Vox Gremlins (90's AC15 TB).

[Sea Chief]

Hi chaps- got a reverb gremlin in my AC15 TB I wonder if anyone can shed some light.

I get the reverb happen 1 strum in 30 lets say. So its on, but only very occasionally works. I've tried a new tube in V3 which I believe is reverb send/ return some part of the circuit afaict, but no difference.

Schematic (thanks to pdf64 for this on my other thread).. https://el34world.com/charts/Schemat...x_ac1596pa.pdf

Thanks for reading- SC

[pdf64]

Needs the preamp scheme too https://el34world.com/charts/Schemat...x_ac1596pr.pdf
V3 both sections is the send AKA drive, one section of V2 is the recovery.
Maybe a dodgy connection, try reseating all cables and valves a time or 3.

[garytoosweet]

Possible bad connection.

I would first inspect the cables and verify continuity with your meter. Move the cables around a bit with the meter leads clipped on to see if the connection is intermittent.

If the cables are ok, check the RCA jacks on the reverb tank and on the amp.

[Mick Bailey]

The leads inside the tray that connnct the reverb transducers to the phono connections frequently fail.

[Sea Chief]

Needs the preamp scheme too https://el34world.com/charts/Schemat...x_ac1596pr.pdf
V3 both sections is the send AKA drive, one section of V2 is the recovery.
Maybe a dodgy connection, try reseating all cables and valves a time or 3.

Hi pdf. If one section of V2 is recovery, is it not sort of 1st port of call to try my new tube in V2? I mean it's a sod to get to.. gotta take whole amp out.

I have tried the phono leads, resitted all valves including a contact clean for each socket. No difference. I will look inside the tank to see if any wire is badly connected next tho.. I recall some thin wire goings on in there.

Thanks chaps. SC

[g1]

Disconnect the cable that connects to the 'out' jack of the tank. Hold your thumb on that cable and turn up the reverb control. Is the hum from your thumb constant, or cutting in and out?
If the hum is nice and constant, we can ignore the recovery part of the circuit. If the hum cuts in and out, we can ignore the drive side.

[Sea Chief]

Disconnect the cable that connects to the 'out' jack of the tank. Hold your thumb on that cable and turn up the reverb control. Is the hum from your thumb constant, or cutting in and out?
If the hum is nice and constant, we can ignore the recovery part of the circuit. If the hum cuts in and out, we can ignore the drive side.

Hi there g1. Just got round to doing this sunday job. Now what I found taking the reverb tray out just before I'd planned to do your test , was in fact 1 broken tiny 1 of 4 springs (holding the middle steel section floating). So this seemed logical why I only got 1 strum in 30 reverb working. Refashioned half of spring (a minature pesky job) & now just about holding it in place. And reverb back working! I dont suppose these springs are available separately, so will hopefully do as is.

Great many tanks (that's my best reverb joke..).

Ok 1 other Gremlin. Input 1: when I plug into this I either get no sound, or most often a nasty loud !!GRRRRRHUMMMM!! without any guitar heard with it. If I waggle the plug rocking it L to R.. I get intermittant between these two symptoms. Input 2 is fine. I did contact clean n0.1 input (the marshall black plastic jobs, not the switchcraft fender types) plug/ unplug 10x thoughrally but has made but a fart of difference.

Thanks SC

[Chuck H]

Now what I found taking the reverb tray out just before I'd planned to do your test , was in fact 1 broken tiny 1 of 4 springs (holding the middle steel section floating). So this seemed logical why I only got 1 strum in 30 reverb working.

Sorry to piss in the punch bowl, but that broken spring connection wouldn't cause intermittent reverb, it would cause consistently weak reverb. I expect that during your work you may have shifted things (leads, physical stress on transducers or jacks, etc.) enough to temporarily correct a fault or short that was causing the intermittent problem. That there was a center spring connection broken would indicate that the pan was physically stressed at some time (knocking around, dropped) and that was the cause of both the busted spring and the intermittent fault. Which would be two separate things IME. I hope it continues to work for you. And all is fine if it does of course, but I foresee a new pan in your future.

Ok 1 other Gremlin. Input 1: when I plug into this I either get no sound, or most often a nasty loud !!GRRRRRHUMMMM!! without any guitar heard with it. If I waggle the plug rocking it L to R.. I get intermittant between these two symptoms. Input 2 is fine. I did contact clean n0.1 input (the marshall black plastic jobs, not the switchcraft fender types) plug/ unplug 10x thoughrally but has made but a fart of difference.

