Intermittent output Fender Vibrolux Reverb

[shzmm]

Hello all,

I have been working on the said Fender Vibrolux Reverb for awhile now as it has an annoying problem of losing power with vibration and heat. The amp appears to be operating normally most of the time but after some time at loud output level the sound diminishes and becomes distorted. Interestingly, I can replicate this situation by allowing the chassis to sit in the cabinet at an angle. When I bring the chassis up slightly the level comes back. I canīt replicate the scenario when the chassis is out of the cabinet unfortunately, no matter how I shake it, or tug on the connections. I even tried to place the chassis directly on a speaker outside of the cabinet to see if it was related to vibration but still canīt get anything to go wrong.

So far, I have resoldered all the connections within the amp.
I have replaced the speaker cable.
I have re-tightened all the jacks.
I have checked the voltages within the amp-- everything is running normally even when the volume dips.
I have scoped the signal path all the way through and the only place there is a diminishing signal is on the output transformer secondary.
I have replaced the output tubes.

Is there anything else that I can check before going through the final stage of removing the output transformer from the equation? I have never had an intermittent output transformer before so I am not sure if this is even the case or possible for that matter given the circumstances.

Thanks for all your help!

[52 Bill]

Have you done any real measurements on this like voltages before and after failure?

[shzmm]

Hi and thanks for the reply,

Yes, I have checked all the voltages while the amp is on and while the output level decreases. As I wrote, there is no difference when the output decreases. As well, I have scoped the input all the way from the input to the grids of the output tubes with no decrease in level. The only place a difference is notable is on the output transformer secondary.

Is there another type of measurement you would recommend?

[52 Bill]

The part that I don't understand is the angling the chassis symptom and the fact that it can't be replicated out of the cabinet.

Output transformers do fail, so if that is the last thing on your list pull it and sub in a different one. Or at least inspect the original one, maybe something shorting the output winding to the case? Try R.G.'s shorted turns tester on it.

[Leo_Gnardo]

How are all those ground wires between circuit board & brass bus plate? Sometimes there's an intermittent connection where one pops up thru an eyelet, sometimes corrosion has done its damage at the bus bar end even though the solder looks good. Give each one a good tug with your heavy duty needle nose to check it's not loose. And that pesky feedback node R, 100 ohms, they like to work loose too, a very common thing & I'll be danged if I can figure out why.

There's also the errant pre tube socket. Had a similar runaround with an Ampeg Gemini VI, OK out of the box but lost signal at random when mounted.

Currently having a similar problem with another VLux. Lucky the customer is very patient. On this one tilting doesn't matter but it won't show its symptoms removed from its box. So I'm watching here, who knows might be the same solution as yours when we find it.

[g1]

Just because it seems to be after the power tubes does not necessarily mean you have to suspect the OT first. There is also the wiring, and the speaker jack and the speaker cable.
What you said about the angle of the chassis made me think of the speaker cable, or maybe the jack. When you are running it out of the cab, are you still using that same speaker cable?
And speakers?

[Justin Thomas]

The other guys here know loads more than me, but... Can you post a schem? What about the cap doghouse? Seems to me there's a bad connection somewhere, thermally related, and tipping the chassis is enough to let gravity take effect without needing to heat up first. I am thinking freeze spray/soldering iron and chopsticks might be in order...and PLEASE don't suspect the OT first!

Justin

[Enzo]

First, if you can't make it do it out of the cabinet, how did you take all those voltage readings while it was doing it?

Tilting the cab makes it fail? UNplug the internal speaker and plug another cab into the amp, now tilt it. Still does it? I am checking the speaker itself by this.

Tilting cab problems often involve flexing of the chassis. Try taking the wires off the speaker jack, and use clip wires to a speaker.

[shzmm]

Hello again and thanks for the replies.

The neon trick was especially useful, I hadnīt seen that one before!

So, after I pulled the wires to do the neon test, I decided to reduce the length of the output secondary wires. This didnīt make any difference unfortunately, however then I removed the output jack from the equation-- this may have done the trick. I will replace the main speaker jack, sand a little of the metal to make a better ground and see if that makes a permanent difference. It does make sense at least whereas a faulty output transformer was difficult to accept.

[shzmm]

Ok, replacing the speaker output jack did nothing. I also tied all the wires to the new jack just to make sure-

To do my readings, I carefully attached clips to the point I wanted to measure and attached the device to these.
I have checked using an external cab and there is no problem. However, my friend who uses the amp says after playing for a long while and encountering the problem he tried an external cabinet and the problem persisted but I havenīt been able to verify this.
I am tilting the chassis not the cabinet. I agree there seems to be a flexing problem although i can bend and flex the chassis all I want out of the cabinet but I canīt reproduce the trouble. Only by inserting the chassis all the way into the cabinet and then letting it hang down do I hear the loss of volume. The output comes back when the chassis is lifted slightly towards the top of the cabinet.
As I wrote earlier, I have changed the speaker cable already.

