SF MV Twin Reverb - Noisy vibrato channel troubleshooting advice

[clinkous]

Hi guys,

I'm trying to diagnose the cause of a noisy vibrato channel. With the MV on 10 the NORMAL channel is dead quiet with its volume on 10 and all tone controls maxed with no input.

With MV on 10 and volume on VIBRATO channel on zero there is no noise. The buzz or hum gets progressively louder as the volume is increased to an obnoxious level at full volume. No buzz with tone controls off. I've cleaned the pot and the pot ground as well as changing tubes around to no avail. The vibrato is disconnected at the intensity pot.

Have pulled v1, v3, v5 with no change.
Pulling v2, v4, or v6 stops hum.

The preamp volume pot is scratchy when turned. I believe that the problem is something happening at the beginning of the chain and being sent through the tone controls to the power section. I've read about DC being on the pots from a bad coupling cap causing this problem and making a scratchy pot like a presence control.

Am I on the right track here?

If so, how do I check for the DC on the controls. Do I ground at the chassis and check with DVM at the wiper?

Thanks in advance! Trying to get better at this.
Chris



Any help would be appreciated.

[g1]

No input is not a realistic scenario.
Plug a guitar in with volume set to zero.
Or a cord with tip shorted to sleeve.
Still noisy?

[clinkous]

Noise is significantly less with guitar plugged in and turned down. Still kind of noisy but no more so than my other Fenders. Gets noisy again when I turn up the guitar's volume. It's still noisy with no input. Maybe I don't HAVE a problem?

I just thought it was weird that the normal channel has no issues.

[lowell]

I think what G1 is leading you towards is a faulty input shorting mechanism. The input jacks on most amps, this one included, short the tip to sleeve when nothing is plugged in. Old or oxidized jacks can cause this connection to fail. If plugging in your guitar and turning it all the way down eliminates the noise then your jacks most likely aren't shorting. Not a big deal at all. It serves no purpose other than quieting that channel if its unused. Could also just turn that channel volume to zero. Or, replace the jacks if you're so inclined.

[g1]

Yes, that's what I was thinking. The jack might just need to be cleaned, or maybe replaced if the noise with no input bothers you. That's what I meant about "real world scenario".
How do the noise levels of the 2 channels compare when set to equal loudness by ear? (Don't go by the numbers on the knobs). Now looking at the rear of the amp, swap the 2 tubes that are furthest to the right. Does it have any effect on the noise levels or scratchy volume control?
The scratchy pot may also just need some cleaner.

[Enzo]

My first thought too, a non-working shorting contact on the jack.

A dead giveaway is when you can turn it up some and bring your hand near the jack, and the hum increases.

[clinkous]

Thanks everyone,

I originally cleaned all pots and shorting jacks with a slight re-tension on the jacks. Replaced a broken star washer on one of the jacks and cleaned both contact points after removing the jacks.

The cleaning had no effect on the noise level. I swapped the v1 and v2 tubes originally to no avail.

Enzo, the noise doesn't change when my hand is brought into proximity of the jack.

G1, the noise level on the normal channel is zero in relation to the vibrato channel when set at same volume. I've never actually had a fender amp that was as quiet as this normal channel is. Amp sounds excellent except for this problem.

However, I will re-check all this evening after work.
Thanks again

[cbarrow7625]

Have you tried using the chopstick method to move around the input wire between the jack & first tube? It could just be it is picking up noise due to the routing.

The other thought that occurred to me was that it could be possible the input jack is not making good ground contact, leaving the grid of the first stage floating at a higher than intended impedance since the 1M grid leak resistor is soldered directly to the input jack. Might be a good idea to connect a real ground wire to that input.

[g1]

Another possibility is that the "matching" or hum balance pots were set up for the clean channel, not the vibrato channel.
The output tubes matching (balance) control should be set for minimum hum with volumes down.
The hum balance should then be set for minimum hum with volumes set full.
The matching pot may be on the underside of the chassis.

[clinkous]

Thanks everyone,

With the chopstick I was able to see that the input jacks were not able to be re-tensioned. All is well when I push them closed. Once again my problem ended up being a simple one. The KISS principle serves me again!

I really appreciate the help. I'm trying to get better at this. I'm ordering a few sets of input jacks and resistors. Should I just use carbon comps like it came with? Not to start a great debate I just figured they sounded fine before...

Anyway, I'm grateful for you guys
Chris

[Jazz P Bass]

It can be a real bitch retensioning the normally closed contacts.

[Leo_Gnardo]

It can be a real bitch retensioning the normally closed contacts.

Agreed! If they don't come around fast, I mean a minute or 2 of fiddling, I just replace 'em with new Switchcraft parts. Clinkous, I've tried reducing noise to the preamp by using metal film 68K's, never did make much difference over carbon comp unless you happen to have some very noisy carbon ones. Generally cc's with voltage across them, as in preamp plate circuits, are more likely noise offenders. And years of fixing Fenders has taught me random DC leakage thru the black card circuit board generally is more bothersome than noise from good quality carbon comps.

[clinkous]

There was just no spring left in the metal. I see the normal channel pots have been replaced already. I ordered enough stuff to do a few sets of jacks for Fenders.

[clinkous]

Thanks everyone for the input. New input jacks wired with new resistors fixed the problem. I'll try not to miss the same problem twice in the future!