V4B Ampeg, trying to diagnose over the net for a friend

[reddart]

Tubes glow, but no sound. Not even hum. New tubes, still the same.

Is there a secondary fuse inside? I dont see one on the schematic, and there is only one externally accessible fuse.

[Jazz P Bass]

Quote:" I dont see one on the schematic "

I don't see a schematic.

[pdf64]

Maybe the power tubes have lost their screen grid supply, eg as a secondary failure due to their current limiting resistor/s fusing?

[g1]

If the tubes glow, the fuse is ok. Some have an internal back up fuse, but if it were blown, the tubes would not glow.
Sounds like you are trying to find things you can do without opening the unit and checking voltages.
Have you cleaned the tube sockets? They are often problematic in these and could cause your issue.

[reddart]

Quote:" I dont see one on the schematic "

I don't see a schematic.

http://www.ampegv4.com/images/schematics/V4B.jpg

[g1]

The 'Ext. Amp' jacks (not Ext.spkr.!!) can be used as pre-amp output, or power-amp input jacks.
Try running a signal into there and see if you get any sound out to the speaker. If so, the problem is in the preamp section.
If still no sound, try a cord from 'ext.amp' to another amp. Connect a signal into the regular input of the ampeg. Any sound from the other amp?

[reddart]

New info...
There is an internal 10A fuse soldered to the external fuse tab...and it was blown!

C205 (a painted cap) is cracked on the outside, though I doubt would be the cause of the failure if it was bad.

R54 is looking bad...could be something there.

[nsubulysses]

Those tropical fish caps (C205) always seem to crackle and disintegrate. I removed two yesterday from a V4. I think it's just the paint and the cap is probably fine, but it's a $1 part so I usually just replace it if it looks pretty bad, just in case.

R54 is a 180K resistor that drops the voltage for the pilot light if I remember correctly. You might check D5 or actually all the diodes in the rectifier and bias supply to make sure they aren't shorted/open.

Check R55. Always fails from overload. Replace with a 10W if it is.

This is just some random thoughts on things I would check If I couldn't just turn the amp on and check voltages.

If you have a current limiter or variac and can turn the amp on in some sort of limited power consumption way you can test voltages in the PS to see whats the deal. Of course the amp is distant so whatever.

[reddart]

Thanks for the hints. Yes, the amp is 3000 miles away from me. The owner doesn't have much electronics experience, but is a smart guy, so I'm tutoring him via text messages.

[Leo_Gnardo]

Not mentioned yet:

One of the common places to lose signal in this series of Ampegs is the output impedance select switch. Many's the time I've encountered the "silent amp" complaint, swiped this switch back n forth a few times, shot of pot'n'switch cleaner spray optional, & presto there's your sound.

[g1]

Tubes glow, but no sound.

New info...
There is an internal 10A fuse soldered to the external fuse tab...and it was blown!

If you look at the schematic, you will see the 2 fuses are in series. So if it was wired correctly, and the internal fuse was blown, there is no way the tubes could glow. I would guess the internal fuse has been bypassed somehow, or the fuse has blown since the first post.
I didn't realize you do not have the amp and are trying to fix it remotely. I'm assuming the owner has a multi-meter and will be able to locate components we call out from the schematic ?

And most importantly, is aware of the lethal high voltages present and necessary safety precautions?

[reddart]

If you look at the schematic, you will see the 2 fuses are in series. So if it was wired correctly, and the internal fuse was blown, there is no way the tubes could glow. I would guess the internal fuse has been bypassed somehow, or the fuse has blown since the first post.

Yes, good observation. Since the first post, he cleaned the pots and switches, reseated the tubes, and got the amp to work. He was jamming along for a while, then the fuse blew! Hence the current condition...sorry for not clarifying that.

I didn't realize you do not have the amp and are trying to fix it remotely. I'm assuming the owner has a multi-meter and will be able to locate components we call out from the schematic ?

Yes, he seems to be doing well with that. It helps the components are all numbered on the board. He has access to a lab of basic electronics equipment (oscope, signal generator, etc), but not much experience with using them, yet.

And most importantly, is aware of the lethal high voltages present and necessary safety precautions?

Yes, the first thing I told him. We are tying to do the maximum diagnostics we can do unplugged for now. When he's replaced the suspect components, I'll probably have him monitor the power supply ramping up with variac and no tubes at first.

[g1]

Most likely cause of fuse blowing while in use would probably be faulty power tube or loss of bias to power tube(s).

[52 Bill]

What value fuse is in the outside fuse holder? That 10 amp inside fuse is there to protect from tin foil fuses.

[reddart]

What value fuse is in the outside fuse holder? That 10 amp inside fuse is there to protect from tin foil fuses.

I think it is a 6A slow blow.