Vox Gremlins (90's AC15 TB).

[DrGonz78]

I would do some voltage checks on the plates and the cathodes of V5. At least establish the voltages are what they should be and check the resistance readings of the plate resistors. Also you reposted only the power amp schematic. Here is preamp schematic.

https://www.korguk.com/voxcircuits/c...s/ac1596pr.jpg

[Sea Chief]

Ok, so is there continuity between an 0V common node on the circuit board, eg the bottom of the input stage’s cathode resistor, and the chassis?

Ok I got to pick this apart. A 'common node on the circuit board' means what though, a 'general area of ground'? (I think this might mean a bunching on the pcb of ground lines..?) but you mention a very specific place, the "input stage's cathode resistor": ok I'm googling these words to refresh myself on the anode > cathode, but get nothing definitive as to where you're referring to (there are 2 cathode 'triodes' on a 12ax7, pin 3 and pin 8.. I presume you mean somewhere within one of these areas: or either? & before or after the resistor? with amp on or off?).

Sorry I'm dreadfully lost pdf.

[Sea Chief]

I would do some voltage checks on the plates and the cathodes of V5. At least establish the voltages are what they should be and check the resistance readings of the plate resistors. Also you reposted only the power amp schematic. Here is preamp schematic.

https://www.korguk.com/voxcircuits/c...s/ac1596pr.jpg

Hi Gonz.. aha- thanks for putting the preamp area up again!

Ok so my voltages for V5 are:

pin 1 = 187v
pin 3 = 1.23v

pin 6 = 235v
pin 8 = 2v

And when I did these readings.. no GRRRR!! no idea why. ( edit.. And all seems ok.. but for no guitar signal whatsoever.

So the GRRR!! has now dissapeared, but amp just not working, though typical hum coming from speaker. )


Thanks, SC

[Sea Chief]

pdf64 When you say 'bottom of the cathode resistor'.. are you referring directly onto the schematic, when you use this word "bottom"?

IE is this simply a way one person talks to another, using physical top/ bottom (& left/ right perhaps too?) to pinpoint a location on a schematic-?

I can't see any other way, that you might choose this word. Btw I've put up voltages.. but damn thing behaved itself during them.

thanks. SC

[Sea Chief]

R24 both sides of it read continuity to chassis ground.

R35 one side reads continuity to chassis ground.

[Helmholtz]

R24 both sides of it read continuity to chassis ground.

If continuity means zero Ohm, then C12 is shorted.

[Sea Chief]

If continuity means zero Ohm, then C12 is shorted.

But continuity means there's a continuous connection. How can this mean the opposite-?

[Helmholtz]

But continuity means there's a continuous connection. How can this mean the opposite-?

Zero Ohm means no resistance = perfect continuity.

[Sea Chief]

If continuity means zero Ohm, then C12 is shorted.

Hi Helholtz,

I think a language barrier perhaps? no when I say continuity I imply a connection, IE ohms are measured 10 ohms or so. Or maybe 'continuity' is incorrect language of mine-?

I wish you were right &, I could have just replaced c12 if you'd worked it out, but unless I'm mistaken I don't think c12 is proven to be shorted.

Thanks. SC

[Sea Chief]

Zero Ohm means no resistance = perfect continuity.

Right then I'm thoughrally confused. Please just forget this word it's not helpful then/ my fault.

-- Start again.

I measure the resistors pdf has pointed to. I don't know which side to measure (because "bottom of" is not distinguished to me yet).. so, I measure both sides.

Both, I believe are 1k5. Ok so:

R24 = both sides show a connection of 10 ohms to chassis gnd.
R35 = one side only shows a connection of 10 ohms to chassis gnd.

Thanks again-SC

[Helmholtz]

Hi Helholtz,

I think a language barrier perhaps? no when I say continuity I imply a connection, IE ohms are measured 10 ohms or so. Or maybe 'continuity' is incorrect language of mine-?

I wish you were right &, I could have just replaced c12 if you'd worked it out, but unless I'm mistaken I don't think c12 is proven to be shorted.

Thanks. SC

Well, zero Ohm is the lowest resistance you can have. It typically means a short.

It's always better to measure and post resistance. Continuity is rather vague, it just means that there is a path for current, so actual resistance could be e.g. zero Ohm or over 100 Ohm.

If you measure zero/low resistance between V5 pin3 and ground, something must be shorting/shunting R24.

[Helmholtz]

I don't trust your 10 Ohm readings.

What resistance does your meter show, when you short the probe tips? Should be below 1 Ohm.
If not, check meter battery.

[Sea Chief]

Well, zero Ohm is the lowest resistance you can have. It typically means a short.

It's always better to measure and post resistance. Continuity is rather vague, it just means that there is a path for current, so actual resistance could be e.g. zero Ohm or over 100 Ohm.

If you measure zero/low resistance between V5 pin3 and ground, something must be shorting/shunting R24.

Well the resistors I measured were with amp off. I assumed it was to test if there was a link between these two cathode resistors & chassis gnd (I assume this is what "measure bottom of R.." means). I was unsure, I measured -both- sides of R24. With amp off.

If I meaured 1.23v at V5 pin 3 with amp on, I would assume I'd measure the same on this "top" side of R24 too.

--

Ok what am I meant to find at V5 pin3, in voltages, & in resistance to chassis ground-? would you be able to give me an approximate idea? because I'm as usual totally confused.. here by what you are saying/ or might be suggesting at this precise location.

Thanks, SC

[Helmholtz]

Resistors are always measured with amp off.

You should get 1.5k from pin 3 to ground and 6.8k from pin 8 to ground.

[Sea Chief]

Resistors are always measured with amp off.

You should get 1.5k from pin 3 to ground and 6.8k from pin 8 to ground.

Ok I get 6.8k @ pin 8.. but only 10 ohms @ pin 3. thx SC