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PeeCeeBee V4H GB

Two days ago, the right channel blew. While playing at a soft volume at night, I heard a "pop" and then total silence. Good thing the RTR SSR and SMPS shut things down.
Upon repairing, I found the output mosfets shorted, but no sign of any punctured or burn marks on them.
I replaced those outputs and readjusted the bias. Nothing else was found to be faulty.

The amp is singing now!

My pcb is the REV1, may I know if I should do the R39 and R40 mods?
a) How important are they? And what do they do?
b) Any reason why mosfets fail while playing at soft volume? I am on +/- 53Vdc rails.

Thanks
 

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Hi Meanie, sorry to hear about your damaged V4H.
I had an oscillation issue when my V4H
was a newborn. I performed this mod (ver1 pcb’s) outlined by Shaan and haven’t had a problem in over 3.5years of almost daily service. For peace of mind I did install speaker DC Protection boards.
I hope you have a speedy and successful repair! ;)

From Shaan’s post# 625:

The Solution:

R27,R29 and R28,R30 i.e. the gate resistors (initially 330R and 220R), all increased to 560R (and gate-source capacitors removed).
 
I am using SMPS which provided me exactly +/-55Vdc.

As for linear PS, your transformer should be 40Vac (multiply 1.4142) which will give about +/- 56Vdc.
Please check your line input AC voltage, most transformers will output much higher secondary than rated.
So final voltage may be a lot higher than +/- 56Vdc...
 
Hi Meanie, sorry to hear about your damaged V4H.
I had an oscillation issue when my V4H
was a newborn. I performed this mod (ver1 pcb’s) outlined by Shaan and haven’t had a problem in over 3.5years of almost daily service. For peace of mind I did install speaker DC Protection boards.
I hope you have a speedy and successful repair! ;)

From Shaan’s post# 625:

The Solution:

R27,R29 and R28,R30 i.e. the gate resistors (initially 330R and 220R), all increased to 560R (and gate-source capacitors removed).
Hi, isn't clear to me if I must use Shaan's post# 625 or post 644 solution "Get 6 nos. 47pF/500V capacitors, ceramic or silver mica (12 nos. for two channels). These can be the same caps used for C5 and C6. Also get four 100ohm / 0.25W resistor for each channel."