Fender 5F1 Champ dead

[2TMike]

Hi, this is a 10 year old reissue version of the 5W Fender Champ. It's had very, very little use but last week it died. No output and an electrical burning smell. I looked inside and a 250mA fuse had blown, so I replaced that and now all I get is a loud hum and no response at all to the input jacks or the vol knob. I had a spare preamp valve, so just on the chance I swapped that but it made no difference. There are no repair shops near me that are open so could anyone have a reasonable guess at the cause of this fault?
Thanks, Mike

[Chuck H]

Not enough information for a diagnosis. FWIW fuses blow for a reason. Unless that reason has been discovered and corrected there's a danger of doing more damage by simply plugging another fuse in. Especially if there was a burning smell associated with the initial failure. So you shouldn't do that.

The amp needs to be opened up and inspected by someone that knows what they're looking at.

[Marshtech]

The 6V6 or rectifier tube are more common failure components than a preamp tube. Purchase a new 6V6 and new rectifier tube. Pull the old 6V6 and rectifier, replace the fuse, turn it on with the tubes out. If the fuse holds, shut it off and insert the rectifier tube. Power on, check the fuse. If fuse holds, power off and insert a new 6V6. Tell us how this goes.

[Chuck H]

The 6V6 or rectifier tube are more common failure components than a preamp tube. Purchase a new 6V6 and new rectifier tube. Pull the old 6V6 and rectifier, replace the fuse, turn it on with the tubes out. If the fuse holds, shut it off and insert the rectifier tube. Power on, check the fuse. If fuse holds, power off and insert a new 6V6. Tell us how this goes.

IMHE a burning smell is almost never tubes. It may be caused by tube failure, but whatever has been compromised should be discovered before going further. It may be a baked screen grid resistor or a melted lead jacket. Purchasing both a rectifier tube and a power tube arbitrarily without further knowledge of the failure could be an unnecessary expense.

[2TMike]

Understood, I only swapped out the preamp tub because I happened to have a spare one. And the 250mA fuse that I replaced is holding ok. Visually there is no sign of distress/heat damage to any of the components or cables, in fact it looks like a new amp inside.

[Chuck H]

Ok. But please reinspect anyway. Especially the power tube resistors. I'm thinking the screen resistor. Sometimes just the color bands will darken. Otherwise,..

That fuse you indicated would be the HT fuse. The schematic is hard to locate with an image search and all older Champs don't have secondary fusing. So I had thought you were talking about the mains fuse in the opening post. The HT fuse blowing is likely indicative of what Marshtech was saying.

Do you have any electronics experience? There are safety issues regarding the service of tube amplifiers that you should be aware of. Do you have a DMM?

EDIT: Oh, Also the power tube. Most power tubes in newer Fender amps have markings that brown when overheated. So how does the paint look on the power tube?

[2TMike]

Ok. But please reinspect anyway. Especially the power tube resistors. I'm thinking the screen resistor. Sometimes just the color bands will darken. Otherwise,..

That fuse you indicated would be the HT fuse. The schematic is hard to locate with an image search and all older Champs don't have secondary fusing. So I had thought you were talking about the mains fuse in the opening post. The HT fuse blowing is likely indicative of what Marshtech was saying.

Do you have any electronics experience? There are safety issues regarding the service of tube amplifiers that you should be aware of. Do you have a DMM?

EDIT: Oh, Also the power tube. Most power tubes in newer Fender amps have markings that brown when overheated. So how does the paint look on the power tube?

A job for tomorrow then (evening now in the UK) . Yes, reasonably clued up and have test meter. I also have the schematic but it only shows the main power fuse not the 3 secondary ones so I was unsure which circuit the 250mA one was protecting. I'll do another visual on the resistors first in the morning as suggested.

[Pedro Vecino]

This can be the schematic (it contains four fuses and one is 250mA):

https://www.thetubestore.com/lib/the...atic-Rev-B.pdf

[Chuck H]

Ok. Since you'll be in there...

Do as Marshtech suggested, but don't include the power tube in the test. It's my first suspect. So, if the visual goes well pull the tubes and turn the amp on. If that goes well then power down and insert just the rectifier tube. Power up again and take a DC voltage reading from pin 8 to ground. I think you'll see around 390V.

And that's as far as we can take it before you have a new power tube. I wouldn't plug the old one back in at this time.

And I still have to wonder what got hot enough to smell.

[Chuck H]

That's awesome Pedro I was only able to find a fuzzy version of it. I'll repost it so it's visible on the thread.

[g1]

Ok. But please reinspect anyway. Especially the power tube resistors. I'm thinking the screen resistor. Sometimes just the color bands will darken. Otherwise,..

I think the 'Fr' marking on the schematic means flame retardant? So it may well be blown without any obvious visible clues. And if the power tube is bad and the screen resistor now open, it will not blow the new fuse til the resistor is replaced. Then if the power tube is defective it may blow the screen resistor again.

2TMike, check that R11 mounted on the power tube socket measures 470 ohms.

[Enzo]

I agree that FR means flame resistant. I'd just pull the power tube, turn it on and measure voltages at the socket. Got B+ at pins 3 and 4?

[Chuck H]

2TMike, check that R11 mounted on the power tube socket measures 470 ohms.

Ah! I was going to EDIT earlier with that and forgot. Good looking out.

But I'm not suspecting that resistor anymore because he mentioned the loud hum. I don't think that would be present if the screen resistor were blown.

[Enzo]

Unless an internal short in the tube allowed plate current.

[Chuck H]

Unless an internal short in the tube allowed plate current.

Ok. Yep. Did I mention that I wouldn't plug the power tube back in