Ye Olde Vox Kensington Amp Quandry

[fdesalvo]

Having worked on a several old Vox amps, it seems the underlying design decision always revolves around how to electrocute unwitting techs. Not today, satan!

I'm not getting any input signal through to the power amp. Volume and tone controls affect sound and level of background noise. All caps and resistors leading to Q1,2 are good, so I suspect the issue lies around this area. Actually, there are a couple caps I haven't been able to test yet, bc I haven't pulled the board yet, but I'm hoping ot get a grasp of these transistor part #s in the event they are failing. There's a dearth of info around, so any help ID'ing these parts would be much appreciated!

[The Dude]

I'd inject a signal and trace it through the amp. Do you have a schematic? If not, it is available here:

https://www.audioservicemanuals.com/...ox-kensington/

[fdesalvo]

Sage advice and a referendum on my lack of test equipment. I need a sig gen and tracer amongst other things.

[Enzo]

A signal tracer is a useful piece of kit. But you realize it is nothing more than an amplifier with probe plugged into its input. I use one of those little practice amps that come with cheap guitars. A guitar cord ending in a probe, or even just a bar wire. We want a cap in series with the probe to block any DC. Tha tallows you to probe a plate pin for example.

[The Dude]

^^^^ That, or even a DVM set to ACV. A scope or signal tracer is great, but you can work with what you have. There are many days I don't even turn my scope on.

[nosaj]

Since injecting signal at the input produces no results you can jump to injecting signal at the volume pot. If you have noise problem is before the volume, if no noise then after.
nosaj

[Jon Snell]

A signal generator is not essential. It has an oscillator that is used as a tuner. Set to G.
Use that and trace the audio until it stops. There will be the fault.
Take the G tone and inject it to wherever you want to test the amplifier.
OC28s are very hard to come by now and if you choose to use silicon transistors, bias adjustment is needed.

[fdesalvo]

Thanks for all of the help and comments - I'm learning something here. I constructed a signal probe and have confirmed that Q1,2 have progressively louder signals at their outputs, but there's nothing on the input of q3. There's a .22uf cap feeding its input immediately after the volume pot that isn't giving me anything but a buzz when I probe it. Presently this is my chief suspect. There are a couple of oddities I've discovered:

1. With 1/4' cable at input, I can get noise that I can shape with the volume and tone by touching the tip
2. I am not getting an audible signal from the volume pot

TIA for reading.

[Jon Snell]

If you connect the wiper of the volume control to the signal side of the pot, you should get sound, if not, it could be C13 but unlikely.

[fdesalvo]

Thanks for jumping in there, Jon. There's nothing happening by jumping the terminals. Only getting some antenna effect by letting the lead hang from the wiper. Maybe I should be looking at C10?

[Jon Snell]

No problem, I have seen many in the early years but not many left now so important to get them going again.
If you have good signal at Q2 collector, there is a suspect dry electrolytic ... C6. If the tone conntrol is totally useless, you will still get sound but no tone control.
C10 only gives the treble boost so I doubt it is that.
You stated that you get a buzz on C13 so the output probably works.
The issue as far as I can tell from your description is between C6 Q2 side and Q3 base.
C6 is an electrolytic, it has been there for a number of years and probably weighs in at 200pF!

[The Dude]

I'll add: If you don't have a signal generator, a quick YouTube search will find you all kinds of test tones- even sweeps . You can use them with your phone, computer, tablet, whatever as a source for tracing signal. Here's just one example.

[fdesalvo]

Thanks guys. I figured out how to do it through my daw/interface. I have one input from the tracer and one output with test tones. I’m still hung up before the volume pot. Not a fan of Vox at this point.

[The Dude]

Have you checked or changed C6 as Father Jack suggested?

[fdesalvo]

I don’t have the proper value. I bridged the existing part with a .47 and didn’t get any change.