Vox Gremlins (90's AC15 TB).

[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.

I'll remember that 1st line Helmholtz thanks.

So should they all -always- measure their value, measuring across them, within a circuit? or are there occasions when something interupts the reading, pulling it lower-?

I measured R25 (the bigger 10k 1/2 watt one, closest to the inputs) both across it = 0.02ohms, also from either side to chassis ground = 0.02ohms.

Seems weird-?

[Helmholtz]

I missed R25 which separates the preamp ground from chassis ground by 10R. So your meter seems fine.

[Sea Chief]

I missed R25 which separates the preamp ground from chassis ground by 10R. So your meter seems fine.

So measuring across 10k R25.. I should read only 10 ohms-? (why wouldn't I expect to read 10k?).

And I get only 1.1 ohms now at pin 3 of V5. Measuring across it, I also get 1.1 ohms.

[Helmholtz]

It's always safest (i.e. most reliable) to measure resistors with one ened lifted or out of circuit. When measuring in circuit, other circuit components may lower the reading.

R25 is a 10 Ohm (not kOhm) resistor.

[Sea Chief]

It's always safest (i.e. most reliable) to measure resistors with one ened lifted or out of circuit. When measuring in circuit, other circuit components may lower the reading.

R25 is a 10 Ohm (not kOhm) resistor.

I understand if you're measuring from one leg to say chassis ground, but if measuring across from one leg to another over 10mm of the resistor.. surely it cannot be physically possible for the DMM to read anything other than the resistor's value-? The resistor cannot physically change it's value (unless damaged). If I put one probe at a point one component onwards, I would expect perhaps, this component to possibly have affected the reading... but not the resistor itself.

I'm still bewildered by reading 1 ohm instead of 1.5k.. especially so, because you did mention I should read 1.5k from pin3 to ground. Doesn't this just say it is not correct-? or am I missing something obvious?

If the resistor is 1.5k, how can it be affected so much, as to drop to 1 ohm-? & a cathode cap bypass resistor i believe, or cathode bypass resistor or something like this: so its just effectively piggyback-soldered onto the capacitor I think.. there isn't a whole lot going on here.

[Helmholtz]

I'm getting tired of this.

Lift one end of C12 and see if you're getting 1.5k between pin 3 and ground. If C12 is leaky, it acts like a parallel wired resistor. Two (or more) resistors wired in parallel always result in a resistance lower than each.
So if you see low resistance with C12 in circuit and 1.5k with C12 out of circuit, C12 must be bad (leaky or shorted).

[DrGonz78]

I understand if you're measuring from one leg to say chassis ground, but if measuring across from one leg to another over 10mm of the resistor.. surely it cannot be physically possible for the DMM to read anything other than the resistor's value-? The resistor cannot physically change it's value (unless damaged). If I put one probe at a point one component onwards, I would expect perhaps, this component to possibly have affected the reading... but not the resistor itself.

I'm still bewildered by reading 1 ohm instead of 1.5k.. especially so, because you did mention I should read 1.5k from pin3 to ground. Doesn't this just say it is not correct-? or am I missing something obvious?

If the resistor is 1.5k, how can it be affected so much, as to drop to 1 ohm-? & a cathode cap bypass resistor i believe, or cathode bypass resistor or something like this: so its just effectively piggyback-soldered onto the capacitor I think.. there isn't a whole lot going on here.

Since R24 is in parallel (piggybacked) to C12 you are not going to get an accurate reading on that resistor. Notice you did get an accurate reading on R35 since it is not in parallel with any other component that might confuse the meter's readings. However, if you are reading 1ohm across R24 this would seem odd. I would expect a very high resistance or open line reading on the meter measuring across R24. This is why removing one leg of R24 is important to get an accurate reading in this instance. Could be that C12 is shorted.

Also if the noise is intermittent then it could be a bad connection somewhere that triggers the noise. I recommend to use a chopstick or something non-conductive to tap around in circuitry around V5 to see if tapping in certain areas can trigger the noise.

Edit: Simulpost... Like Helmhotz suggested lifting one leg on C12 seems the better option actually. Lifting one leg on either component is an option but lifting 1 leg on C12 will leave the resistor in place. So measure resistance on R24 and run the amp to see if noise has gone away. Note that you said the noise went away earlier... My thinking is that while probing on certain components this applied movement that made the noise go away. Much like tapping the board with a chopstick it maybe something consider too.

[Helmholtz]

It should be noted that if C12 were good, the Ohm reading in circuit between pin 3 and ground should be 1.5k.

[Sea Chief]

I'm getting tired of this.

Lift one end of C12 and see if you're getting 1.5k between pin 3 and ground. If C12 is leaky, it acts as a parallel wired resistor. Two (or more) resistors wired in parallel always result in a resistance lower than each.
So if you see low resistance with C12 in circuit and 1.5k with C12 out of circuit, C12 must be bad (leaky or shorted).

Ok will do tomorrow thanks, with Dr.Gonz' explanation too I can see a bit more clarity on this 1k5 resistor: I never knew a capacitor ever changed the resistor value/ totally new on me that. But please be patient.. I find circuits very hard to understand, even a basic section as this thankfully is (not two channels like a fender). It is on the other hand very simple for you.

Maybe I've stumbled upon the fault, but, never witnessed such a situation before.. it's a total new anomally to me, hence not understanding the 'wild' reading on this resistor.

[Sea Chief]

It should be noted that if C12 were good, the Ohm reading in circuit between pin 3 and ground should be 1.5k.

Yup that good to know thanks- this is cause for optimism that it might indeed be the problem. Just got to get the damn thing apart to lift c12 leg.. a sunday job.

Will post results. Appreciated Helmholtz.

[DrGonz78]

It should be noted that if C12 were good, the Ohm reading in circuit between pin 3 and ground should be 1.5k.

So healthy cap would measure OL resistance and in parallel with the resistor we would be essentially measuring the resistor by itself, got it. I was confusing measuring resistors that are in parallel with other resistors and getting inaccurate readings. Thanks for the clarification.

[Helmholtz]

Ideally caps don't pass DC current. Not being able to pass DC means infinite DCR.
A cap that passes DC is leaky.

[Sea Chief]

Ideally caps don't pass DC current. Not being able to pass DC means infinite DCR.
A cap that passes DC is leaky.

Lifted c12.. & now I -do- get 1.5k from pin 3 of v5 to ground. So I think you've located my problem Helmholtz! I think..

I presume having the preamp board out, obvioulsly needed to get underneath, is still ok to do this test? I don't need the tube in place & board back in amp-?

If so- great. So c12 was a 22/50. Would a 22/16 be ok..? it's all I have.

Thanks alot- SC

[Dave H]

So c12 was a 22/50. Would a 22/16 be ok..? it's all I have.

22/16 is fine. It will never have more than a couple of volts across it.

[Sea Chief]

22/16 is fine. It will never have more than a couple of volts across it.

Good, I think the 1.2v I measured at pin3 must be relevant. Ok I've replaced c12.

But unfortunately the GRRR! remains. Dammit I had my money laid down c12 must be the cause.

Ok so re-checked by pulling v5.. silent/ no GRR. So I'm back to square 1. All that is known is issue is in the input area.

Thanks. SC