Yamaha YPA-720 problem

[Perry Babin]

That's setting the rail voltage.

 

[azvrt]

Thanks.
What do you think about the relays ?

 

[azvrt]

By the way, that's not very common, is it, being able to adjust the rail voltage ? It's the first amp I've had on which there's pot for that, I believe. Any reason why they did it and other manufacturers don't ?

 

[Perry Babin]

Nothing yet. You haven't determined where the signal is being lost.

If you have to clean the contacts (which will only work temporarily), saturate a piece of paper with contact cleaner. Place the paper between the contacts. Push the contact together and pull the paper out while they're clamped on the paper. Do this several times.

 

[Perry Babin]

Not too many amps have adjustable rail voltage. You can get very precise voltage by using just a few 1% resistors in place of the pot.

 

[azvrt]

Thanks for the tip (piece of paper).

Yes we will have to perform more tests I guess. I'll look in the service manual whether these relays are common parts or custom made for these amps (thus hard / impossible to get).

Okay. Well it's good we checked as it was off, so I'll do the same on the other amp once I get it out of my car to have it re-capped along with this one. Bought a third one but it'll take some time before it is shipped to me. I like them a lot.
Very smooth non-fatiguing sound which is good for my damaged ears.

 

[azvrt]

This amp uses relays for muting. Unless the contacts are dirty, the audio output level should be the same as it is in the other channels. Connect speakers to the channels that aren't working properly and, when the audio is low or non-existent, compare the audio level on the output transistors to the level on the speaker terminals. Are they the same?

Hi Perry,

This is an old topic, due to some circumstances we haven´t had much time to work on amps lately.
Anyway I wanted to thank you and everyone else for helping me with this amp as we got it working. Actually, I have three of these amps and two had the same problem, so we got both working.

The relays were indeed dirty. We cleaned them thoroughly and now we can hit the relays with the back of a screwdriver and the music keeps playing.

So thanks again guys ! I am so happy we were able to save two more oldies from ending up in a trashcan. Apart from the relays, they are pretty bulletproof. Simple and solid design.

Fred

 

[azvrt]

This pic shows the copper-colored relays near the speaker terminals.
They have transparent covers so I find it strange dirt got in there but it did.
Cleaning the relays with contact spray alone did not do the trick, we had to use smooth sanding paper as well.

http://hugi00.comoj.com/Yamaha/Yamaha.YPA.720/yamaha_ypa-720_02370.jpg

 

[Perry Babin]

When cleaning the contacts, use paper (printer, notebook, business card...) instead of sandpaper (item #32 on the audio troubleshooting page). The top of most contacts are coated with a very thin layer of material and sandpaper will remove that layer. This will make the contacts more likely to oxidize and require cleaning in a very short period of time.

 

[azvrt]

We used ultra-smooth sandpaper (least agressive) and could not apply any force due to space restriction so with a bit of luck we did not damage the coating.
Next time we will use regular paper, thanks again !

 

[AKHeathen]

what about if there is arcing on the contact tip? i have sanded that down many times to reduce heat.

 

[Perry Babin]

If the top surface is damaged, the relay generally needs to be replaced. It may work but it's not as good as it was before the contacts were burned, especially when the relay contacts originally had a special top layer to improve reliability/conductivity.

 

[Ykarim]

YAMAHA YPA 720

Last week I bought a very old Yamaha YPA-720 car amplifier. I liked it so much I immediately bought an other one for sale in my country. If I find another I would not hesitate for a second.

Anyway, one of both amps has a malfunction.
When powered on, no sound comes out of the right channel (channel 2 of 4, haven´t tested 3+4 yet). Also, it seems the left channel is weak.

Then, after half a minute or so the sound suddenly comes alive, coming from both speakers and strong. It plays perfectly from then on.

I think that when powered off and powered quite soon, the problem is not there.
Can´t remember for sure.
If powered off for a longer time, then powered on, the problem reappears for half a minute.

The strange thing is that the last time I powered it on, it was the left channel that wasn´t playing and the right channel that was weak
After half a minute, it played great once more.

Anyone have an idea

The service manual can obtained for free from here

YAMAHA YPA-720 SM Service Manual free download, schematics, eeprom, repair info for electronics

Can´t remember where you have to click on this page to get it (maybe the picture), but it´s there, somewhere.

Before I forget, the amplifier shows the same malfunction when bridged for 2 channel operation.