Top-boost switch not working on Vox JMI Conqueror

[seven]

So I have this ca. '67 JMI (UK) Vox Conqueror solid-state head, mk 1 version (i.e., OS/136 schematic).

I've just had it gone over, recapped as necessary. Don't think anything was done wrong as the rest of the amp works fine but the top boost switch apparently has no effect... ?!? Couldn't determine/forgot to check if the top boost worked before the recap as it still sported original electrolytics prior to that and so was not fully operational (hence the recapping!).

Apparently, top boost in these amps works by shorting R46 (1k)... and the switch has been tested/works, as does C25 (1uF). FWIW, another well-regarded Vox tech says his experience is similar with one Defiant he encountered...

Pre-amp schematics for reference. Photocopy of an original:



A slightly cleaned up version:

https://www.voxshowroom.com/ct/schematics/SS.pdf

Normal channel in question:



Presuming it's not a factory/schematic error, outside of checking the switch, how do I go about troubleshooting and ultimately resolving this?

[Chuck H]

Since components test ok it seems straight forward enough to determine where the fault in the circuit is, IF there's a fault. You can simply check continuity along the circuit path. That said, there's also the possibility that the top boost is always on rather than not turning on. So you also need to check for solder bridges or anything else that might engage the circuit. With both possibilities in mind it should be trouble-shootable with continuity tests.

EDIT: Another possibility, regarding your well regarded tech's observation, is that the effect is so subtle you can't tell the difference with it on or off. If you're testing with a sine wave that might be the case. It might be more noticeable with a guitar and higher frequency harmonics.

P.S. Nice reporting with a schematic and even a close up of the circuit in question

[Helmholtz]

Just measure the resistance between the switch contacts.
Should be 1k with switch open (no top boost) and zero with switch closed (top boost on).

[Chuck H]

Yep. This^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

That will tell you if the "circuit" is working. Because I'm thinking now that even though you've tested C25 as "good" (as in not leaking DC?) it may still be of a higher value such that it's passing all frequencies. So it would just be a small boost rather than a treble boost.

[seven]

993 ohms with switch off, 0.3 ohms with switch on. C25 is new, 1uF.

I'm testing with a guitar and I guess it could be so subtle to be imperceptible?!?

[Helmholtz]

Is the guitar connected to a Normal channel input?

Are transistor voltages good?

[g1]

I think C24 should also be a factor.

[seven]

Is the guitar connected to a Normal channel input?

Yes.

Are transistor voltages good?

TR12 base: 1.3V, emitter: 0.6V, collector: 9.5V

[seven]

I think C24 should also be a factor.

Swapped in a new one to verify. No change...

[Chuck H]

993 ohms with switch off, 0.3 ohms with switch on. C25 is new, 1uF.

I'm testing with a guitar and I guess it could be so subtle to be imperceptible?!?

I'm unable to find any reports about the "top boost" function on that particular amp. It doesn't get much mention for that model it seems. I had hoped to find something like a *uoutube video rundown where you get to hear it with TB off and then on. But no dice. If this is the case it wouldn't be the first time an amp had a feature that didn't reallly do anything noteworthy. But it is rare and it does seem odd.

[Helmholtz]

TR12 base: 1.3V, emitter: 0.6V, collector: 9.5V

So nothing wrong with the circuit and the switch.
Can't you even hear a slight change in volume?

Using a square wave input signal and a scope should show some waveform change with the top boost on.

You could try larger values for C24 and C25.

[Chuck H]

Using a square wave input signal and a scope should show some waveform change with the top boost on.

That's clever. I hadn't thought of it

[Chuck H]

Found it! This guy demonstrates the top boost switch at about :40 and you can clearly hear it.

[seven]

Found it! This guy demonstrates the top boost switch at about :40 and you can clearly hear it.

Yup, had already seen this video. Thank you for posting anyway, you never know. I reached out to other owners of these amps and they said they could hear it working.

Ultimately, I got an answer from R.G. Keen on another forum. Apparently, "[JMI] put the boost point way too high in frequency".

I strapped a 4.7uF cap over C25 and... TOP BOOST!

Hope this helps someone now or in the future if they have an amp with this similar "feature".

[Helmholtz]

See post #11 .