Vox AC15 (1997 UK) – Reverb B+ short, Resistor Overheat

[Grantorino]

Hello again EL34 world.

A 1997 Vox AC15 with no reverb send. Not a reverb tank issue, the RCA cables had a short and were replaced. Return path is fine, amp otherwise sounds great.

Background/Condition

• Reverb send signal missing
• RCA cables were shorted
• Return signal increases with reverb pot as expected (tap test)
• Original R7 (10k reverb resistor next to TX1) was burned and was replaced

Evidence of previous work:

• Lifted/damaged PCB pads
• Traces restored using jumper wires (continuity tests okay to intended nodes)
• Several components altered from schematic (Reverb mod?)

Deviations from schematic:

• C30 (47pf mix cap) was replaced with two 150pf in parallel
• C17 (470pf driver) was missing and replaced with a 6800pf WIMA linking LK7/LK21 near V5 socket.
• These mods were removed and the amp has now been restored to stock schematic values - C30 restored to single 47pf 500v, c17 restored to 470pf 500 V, R7 replaced with 10k 2W

 

The amplifier was restored to stock according to the schematic.


Isolation tests:

• Reverb transformer TX1 removed
• Primary pads for TX1 jumped together
• All tubes except rectifier EZ81 removed, R7 still overheats immediately

This suggests to me the fault is downstream of R7 and is not TX1 or the tubes.


I have attached pictures of the board.
I'm seeking advice diagnosing this issue.

Thank you.

[SEL49]

All tubes except rectifier EZ81 removed, R7 still overheats immediately [/li][/list]

I would check/replace C1.

[Grantorino]

All tubes except rectifier EZ81 removed, R7 still overheats immediately [/li][/list]

I would check/replace C1.

Thank you Sel. I was suspicious of C1, as R7 has a ground path through here. Having just tested the cap, I can confirm it’s within spec at 10.12uf, 2.9ESR, 0.1V loss. Seems to be no issue here.