Musical Fidelity P170 Power Amp - Page 2

maf_au · 28th June 2005, 02:00 AM

Quote:

Originally posted by amplifierguru
Hi maf_au,

I would definately replace all the electros near the heatsink/MOSFETS/power resistors or anything else that gets hot, with 105C types.

On replacing the 318 with the OPA627 (or even better OPA637 if it can handle the decomp BW), I would check for method of offset adjustment in the cct as it may utilize the chip offset pins which may be different and/or cause problems if it activates a non-active pin.

A schematic would clarify. Better safe than sorry.

Cheers,
Greg

Hi Greg,

How would I get hold of a schematic? (sorry, newbie at this stuff) or are you saying I should draw one from the circuit board?

Given the age of the amp, I have ALL the electro caps down for replacement - tho only costly ones are the 4 22,000uf/63v.

Heh. By the time I have done the caps and opamps, I will have spent more on the amp than I paid for it! Once this is done, it will be the P150's turn. It's in the A1 chassis with the heatsink case - not looking forward to dealing with the heatsink goo and tight space there...

I've had the P150 since new, and it hasn't missed a beat. I bought it just after they were discontinued - must have been around 1990, but my memory for dates isn't so hot, it came with a preamp 3A, which has an active and passive output - always had a lot of noise in active, so I used passive, which was at least quiet. It doesn't get much use these days.

Michael

maf_au · 29th June 2005, 03:54 AM

Hi.

Ok, I've searched, but can't find a schematic.

Hunted on my system and found a program called 'oregano' for capturing electronic circuit schematics, I'll have a go at it while I have it apart to replace the caps.

Is there someplace that collects schematics that I can upload it to when done for the next person to use?

Thanks,

Michael

amplifierguru · 29th June 2005, 03:57 AM

Ideally you need to hunt down a schematic. However you could access the area of the board around the chip and check for pin connections to the cct from pins 1,5 I think it is to +Vs for the OPA627. LM318?

Your quest.

Cheers,
Greg

maf_au · 29th June 2005, 04:10 AM

Quote:

Originally posted by amplifierguru
Ideally you need to hunt down a schematic. However you could access the area of the board around the chip and check for pin connections to the cct from pins 1,5 I think it is to +Vs for the OPA627. LM318?

Your quest.

Cheers,
Greg

Hi.

I hunted google and this site, but couldn't turn one up. That's why I was wondering if anyone is creating an archive of them.

They're LM318's - it was suggested some improvement might be had by swapping them for 627's, which I have coming with the caps, as I'm sure they will get used elsewhere if not on the P170... I think I've been infected with DIY..

I'll have a go at creating a full schematic of the amp, and I'll upload it here and send it to Mr Hennesy, maybe he'll archive it on his site...

Maybe on the weekend...

Michael

A 8 · 29th June 2005, 08:31 AM

Hennesy should have the schematics, I think I sent it to him some time ago.
Regarding swapping the 318, I seem to remeber that there are no dc ofset adjustments only a freq-compensatio pins/curcuits/components that you need to disconnect. Other then that it should work as is. The 637 will probably make the amp go into oscillation.

maf_au · 1st July 2005, 01:08 AM

Ok, I have the schematics, thank you Mark and A 8

Here is what I see on pins 1 and 5 on the 318:

Pin1 via 3K3 and 2n2 to Pin5

So should I just hook them out and swap in the 627?

Regarding replacing the caps, especially the 22,000uf jobs on the PS:

How should I (safely) discharge them before working on them?

Is running the amp disconnected from the power for a while enough to discharge them, or should I ground the + terminal as well? Not wanting to get fried this week, and aware of the potential for stored current in these puppies...

At this stage, I'm going to replace caps, repair board and re-assemble. If everything behaves at that point, I'll swap the opamps.

Michael

amplifierguru · 1st July 2005, 01:20 AM

Firstly, if you have any power resistors around say 5W'ers then work out from the supply voltage stored on the caps whether they would be in their ratings if placed across one C to ground i.e. P=VxV/R. Then just hold it on -the the other(s).

The RC on pins 1,5 sound like comp for the 318. I haven't used one for decades. Can you post the amp schema..

Cheers,
Greg

maf_au · 1st July 2005, 06:24 AM

Quote:

Originally posted by amplifierguru
Firstly, if you have any power resistors around say 5W'ers then work out from the supply voltage stored on the caps whether they would be in their ratings if placed across one C to ground i.e. P=VxV/R. Then just hold it on -the the other(s).

