Scratchy Pot-???

[Zephyr]

I have a Magnatone MP-3. Have a problem with the volume pot on the normal channel. I thought it was just a case of an old or dirty pot, but after cleaning and finally replacing it with several other new pots with no success, I started looking at other things.
Voltages, resistors, and capacitors seem to be ok. The input jacks are of the type that ground the input when nothing is plugged in so the inputs are shorted and anyway the noise also occurs on the pot with a guitar plugged in.
Thought it might be a microphonic tube but rapping on the tube (V1-12AX7)with a dowel at full volume produces very little to indicate this. Nor does tapping the tube socket connections or the rest. Have re-flowed all the solder connections associated with the circuit.
Swapping V1 with a known good tube produces no improvement.
What can produce noise is rubbing the wire from the pot wiper to the tube (V1B) grid with a dowel or plastic TV IF inductor tool. Replacing the wire didn't change things. It's almost as though the mechanical vibrations of the wiper of the pot are being amplified.

The Volume pot on the Reverb/Tremelo Channel (V2) is dead quiet all the way up.

My experience with tube circuits is limited. I can follow the schematics
but am still working my way through the finer points of these types of circuits.
(I have enough electronics background/education so the usual cautions probably aren't necessary.)

Before I start replacing resistors and caps in this part of the amp which might be unneeded, I thought I'd ask for help here. Have learned much lurking here over the last 10 years or so.

[52 Bill]

Check for any dc voltage on the pot. If any of the coupling caps in the tone control section leaks any dc to the volume pot it will cause the scratchy pot sound.

[Zephyr]

re:

Thanks Bill

Had attempted measuring the DC at the wiper. Was distracted by the fact that there was no scratchy sound while turning the pot up/down with themeter connected. Guess maybe the hum picked up by the test leads was masking it. Or the resistance in parallel was shunting it to ground.

Used a DVM and found -107mV on wiper to grid of V2B with pot turned to max. Pretty much 0v at min. Considering V2B has enough gain to pick up hum any time my fingertip gets within 3-4 inches of that wire, a range of 0-107mV DC is maybe more enough to cause what I'm seeing.
Looks like someone has replaced the original tone caps with orange drops sometime in the past.
The caps measured good, but think I'd better replace them and see what happens.

Thanks Again,

Mike

[Alex R]

Whilst not enough to cause much scratchiness, that -107mv ought not to be there. Can you trace it back through the tone controls?If you lift the wiper wire, is it still on the grid?

Couple more things - is the cathode bypass cap (pin 8 to ground) ok? And is the pot ground good?

A faint possibility is RF pickup. Try putting a grid resistor on pin7 to pass the signal through; it makes a filter for RF with the tube's internal capacitance; you lose very little signal. 25K might be enough; 68K is traditional.

[52 Bill]

"Used a DVM and found -107mV on wiper to grid of V2B with pot turned to max. Pretty much 0v at min."

I just looked at the schematic, at minumum the wiper is grounded so that should be 0 volts.

Do any of the tone pots make any noise when turned? Is the ground connection to the volume pot a separate wire or is the lug soldered to the case? If so is it making good contact?

I just thought of this...
Have you compared voltages with the other channel?

[Alex R]

Crossed posts with you, Bill. Cool ideas.

[Zephyr]

Re...

Thanks All,

You've given me some really good advice.

Will lift the wire to see it if there's any voltage on the grid.
Guess if there's none, maybe it's in the tone circuit.

No, there's no noise on the tone pots. When I subbed the new pots I soldered a piece of hookup wire to the pot case and the lug that's to ground which is soldered then to the ground rail to make sure I had a good ground. Didn't seem to have any effect.
Wondered about the grounding, but after doing this, it didn't seem to be the problem since the same ground buss(wire) is used on the other channel.

The pot noise gets somewhat louder as the volume is turned up. Doesn't seem to be any noise unless I'm turning the pot and varying the voltage on the wiper.

...checked the wiper voltages on the Reverb/Tremolo Channel volume pot wiper - at min 0.2mV, at max -7.4mV.
Probably the range I need on the Normal Channel

If I get time tomorrow, I'll try replacing the .022uF and .047uF tone caps. They had been replaced but I don't know how long it's been. Wonder if someone else was trying to solve this problem.
If I can find a 100pF in my parts box, I might change it out also. Seems like I need to get the voltage on the grid reduced.
The DC on the bypass cap is pretty close to what the print calls for at .67V. The cap measured within tolerence on my multimeter.

Wondered about RF. I have an AM station at 1550 about a mile away. While I was jumpering stuff, I could hear it very faintly with the volume at max. in amongst the hum.

Thanks Again... If any have any other suggestions please post them.

Mike

[Enzo]

I wonder what the voltage is across the pot, not just to the wiper.

We need to isolate the problem. Scratchy pots are a sign of DC on the pot often as not, adn aespecially if they are clean. Measure the voltage across the pot element. If the voltage is coming in from the tone stack, the max value will be present at the top of the pot no matter what the wiper setting. If it is coming from the grid, then the voltage at the top of the pot will vary as the wiper setting.

Alternatively, disconnect the wire from the treble pot. Now all you have is the pot connected to the grid. If it is still scratchy, then it is the pot or the tube.

Or pull the tube adn now the pot sits alone, though connected to the tone stack. NOW is there still voltage across it? If so, it isn't from the tube.

