Tracking down DC voltage

[Hoodnight]

Working on my DRRI '65, tracking down a hum and a crackling sound that both increase with the reverb level.

The hum sounds like a 100hz hum (Aus voltages) so I'm guessing it's just DC on the grid of V4B. I am getting DC mV readings on the spots I have marked in red in the diagram below.

I have also replaced the two coupling capacitors from the preamp tone stack recovery stages that are outlined in blue in the diagram (had some spares around so figured might as well).

It seems like the only option left must be the coupling cap on the plate of the reverb recovery triode (C17), at least for the DC that I'm getting anyway. It also appears to be slightly microphonic when flicked with a chopstick.

I'm relatively new to troubleshooting, so I was hoping someone could confirm my logic here. I'm also curious as to how a failing capacitor could produce a popping/crackling sound, or if that needs to be pinned down somewhere else.

Diagram of the relevant schematic portion:


Troubleshooting so far (that I can remember...):

• Pulled all preamp tubes, one at a time, and replaced with known working tubes. Hum/crackle dissappear when V4 is pulled.
• Tested with known working power tubes.
• Tested with the reverb tank disconnected (Hum/crackle still present, but I inadvertently busted my reverb transformer in the process...)
• Verified that the resistance is within tolerances for all resistors in the reverb recovery circuit, the phase inverter, and the output sections.
• No visible defects on the filter caps, and are only 7 years old at this point anyway.

Have I missed something obvious? My understanding of grid and cathode circuitry is a little lacklustre at this point. Planning on replacing C17 once my city's not locked down anymore.

[shooter]

that needs to be pinned down somewhere else.

pop-crackle is typically a "break" in the circuit, bad solder ,sloppy socket pins
If the re-issue is PCB based, there's a good chance the wave soldering was calm that day 

[Willabe]

pop-crackle is typically a "break" in the circuit, bad solder, sloppy socket pins

That "break" can be dirty sockets too. Clean them if you haven't already.

[Latole]

that needs to be pinned down somewhere else.

pop-crackle is typically a "break" in the circuit, bad solder ,sloppy socket pins
If the re-issue is PCB based, there's a good chance the wave soldering was calm that day 

100% right,

Bad solder in power supply filter caps circuits = crackle = hum

[PRR]

> sounds like a 100hz hum (Aus voltages) so I'm guessing it's just DC on the grid of V4B.

DC, by definition, is dead steady. Not a hum.

> I am getting DC mV readings on the spots

What is "mV"? Three mV? 900 mV?

When I was young we did not have mV. We learned to expect several tenths of a volt stray on any tube grid. So 200mV is probably not a problem.

If the hum rises just as fast as the reverb knob then it is in the reverb recovery. Wires/parts exposed to dirty DC (which does hum/buzz). Poor B+ filtering on the recovery stage.

[Hoodnight]

That "break" can be dirty sockets too. Clean them if you haven't already.

Yeah I've used some deoxit and some vigourous remove/insert action on the tube socket for V4, no change.

100% right, bad solder in power supply filter caps circuits = crackle = hum

Haven't heard any change in the crackle when poking around with the filter caps or the dropping resistors, but will take the board off to check the solder visually when I get some time.

Additionally, it's only coming through V4, which I assume means it can't be an issue in the filter cap section given that V1 and V2 also share this supply?

What is "mV"? Three mV? 900 mV?

From what I can see it appears to only be peaking at ~60mV, would this not be enough to cause issues?

Thanks for the input all, much appreciated.

[Willabe]

From what I can see it appears to only be peaking at ~60mV, would this not be enough to cause issues?

When I was young we did not have mV. We learned to expect several tenths of a volt stray on any tube grid. So 200mV is probably not a problem.