The RC on pins 1,5 sound like comp for the 318. I haven't used one for decades. Can you post the amp schema..

Cheers,
Greg

Thanks for the reply!

Ok, bear with me, I'm new to this. I've got the bit about grounding the cap to earth via a power resistor, but calculating the load:

P=VxV/R

Am I on the right track here:

I'm thinking that V= the tranny output volts, R= the power resistor value ohms, so assuming I had a 5k/5w resistor, and the tranny was putting out 63v:

P=63 x 63/5000 = 0.79 (watts?)

So, my 5watt/5k resistor would be fine, and turning the eq around, I'd need a 793.8ohm/5w resistor or better?

I've asked A 8 if it's ok to post the schematic...

Thanks,

Michael

A 8 · 1st July 2005, 06:27 AM

They are for comp so yes desolder them, there should also be a 5p Mica between output and pin 8, disconnect pin 8 and do the swap. You may want to keep it but connect the other end to the inverted opamp input. Cant remember what I did.

I think I also added filmcaps to the opamps powersupply.

A 8 · 1st July 2005, 06:39 AM

I just sent you a private message.

BTW my unit had the K1058/J162 mosfets in them and it worked great with the 627, your unit seems to have a lot of components replaced which theoretically can change how it behaves with new opamps. Be watchful of dc level on the output of the amp and current drain when you first power it up after placing the 627 in there.

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29th June 2005, 03:54 AM	#12
maf_au diyAudio Member Join Date: Jun 2005 Location: Adelaide	Hi. Ok, I've searched, but can't find a schematic. Hunted on my system and found a program called 'oregano' for capturing electronic circuit schematics, I'll have a go at it while I have it apart to replace the caps. Is there someplace that collects schematics that I can upload it to when done for the next person to use? Thanks, Michael

29th June 2005, 03:57 AM	#13
amplifierguru Account Disabled Join Date: Aug 2004 Location: Earth	Ideally you need to hunt down a schematic. However you could access the area of the board around the chip and check for pin connections to the cct from pins 1,5 I think it is to +Vs for the OPA627. LM318? Your quest. Cheers, Greg

29th June 2005, 08:31 AM	#15
A 8 diyAudio Member Join Date: Mar 2003 Location: Gothenburg	Hennesy should have the schematics, I think I sent it to him some time ago. Regarding swapping the 318, I seem to remeber that there are no dc ofset adjustments only a freq-compensatio pins/curcuits/components that you need to disconnect. Other then that it should work as is. The 637 will probably make the amp go into oscillation.

1st July 2005, 01:08 AM	#16
maf_au diyAudio Member Join Date: Jun 2005 Location: Adelaide	Ok, I have the schematics, thank you Mark and A 8 Here is what I see on pins 1 and 5 on the 318: Pin1 via 3K3 and 2n2 to Pin5 So should I just hook them out and swap in the 627? Regarding replacing the caps, especially the 22,000uf jobs on the PS: How should I (safely) discharge them before working on them? Is running the amp disconnected from the power for a while enough to discharge them, or should I ground the + terminal as well? Not wanting to get fried this week, and aware of the potential for stored current in these puppies... At this stage, I'm going to replace caps, repair board and re-assemble. If everything behaves at that point, I'll swap the opamps. Michael

1st July 2005, 01:20 AM	#17
amplifierguru Account Disabled Join Date: Aug 2004 Location: Earth	Firstly, if you have any power resistors around say 5W'ers then work out from the supply voltage stored on the caps whether they would be in their ratings if placed across one C to ground i.e. P=VxV/R. Then just hold it on -the the other(s). The RC on pins 1,5 sound like comp for the 318. I haven't used one for decades. Can you post the amp schema.. Cheers, Greg

1st July 2005, 06:27 AM	#19
A 8 diyAudio Member Join Date: Mar 2003 Location: Gothenburg	They are for comp so yes desolder them, there should also be a 5p Mica between output and pin 8, disconnect pin 8 and do the swap. You may want to keep it but connect the other end to the inverted opamp input. Cant remember what I did. I think I also added filmcaps to the opamps powersupply.

1st July 2005, 06:39 AM	#20
A 8 diyAudio Member Join Date: Mar 2003 Location: Gothenburg	I just sent you a private message. BTW my unit had the K1058/J162 mosfets in them and it worked great with the 627, your unit seems to have a lot of components replaced which theoretically can change how it behaves with new opamps. Be watchful of dc level on the output of the amp and current drain when you first power it up after placing the 627 in there.