Line6 Bogner Dl50 Blown Bias cap

[tdlunsfo]

Seemed like a simple fix.. bulging bias cap so owner changed the cap (did a decent job). Start up the amp and it bulges and vents gain, strange. I get it and put my meter across the cap at idle and I have over 200v dv across the cap. I change the cap to a higher voltage rating but bias voltage is still through the roof. This amp has a switchable class A/B and class A mode. In class A/B there is no plate current draw even when reducing the bias down to resonable levels with the decade box. Class A mode, the plate voltage drops down to 65v and I only get 8Ma plate current and 3 watts output. Relays are everywhere, so I jumpered pin 1/8 to the chassis to rule out on open cathode issue but same result. I'm thinking this amp has a failed PT (known issue) but my tests are inconclusive using my neon tester. Thoughts on additional tests?

Previous issue - https://el34world.com/Forum/index.php?topic=25634.0
schematic - https://docdro.id/gs02neF

[nosaj]

What are your voltages on the PT with the secondaries disconnected? also what should the voltage be across the cap that is blowing?
nosaj

[tdlunsfo]

Well nvmd, I'm a dumbass and found a blown F2 fuse hidden below some wires. Amp is fine now but interesting the voltage across the bias cap when F2 blows.

[nosaj]

Great.!

nosaj

[g1]

Which one of the bias caps, C21 or C15 ? Is the diode D15 good? And R18 & VR1 ? You can measure resistance across C15, should measure approx. 20K to 50K as you sweep through the pot.
Are other voltages in correct range, and approx. 70VAC feeding the bias diode?

edit: never mind. Strange that this voltage would blow a cap if F2 opens. I guess the 70VAC is derived from the HT winding. Seems a poor design if this is what happens whenever F2 opens.

[g1]

Oh look. There is a note on pg.14 of the manual pdf, to check for blown cap C21 if F2 blows before the panel fuse does. No service bulletin to address the issue. Bush league.

[tdlunsfo]

I completely missed that note. Good catch. Now to figure out why F2 blew. It wasn't a hard blow.. fuse looks like it is barely open. I'll put it on the dummy load tomorrow and see how it performs.

[Helmholtz]

Oh look. There is a note on pg.14 of the manual pdf, to check for blown cap C21 if F2 blows before the panel fuse does. No service bulletin to address the issue. Bush league.

Also see the note above that one. Obviously the fuse was changed for a somewhat slower type - making it less likely that F2 blows before the panel fuse.

[tdlunsfo]

Yep timed 1.6A. A gauranteed special order for me.

[g1]

Also see the note above that one. Obviously the fuse was changed for a somewhat slower type - making it less likely that F2 blows before the panel fuse.

I didn't see the change as very significant (31 vs 23 seconds?) but I could be wrong.
I was more curious about why losing the CT connection of the HV winding should cause a 160VDC bias cap to blow ever, even after that fuse blows. Why don't they just fix that instead of a band-aid extra time-delay? Any ideas?
That cap should normally be running only around 100VDC, so that is quite a surge.

[tdlunsfo]

I saw as high as 220v when playing with the bias pot. I put a 250v cap in there in case the fuse blows again.

[Helmholtz]

I didn't see the change as very significant (31 vs 23 seconds?) but I could be wrong.

The I²t value of a fuse is not a time.
Rather it corresponds to the max. single event energy the fuse can take. It's given in Ampere squared times seconds.
A higher value means the fuse can either stand the same current for a longer time or a higher current surge with the same duration.
In either case this might give the panel fuse a chance to blow before before F2, thus avoiding the critical voltage increase on C21.

I was more curious about why losing the CT connection of the HV winding should cause a 160VDC bias cap to blow ever, even after that fuse blows. Why don't they just fix that instead of a band-aid extra time-delay?

Of course I'm wondering myself.
From the schematic it looks like there's a separate 70V bias winding (but can't be sure as the PT itself doesn't show).
Maybe the OP can clarify.

The essential question is what caused F2 to blow, i.e. the root cause.
Knowing that and the complete bias wiring should help to explain things.

[tdlunsfo]

I ran the amp on a dummy load at 20w for hours today and it performed flawlessly. I'll have to contact the customer to figure out why F2 blew. There is a separate bias winding and the bias circuit is pretty standard issue minus the relays swapping between A/B and A.

[g1]

If it's a separate winding, my guestimate is there was at least 160% voltage on that C21 cap when the F2 was open and the HV winding unloaded.
If that same 160% appeared on the heater winding and LV winding, I'm surprised it's not blowing the 16V LV supply caps as well.

But maybe the '70VAC' source is mis-labelled on the schematic and actually higher than that.

[Helmholtz]

If it's a separate winding, my guestimate is there was at least 160% voltage on that C21 cap when the F2 was open and the HV winding unloaded.

The OP reported measuring 200VDC across C21 with F2 open. Not clear if this is positive or negative voltage wrt ground.

70V AC makes sense with a nominal -100VDC at C21.
I have no clue what could cause the bias winding voltage to increase by more than maybe 10% if F2 blows.

Too little info for promising speculation.