Help Please! Tone stack issues.

[StingRay85]

I would still try to use the DMM and measure continuity of each soldering point. Sometimes you can have components that have some oxidation layer on the leads, that can cause bad connections that you can't visually see.

 

[AndyD]

I would still try to use the DMM and measure continuity of each soldering point. Sometimes you can have components that have some oxidation layer on the leads, that can cause bad connections that you can't visually see.

Will do!

 

[Pete Farrington]

At a certain point on its rotation, the amp would give an audible "thump" which was the point at which the oscillation started and stopped, depending on which direction you were turning the control

Just to expand on that, the thump is a step change in the DC level of at least one stage inside the positive feedback loop (that’s oscillating). Caused by the stage/s flipping from idling to oscillating. It’s useful to identify the stages where it occurs, eg check cathode voltages / currents at all stages either side of the thump.

The bass pot acts as a variable resistor so no need to connect the remaining tab to anything

When wiring up pots as a variable resistor, as mentioned, it’s good practice to link the wiper to the unused lug. Then for several failure modes, if the pot ever fails open circuit, eg crud on the track, the resistance can’t increase any higher than the full track resistance.
There’s still some failure modes where a total open circuit can occur, and whatever, if the bass control goes open circuit there’s going to be barely more bass than full bass anyway.
There’s more actual benefit with other applications eg bias trimmers.

 

[TheKman76]

The bass pot acts as a variable resistor so no need to connect the remaining tab to anything.

Pete beat me to it. In this circuit it hardly makes any difference, but it is good practice.

 

[Spanngitter]

Am I the only one missing the GND bus bar behind the pots?
For me, the symptom still denotes a loss in GND connection somewhere, eihter bad solder joint or corrosion on the tooth washers used for pot mounting...

 

[AndyD]

Pete beat me to it. In this circuit it hardly makes any difference, but it is good practice.

I don’t mind learning new things! Cheers.

 

[AndyD]

Am I the only one missing the GND bus bar behind the pots?
For me, the symptom still denotes a loss in GND connection somewhere, eihter bad solder joint or corrosion on the tooth washers used for pot mounting...

Everything is grounded, trust me. I’ve used the Larry grounding scheme.

 

[Spanngitter]

Everything is grounded, trust me. I’ve used the Larry grounding scheme.

..which could be a point of discussion, especially as there is variations out there in the wild.
Nevertheless, I am used to see the bus bar behind the pots (if you use dedicated GND split up into Volumes, ToneStack, Presence) and I also miss the GND for presence (pictures do not capture this pot). So said, I would focus mainly on the GND going to TS and Presence, the solder joints on the lugs are not 100% convincing to me...

 

[Gene Ballzz]

@AndyD ,
It may be high time to scratch that itch on your "wish list" for an O-Scope! There were a couple fairly recent threads comparing some VERY inexpensive units that blew me away with the feature sets in the $150 price range!

Oscilliscope Recommendations?

Something new, something modest (I'm not building a space shuttle - not this week anyway) off Amazon, for checking bias settings, clipping, etc. Yes. I do know how to use one. A wave generator would be useful too, guessing 500 Hz to 5 kHz range or fixed 1 or 2 kHz. If they can come as a...

www.marshallforum.com

Okay. So now ive got an oscilliscope...

What do I do with it?:rolleyes: And how do I interface its USB out with a laptop? One of these BTW: https://www.ebay.com/itm/402961985497?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=7F3MjnO2SMG&sssrc=4429486&ssuid=pDK-7zTlTl2&var=&widget_ver=artemis&media=COPY

www.marshallforum.com

Adding Fuel To The GAS fire!
Gene

 

[AndyD]

..which could be a point of discussion, especially as there is variations out there in the wild.
Nevertheless, I am used to see the bus bar behind the pots (if you use dedicated GND split up into Volumes, ToneStack, Presence) and I also miss the GND for presence (pictures do not capture this pot). So said, I would focus mainly on the GND going to TS and Presence, the solder joints on the lugs are not 100% convincing to me...

 

[mickeydg5]

@AndyD
I agree with @StingRay85
Start checking resistances from component lead to component lead for all parts around the TREBLE circuit.
If you swapped out the treble capacitor and both acted the same then it seems the issue would be in the connections or wiring.

You mentioned the TREBLE potentiometer seemed fine but did not mention how it was tested nor any findings. Well .

 

[AndyD]

Success! Did all that @Pete Farrington suggested and moved some wires around. Double checked all connections for continuity and grounding etc etc. Played for 10 minutes at -24 db attenuation (8.30 pm!), volume way up and proceeded to turn every control and not a hint of trouble! I will try it again for longer in the next day or two, but I am confident that Pete was correct in his diagnosis. I will, as I say, give it some more time before I start handling out awards! Thank you all for your help.

 

[Pete Farrington]

Brill

Putting the chassis into a cab that puts shielding / screening over the open top / bottom may also help to calm the ultrasonics down a little.

I think stability in most Marshall type builds would easily be improved by moving the board along 1”, and swapping the normal and bright input sockets around.

 

[Marcomel79]

Glad you got it sorted Andy, that amp sure is a beauty! May i ask why you decided to change into a 1987?

 

[AndyD]

Glad you got it sorted Andy, that amp sure is a beauty! May i ask why you decided to change into a 1987?

Too many bass amps!

 

[Gene Ballzz]

Success! Did all that @Pete Farrington suggested and moved some wires around. Double checked all connections for continuity and grounding etc etc. Played for 10 minutes at -24 db attenuation (8.30 pm!), volume way up and proceeded to turn every control and not a hint of trouble! I will try it again for longer in the next day or two, but I am confident that Pete was correct in his diagnosis. I will, as I say, give it some more time before I start handling out awards! Thank you all for your help.

Yeah, that Mr @Pete Farrington guy is quite the sharp cookie, indeed! I can't express enough the level of respect and admiration I have for his skills, patience, experience and knowledge!
A Genuine Gentleman!
Gene

 

[Pete Farrington]

Aw, shucks

 

[dtier]

I think I would want to back walk to find the actual cause for reference, but then again if I was really enjoying playing it I would just box it up and play!

Nice build and I like your chassis stand.

 

[AndyD]

Nice build and I like your chassis stand.

Got fed up off balancing my chassis’s on piles of books!!!

 

[neikeel]

Late to party!
I agrée with ultrasonic oscillation of selected frequencies, particularly as it worked fine as a JTM
But your tone stack, vol bypass cap and the cathode bypass caps will significantly boost certain frequencies leading to instability.
Often you will hear a chirp or Twitter from the speakers when you turn the pot before you lose sound.
More common with gain pots but both treble and presence pots can do the same. At least you did not need to add snubbers on the plates or input jacks or reach for shielded wire (all band-aid measures!)