Ampeg V4 ---blowing power fuse when standby engaged.

[randomair]

I am working on an Ampeg V4 for a friend and having some issues with the main power fuse blowing. Apparently at some point the OT was replaced and after some time (months) of the V4 working as it should, there was a loud buzz through teh speakers, then the main fuse blew. The fuse was replaced and when turning the power switch on there was a buzzing sound. ((The buzz was coming through the speakers not from the chassis, it is not a loose component.)) The standby switch was engaged and the main fuse went again. Now the amp is in my hands and I am trying to track down the culprit. I pulled all of the power tubes and fired the amp up, but the buzzing sound still exists, in fact, with all of the tubes out of the buzz is still there, i tried the standby switch for the first time since it has been with me and the main fuse blew again. I have checked the diodes in the rectifier and they all read fine. I also checked the grounds to the filter caps, they are fine. Any ideas on this? Thanks!

[randomair]

Can caps verified

I checked the can caps C17,C18,C20 in the power supply and they are all taking an infinite charge. At this point, I am at a loss being the rectifier is reading ok as well.

http://www.drtube.com/schematics/ampeg/vt22-v4-74.gif

Above is the schematic I am using.

[Enzo]

See the four diodes D7-10 connected to the ends of the OT primary? They go to the neg terminal on the B+ bridge. ANy shorted?

Troubleshooting now: the only way for something to come out he speaker is for current to flow through the OT primary. (Well, or the NFB line connecting to some source of current. Not likely) The tubes are out. The only way for current to flow through that primary is for some part of it to find its way to ground. Or at least back to the B+ return. IN AMpegs, that is not the same thing. Note your standby switch is in the gournd leg of the B+ supply.

Potentially we could have a OT primary winding short to frame or to secondary, or those diodes are shot. Turning on the standby completes the circuit to ground. Is that +545v across the main filters while in standby? Or is it loaded down? Across the bridge diodes or across R56 and R57.

[randomair]

See the four diodes D7-10 connected to the ends of the OT primary?

Hey Enzo, thanks for the reply. Funny you mention those diodes, because I honestly cannot find those in this V4, I must be blind but they are not there. I replaced diodes 1-6 and looked for d7-d10 but no luck. Typically if I have had this problem with amps in the past , it has been in the rectifier so this problem is a new thing for me.
I am getting 545 at C18, but I am still baffled on where those D7-d10 are, because I feel , if they are there, they could be my problem.

[Enzo]

FInd sockets for V5,6. From pin 3 of each find R39, R46, the power resistors in the plate leads. If they have been removed, there should be a place for them. That all connects to the blue wire from the OT. The diodes should connect there too. Or there should be somewhere they were removed.

I don;t recall, is all this on the circuit board or are the power tubes in wired sockets?

[randomair]

Mystery solved, for the missing diodes at least. It appears whoever worked on this V4 previously, removed D7-D10. They were running from a small terminal strip that is mounted near the rectifier on the circuit board. (V5-V8 are separate off-board tube sockets by the way). D7-D10 were soldered to a terminal that has one end of the standby switch wired to it. From there D7-D10 ran to the circuit board where they were ultimately soldered to a trace where the blue and violet wires come in from the OT. Being they are not there and they were not when I got the amp, its tough to say if this is causing my problem? or is it? because the owner was running the amp without them in there until it finally started blowing fuses. I will ask him how long he ran it before this happened, although it may not make sense that it would work if they were not there. Back to square 1.

I did find this photo of a V22, which is essentially the same circuit, and as you notice where the purple and blue wires come in to R45-R40 and R46-R39 , I see no D7-D10, dont get it.

http://www.geocities.com/flintstudio/Ampeg2_recap.jpg

[Enzo]

They are protective devices. The amp works fine without them, but it no longer has their protection. COnsider your car will run just fine if you remove the seat belts. But in a crash they won't be there to protect you. If they are gone, then they cannot be the cause of the problem.

Disconnect the OT center tap from the board - the orange wire on the drawing. Does that have any effect on the hum in standby? Does that have any effect on the fuse blowing thing?

[randomair]

Got it, fine way of stating it Enzo. I will check the OT and thanks again for all of your help! Much appreciated! I will keep you updated.

[randomair]

Disconnected OT orange center tap wire going to board and buzz is gone and no blowing fuses! Where does this leave me?

[randomair]

Using center tap as reference I measured resistance to both blue and violet side(pin 3 all output tubes) of OT. Center tap to both blue sides 2.7k and center tap to violet sides 50 ohms. Huge difference there.Also, checked secondary (switches,jacks,etc) to pin 3 of blue and violet sides: blue(pin 3) to secondary, 2.2k (all points), violet(pin 3) to secondary, 80 ohms (all points).

[Enzo]

It leaves you with a bad OT.

[Diablo]

Enzo, is it likely that the missing diodes D7-10 lead to the failure of the OT?

[Enzo]

Those "flyback" diodes - and many amps have them - are there to squash transient spikes. Those spikes are a common result of the amp running without a speaker load. I see that as their main purpose. So if someone played the amp without a speaker AND those diodes were already removed, then maybe they were involved.