ORANGE AD-140HTC V8 socket causing problems?

[evil79]

I’ve got an AD-140 on the bench. First of all, getting to underside of the pcb is ridiculously difficult and time consuming. Anyway, friend initially brought it to me because it was noticeably putting out about half the volume. All voltages checked out. I didn’t wanna deal with it so I gave it back. Then he blew both HT fuses and V8 blew an EL34. I put new tubes in, but now V5 is pulling 32mA and V8 pulled 23mA, and tubes were matched. I tried a different tube in V8, and it pulled less than a mA. What is suspect here? Screen grids measure fine, all coupling caps in both PI’s test good. All power tube sockets were really loose, so I retentioned them. Maybe I made the socket too tight? Maybe when the original tube blew it left carbon? I’ll post some pics of what looks odd to me. Please help.

[evil79]

What is this goop on the pcb? Is it supposed to be there? It’s the same waxy stuff that the OT leads are coated with.
https://photos.app.goo.gl/kaiovRE3Zqmb5UV66

[evil79]

Are the leads supposed to be missing from this part of the OT? I’m guessing so and that they wax potted it to seal it off? I don’t know. My brain hurts
https://photos.app.goo.gl/Wg573N6LsfxSNg7g8

[Mick Bailey]

The additional hole in the transformer is there due to it being a stock part that can be used in a variety of configurations and on different transformer types. The wax can drip from a transformer when it warms up. If there's no problem with overheating I'd say its a characteristic rather than an outright fault, but it's prudent to make sure there's no fault. I would clean off the board to make sure there's nothing going on underneath the wax.

Swapping known-good tubes into a location and getting extremes of variation suggests a socket or connection problem with that location. Did the tube you swapped into V8 to replace the one drawing 23mA come from the same set? What happens if you exchange V8 with V5 - does the fault move with the tube?

[evil79]

well, seeing how big of a PIA it was to get that board out, i'm gonna shotgun some stuff in hopes that i won't ever have to take it apart again. i'm gonna hit all solder joints in that area with fresh solder, get rid of the 10 ohm cathode resistors because i have a nice dual bias probe, and Orange makes it extremely difficult to access anything on V6, V7 & V8. Should i replace any other potential parts while i'm in there? 1K Screen resistors, 1.5K's on pin 5? R27 and R26 are both 180K and on both sides of the bias voltage. maybe R27 that goes to V7 & V8 is affecting my bias voltage? What do you think? My gut tells me it's probably the tube socket, but $5 of parts is worth it just to not ever pull out that board again for the rest of my life. Seriously, in 15 years I've definitely had my share of amps that make you rip your hair out trying to get to the pcb (cough, mesa, cough), but this one makes the top 3. Hit me with whatever you got. Much appreciated..

[evil79]

Oh, and I didn’t swap the tube in V5 with V8. Was going to, and then I accidentally reached the point of no return trying to get that board out. But I will definitely swap tubes around when it’s reassembled, though I’m hoping I fix it before that

[evil79]

Here’s the schematic

http://www.orangefieldguide.com/OFG_...Cmainboard.pdf

http://www.orangefieldguide.com/OFG_...mainboard2.pdf

[Helmholtz]

get rid of the 10 ohm cathode resistors

Why this?
A bias probe that measures the voltage drop across an internal 1R resistor won't be bothered by the cathode resistors. Also they might have a fusing function.

[evil79]

I just assumed that the 1 ohm resistor in the bias probe would get added in parallel with the 10 ohm. Would that not alter the bias probe's readings? I've always been unsure about this, and have been meaning to do an experiment at some point to see if it affects the bias probe's current reading. If anyone knows for sure either way, please enlighten me. And I'm pretty sure the 10 ohm's are in the AD-140 for measuring bias. There are diodes on the plates for protection. What else would a cathode resistor do for EL34's in a fixed bias amp?

[Helmholtz]

I just assumed that the 1 ohm resistor in the bias probe would get added in parallel with the 10 ohm.

The 1R resistor will sit on top of the 10R resistor in a series configuration both carrying the same cathode current. The bias probe only measures the voltage drop across the 1R. Otherwise it wouldn't work with cathode biased amps.

There are diodes on the plates for protection. What else would a cathode resistor do for EL34's in a fixed bias amp?

The diode protects against high voltage spikes. Fusing cathode resistors can protect against tube over-current.

[DrGonz78]

Man those solder joints are really bad in places on that board. I would fix up all the joints I could while the board was out. Were these solder joint like this when you took the board out?

[evil79]

Oh yeah. I haven’t soldered anything yet. There’s all kinds of cold solder joints and dull blobs. The chassis was so filthy I had to wipe it down with paper towels before I could hit it with duster cause that filth would’ve gone everywhere and into my lungs and eyes. I cleaned up the worst looking solder joints with my small steel bristle brush and checked for hairline fractures and bad connections etc. Just getting my ducks in a row so I can hit it hard and fast tomorrow. I’m taking today off!