Help Troubleshooting volume loss PLEASE

[Khron]

Ok, so that would be connecting the probe between v1 pin1 and GND, right?

Yup

It's a One Spot 9v/1700mA filtered PSU

Even with horrible efficiency on the boost converter, that should be plenty. The tubes shouldn't draw more than 1-2mA; you said something about having measured 190V DC, that should be well under 1W, so some hundreds of mA from 9V, allowing for lots of loss. The two opamps shouldn't draw more than 10-15mA in total.

That OneSpot is a switchmode one, though. How old is it? Even if it's new, i wouldn't be surprised if it had crap Chinese electrolytics which are... not trustworthy. At least / especially for testing, a bench/lab supply would be preferable (that way you could also keep an eye on the current draw).

That part of the circuit belongs to the cabsim and right now is being left out.

That still doesn't rule out one of the opamps oscillating and messing with the power supply. Unless you also physically disconnected the 9V going to the opamp sections (too)...

One question: what signal shall I use? Still my guitar or a sine wave? If a sine wave would 1k be fine?

A sine wave would indeed be a known, stable "control" quantity.

 

[sonolink]

That OneSpot is a switchmode one, though. How old is it? Even if it's new, i wouldn't be surprised if it had crap Chinese electrolytics which are... not trustworthy. At least / especially for testing, a bench/lab supply would be preferable (that way you could also keep an eye on the current draw).

These PSU are supposed to be high quality (at least for guitar).
The other option I have is this:


It's Chinese too

That still doesn't rule out one of the opamps oscillating and messing with the power supply. Unless you also physically disconnected the 9V going to the opamp sections (too)...

You're right!! BUT the opamps are on sockets, so I can pull them out and that would be complete isolation, right?

A sine wave would indeed be a known, stable "control" quantity.

Excellent!

Thanks a lot for your help
Sono

 

[Khron]

It's Chinese too

I guess, but at least you have SOME metering available. Not so much with a plug-pack...

BUT the opamps are on sockets, so I can pull them out and that would be complete isolation, right?

Very much so, yes.

 

[sonolink]

Excellent. I'll set everything up and measure this afternoon and report back.

Thanks a lot for your time and help Khron!
Cheers
Sono

 

[usbdevice]

Hi Sono, sorry, I wasn't able to reply sooner.
Khron makes some very good points.

Would be interesting to monitor the HV DC supply to the tube, when it sounds ok versus when it doesn't. May well be that boost-converter "runs out of steam".

This. Since you posted yesterday that the volume drop happens even with the preamp alone, this would also have been my suggestion. And measure it directly across C1, common (black) probe of your multimeter to the negative terminal of C1 and the other (red) probe to the positive terminal of C1. Leave the multimeter connected and play guitar through the preamp either into your amp or the interface. Note the voltage right after you power up the preamp, and observe what happens during playing and when the volume drop occurs.
You can even go further and before you do that, measure your HV B+ but without the tube inserted (no guitar playing required for that). Is the B+ across C1 much higher without the tube inserted?

That being said, I'm still trying to make sense of the voltage measurements you took. They change with every new measurement and I'm starting to think your probe tips (actually, what kind of probes are you using?) may not be making a good connection or, even worse, you have a bad ground connection point somewhere that you're using when you're measuring DC volts.

It might also help us if you could provide pictures of the actual circuit board (both sides if you can).

That still doesn't rule out one of the opamps oscillating and messing with the power supply.

Couldn't this also be the case with the tube in the preamp circuit?

L

 

[sonolink]

I will try to provide all the info this evening. Thanks to both for your help

Cheers
Sono

 

[sonolink]

Hi again,
I've had a bit of trouble dealing with my scope TBH....

But first of all the pics you asked:

My meter with its probes:


The borrowed scope checking the oscillator signal on the osc output:



The oscillator:





The board top side:
(just in case you're wondering, the yellow cable is the mod suggested by usbdevice to properly terminate the unused part of IC3


The board bottom (components) side:



So I connected the multimeter across the positive side of C1 and GND and the scope across V1p1 and GND and fired the PSU up. The multimeter went up to +/-170VDC and kept dropping for the entire 10 mins I measured (see attached accelerated vid), and the scope, after pressing the "autoset" button wouldn't show anything except Vpp=?

View attachment lv_0_20240416210304.mp4

I seem to be kinda useless with the scope so I did a second vid with the multimeter across V1p1 and same thing: voltage drops continously:

View attachment lv_0_20240416210640.mp4

I hope this provides some useful info...
Thanks for your help and advice
Cheers
Sono

 

[Khron]

Yeah - no power to the tube -> no signal through the tube...

 

[sonolink]

So the PSU is loosing power although not all of it, right? What could be the cause and where to begin looking?

One question: I couldn't get hold of 120uH/2A coils so I used a 120uH/1A. Could that be the cause?

