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Old 15th February 2012, 05:10 PM   #1
azvrt is offline azvrt  Netherlands
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Default Yamaha YPA-720 problem

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, havenīt 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.
Canīt 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 wasnīt 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

Canīt remember where you have to click on this page to get it (maybe the picture), but itīs there, somewhere.

Before I forget, the amplifier shows the same malfunction when bridged for 2 channel operation.
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Old 15th February 2012, 06:05 PM   #2
azvrt is offline azvrt  Netherlands
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Download the Yamaha YPA-720 service manual for free - Hifi Manuals
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Old 15th February 2012, 09:12 PM   #3
azvrt is offline azvrt  Netherlands
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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-UPC1...item43aa2abdcc
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Old 16th February 2012, 01:24 AM   #4
RiLoWa is offline RiLoWa  United States
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Just a random thought, Yamaha hasn't made car audio since around 1992. I would re-cap the entire amp and then come back and see if there are any problems after that. You probably should do the other one too.
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Old 16th February 2012, 02:21 AM   #5
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Send me a copy of the manual. The site requires registration.

babin_perry@yahoo.com
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Old 16th February 2012, 07:56 AM   #6
azvrt is offline azvrt  Netherlands
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Quote:
Originally Posted by RiLoWa View Post
Just a random thought, Yamaha hasn't made car audio since around 1992. I would re-cap the entire amp and then come back and see if there are any problems after that. You probably should do the other one too.
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

Quote:
Originally Posted by Perry Babin View Post
Send me a copy of the manual. The site requires registration.

babin_perry@yahoo.com
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.
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Old 16th February 2012, 03:12 PM   #7
azvrt is offline azvrt  Netherlands
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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 ?
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Old 16th February 2012, 10:27 PM   #8
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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?
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Old 16th February 2012, 10:38 PM   #9
azvrt is offline azvrt  Netherlands
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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 ?
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Old 16th February 2012, 10:48 PM   #10
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Using the scope will be the easiest. You'll check the level on the emitter of the output transistors.
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