002
Paid Member
I did that too! On my PM665. I'll see if I havea copy of the manual but I got mine from a very kind member on this site, maybe if you did a search...

I remember we had to replace the driver transistor. The main power supply caps would like to be changed after this sort of a thing, they were OK then but show accelerated aging...

The transformers should mostly be OK, they could survive the x2 voltage. I changed out the transformer to custom units. The manual specified 53 volt rails, so I used 38 volt transformers...
 
Hi,

I don't have schema for this amp, but I serviced one years ago adn, is same like pm68 inside exept some wiring and resistors which turns to class A.
Regarding transformator, I think is 45volts per rail.
Output trannies are toshiba 2SA1941/2SC5198 and, you have to be careful with those small metal boxes called hdam. Might happen sometimes any transistor to misbehave, so you have to check one by one.

I agree, you better need schematics in order to fix it, it took my 1 month and half until i fix it... :whazzat:

bekim
 
Hi Sangram and Bekim,

thanks or your input. I would have thought that the outputs and maybe drivers have failed. I hope not the hdam:(
Bekin, is the transformer 45V or the rail voltage 45V?
Maybe PM68 schematic is readily available on web. Thanks for the tip:)


regards Arthur
 
well I have had a look at it now. One channel plays but its very distorted. The drivers test fine, outputs too. But it doesnt get warm like the good channel?
Any ideas whats wrong, still no schematic:(
Where can I get quality caps tp replace they look like they have been stressed, 12000uF 63V Elna for audio. But I cant find a good cap at the right price, ie Panasonic FC dont have this voltage and value.

seeya ab
 
Administrator
Joined 2004
Paid Member
Hi Luke,
Don't worry about the filter caps yet. Let's get it working first.

Cool heatsink = no or very little bias current. So that tells you something.

There should be a small transistor mounted on the heatsink. Sometimes there is a capacitor across it. Discharge the filter caps first with the unit unplugged and off. Measure from the collector of the bias transistor to the driver bases. One should have a low resistance, do the same with the emitter of the bias to the other driver. You want to test the voltage between these two points (the two driver bases) but measure the resistance first.

There may be a couple signal transistors that lead back to the emitter resistors (check those), test these transistors for shorts.

Let us know what you find.

-Chris
 
Hi Chris,

The base to collector conducts one way, and base to emmitter conducts one way. The two drivers test the same in both channels.
The small signal transistor that is connected to the emmitter of the driver (thats the one you mentioned in your last post?) measures open circuit, so do all the other small signal transitors I have tested. But this is the same on both channels?

With class A bias switch on, the voltage on the base of the good driver is 1.2 V steady. On the bad channel, its 1.0 volt but it drifts.
Its about .2V less on both channels in class AB mode.


whats up here?

seeya arthur
 
Hmmm, everything tests out ok, cos it is. I think its a dry joint. After some tinkering it now works:) Thanks to all who helped.
Havent done critical listening, but low level first impressions are its second rate compared to alpeh 2s, krell clone, jlh and gainclone. Will have a better listen when our baby is not asleep and see if it warrants tweaking.
 
Administrator
Joined 2004
Paid Member
Hi Luke,
The optocoupler is for the "class A" position. Measure the bias, if it goes up when the "class A" switch is pushed in, the opto works fine.

So if the optocoupler was open you would have normal bias current, if it were to short the bias would read high.

Just one note, this only increases the bias current and runs everything hot. Don't do it. There will be little to no difference in the sound quality and you will shorten the life of the amp and power supply. Why bother?

-Chris
 
Administrator
Joined 2004
Paid Member
Hi Luke,
I heard the high end models in England in 2004 at the Hi Fi show. I was very impressed. I also really liked the cosmetics.

I got a chance to speak briefly with Ken I. Interesting. He does not like the earlier stuff very much. We disagree on those points.

-Chris
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.