pm78 schematic or service manual

Luke · 3rd March 2006, 09:38 AM

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:-)

Luke · 4th March 2006, 04:14 AM

anyone??

anatech · 5th March 2006, 12:06 AM

Hi Luke,
Sorry I can't help with the schematic, but what makes you think the power transformer is bad?

Sometimes you can replace the thermal fuse if you can get to it.

-Chris

Luke · 5th March 2006, 12:55 AM

Quote:

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

anatech · 5th March 2006, 01:27 AM

Hi Arthur,
Sounds like a plan. With any luck the guy just popped the fuse. When transformers overheat they generally make a sickly sweet / burnt smell.

So you're making an amp for your computer speakers?

-Chris

Luke · 5th March 2006, 07:07 AM

Quote:

So you're making an amp for your computer speakers?

lets just say its a reality check

Maybe it will give me the motivation to finish the Aleph-X.

seeya arthur

sangram · 6th March 2006, 11:32 AM

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...

bekim · 7th March 2006, 09:50 AM

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...

bekim

Luke · 7th March 2006, 08:09 PM

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

bekim · 13th March 2006, 02:34 PM

45 volts is per rail, to be sure now, hmmm ... frankly, I forgot but, usualy this models have 45v per rail - pm68, 78, pm7000 & 8200..

be careful with zeners in preamp pcb..

bekim

3rd March 2006, 09:38 AM	#1
Luke diyAudio Member Join Date: Sep 2001 Location: Wellington NZ	pm78 schematic or service manual 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:-)

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4th March 2006, 04:14 AM	#2
Luke diyAudio Member Join Date: Sep 2001 Location: Wellington NZ	anyone??

5th March 2006, 12:06 AM	#3
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	Hi Luke, Sorry I can't help with the schematic, but what makes you think the power transformer is bad? Sometimes you can replace the thermal fuse if you can get to it. -Chris

5th March 2006, 01:27 AM	#5
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	Hi Arthur, Sounds like a plan. With any luck the guy just popped the fuse. When transformers overheat they generally make a sickly sweet / burnt smell. So you're making an amp for your computer speakers? -Chris

6th March 2006, 11:32 AM	#7
sangram diyAudio Member Join Date: Sep 2002 Location: India	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...

7th March 2006, 09:50 AM	#8
bekim diyAudio Member Join Date: Jul 2004 Location: Prishtina, Kosova	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... bekim

7th March 2006, 08:09 PM	#9
Luke diyAudio Member Join Date: Sep 2001 Location: Wellington NZ	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

13th March 2006, 02:34 PM	#10
bekim diyAudio Member Join Date: Jul 2004 Location: Prishtina, Kosova	45 volts is per rail, to be sure now, hmmm ... frankly, I forgot but, usualy this models have 45v per rail - pm68, 78, pm7000 & 8200.. be careful with zeners in preamp pcb.. bekim

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