Yamaha AX-900U

[Philster]

Q313
E= -47vdc
C= 48vdc
B= -46vdc

Replaced caps, still no relay engagement

 

[Philster]

Q341
E= 48vdc
C= -47vdc
B= 48vdc

Its getting late for me. Ive had a lot to do this evening, Ill pick it up tomorrow. I have a sneaking suspicion that most of the rest of the transistors I need to check will have 48vdc on them.

 

[Mooly]

I'll look again later. We really need all the remaining voltages adding from the other transistors.

 

[Mooly]

For anyone interested. First sim has the above measured voltages added, second sim a fully working one:

 

[Philster]

Yes sir, Im going to get those tomorrow. I was in and out today with the snow storm and had my water heater crap out. Can't be with out hot water when the temps are below zero.. I like the software you use to draw your circuits, nice

 

[Mooly]

I like the software you use to draw your circuits, nice

It does more than that 😉 Its a fully working simulation where you can look at any currents and voltages on any component. Click my signature line in any post for details how to install and use it.

Click any node to see the voltages in the simulation.

 

[Philster]

Q327
e=-47vdc
b= -46vdc
c=49vdc

Q319
E=-47vdc
C=49vdc
B=-46vdc

Q323
E=-47vdc
C=50vdc
B==47vdc

Q311
E=-46vdc
C=-47vdc
B=-47vdc

Q325
E=-47vdc
C=-47vdc
B= -47vdc

Q321
E=-46vdc
C=-47vdc
B=-47vdc

Q329
E=-47vdc
C=-47vdc
B=-47vdc

Q313
E= -47vdc
C= 48vdc
B= -46vdc

Q341
E=38vdc
C=-47vdc
B=39vdc

Q301
E=-5.81vdc
C=-49vdc
B=2.6mv

Q303
E=-4.3vdc
C= -47vdc
B=-4.7vdc

Q305
E=0
C=-48vdc
B= -47vdc

Q307
E= -43.27vdc
C= -47.84vdc
B= -48.14vdc

Q309
E= -49vdc
C=-44.18vdc
B= -49.60vdc

Q315
E= -47.9vdc
C= 51vdc
B= -48.5vdc

Q317
E=-47.4vdc
C= -48vdc
B= -47.6vdc

Q335
E= 0
C= .6mv
B= -.7vdc

 

[Mooly]

That great 👍

There are a couple of anomalies to investigate. Q301 and Q303 emitter volts. Make sure there is continuity between the emitters of the two. Recheck that result.

Q305 needs investigating. Where has that 0 come from. Recheck around that area.

Q335 I'm not sure which that is 🙂

 

[Philster]

q335 is up in the power supply/reulator area

 

[Mooly]

Found it. That looks to just be a 'switch' to turn on or off all the front panel LED's.

Its not related to the power amp as such but that transistor doesn't 'compute'. Drawn as PNP, listed as a 2SA device which is PNP and shown with emitter grounded. That's all fine but the collector and LED supply would then be a negative rail and its drawn as a positive supply.

Positive rail in red, return to transistor in blue.

The diagram has to be wrong. Emitter and collector must be reversed on the drawing.

 

[Philster]

do a continuity check between Q301 and Q303 emitters?

 

[Mooly]

Yes, your readings show a 40 volt difference between the two points.

 

[Philster]

I( rechecked the emitters of Q301 and Q303 ansd both read -4.6vdc. And -48vdc on the Q305 emitter. Sorry about the missed readings I was tired last night.

 

[Philster]

Very perplexing issue. Makes me wonder if the issue is in the output stage somewhere

 

[Mooly]

It might be worth looking around Q313.

All the negative voltages on the driver and output stage suggest Q341 is not turning on. Check the print from Q313 collector to the base of Q341. Your readings shows a significant difference between those points and an open circuit in that line would actually give all these negative voltages you are seeing.

 

[wg_ski]

Somehow current thru Q313 is insufficient to activate the VAS current mirror/source. In AC operation, C313 provides coupling to allow the emitter to swing more negative than the supply, keeping its collector current high enough to properly bias Q341. When stuck to the negative rail, it’s not. It’s possible that if the supply voltage is insufficient the VAS stage might NEVER come up properly. Q313 may have a critical current. To test that theory, pull Q307 (or short it base to emitter). If it snaps positive, you’ll know. If it stays snapped negative, you’ll know you still have the original problem (whatever it is) and the rest of the front end is probably good.

 

[Philster]

Where do I look for positive voltage if I short Q307?

 

[Mooly]

The voltage on the 0.22 ohm's would come up to a positive voltage the value of which depends on how much current flows in Q341 if you short base to emitter of Q307

 

[wg_ski]

It should try to drive it to the positive rail, as it would when the incoming signal drives it there. In DC conditions it won’t quite go that far because of R341, which is AC boot strapped. But it should not stay stuck at the negative rail, either.

 

[Philster]

It stayed negative