Yet Another M-80 Repair Thread

[Wayniac]

I bought this amplifier new back in the 1980s. I knew it was a time bomb, but I continued to use it as as my second string backup amplifier for use when my main amplifier quit.

Fortunately, without fire or smoke, it clicked into protection one day and that was that. I swapped in my third string amplifier (M-50), and took the M-80 into the shop.

When I lifted the M-80, one of the binding posts fell off onto the table. Great. Luckily, I had a set laying around for a Pass F5 I'm building.

I tore it down and found one of the (6) 330 ohm resistors had succumbed to the corrosive capacitor glue. I hope this is the only problem.

I've ordered a full complement of electrolytics (except the main PS filters) including those on the display PCB. I also ordered new pots, a few resistors and zener diodes also corroded by the glue.

The glue has been removed. Much to my relief, the heat sink transistors all checked good.

I addressed the binding posts by fabricating a small panel from a scrap piece of 1/8" aluminum. I drilled and tapped the mounting holes to accept a 6-32 screw.

I removed and cut down the original bus bars, then soldered them to the new binding posts.

Tomorrow, I plan to replace the cheesy 20 AWG power cord and also replace the heat sink compound.

 

[avionic]

Might want to reflow the solder on all eight TO-126 transistors on the main board As well as all the transistors on the two narrow upper APS boards.

 

[avionic]

Don't forget. There is one non-polar capacitor..Its a 47uf/25v in the front row of caps.Nichicon ES or EP. The silkscreening on the circuit board and the schematic are not correct.Parts list is correct.

 

[avionic]

I tore it down and found one of the (6) 330 ohm resistors had succumbed to the corrosive capacitor glue
.

Those are 330 KΩ. Not 330 Ω. (orange/orange/yellow/gold)

 

[Wayniac]

Those are 330 KΩ. Not 330 Ω. (orange/orange/yellow/gold)

Understood, I was a bit bleary-eyed when I typed the post last night. I checked my order and I ordered 330k resistors.

Thanks for your comments, your input is both welcome and appreciated!

While messing around with the amplifier today, I examined the RCA input jack assembly. What a piece of crap.

I plan to replace the jacks and both ribbon cables (to and from the input attenuators) with some high quality Belden low noise coaxial cable I have laying around.

 

[avionic]

Was D119 and D120 all corroded (3v zeners) ?

 

[avionic]

How about the jumper wiring between the the copper busses ?

 

[Wayniac]

Both diodes are badly corroded, one of them checked good, the other fell apart. Replacements have been ordered.

Two bus bars had mild corrosion, these have been removed and cleaned.

One jumper has been affected, it will be replaced.

 

[avionic]

Super..

 

[Wayniac]

I've completed the passive component replacements. No problems.

The power cord has been replaced.

Whenever an APC UPS fails at work, I salvage the power cord for use on my stereo. The cords are 2 meters long, 14AWG with a foil shield and a drain wire. I added some braided sleeving and heat shrink. A cable clamp is installed for strain relief. As you can see, I added a safety ground to the chassis. A star washer is installed under the ground lug. A rubber grommet was used for chafe protection where the cable enters the chassis.

The RCA input jacks have been upgraded with sturdy Amphenol replacements. A low noise Belden coaxial cable replaces the factory ribbon cables. Copper shielding tape was used to secure the coax under the lip of the chassis for the run to the input attenuators.

 

[avionic]

Is that amplifier now "earth grounded "?

 

[Wayniac]

Is that amplifier now "earth grounded "?

Just the chassis. The signal path is still isolated. The Power supply is also still isolated.

 

[avionic]

Are there any black wires connecting the power supply to the main board and/or the APS boards ? About 6 of them on the underside of the mainboard.

 

[Wayniac]

There is one black jumper wire which runs from the front left of the main board to the general area of the 1000uf caps. I know why you're asking as I initially assumed this was a ground connection, but when I rang it to the chassis, it wasn't. I don't recall what value I measured...

 

[avionic]

Are the two brass grounding plates still soldered to the main board at the two rear circuit board screws?

 

[Wayniac]

Yes. Grounding the chassis is not a problem. The safety ground I added is electrically the same point as the ground stud on the rear of the amplifier.

From the manual:

 

[avionic]

All the power supplies reference "chassis ground". And are isolated from earth ground. Same goes for the control amplifier.

 

[Wayniac]

If only two conductors are present in the power cable, I see how this can happen!

Grounding the chassis is just a good practice.

 

[avionic]

Your amp. Your project.

 

[avionic]

BTW make sure you have the Grey and brown AC mains wires connected to the correct spades on the IEC cord.