Fender 6G4-A Super: simple device with elusive problem

[Tubetec]

Ive seen the contact resistance of the wiper itself go drastically high , that explains how the two measurments add up higher than the total resistance across the pot itself .

 

[Tubetec]

I found a pot once , a verdigris crust had formed on the wiper tip ,it was Mohms

 

[soapfoot]

Just measured the (date coded 47th week of '60) Stackpole that came out of the amp.

1.054 MΩ total
125.8 kΩ at mid-rotation.

 

[CJ]

pots nowadays seem to have two different carbon tracks, one for the low range, and another with more ohms per mm for the high range. so you have that point where things seem to jump up in an un-natural manner.

conductive plastic like Alps seem to do better , maybe the artwork for the track can be tweaked for a smoother taper.

what about a P and G Neve fader for a guitar amp? might raise a few eyebrows,

i had Bourns make me a 4 turn trim pot with a reverse log taper. we used tons of their stuff so no setup fee. i even remember the engineer's name from back in 86. Her name was Frank. you don't forget a name like that.

 

[abbey road d enfer]

Most affordable (read cheap) pots are two-section, so the log law has a sharp corner at mid rotation.

AFAIK, no commonly available pot have more than two sections. An exception is S-law, that has two kinks, each close to one or the other end.
I believe the gain pot that JR had custom-built might have more than two sections.
High-quality faders are a different story; the track has multiple taps fed via branches that are laser-trimmed. As a result, their output impedance is not varying in a monotonous way, it jumps up and down.

 

[JohnRoberts]

Most affordable (read cheap) pots are two-section, so the log law has a sharp corner at mid rotation.

AFAIK, no commonly available pot have more than two sections. An exception is S-law, that has two kinks, each close to one or the other end.
I believe the gain pot that JR had custom-built might have more than two sections.
High-quality faders are a different story; the track has multiple taps fed via branches that are laser-trimmed. As a result, their output impedance is not varying in a monotonous way, it jumps up and down.

Not literally two or more sections but multiple passes with the screened resistive ink overlaid. It is difficult to realize the ideal taper for popular mic preamp topology. You want good resolution/adjustability down at the low ohms range, while delivering tens of kOhms at the high ohms end (for decent headroom and higher signal handling at low gain). This was accomplished by using inks with different resistances to screen on the low ohms and and high ohms end.

For todays's TMI about pots these screened resistive inks get heat cured after screening to stabilize bulk resistance values and even tweak resistance values a few percent one way or the other. Last century I had to deal with a higher than normal pot breakage problem during final assembly on my factory production lines. Digging deeper to learn more than I wanted to know about manufacturing pots, it turns out our pot vendor had recently moved their production to China. The Chinese factory didn't mix the resistive ink properly and cooked the finished substrates too long trying to pull the bulk resistance into tolerance, and these were 20% pots. Over cooking the pot substrates made them brittle and explained our higher than normal breakage problem. The vendor dropped Peavey as a customer saying that we were too picky.

For TMI#2 we made one custom pot that had two screened resistive elements side by side on the same substrate. The pot wiper fingers shorted between the two resistive tracks. This pot took the place of a dual pot in a nice Wien bridge equalizer circuit. We actually approved two vendors for this one part (71190557). One vendor's parts worked better than the other's because they used more fingers on their wiper. Wiper finger bounce caused a scratchy pot noise in that eq if that EQ circuit op amp momentarily lost NF (the alps pot was the better one of the two).

JR

 

[soapfoot]

Well, sorry to say this problem recurred today. It stuck around long enough for me to gather some data.

I was plugged into the "normal" channel and playing normally. The volume pots on both channels are new.

1. After the sudden massive volume jump (and bright timbre), I turned both "normal" and "vibrato" channel volume controls to zero. I still had appreciable signal
2. With both volume controls at zero, I turned up tremolo intensity. Even though I was plugged into the 'normal' channel, I heard tremolo loud-and-clear
3. Both channels' volume controls seemed to impact the level. On the normal channel, it was almost as though turning up the volume knob was "supplementing" the signal with additional level.
4. On the "vibrato" channel, increasing the volume knob seemed to create a null that reduced level a little bit (as though the mystery signal was opposite-polarity of what the knob was controlling).
5. Presence control had an impact, but perhaps not as great as under normal circumstances
6. Similar with the tone controls on both channels... an impact, but perhaps not as great as normal?

Now that we've got a little more data... anyone have any leads on where to look?

Thanks again!

