Yamaha AX-500U Integrated Amplifier Repair

[Smoky]

The third project I have open is this damsel in distress, a Yamaha integrated amplifier.

Over the past week I've been able to get the protection relay to stay on. The glue holding down the bulk capacitors spilled over some of the zener diodes and transistors in the protection circuit which appeared to become conductive and causing them to burn.

Now with it running, I can here a hum from both the left and right speakers. The funny thing is, when I touch the chassis with my hand, the hum goes away

Anywho, the chassis and faceplate is in perfect shape and I think it's worthy of this TLC.

The TDS460 is warming up and now it's time to check the vitals 

[Smoky]

Here was some of the carnage. It almost looks like you can see an arc trail in the glue between components










And here is the underside of the top cover showing the smoke and heat damage residue and the deteriorated plating:

[tautech]

That amber hard setting hygroscopic semi porous stuff ? 

[andy3055]

Was it the glue or did the cap leak out? Those old electrolytics were notorious for leakage.

[Smoky]

But now the main board is back and as clean as a whistle. The protection circuit functions properly and it's off to the re-capping:





I also matched the new Zener diode pairs for the opposing (+) and (-) rails. These two both break down at 16.79v:





On a side note, one of the flat ribbon cables seemed loose in its connector. When I touched it with a wooden stick, you could hear popping and crackling through the speakers, so I removed the darn thing and soldered the cable directly to the main board. Super-solid. The cable is the one in the upper-left corner of the picture above going straight down into the board.

The connector is really cheesy and the metal fingers that are supposed to grip the wires lose tension.





I couldn't tell if the caps were leaking, they're Elna's. I think the amp has been sitting for a long time.

[KevinA]

Prior to retirement, I had a similar Yamaha in for repair, it had been given a couple of pints of lager at a party. A quick check over told me the outputs were shorted, and after the owner had accepted my estimate, I started on it.

Stripped down, I washed the affected boards with IPA, and as I scrubbed, the shorts disappeared. No components were faulty, it was just gunk on the boards causing leakage. Once cleaned, I checked the bias, and it passed all its tests with flying colours.

[Smoky]

It's definitely glue and similar to what was found in a previous NAD amplifier repair.

This amp has an A/C hum that varies in amplitude depending upon on how close I bring my hand closer to the chassis , and that's with the "A" speakers on and it doesn't matter if Tuner, Aux, Tape, etc. is selected. If Phono is selected, the noise gets louder and even more so if moving-coil is selected. The volume is at the middle and the balance control can move the noise more towards each channel but the Left side seems to never lose it.

With my DMM connected to the speakers terminals and set to AC volts, the voltage starts at ~30mV and goes as high as 140mV the closer I bring my hand to the chassis. When I touch the chassis, the voltage reading drops to around 2-3mV 

On the Phono MC/MM Amp board are several other capacitors glued down with a few discolored resistors buried too. I'm going to clean and re-cap this area first and then do sound tests again. I'll know when I find the problem since it's easily heard through the speakers

This amp has probably had a very interesting past!

The schematics available online are chopped up and voltages can't be read since they were printed so lightly on the drawings.

A good switch cleaning will be done too 

Here's a shot of the ripple riding on the two original 12,000uf filter capacitors. ~48mV on one and ~40mV on the other:

[Smoky]

This amp was a good learning experience

Rule #1, don't bother to test amplifiers with most of their covers off! It seems that the hum issue was due to having the bottom metal ventilation cover and the top cover removed. The bottom cover especially, it shields the Phono Amp board heavily. As soon as I put them on, the hum vanished!

Anywho, the one thing I did notice was while it was humming, it was louder in the Left channel than the Right. Fast forward, the bias voltage for each channel should be 10mV +/- 2 DC. I discovered the Left channel read nearly 150mV and the Right 10.8mV!

I cleaned the switches and sent test tones through the Aux inputs with the Siglent waveform generator and now the channels track very close to the same. Maybe a difference of 3-5mV of AC voltage at the speaker terminals.

To adjust the Left channel's bias, hook your DMM to each side of resistor R229 and adjust VR107:



The Right channel is R230 and VR106:



This audio repair stuff is a hell of a lot of fun!

[Smoky]

I also found the full schematic for the AX-500 U on a European website. Someone painstakingly pieced together all of the PDF pages into one

It's too large to post but here's the link:

https://obrazki.elektroda.pl/2520930200_1505236229.jpg