Basket case Ampeg SVT2 pro

[g1]

I think there is a time and number of posts combination before you can upload. It's not the size. Meantime if required you can upload them to some free image hosting service and post a link.

[Climberkalen]

Alright, so I backtracked from the screen grid and think I found one of the issues. On one leg of R57 it has high voltage DC and the other is low mA's. I pulled one leg and checked it. Tests within spec, its not open. So im thinking some of the filter caps might be on their way out. Maybe c17 or c18 is letting my 380v to ground instead of down onto c21and c22? I'm getting 55Vdc at J18/J13,so somthing is going on there. It it possible to check the filter caps for dc leaking with a normal multimeter? I have a Klien mm400. I mean, the caps are as old as I am so its highly likely their past their prime like me hahahaha.

[The Dude]

One side of R57 is ground, so there would be no voltage there. I have no idea what you mean when you say, " the other is low mA's". Milliamps is a current measurement- not voltage. Your problem is not filter caps, or anything else in the power supply. You have voltage at R58, which is the screen supply. You do not have that very same voltage (which exists because you measured it) at the screen grids of the output tubes. You have a connection issue.

[Climberkalen]

Thanks for letting me know The Dude, regarding the MA reading i mistyped. I was following the traces from R58, towards the brown quick connector that goes from J18 on the poweramp board to J13 on the tube board. When I hit R57 one side shows 272 Vdc, (I didn't write it down at the time my bad). The other leg (that looks like it goes to the negative side of c18) shows .7ish mV. My bad on saying milli-amps, I had a brain fart and typed ma instead of mv. But that makes sense it would be low its going straight to ground

If it helps ill try to describe how i have it hooked up. So on the power amp board, j17/16 both have the blue and yellow from the PT. J18 is going to j13 on the tube board. J13 is the red and white wire coming from the OT. J14/15 are the red wires from the from the PT. J19 is the 5 pin plug for the filimant transformer. J20 is going to the preamp board. J12 is going to the ac mains board j35. The 14 pin at j1 has the red line closest to R50 on the poweramp board, its on the side closest to the pair of 2 pins on the preamp board. The 16 pin ribbon cable as the red line facing JW20/R34 on the poweramp board and the red line is on the side closest to powertube V5. Oh and j10 is the blue 2 prong going to the output jack board. I was trying to upload the photos to avoid typing this confusing ass paragraph.

I appreciate the patience in helping me along the way. This ampeg is by far the most complex thing I've torn into, so it's a little overwhelming. I'll keep working my way from R59 towards the screen grids. Am I interpreting the schematic correctly in assuming that J18 is supposed to be 380v?

[The Dude]

I don't have the amp in front of me, so I assumed that the 345V supply connected to the 345V input to the tube board since they are both labeled the same voltage. That is why I had you measure voltage at R58. Possibly this is not correct? If you have J18 on the supply board going to J13 on the tube board, you'll need to check that supply. What voltage do you have on both sides of D26?

[Climberkalen]

The Dude that is a mega huge oversight on my part if thats the case. Ill swap it and see what changes. Now here's the real question, what did I possibly fry hooking that up wrong? I'll follow up with voltages from D26 in a few minutes.

[g1]

Yikes. J13 on 'power amp' (supply board) is one thing (660V) to OT primary, but J13 on 'tube bd.' is 345V screen supply.

[Climberkalen]

Thanks for that little juicy nugget of knowledge @g1 I would have never figured out the box around the voltages indicates Vac. I've never came across that Inna schematic before. I'll get the readings on that diode and edit this post shortly.

Edit: Reading back my post here later, I sound like I'm being a smartass. So sorry if it came across as such,I was genuinely saying thank you.

[The Dude]

It's a 6550 amp, so tubes should be able to handle the plate voltage. And, my recollection is that actual screen voltage does not jive with the schematic. We'll see once we get it figured out. At any rate, there is no voltage at the screens at all right now, so the amp certainly won't have any output until we figure out where it comes from and why it's not there.

[Climberkalen]

So I have a head-scratcher. Amp off On D26, I set my meter to diode test mode and it reads .725 from one leg to the next. Now here's were it gets goofy, amp on, switching the meter to vdc the negative lead of the diode reads negative 15Vdc. The positive lead is jumping all over the place and not stopping.

[The Dude]

What AC voltage do you have from J16 to J17? Are those transformer wires well connected to the board/rectifier diodes? Are diodes D14-D17 good?

[g1]

I'm not clear on post #40. Diode check should only be done with amp turned off, voltage readings are done with amp turned on.
D26 'the negative 15V'? D27 and D28 are for the 15V rails. D26 should have approx. +380V on anode (from D14 cathode) and 0.7V less than 380V on D26 cathode.

[Climberkalen]

@g1 I didn't have the amp on when testing in diode test mode. Ill go through the diodes you and The Dude mentioned to see If any are failed, then fire it up and take some voltage readings. And for clarification, cathode side of a diode is then negative with the stripe, and anode is the positive side.

[Climberkalen]

Alrighty then The Dude I'm getting negative .621Vdc on J16 and j17.
@g1 D14 through D17 all have low negative voltages. Like negative.1.5Vdc. In diode test mode, D26 is showing .725v checking across it in either direction, so maybe it's failed,

D14,D15,D16 shows the same .725V.

D17 shows .725 one direction and .5 switching sides with the probe, im assuming that's the amps main bridge rectifier.

Update: now The green standy light won't come on. It stays red around 30 seconds then turns off. Fan and filimants are still on when this happens. I have work in the morning so I'm probably done messing around at the bench for the night.

[The Dude]

I asked for the AC voltage from J16 to J17 (not DC). That is the AC supply for the rectifier circuit that makes screen voltage.