Tweed Deluxe volume drop problem

[macula56]

A friend brought a Fender® 57 Amp to me a few days ago with a severe volume loss problem.  He said that normally with the instrument volume on about 4 it is loud and pretty clean. After that of course it gets dirty. As it is now it's very distorted at 2 on the volume and there isn't any noticeable increase in volume as you turn it up. Just more dirt. And this problem  was not a gradual thing. It went from regular volume to really low volume very quickly according to my friend. I have checked around for any loose connections and found none. I have substituted tubes that I know to be in excellent working order and no change. The schematic has voltages  listed at many different spots and they all check out like they should. I also tried a different speaker and no change there either. Anyone have any suggestions as to what I should be checking next? I have a OT I could sub in if necessary. I am attaching the schematic. Any ideas are appreciated as I have checked the usual suspects already and am not sure what to look for next. Thanks as always, JMac.

[HotBluePlates]

Could it be as simple as a failed/failing volume pot?

[macula56]

The problem affects both volumes. I'm gonna see if I have some parts lying around for a listening amp as I have an audio generator.

[HotBluePlates]

The speaker is plugged into the jack closest to the output tubes, correct?

The symptoms described sound like a classic "loose connection" although plugging the speaker into the wrong jack also results in low, distorted output.

[macula56]

Yessir. everything is where it is supposed to be. I never get amps with obvious problems that are easy to fix. For some reason people only bring me these weird ones.

[labb]

I would try the OT.

[mresistor]

I would check to make sure all the ground points are tight and good. And if this amp is on a green substrate  (pcb) I'd shine a light underneath it and make sure there are no burned spots between traces or terminals, especially ones that are close together. It is not unheard of for Fender pcbs to start conducting between points on the pcb. 

[macula56]

no pcb. this is a fairly nice eyelet board.

[Redfishbum]

I'm a rookie on this, but could this be a output bias resistor issue?  This could affect headroom and volume, right?

Bart

[macula56]

Yes that could affect it but the voltages on the bias resistor/cap combo are in line with the schematic voltages so I don' think that is the issue.

[mresistor]

can you post up a picture of the eyelet board... please ..  thanks

[HotBluePlates]

I think you probably have the right idea: d.c. volts were verified, so on to signal tracing.

[macula56]

mresistor, I would but I don't have a working camera. if you click on the schematic link there are a couple of page that show the eyelet board and all wiring. the board in this amp looks exactly like that if that's any help.
HBP, I never got around to building a listening amp although I do have a couple of small wattage amps that would work just fine I think. I wonder if my Oscope still works.

[HotBluePlates]

HBP, I never got around to building a listening amp ...

Bottom of this page

• Shielded wire with a probe on one end.
• Add a blocking cap somewhere (old VTVM might put the cap in the probe; Hoffman put the cap in the listening amp; you could put a cap is a "Listening Box").
• Add a 1MΩ pot from post-blocking cap Hot to Ground.
• Add provision to connect Listening Amp/Probe Ground to Amp-Under-Test Ground

This could be a dedicated test amp, or it could be an interface box between the Amp-Under-Test and some other small, clean amp you have.

[labb]

Could give this a try..Just make sure the cap is 400 or 600 volt.  http://www.diystompboxes.com/pedals/debug.html


I actually plug it into a CD player and use it to inject the signal where I want to...Works pretty good.

[tweedpro]

A friend brought a Fender® 57 Amp to me a few days ago with a severe volume loss problem.  He said that normally with the instrument volume on about 4 it is loud and pretty clean. After that of course it gets dirty. As it is now it's very distorted at 2 on the volume and there isn't any noticeable increase in volume as you turn it up. Just more dirt. And this problem  was not a gradual thing. It went from regular volume to really low volume very quickly according to my friend. I have checked around for any loose connections and found none. I have substituted tubes that I know to be in excellent working order and no change. The schematic has voltages  listed at many different spots and they all check out like they should. I also tried a different speaker and no change there either. Anyone have any suggestions as to what I should be checking next? I have a OT I could sub in if necessary. I am attaching the schematic. Any ideas are appreciated as I have checked the usual suspects already and am not sure what to look for next. Thanks as always, JMac.

[tweedpro]

Hi, was there any happy issue for this post, ...solution?