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.
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.
I checked, the malfunction does show itself even after powering on after a very short power off.
Sometimes one channel is dead and the other weak or vice versa.
One time I had sound out of both channels but even weaker.
This is all for about 30 seconds or so.
The unit smells like old components so we will have a look inside next week and check the capacitors and everything.
We can also do comparative measurements by opening the good one as well.
In the meantime, if someone happens to have an idea what could cause this kind of malfunction, any thoughts would be very much appreciated.
I see it uses obsolete parts like these
http://www.ebay.com/itm/NEW-NEC-UPC...652?pt=LH_DefaultDomain_0&hash=item43aa2abdcc
Sometimes one channel is dead and the other weak or vice versa.
One time I had sound out of both channels but even weaker.
This is all for about 30 seconds or so.
The unit smells like old components so we will have a look inside next week and check the capacitors and everything.
We can also do comparative measurements by opening the good one as well.
In the meantime, if someone happens to have an idea what could cause this kind of malfunction, any thoughts would be very much appreciated.
I see it uses obsolete parts like these
http://www.ebay.com/itm/NEW-NEC-UPC...652?pt=LH_DefaultDomain_0&hash=item43aa2abdcc
Thanks my man. Yes I woke up just now and the first thing that came to my mind were re-capping both amps. There aren't many in there anyway, as you can see here:
Index of /Yamaha/Yamaha.YPA.720
Sorry for that Perry. I'm sure I got it without registration but I can't remember from which site. I have forwarded it by email.
I am going to a town tomorrow where there is an electronics store.
Do I only need to replace the four 2200 uF 35V and two 3900 uF 16V caps as can be seen on the pics here:
Index of /Yamaha/Yamaha.YPA.720
Or do I have to replace all electrolyctic caps ?
Or do I have to replace ALL caps, including ceramic and film caps ?
Do I only need to replace the four 2200 uF 35V and two 3900 uF 16V caps as can be seen on the pics here:
Index of /Yamaha/Yamaha.YPA.720
Or do I have to replace all electrolyctic caps ?
Or do I have to replace ALL caps, including ceramic and film caps ?
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?
Okay.
How do I measure the audio level on the transistors ? With the oscilloscope or multimeter ?
Which legs ? Where should I put the positive and negative probes.
Thanks. The strange thing is that it takes the exact same amount of time for the sound to play properly. I know this as it is always at the very same progress in the audio track that I start playing when I power it on. Coincidental (or not ?) it is when the audio track starts to be louder. I mean the audio track is a guitar playing soft, then at some point suddenly loud, and I noticed a couple of times it was at that exact same instant, which shouldn't be as I first press play, then connect the remote, sometimes there's two seconds in between play and remote, sometimes three, while a couple of times at least (maybe every time) the amplifier starts to work properly just when the audio track starts to get louder, I mean right at that moment, not half a second earlier or later ?
How do I measure the audio level on the transistors ? With the oscilloscope or multimeter ?
Which legs ? Where should I put the positive and negative probes.
Thanks. The strange thing is that it takes the exact same amount of time for the sound to play properly. I know this as it is always at the very same progress in the audio track that I start playing when I power it on. Coincidental (or not ?) it is when the audio track starts to be louder. I mean the audio track is a guitar playing soft, then at some point suddenly loud, and I noticed a couple of times it was at that exact same instant, which shouldn't be as I first press play, then connect the remote, sometimes there's two seconds in between play and remote, sometimes three, while a couple of times at least (maybe every time) the amplifier starts to work properly just when the audio track starts to get louder, I mean right at that moment, not half a second earlier or later ?
Single-sided means that the copper traces are only on one side of the board.
Cold solder joint is used for many different types of failed solder connections. Cold solder joints are joints where the solder didn't flow because the components being soldered together weren't hot enough to melt the solder. These are rare on OEM solder connections. It's also used to refer to joints that have overheated and oxidized or those that failed because the solder cracked (generally due to a through-hole that's too large - generally only a problem on single-sided boards).
Cold solder joint is used for many different types of failed solder connections. Cold solder joints are joints where the solder didn't flow because the components being soldered together weren't hot enough to melt the solder. These are rare on OEM solder connections. It's also used to refer to joints that have overheated and oxidized or those that failed because the solder cracked (generally due to a through-hole that's too large - generally only a problem on single-sided boards).
I am buying up all '20' model Yamaha YPA amplifiers I can find, so I'd better wait until I have many and then buy lots of caps for all of them from Ebay or Farnell or something, at a much cheaper price than buying them from the electronics store, which is only good if you need a small number of parts.
I don't know what's causing it the problem. I wouldn't rule out the capacitors.
It varies. Many times the smaller caps fail and cause strange problems. The larger capacitors are less likely to cause strange problems unless they leak electrolyte onto the board or open internally. The film and ceramic capacitors rarely fail.
It varies. Many times the smaller caps fail and cause strange problems. The larger capacitors are less likely to cause strange problems unless they leak electrolyte onto the board or open internally. The film and ceramic capacitors rarely fail.
Alright Perry, we were not able to finish your test as we ran out of time, but this is what we did:
We first checked the solderings once more, and they look fine to us, and my father knows what he's talking about when it comes to solderings, so I trust him on that. They look pretty good to my lesser trained eye, too
Anyway, we hooked up the RCA's to channels 1 + 2 (4 channel amp) and speakers to the speaker terminals and checked AC with the oscilloscope using your test tone CD.
The scope clearly showed than the signal on channel 2 was very weak for the first X number of seconds.
Tapping the relay responsible for channels 1 + 2 with the back of a screwdriver (once the amplifier was fully functional, after an X number of seconds) caused momentary problems on the speaker hooked up to channel two, so the contacts might be dirty. We haven't had the time to measure at at the outputs to compare with the speaker terminals by the way.
We checked the solderings on that specific relay and they look fine.
We will try and clean the relay next time with contact cleaner spray and something tiny to rub the contacts with.
We will also check channels 3+4 next time (haven't tested them at all) and also tap their relay.
By the way we performed the adjustment mentioned in the service manual, have you seen it ? It involves a potentiometer and setting the voltage on a certain leg of a certain output to 23.5 V while feeding the amp 14.4 V.
The voltage was a bit off so we corrected it. Can't remember what it was before adjustment, maybe 22.6, maybe 20.6, not sure.
Can you tell me what this adjustment does ? Is it like settin the bias or something ?
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