If you disconect the wire from thw tube grid, then teh grid will be unterminated, and the space charge from the cathode will accumulate a charge on the grid. The only termination for the grid would then be the impedance of your meter. The typical 10meg of e meter sounds really high, but it is about a perfect value for a grid leak bias settting.

And for that matter, -100mv on a 12AX7 grid sounds unmysterious. Grid leak voltage can build up after all. But why not both channels? Ground your meter to the bottom end of the cathode resistor for that tube, now measure voltage to teh grounded terminal of the volume control. If the grounding is good, they should be at the same potential. If however you find a difference voltage between the two "ground" points, then I'd say you found your voltage. Don't assume a ground bus is one big happy everyplace is the same ground system. Two channels can be grounded to the same bus, but in between them the return for come other stage might be connected and then that bus shares the current from the two stages. That can create a voltage drop, and all we need is a drop of a few tens of millivolts and we have a voltage symptom.

[Alex R]

If you disconect the wire from thw tube grid, then teh grid will be unterminated, and the space charge from the cathode will accumulate a charge on the grid. The only termination for the grid would then be the impedance of your meter. The typical 10meg of e meter sounds really high, but it is about a perfect value for a grid leak bias settting.

- of course, stoopid of me. Don't disconnect the wiper, pull out the valve and see if the volts are still on the grid pin. Best general advice remains: do what Enzo says.

[Zephyr]

test readings

Thanks All,

Tonight the grid voltage was in the -70mV to -80mV range.

Checked for a voltage across the cathode grounds for both channels, meter read 0.1mV. Likewise the other grounding points for pots.
Doesn't look like grounding is an issue.

Checked voltage across the pot and the voltage did vary from 0 to the max grid voltage with the wiper.

Pulled the tube. The grid pin voltage was +.2mv at vol pot min and +2.2mV at wiper max.

Guess this probably eliminates the tone circuit as the source of the DC.

Took another look at the tube circuit.
Checked the voltages at the plate on both channels.
The voltage on V1 where I'm getting noise is somewhat low at 68vDC.
It's more or less this value no matter which 12AX7 I install.

On V2 where there is no noise the voltage the plate voltage is 78vDC.
Likewise pretty much independent of which 12AX7 is installed.

Looks like the problem's narrowed down to the tube circuit.

[oc disorder]

52 Bill mentioned checking the earth which reminded me of a Fender Twin
which endorsed my membership to the "weird wild and wacky " club hopefully
no one has let their membership lapse..
The treble pot on the trem/rev channel had a nasty scatchy sound so
I naturally assumed it needed replacing.
On opening it up I saw where a previous repairer had replaced it but obviously
to no avail.
Cutting a long story short by changing the earth point around the second
input valve to a common point got rid of it.
So I would suggest if all else fails try altering the earth points ,resoldering
them and maybe even just putting a piece of wire on the earth lug of the pot
and try earthing it in different places.
Be adventurous 'cause you can always return it to what is was but this is my suggestion....

Who let OJ out of the fridge.... current WW&W club topic......
good luck
O.C.Disorder

[Alex R]

Cathode bypass cap passing DC or low value cath resistor might pull that voltage down and stress the tube and make the noise.

[Zephyr]

results-long-ish

Finally got time to get back to this problem.

I've had some success but I'm not sure which change helped. The plate and cathode resistors both measured high - 3.02kohm for the 2.7kohm and 301kohm for the 270kohm. Just barely out of tolerance but I replaced them with RadShack off the shelf 1/2w. Maybe not the best choice, but it's what I had.

The bypass cap measured ok in circuit but when I removed it, it read open on the meter. Used a heat sink and didn't stress it physically so am thinking it was probably borderline at best. Replaced it.

The wiper voltage max was then in the -50mV -60mV range. That's
where it's remained.
At this point I still had a scratchy pot.. somewhat improved but still scratchy.
Wasn't much noise until it was turned up well past halfway.

Tried putting a 56k resistor between the wiper and pin 7 - no change.

Swapped out the pot and had no improvement. The wire from wiper to pin 7 produces noise when rubbed with the dowel I use to probe for loose wires etc so I took some solid insulated hookup wire and twisted it a couple of times around the wire and soldered it between the ground lug of the low gain input and the pot ground lug. The wire is a little bit stiff and pretty much immobilizes the wiper to pin 7 wire. Might also act a little bit like a twisted pair.
While doing this, the wiper wire broke off just above the pin 7 socket. Resoldered and had a noisy pot but only near max. Wiper wire seemed solid when I checked it before, but all the testing I'd been doing might have weakened it. Can't say for sure. Might have been the problem... not sure.

Compared to the noise I heard before the pot is quite a bit quieter now.
Won't know if it's quiet enough until I have a chance to put the amp through it's paces.

Thanks to all who helped. Been a long, long time since I last encountered tubes in Tech School and the advice and suggestions were very helpful.

Seems like I had/have several problems that need attending to.
This 40+ year old amp is nearly all original, but she's starting to show her age.
Looks like I should begin replacing the electrolytic caps and carbon comp resistors.

What resistors should I replace with carbon comp and which should I replace with resistors that aren't as prone to some of carbon comps weaknesses? Likewise, capacitors. And, is there source for high quality 1M Audio Taper Pots?
I've tried Alpha and CTS, But am still wondering if some of the noise is in the pot.

Thanks All, the info was much appreciated.