Yamaha B-2 power amp - possible main caps issue?

[spaghetti]

I recently purchased this great amp on the famous auction site, I spent a few days disassembling it and cleaning each and every part, putting everything back together, and finally today I decided to test it... with VFETs safely removed, of course

Voltages are quite fine, +85V is slightly lighter at 82.5, but being spot on should be just a trimmer adjustment away. I noticed that, after turning the amp off, the VFET +59V and -59V rails on the left channel bleed out quite fast, while the other channel rails stay high for a very long time. I think that the right channel is behaving correctly, as there is no load attached to the wires (no VFETs), so they should not discharge.

I may start my troubleshooting by swapping the big caps, and see if the issue moves with them... or am I overlooking something more obvious?

 

[rottalpha]

Why don't you replace them? You will have to do that sooner rather than later
Be careful, you need to drain them first...everytime

 

[Oilmaster]

I noticed that, after turning the amp off, the VFET +59V and -59V rails on the left channel bleed out quite fast, while the other channel rails stay high for a very long time. I think that the right channel is behaving correctly, as there is no load attached to the wires (no VFETs), so they should not discharge.

If you look carefully at the schematic, then you can see that the VU-meter board is powered by the left channel VFET power rails. Therefore the left channel is being discharged faster than the right channel. There is no reason why the left channel caps would be in a different condition than the right channel caps.

 

[rottalpha]

Like he ^^^^^said

and if you wanted to put the worries to rest, plant some drain resistors in there until you are done experimenting




Choose smaller value if you wanted to drain the caps faster

 

[spaghetti]

@Oilmaster thank you very much, I missed the circuit connection to the VU meter board! That explains everything.

@rottalpha Yes I am planning on replacing the caps, but I think I will do that later. I would like to use the amp a few days now, I've never head it play so I really can't wait much longer. I am taking some risk maybe, but if voltages seems good right now I think the amp won't fail in the next few days.

 

[Mr. Yamaha]

Don't forget to follow the adjustment steps in the SM. Both bias pots fully counter clockwise, adjust -85 and +85 spot on, adjust bias to 40 mV and adjust DC offset around 0 mV.