CRATE BV amp Ch1 prob- "Crunchy" Noises.

[Steve Conner]

In troubleshooting you never assume anything, except logic and the laws of physics. You are going in already assuming a bad OC1, we want to challenge that assumption and test it by experiment.

[Enzo]

I am trying to get across a concept, not a literal description of physical reality. You can see what a grid looks like by reading any basic tube text. But the point is that it is a controlling element. Like my gate or a valve or whatever. HOW it controls is not the point, it is sufficient that it does. I am trying to see how far upstream I have to go to get before the source of pollution.


Here is where we are drifting apart. You have decided OC1 is bad in your mind - at least that is the impression I get. ANd you may be correct. All I am trying to do is to verify that. I am not going for anything more than that. Nothing has so far proven to me that OC1 is guilty. Whether OC1 is good or bad, shorting R12 will stop any sound noise or signal that comes through OC1. So if we short R12 and the noise continues, then we know OC1 couldn;t be the cause. Right? If OC1 is good, then it is not making the noise. SO shorting the signal path right after it won;t kill the noise unless the noise is from before even OC1 itself.. Right now you are assuming the noise is coming through OC1. I am not assuming that. It may be true, but I won;t assume it, I need proof. If for example a noisy Q5 is the cause, then shunting R12 will have no effect on it.

So whats the point of this shorting test, just to see if the noise is before the tube? (but surely i know this as ive swapped V1 for a new one). It is known is it not that the noise is before V1? I thought that has been established.

Not in my mind, what have we done that proves the noise is from before the tube? Yes, that is exactly the point of the R12 test - to see if the noise is from before the tube. Unless I missed something I see assuming the problem is in that branch with the OC1. I don;t recall any evidence. I outlined a number of parts after that tube section that could inject noise only during one channel.


Steve, I like water analogies for current to get that started, but in this case I wasn't going for that, I was only going for signal flow. I regret any confusion between analogies. I see it didn't work, but that is another story...


A jumper wire is just that, a piece of wire. I have a bunch of them on my bench with little clips on the end, but even a hunk of regular insulated wire with the ends bared would do. All we need to do is touch it to the points long enough to see if it kills the noise.

Test wires with clips:

[Sea Chief]

Gotcha- ok Im on board now. Sorry I thought the R12 was to determine my (assumed) OC1. Ok will use my MM croc clips, take them out of the MM body, and link them s'how. The only croc clip jumpers I have are great big mo-fos for my car battery. I think these might be a little big big for the job?

Do I need to drain the filter caps?? or can I just carefully attatch the jumpers seeing as R12 is in the preamp area/ the badass caps with their biznis ends are under board t'other side so no probs there. I guess were talking small V or even mV at R12 area are we, before its amplified/ hits the 1st tube?



I know its tiresome JazzP but I am genuinley just trying to establish why Im doing the test. I just dont like 'go ahead and just do it already' approach. Im quite determined you see to grasp what I can of another language. Then I can order a coffee and expect (some sort) of coffee thing in rtn, & not be presented with a sausage sandwich.

[Steve Conner]

There's a troubleshooting technique sometimes called "divide and conquer" where you divide the system into two roughly equal parts, and force it to reveal which one of the two halves the problem is hiding in. Then you divide that half into two roughly equal parts and repeat the process until you get it down to a single component.

This is what you're doing with the shorting link. Since no signal can get past it, it divides the system into two parts. In the speaker you only hear the output from the stages after the link. If the noise stays, you know it is coming from somewhere after the link, so you move the link later in the signal path and try again. If the noise goes away, you know it came from before the link, so you move it earlier in the signal path and try again.

It might sound like a lot of work, but it isn't, you could find the problem in a system of 64,000 components with just 16 carefully chosen tests.

[Sea Chief]

Got that (as far as a theory goes) as well Steve.

[Sea Chief]

Enzo- Ive done two grounding tests so far, after re-reading your reply #37 ("if not R12 then go on to R16, then R28.." etc).

R12 (V1b pin 7) shorted: noise still there- although this is difficult to hear as the shorting introduced a louder squealy/ whiny noise ontop.
R16 (V2a pin 2) shorted: the same noise all " as above..

Now I assume this even worse noise (squealy/ whiny 'Im not happy with that short you did' noise) is normal for such tests? I dont see how if so, Im meant to hear the noise underneath Im trying to isolate if so.

Should I be hearing a dead silent amp via the spkr doing these short tests?? At the moment Im on hold before I continue onto R28 (V2b pin 7). More confusion, but at least Im progressing, of sorts.

[Sea Chief]

Ok I've eliminated the extra whiny-noise my short leads seemed to introduce (I used a much smaller jumper clip).

Here's what i find:

R12 shorted: ch1 noise remains.
R16 shorted: ch1 noise remains.
R28 shroted: ch1 noise remains.

Any idea where I go from here? obviously my numptys-hunch of a bad OC1 was completely wrong.. sorry about that. Thx.

[Jazz P Bass]

So move on down the line.
Have we proven that the Ch2 Tone controls & Master Volume have absolutely no affect on the noise?
With Ch 1 selected, no applied signal, rotate those controls & see what happens.
Q5 & Q6 jfets are supposed to turn these controls Off when Ch 1 is On.
My next suggestion would be to jumper R38 to chassis ground. (either end of the resistor to ground)

[Sea Chief]

Hi JazzP-

Just tried Ch1 selected, no signal, rotated ch2 tone & MV knobs: no difference to the ch1 noise (so that rules out also Q5,Q6 as culprits then?).

Ok so onto R38. Ill report back. Would love to get this sucker/ culprit nailed today! thanks Chief.

[Sea Chief]

Ah hang on: just looking at the schematic, R38 is adjacent to the MVol.

The MVol -only- affects the Ch2. I guess that makes testing R38 n/a?

The facia knobs are bunched L to R ["Ch1" Vol, L, M, H] .... ["Ch2" Gain, L, M, H, Master] ... [Rev,Pres].

[Jazz P Bass]

Humor me & ground out R38.
In fact, in case there is an unknown leaking voltage on it, ground the MV side of it.

[Sea Chief]

Humor me & ground out R38.
In fact, in case there is an unknown leaking voltage on it, ground the MV side of it.

Ok sorry! (more of my numpty-bollocks assumptions there then!).

R38 MV side grounded to chassis: noise ceases! (but some as-before crackly noises to start -but much much fainter- then gone & both ch1&2 same background hum!).

Good stuff. ok Im not going to ask why R38/ etc now, Ive tried and learnt alot so far but this bit now is above me. Just tell me where to poke!

[Steve A.]

Yeah, I been using the Chrome lately to get around that.

Speaking of browsers I have been very happy with Pale Moon which is an optimized version of FireFox. I'm not sure how it was done but it is less bloated and with a smaller RAM footprint but it still works with FF add-ons and is recognized as being FF. (FF had gotten so buggy after version 10 that I reverted back to an old version (3.15 as I recall) just to avoid the continual crashes and malfunctions.)

The Pale Moon Project homepage

Steve Ahola

[g1]

Put the amp on Ch.2 and crank up the master (with gain down). Is the same noise present? If so, the noise problem is not isolated to Ch.1. A noise problem that is common to both channels would not be heard on Ch.2 if the master was turned down.

Did you ever try this? I have the feeling the noise has been there in Ch.2 all along. You wouldn't have heard it unless you had the master up high.

[Sea Chief]

Did you ever try this? I have the feeling the noise has been there in Ch.2 all along. You wouldn't have heard it unless you had the master up high.

No I didnt. Oops guess what.. just cranked the MV & the problem/ noise is also there on ch2. Another apology is in order then.