Yamaha A720 amplifier not working

[jlbosch]

Hi all: New to this board and here's my first question. I have a Yamaha A-720 integrated amp that doesn't put out any sound any more. It seems to power up ok, but will not make any sound.
I'm "somewhat" familiar with electronics, but am far from an expert, and I'd like to fix this myself. Can anyone point me in the right direction?

Also if schematics (ie service manual) are available for this amp I'd appreciate getting my hands on good quality jpg's or bmp's of it!

Thanks in advance

[djk]

Does the pilot light work?

The lamp is used as a current source for a voltage regulator.

No lamp, no voltage.

[robbie b]

I've had a A1020 and experienced 2 problems.
Too high dc offset on the output, then the some safety system came in and blocked the output, how it was exactly I dont now annymore over 10 years ago. check the dc offset directly at the outputtransistors and keep it below 20-50 mV, mine had over 700 mV.
Another problem was to high bias current in classA mode, a very silent poeff it said and gone were the outputstages of both channels. After installing new outputdevices, Check the bias current if possible or else turn the trimpot as high as possible, this gives very low bias current , and try again.

For manuals go to this website of Yamaha and look for services manuals or manuals. they have quite a lot for free downloadable. Or call the distributor of your country, mostly they are very willing to help. At least in Holand they were

good luck

Rob

[jlbosch]

Thanks for the replies so far! I haven't torn the amp apart yet..... I have an outdoor project on the go right now that my wife and I want to finish. Gotta get that stuff done while the weather is nice.

Anyway, by pilot light, I presume you mean the light that comes on when you push the power button? If so, it does indeed light up.

When I get a chance I'll take it apart and check the things that you've said .

[hjelm]

If you have DC on the output then you will not hear the output relay click, it was quite distinctive if i remember correctly.

[jlbosch]

When I turn the amp on, it lights all up as normal, then after a few seconds I hear a distinct click. I recall that the amp always did that........still haven't opened it up and put a meter to anything

[hjelm]

Sorry to ask but have you checked the obvious like speaker switches and tape selector. If the relay pulls and you still do not hear anything it is strange. Do you hear a faint noise in the loudspeakers if you turn the volume up full?
Have you tried all sources if it is just one input that is wrong.

[robbie b]

Originally posted by jlbosch
When I turn the amp on, it lights all up as normal, then after a few seconds I hear a distinct click. I recall that the amp always did that........still haven't opened it up and put a meter to anything

If the relay pulls with a click then there is no DC-offset present, at least not enough to activate the protection. But when nothing is heard than there is something strange happening. A possibllity is that you have a faulty selector switch for input or speaker. I would check and push all switches and check your tape selector.
Did you check the mute switch.

Is there a signal comming out of the preout plug, at your amp it is called accessory send, if so than the preamplifier is working correctly if not than someting in the preamp is causing the signal not to come through. When connecting the variabel output of a cdplayer to the accessory recieve plug do you hear something? if not then the output stage or a switch is faulty.

Does the amp get warm after let say 20 minutes? If so then somewhere in the signal path their is a crack. If not then there is no power to the outputstage, which means that either a fuse is blown or in the powersupply is something wrong.
You can check a lot before opening an amplifier, nice hè.

As I know the protectionsystem and the rest of the amplifier don't share the same powersupply. Also there is a different powersupply for the preamplifier. I'm not sure if they have separate fuses but check it.

I also included a link to yamaha site with the ownersmanual of the 1020, it looks a lot like the 720 only the outputpower is lower
Look at page 9 there you can see an overview of the signalpath and the important buildingblocks of the amplifier.

good luck

Rob


http://www2.yamaha.co.jp/manual/pdf/...ntA/A-1020.pdf

[jlbosch]

Thanks Robbie B. I see that you're right about being able to check lots of things before opening up!

Anyway, the first rainy day I get, I'm going to do all the things you suggested and see where I'm at!

Thanks also for the link to the manual: I see it has a schematic which gives me a starting point!

[Sveasvea]

Hi guys,
I don't have a solution for the mentioned problems but I was wondering if anyone could help me with a similar problem I have with my Yamaha A1020.
From time to time it just stops producing output and when I firmly close the lid over the controls I hear a clicking sound like a relay and the amp switches on again.
Anyone who has seen this and solved it? I would really appreciate a reaction, thanks in advance.

Bert