I pulled out the plate resistors to isolate the PSU and measured the HV output voltage: same thing. It raises to 170vdc and slowly drops down

Thanks for your help
Cheers
Sono

 

[Khron]

I couldn't get hold of 120uH/2A coils so I used a 120uH/1A. Could that be the cause?

Not impossible that the inductor saturates. 100uH / 2-3A, if you can get your hands on one?

 

[usbdevice]

Yes, looks like the HV supply is not working properly. I would also follow the schematic and get the proper inductor.
Since you have a scope now, you could start by looking at the signal at the gate of the mosfet. See if autoset works, and if not, set the horizontal to 20 or 50us or even 100us, adjust the trigger level until the waveform is stable and adjust the vertical until you can see the entire waveform. Take a picture or video.

@Khron What would be the effect of the inductor saturating?

 

[Khron]

@Khron What would be the effect of the inductor saturating?

https://www.ti.com/lit/ta/ssztav7/ssztav7.pdf
https://www.monolithicpower.com/en/how-to-avoid-inductor-saturation-in-your-power-supply-design
https://www.zxcompo.com/inductor-saturation-current-the-key-factor-in-inductor-selection/

 

[sonolink]

I swapped the inductor. Now power goes up to 190vdc but after drops down again slowly

I'll try to check the Mosfet with the scope and report back

Thanks!!
Sono

 

[sonolink]

Yes, looks like the HV supply is not working properly. I would also follow the schematic and get the proper inductor.
Since you have a scope now, you could start by looking at the signal at the gate of the mosfet. See if autoset works, and if not, set the horizontal to 20 or 50us or even 100us, adjust the trigger level until the waveform is stable and adjust the vertical until you can see the entire waveform. Take a picture or video.

@Khron What would be the effect of the inductor saturating?

Does this make any sense.
The scope is across the mosfet gate and GND. I set it with the autoset button:

 

[Khron]

Does this make any sense.
The scope is across the mosfet gate and GND.

Well, it functions - that's about all that can be said. What MOSFET is it?

 

[sonolink]

Well, it functions - that's about all that can be said. What MOSFET is it?

IRF740
Any suggestions/ideas?
Thanks!
Sono

 

[Khron]

https://uk.farnell.com/w/c/semiconductors-discretes/fets/single-mosfets/prl/results?channel-type=n-channel&drain-source-voltage-vds=400v_to_950v&drain-source-on-state-resistance=0.00125ohm_to_0.295ohm&transistor-case-style=to-220fi_to_to-220sis&range=inc-in-stock|exc-delivery-surcharge|inc-in-stock|exc-delivery-surcharge&sort=P_PRICE

"Just in case" - one of these should net you (at least in theory) lower losses, but if the current IRF740 doesn't even get warm (which it kinda shouldn't), that's just a "maybe"...

the multimeter across V1p1

Is this supposed to be the plate of the first triode?

D2 is definitely a UF4007 (not a plain 1N4007), right?

Just to be on the safe side, i'd even consider replacing the 2N3904. Might also want to triple-check the values of R2, R5 & R6, and that they are indeed connected (properly) to the nodes they're supposed to.

Videos show about 0.5A draw @ 9.1V - where's that 4.5W getting burned off? Inductor and/or MOSFET? Some heat should be noticeable, one way or another.

Just be careful when poking around, the exposed tab of the MOSFET will have 100-200V on it...

 

[sonolink]

Actually, I kinda remember after disconnecting power, feeling quite a bit of heat coming from one of the TO-92s. I guess in the back of my head I assumed it would be the 7806 reducing voltage but maybe it was the IRF740.

I'll check everything and report back.
Thanks for your help Khron
Cheers
Sono

 

[usbdevice]

I swapped the inductor. Now power goes up to 190vdc but after drops down again slowly

What did you swap it for? I thought you said you couldn't get a 2A inductor.

Videos show about 0.5A draw @ 9.1V - where's that 4.5W getting burned off? Inductor and/or MOSFET? Some heat should be noticeable, one way or another.

Part of that is probably the 6V filament supply maybe?

Actually, I kinda remember after disconnecting power, feeling quite a bit of heat coming from one of the TO-92s. I guess in the back of my head I assumed it would be the 7806 reducing voltage but maybe it was the IRF740.

Or the inductor?

L

 

[sonolink]

What did you swap it for? I thought you said you couldn't get a 2A inductor.

Sorry for the confusion Lenny

I began populating the board while waiting for the 2A inductors to arrive and put a 1A in the meantime. I didn't think it would be a problem. I thought that at most I would get a bit less voltage and would be able to get a taste of the build in the meantime. But then I forgot about it!!
So now I'm using a 120uH/2A inductor.

Or the inductor?

No, I think it was one of the 2 TO-92. the inductor is covered in some kind of rubber and the feeling was metallic.
Anyway I'm going to check all that in half an hour.

Cheers
Sono