Fender blues DEVILLE repair. Blown R3000 diodes + Insane noise

[VerticalAmps]

First of all a big hello to the musicians and amplifier techs here!
I repair amplifiers part time at home, i mainly want to continue making guitar pedals but Reputation gets around and amp repairs help fill my gas tank. (Vertical Amplifiers is my company name)

A friends Fender DeVille would work and play fine, and then after a few minutes the speakers would go into shock with LOUD buzz.
Replaced the blown R3000 diodes(D5-D4) with 3-1000v rated diodes in series for each side.
Now the buzz is instant as soon as standby is switched on. I was thinking the Output Transformer is shorting (because what had blown D5-D4 may also have damaged the OT) but both Blue and Brown measured 50ohms and 54.5ohms to the Red centertap. Are those normal resistance on the primary coils?

I will check the Phase Inverter plate voltages tomorrow and R57. Also those solder joints like Enzo+52Bill had suggested in previous posts.
Im working 12hrs a night right now (8pm-8am) so im having time trouble to find and fix issues and am glad i found this forum.
BTW- SWR SM900 bass amps are my nitch. Two friends have them and i have fixed many issues and quarks if you are having trouble with them!
-Will

[VerticalAmps]

-UPDATE- pin 3+8 Phase Inverter voltages are at zero. However the voltages are on pins 3+8 for 5 seconds when the amp is cold started!

-UPDATE 2-
Replaced the tube- BAM problem solved.
When the PI (a JJ' ECC83S) Heated up to normal temp for some reason current would stop flowing through the cathodes... Just a bad tube. Hope this helps anyone! he said he had just replaced the preamp tubes. I have had 1-2 JJ' brand tubes be DOA right out of the box. But I love there sound, will just have to buy tested ones.

[Jazz P Bass]

Nice fix!
Which begs the question 'why did the flyback diodes fail?'

[stokes]

Nice fix!
Which begs the question 'why did the flyback diodes fail?'

Thats what I'd like to know.A bad PI tube would not cause that.I think that amp will be back soon.

[Familyortiz]

Did you remove the diodes from the circuit to check them? I know this is silly but I thought I'd ask, given that measuring them in circuit would give a zero diode check or other, due to being across the primary.

[Enzo]

The flyback diodes are not across the primary. They are from each end of it to ground. That essentially puts them across the B+ supply.

If one is shorted, it will reads so and ground the B+ at that point. If the other one is still good, at its cathode we would find the resistance of the primary to ground. Unless your B+ is shorted to ground or your primary is shorted to frame or secondary, ther is nothing to make the diodes appear shorted.

[Familyortiz]

The flyback diodes are not across the primary. They are from each end of it to ground. That essentially puts them across the B+ supply.

If one is shorted, it will reads so and ground the B+ at that point. If the other one is still good, at its cathode we would find the resistance of the primary to ground. Unless your B+ is shorted to ground or your primary is shorted to frame or secondary, ther is nothing to make the diodes appear shorted.

I understand what you are saying... when I looked at the schematic I thought the primary centertap was at ground. Something must have really juiced up those diodes!

[R.G.]

Flyback diodes have nearly 2x B+ across them. The on-side output tube pulls down to within 50-75V of ground when it's turned on hard, and the off-side tube plate flies up to B+ plus the voltage across the other side by transformer action.