Musical Fidelity P170 Power Amp

[maf_au]

Hi.

I have a bit of older MF gear, and I've been looking for some extra power amps to complete the HT setup.

I picked up a P170 the other day, it must be '80's gear, but I can't find a production date on it, so I'm guessing. Anyway, I'm going to replace all the Electrolytic caps in it, and then see how it compares to my other P150 and decide if it will be front or rear...

There is some evidence of previous repair inside, It looks like one of the caps has let go at some time in the past - there's a burn on the circuitboard near one of the transistors, and one of the 100uf/63v caps doesn't match it's other brothers - it's a 100v job, and a different brand - photo attached.

Apart from the cap replacement, is there anything else I should be looking at? The amp runs ok, and sounds good on my test setup of old bits 'n pieces.

Should I be looking to up the spec to 105 degree caps? They're 85 degree in there at the moment.

Any history on the P170? Unlike my other P150, this has a rear heatsink, and it weighs a lot!

Thanks,

Michael

[Netlist]

To me this is a bad repair. The overheated/burned PCB area should have been removed completely as it is highly conductive.
Replacing the caps with a 105°'s is not a bad idea if you plan to renew them.

/Hugo

[maf_au]

Originally posted by Netlist
To me this is a bad repair. The overheated/burned PCB area should have been removed completely as it is highly conductive.
Replacing the caps with a 105°'s is not a bad idea if you plan to renew them.

/Hugo

Amen. I was quite shocked to find it too, I can tell you. I was also surprised that the other caps weren't replaced at the time, along with a couple of resistors that must have sufferred a lot of heat. I'll see what I can do about that while I have it in bits for the cap replacement - maybe cut it away and attach a bit of veroboard in it's place.

Michael

[maf_au]

The amp uses 63v/22,000uf, 63v/100uf, 50v/10uf, and 16v/220uf electros.

I have found these locally:

Farnell 22000uf (EPCOS B41560A8229M)

Farnell 100uf (NICHICON 4153406)

Farnell 220uf (Rubycon 16YXF220MY0811)

Farnell 10uf (Rubycon 50YXF10MY0511)

I really don't know much about Caps, are these all low-esr, and are they suitable replacements?

Thanks,

Michael

[Netlist]

Michael
It's really hard to tell which caps would suit best where.
Other people here will be able to tell you more as I'm not a cap guru myself. And, without a schematic this could be a difficult task.
Anyway, the basic idea of choosing good quality caps is a good starting point and IMO you did.

/Hugo

[A 8]

I had one of those for almost a decade a few years back.

If you want to improve things bigtime...just swap the existing op amps for the BB627.

You'll find that it gives you a completly new amp with a much, much cleaner sound. I found it pretty muddy and ruff or harsh as is. When I first got it i figured it was broken and got it exchanged for another unit but it was the same.

There is a horrible 317..something in there as original

/Michael

[chalky]

I remember repairing one of these once. The mosfets in your photographs aren't the originals, which are now obsolete. I guess that the charring on the pcb explains why the mosfets were replaced. As I recall MF used underrated resistors to drop the main power supply voltage down to +/-15V for the LM318(?) opamp. They get very hot and eventually scorch/char the pcb. They can usefully be replaced with constant current sources, as MF did in the P270/370. Be very careful if you have to desolder any parts from the pcb. It is a very cheap paxolin pcb with very thin copper tracks which lift off when you so much as look at them. I have no idea of whether the model I worked on was an early or late version so things might have improved since the one I had experience of.

[anatech]

Yeah, these were always fun to work on. That looks like a very "fast" repair, read poor. Remove the charred area. You can replace it with Epoxy glue and trim it down wit ha razor. That works for me in these situations. Drill new holes and connect point to point.

-Chris

[maf_au]

Originally posted by chalky
I remember repairing one of these once. The mosfets in your photographs aren't the originals, which are now obsolete. I guess that the charring on the pcb explains why the mosfets were replaced. As I recall MF used underrated resistors to drop the main power supply voltage down to +/-15V for the LM318(?) opamp. They get very hot and eventually scorch/char the pcb. They can usefully be replaced with constant current sources, as MF did in the P270/370. Be very careful if you have to desolder any parts from the pcb. It is a very cheap paxolin pcb with very thin copper tracks which lift off when you so much as look at them. I have no idea of whether the model I worked on was an early or late version so things might have improved since the one I had experience of.

Thanks chalky, A 8, anatech and Netlist for the good info!

I can't see any evidence of overheating resistors, apart from the one near the burn which looks like it was caught in the crossfire Is it likely that they have been replaced already, or that some other change was made to alleviate the problem? I have a photo of the whole interior if it would help, but it's about 1mb - I can upload it to my webspace if it would help or anyone is interested.

The opamp is indeed a 318 - labelled LM318 OP749 - I'll have a look for some BB627's for it - these are socketed, so can I just drop in the replacement with no other change?

I'll be careful with the soldering, thanks for the warning.

Michael

[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