All in all, it appears to be a bad contact that is related to the output transformer. I have tried everything but replacing this part. As I donīt have another on hand, I will have to wait and order which could take some time.

Thanks for all your replies and if anyone has another idea I am certainly all ears.

[Enzo]

I have checked using an external cab and there is no problem...

I am tilting the chassis not the cabinet. I agree there seems to be a flexing problem although i can bend and flex the chassis all I want out of the cabinet but I canīt reproduce the trouble.

Only by inserting the chassis all the way into the cabinet and then letting it hang down do I hear the loss of volume. The output comes back when the chassis is lifted slightly towards the top of the cabinet.

Using an external cab there is no problem? Meaning with an external cab nothing you do will make the problem?

You say you are tilting the chassis, not the cab, yet you also say the only way to make the problem is to insert the chassis into the cab. So how can you tilt the chassis without tilting the cab it is in?

I wasn't concerned the speaker cable was bad, I was more concerned the speaker itself had a bad tinsel wire or something, sensitive to position.

Pulling the chassis up tight stops the problem? OK, sight across the top edge of the chassis. Do ANY wires or anything else stick up above the rim? In other words, could bolting in the chassis push down on a wire?

[Leo_Gnardo]

then I removed the output jack from the equation-- this may have done the trick. I will replace the main speaker jack, sand a little of the metal to make a better ground and see if that makes a permanent difference. It does make sense at least whereas a faulty output transformer was difficult to accept.

Ok, replacing the speaker output jack did nothing. I also tied all the wires to the new jack just to make sure-

Trying to find 'zackly what . . . with wires straight out of the OT secondary to speaker, no problem? But when sent thru a new output jack the problem persists? Is the ext speaker jack wired to the main one? If it is - I'm thinking there's a fault with the ext jack, maybe a flake of metal or stray snippet of wire shorting your output when the chassis is tilted one way and not the other. Another place to suspect is the eyelet where the output signal is brought to the feedback resistor. There may be some metal debris stuck between the boards there.

[shzmm]

Sorry if I was unclear. In this era of Fenders at least, there are guides to lead the chassis into cabinet. However, when the chassis is fully in, it goes past these guides so it can hang down for whatever reason. Just dumb luck that I found I could replicate the problem this way.

As well, I donīt have to lift the chassis-- just pushing upwards by a small amount brings the volume back up to normal. I have also changed out the speakers, tried one at a time, etc. No wires are sticking up over the rim of the chassis, and like I checked earlier, the signal is the same all the way to the input of the output tubes.

I also connected all the wires in question to the new speaker output jack, the external jack plays no role any longer.

This is certainly a mystery!

[Chuck H]

Forgive me if you're already past this... I would follow Leo and Enzo's advice and suspect that there may be a fault or short occurring between the boards or elsewhere when the chassis is flexed. I know you've been pushing and flexing things around and I can only assume you are terribly frustrated by now, but by the description of the circumstances, what else could it be?

If you let it hang in the position that makes it fail AND plug into an external speaker the problem goes away. You can't recreate the problem outside of the cabinet. Once the amp is hot and bothered, the owner has tried another cabinet and the problem persists. All this according to your observation and reports by the amp owner. There is some contradiction between yours and the owners observations. But it's probably true that the owner is subjecting the amp to different circumstances (amp is hotter and used harder?) or even plugged in the alternative cabinet incorrectly for the purposes of the test (left the main speaker plugged in and ADDED an external cab to the ext. jack OR unplugged the main speaker jack and then plugged the other speaker into the ext. jack). The owner isn't a tech and may have missed an important detail in reporting or testing criteria. On this premise I would say that his testimony has to be omitted for now. So this brings me to one other possibility you can examine...

Microphonic output tubes or dirty output tube pin socket holes. It's possible that once the amp is fully warmed up the tubes are more sensitive and choking out physically or losing contact on one or more plug pins. When the chassis is hanging, the tubes may be at an angle that exacerbates the problem. Even if they are relatively new tubes this may be the case. My recent experience with three sets of Ruby selected Shuguang el34's was that all three were unacceptably microphonic for combo amp use RIGHT OUT OF THE BOX!!! But they sound and perform well otherwise, so I've accepted a little clanging and noises. This amp is reported by the owner to have a similar problem to what you describe. Though I've been unable to recreate the issue for myself. Anyway, this may be another avenue to explore. Try thumping on the power tubes with the amp in play mode. If they make a Pvvffft - Pvvffft or cracking sound, they may be microphonic losing pin contact.

[Leo_Gnardo]

No wires are sticking up over the rim of the chassis

And no threads of metal screening hanging down from the "ceiling" of the box either, right? Although they rarely cause a problem I always snip away any loose bits fearing what would happen especially if one contacted hi voltage or AC line.