Since cleaning didn't help I would expect this to be a busted circuit board pad where the jacks are mounted or possibly a sprung jack/plug contact. The only fix would be to replace the jack and/or repair the board as needed upon inspection. It might be possible to avoid a board repair by using chassis mount jacks flipped upside down and then running flying leads to the board rather than having the jacks actually plug into the board. I've done a lot of this to repair bad contacts for board mounted pots and jacks.

[Sea Chief]

Chuck H all leads were solid tho, & I saw that physically 1 less spring meant the floating section had wedged against the enclosure so it couldn't work.. which gave evidence that it very occasional working ok, was when it momentarily shifted free of side. somehow. As its now working does suggest this. But you may be right.

I think I might just change the jack socket then.. as I cant see anything fishy on the board to help.

Thanks- SC

[Helmholtz]

So it was one of the suspension springs - and not a delay line spring.

[pdf64]

As I’ve previously noted, these amps can be prone to failing solder joints. And PCB mounted jacks on any amps are especially prone to solder joint failure anyway, due to mechanical stress. Hence just reflowing the input jack's solder joints may well fix that issue.

[Chuck H]

Chuck H all leads were solid tho, & I saw that physically 1 less spring meant the floating section had wedged against the enclosure so it couldn't work.. which gave evidence that it very occasional working ok, was when it momentarily shifted free of side. somehow. As its now working does suggest this. But you may be right.

I think I might just change the jack socket then.. as I cant see anything fishy on the board to help.

Thanks- SC

I was going to ask what Helmholtz is asking. It sounds like the suspension spring was off. That could be a different matter depending on just what the tray may have been touching I suppose. Glad it's working. And...

Like Pete said. Never assume a connection is good just because it doesn't look obviously bad. These guys that do repairs troubleshoot problems to bad connections at good looking joints every day.

[Sea Chief]

As I’ve previously noted, these amps can be prone to failing solder joints. And PCB mounted jacks on any amps are especially prone to solder joint failure anyway, due to mechanical stress. Hence just reflowing the input jack's solder joints may well fix that issue.

Aha.. so my wiggling the plug in the socket, might actually exacerbate stress on the solders? yup good idea as 1st go/ redo all the solders around the jack & resistors whilst there I guess.

Prior to that getting those 2x jack open-close steel contacts definitely good of course. Is there any trick to getting to them, in their open state, to scrape the contact points clean? I did just this on my cooker hood gu10 bulb sockets, could get a tiny screwdriver up & in ok to get at their contact points.. but access to these jack contacts seems a different business.

Yes sorry if I didnt make it clear.. it was a suspension spring broke, but of course I did scrub the 2x din socket outer areas whilst I was there, so as likely it was a bad contact here, as Chuck H eluded to.

thanks chaps. SC

[pdf64]

Aha.. so my wiggling the plug in the socket, might actually exacerbate stress on the solders? ....

Not really, once a solder joint has failed, it's not going to be right until it's resoldered. The annoyance caused by the failure will increases as the connection further deteriorates, but the same fix will resolve it.

Prior to that getting those 2x jack open-close steel contacts definitely good of course. Is there any trick to getting to them, in their open state, to scrape the contact points clean? ...

I cut a narrow strip of strong paper from an envelope, squirt a tiny bit of cleaner lube on it, open the contacts eg with a plug, put the paper between the contacts, take the plug out, then draw the paper out.
The paper will bring crud around the contacts with it.
Repeat until the paper stays clean.

Avoid scraping or otherwise abrading electronic contacts, as it may damage their surface plating, eg silver, palladium or whatever. Once the plating is damaged or worn away, oxides will form and the contacts will be very unreliable.

[Sea Chief]

Not really, once a solder joint has failed, it's not going to be right until it's resoldered. The annoyance caused by the failure will increases as the connection further deteriorates, but the same fix will resolve it.


I cut a narrow strip of strong paper from an envelope, squirt a tiny bit of cleaner lube on it, open the contacts eg with a plug, put the paper between the contacts, take the plug out, then draw the paper out.
The paper will bring crud around the contacts with it.
Repeat until the paper stays clean.

Avoid scraping or otherwise abrading electronic contacts, as it may damage their surface plating, eg silver, palladium or whatever. Once the plating is damaged or worn away, oxides will form and the contacts will be very unreliable.

pdf.. that is so damn sensible. All these years Ive done the sandpaper scrub to kingdom come approach, which after reading your advice.. is dimwitted & ;proper numbskullery. So maybe I can redeem myself & try this 1st.

thanks.