Help me fix Yamaha R-S700

[zepplock]

I have this amp and it trips power protection.
When I turn power on, I hear it trip protection and desk light in garage (on the same powerstrip) dims for a split second.
I downloaded service manual (attached) but I can't get it to self-diagnostics.

I suspect it has a short in power supply and main board does not even get power.
My usual visual inspection didnt show any bad caps or fried components.

What should I check first?

 

[Doodski]

Sounds like you might have a direct short in the amp stage. Do you have a resister to discharge the power supply. Do you have a digital multimeter with diode test function?

 

[zepplock]

I do have a multimeter, I worked with tube amps before. Long time ago.

 

[Doodski]

Discharge the power supply with a resister. If you have a ~100 Ohm resister or more Ohms that should suffice but it may get hot so watch your fingers. Use the diode test function on your multimeter. It's best if you know how to check a transistor but if you don't then simply look for a short ( zero ohms) on the output transistors. Check between all three legs of the transistors; there are 8 of them. Q122A, Q124A, Q124C, Q122C, Q121A, Q123A, Q123C, Q121C. That's a good start for the first check.

 

[solderdude]

On the amplifier board pull the plug CB104.

See if powers on normally. (the light does not dim shortly)

 

[zepplock]

On the amplifier board pull the plug CB104.

See if powers on normally. (the light does not dim shortly)

ok so that changed a few things.
with CB104 disconnected, if I hold DIMMER and PRESET < to go to self-diagnostics I get the power light to stay on (it was blinking before)
In this mode I still get no light on the screen but the following works: speakers A and B - I can hear relays click, CD DIRECT and PURE SOUND buttons have lights on/off when those buttons are clicked.

If I click on power, I hear click and switched to protection again though.

 

[Doodski]

A heads up the power supply has bleeder resisters of 5K10W and so if you power off, unplug and wait a minute or two the power supply will be discharged and you won't need to do it manually. Makes your life easier.

 

[Doodski]

Solderdude you go ahead. Two cooks in the kitchen will make it confusing.

 

[zepplock]

Solderdude you go ahead. Two cooks in the kitchen will make it confusing.

No worries, I'm good with following directions. Is it ok to test transistors on the board? obviously with the unit off lol

 

[Doodski]

Yes you can check them on the board. Most likely there will be a short that will be obvious; IF there is a short.

 

[zepplock]

Yes you can check them on the board. Most likely there will be a short that will be obvious; IF there is a short.

weird, both Q121s and both Q123s (all on 1 side of the main board) show zeroes, and the other 4 show good ~0.6v and ~0.9v (in diode test mode on my DMM)

 

[Doodski]

Seems you have shorted output transistors at the + rail on the left channel. The multimeter is indicating ohms not volts when in diode check. Unsolder those bad transistors and recheck.

 

[Doodski]

When i said recheck I mean using the multimeter on diode test. Don't try to power the unit ON after you unsolder the output transistors.

 

[Doodski]

OH. I noticed you said Q121S and Q123S. Are you @ the output transistors. I ask because @ the service manual they are labelled 121A or 123A or 121C or 123C. Just making sure we are on the same page of testing the output transistors.

 

[solderdude]

At the moment I cannot react very fast. My idea would also be to check the output transistors (Doodski has got you covered already)

Also do check the diodes in the bridge rectifier. If they were defective most likely the fuse would have blown which isn't the case.
So indeed check the output devices between the C and E (middle and right pin) in both 'directions'
Compare Left and Right channel. A short between C and E means a blown transistor (would need to de-solder it and re-measure to be sure).
Also check the thick resistors between the output transistors.

 

[zepplock]

OH. I noticed you said Q121S and Q123S. Are you @ the output transistors. I ask because @ the service manual they are labelled 121A or 123A or 121C or 123C. Just making sure we are on the same page of testing the output transistors.

I don't see any other transistors with 121 number, only 8 ones with Q. These are with big heatsinks right in the middle of the unit. Those are the ones, right?

 

[zepplock]

Also, what would cause all 4 to go bad?

And what brand/type should I get? (I only have a bunch of spare caps at home.) Any links to ebay or partsexpress?

 

[zepplock]

Like this? https://www.amazon.com/2pairs-2SA1694-2SC4467-A1694-Quality/dp/B07MV67W75
or this https://www.ebay.com/itm/5pairs-10p...250455?hash=item417afc9f57:g:RtsAAOxydlFSwYIL

 

[Doodski]

IF those transitors from ebay are original Sankens those would be good. There's other stuff you should check before ordering stuff. You might need more stuff but we don't know yet till more tests are done. More probing with the multimeter is required to do this properly. We need to identify which transistors you are replacing so we can speak the same terms and identify stuff correctly. If you look at page 61 of 84 @ the, "Main." that will give you the schematic and for the board layout see page 52, "Main PCB." Then tell me which transistors need replacing according to the service manual. That way I can show you via the service manual what to check next.

 

[solderdude]

Also, what would cause all 4 to go bad?

When merely 1 of those transistors fails it often takes 1 (or both) of the complementary ones along with it.
I recommend in such case to exchange both pairs (so all 4 of them) even when 1 or 2 still seem O.K.

You also need to check all the base transistors (they are all 4.7 Ohm safety resistors) and need to be checked to have the proper value.
Those are resistors 148, 149 ... to 155.
Also the 0.22 Ohm emitter resistors (2 in 1 ceramic house)
It is not uncommon the driver transistors also take a hit.
There are ways to test the rest of the circuit without the 4 transistors to see if drivers are defective.

First you need to confirm the transistors are defective (or at least one of them) and check the base and emitter resistors.