 

[JohnRoberts]

check for bad solder connections

JR

 

[soapfoot]

check for bad solder connections

JR

I begun by chopsticking every connection and component in the amp, methodically, to try and excite the fault. No luck.

Then I proceeded to reflow most (if not all?) joints on the board and on the pots on the front panel.

No luck with either approach. Should I also flip the chassis and redo the solder joints on the power supply filter capacitors? I don't think I did those. I may also have neglected to touch the ones on tube sockets.

Or alternatively, do you have any other troubleshooting steps I should try to locate a bad solder connection?

 

[JohnRoberts]

maybe tomorrow when I'm more sober...

JR

 

[DKatz]

Sounds consistent with loose ground connection(s). See if loosening board screws makes it worse, then clean where they contact & tighten.

 

[GussyLoveridge]

Maybe something in the tone circuit? I could be way off the mark here, but a passive tone circuit that is somehow not attenuating or intermittently attenuating could cause both a jump in gain and the brightness you are experiencing. Maybe. I think.

 

[industrialarts]

Check the ground wires that go from the circuit board to the chassis under the controls. I had a Twin Reverb with a similar problem and it turned out to be that one of those wires was broken. I can't remember exactly, I think it was right at the solder joint and it looked good but until I really torqued the chassis I couldn't get it to act up. The Twin had obviously fallen on it's face so I didn't find the problem until I was installing a new volume pot and had to work to get it in place where the front panel had been bent.

 

[soapfoot]

Today's update:

• Reflowed all joints on eyelet board and V1, V2, V3 tube sockets
• Aggressively used chopstick to try and excite every single component and joint (no result)
• Double- and triple-checked all ground connections to brass plate under front panel controls. Mechanically tightened both volume pots
• Reflowed all joints under "doghouse" (power supply filter/decoupling capacitors). Found one in particular that seemed slightly suspect--the component lead wasn't inside the eyelet but rather resting on top (still, the solder connection seemed solid). I remedied that issue

A good, thorough play test accompanied with the same sort of "percussive stimulus" that sometimes excited the problem before did not reveal a fault

But I've been here before, so I'm reluctant to declare it fixed.

The only condition that remains (may be normal) is that high levels of signal (white noise) applied to "normal" channel input is passed to output even with volume control fully down. Tremolo seems to impact it, as does presence control, but neither channel's bass/treble controls have an impact on this.

The phenomenon is subtle enough to be undetectable with regular guitar input, so it may not be a true fault condition.

I'll check back in if/when the fault decides to recur (this generally happens at the least-opportune moment!)

 

[CJ]

if the problem comes back, plug in a signal generator or CD player or some sound source to free up your hands and try heating the circuit board with a heat gun or blow dryer and see if the volume changes. don't et too close with the heat gun or you will fry everything like medium well bacon.

 

[soapfoot]

1 month later, the amp has been in use literally every day for at least one hour, and the fault has not recurred. That makes me feel like I might've gotten it (touch wood).

If I had to place a bet, it was the one resistor between the power supply filters/decoupling that was simply soldered on top of the eyelet (rather than seated inside).

Of course now that I've typed this, I'm sure it will fail tomorrow...

 

[soapfoot]

A problem that's back from the dead, but I think I may have solved it. This is an interesting one!

The symptom disappeared for awhile after reflowing the joints on the power supply caps in the "doghouse," but then returned a bit differently: much more subtle, but whacking the top of the amp would still make a little bit of a grumpy noise (just quieter than before), and playing certain lower notes would cause a vibration-induced bad behavior.

After re-tensioning and cleaning all tube sockets, I thought I'd traced the fault to the output tubes (the only tubes for which I had no spares), so I got a fresh NOS set, installed, biased it up... no dice. Same as before.

Powering the amp up and removing the cover from the "doghouse," I started poking around with a chopstick. Was bemused to find a spot on the eyelet board that clicked in microphonic fashion whenever I poked it with the stick. Pressing down on it further made a sharp clicking noise as I applied more pressure.

So I powered the amp down, drained the caps, unsoldered a couple of leads, lifted the top layer of eyelet board... and out flopped an ancient piece of highly yellowed/acidic white paper.

I cleaned both inner surfaces of eyelet board with some IPA and just for good measure installed some fish paper between. Reassembled, powered up, and the weird behavior was gone.

It's always the last place you look....

 

[abbey road d enfer]

It pays to be persistent...

 

[CJ]

wow, that's one i never come across, thanks for the update!

those black boards do become conductive.

is yours dipped in wax?

 

[soapfoot]

It appears as though it has been, like most of the Fender eyelet boards I've run across