Vox AC4 blowing main fuse. done stuff, need more help

[Enzo]

Editing of posts is only allowed for up to a day, so errors may be corrected. If we allowed editing a month of two later, then entire threads could be rendered unreadable as people would have responses to material no longer there.

[Kenrod]

My bad. The schematic says pin 8, but it's an el84 (palm to forehead emogi). Sorry. Please do the same test from pin 3 to ground.

221 ohms

[Chuck H]

Ok. That's as it should be. Since you had to replace the bypass cap I wondered if something might be wrong with the cathode bias. If the bias was way too hot that could pop the fuse when the tube was plugged in. If that circuit were still shorted you'd have NO bias. Really hot.

With the power tube out and the standby in "play" (I don't even know if that amp has a standby, but if it does flip it to play) is there any DC voltage on pin 2?

[Kenrod]

Well..Crap! I'm back to blowing fuses with no tubes and the circuit side of the OT unplugged.
I couldn't check pin 2

[Enzo]

Have we not mentioned a "light bulb limiter" instead of tossing fuses at it?

So you blow the HV fuse? The OT is disconnected? Just start from scratch. Disconnect the two 220 ohm resistors before the rectifiers. Does the fuse hold? Now reconnect ONE of them, same question. Now connect just the other one, same Q. Put them back in. Is either rectifier shorted? Lift one end of each diode, then test, now reconnect each one alone, and test. Lift one end of the 100uf ccap, still blows? Lift one end of the 470 ohm resistor, still blow?

All this isolates the parts that MAY be involved. other things that will not be shown on the schematic would be inadvertant shorts between PT winding ends or to ground. Measure resistance to ground from either end of the 470 ohm.

The whole idea of troubleshooting is to isolate the problem. Once we know WHERE it is we can usually figure out pretty quick why it is.

[Kenrod]

Have we not mentioned a "light bulb limiter" instead of tossing fuses at it?

I have one and used it but still blow the little 125mA fuse. I tried it once and that's what happened.
I have a 200 watt bulb in there. Should I be using a smaller bulb or does that matter?

[Kenrod]

So you blow the HV fuse? The OT is disconnected?

yes and yes

Just start from scratch. Disconnect the two 220 ohm resistors before the rectifiers. Does the fuse hold? Now reconnect ONE of them, same question. Now connect just the other one, same Q. Put them back in. Is either rectifier shorted? Lift one end of each diode, then test, now reconnect each one alone, and test. Lift one end of the 100uf ccap, still blows? Lift one end of the 470 ohm resistor, still blow?

All this isolates the parts that MAY be involved. other things that will not be shown on the schematic would be inadvertant shorts between PT winding ends or to ground. Measure resistance to ground from either end of the 470 ohm.

The whole idea of troubleshooting is to isolate the problem. Once we know WHERE it is we can usually figure out pretty quick why it is.

I had pretty much done all this in the past and was back to the point of the fuse holding WITH the OT connected, but no tubes. Today I put power tube in and blew the fuse, changed the tube to a new one and blew it again, pulled the tube and disconnected the OT and still blow fuse. Back to square one.


I'll back up to where I remember not blowing fuses. ( just past the 470 ohm resisitor is how I remember it)

I'll document everything and list it in great detail so as to not waste any time in folks wondering what the heck i'm doing, or not doing. I appreciate the patience and help each one of you are contributing.

[nosaj]

I have one and used it but still blow the little 125mA fuse. I tried it once and that's what happened.
I have a 200 watt bulb in there. Should I be using a smaller bulb or does that matter?

I only use a 60watt bulb in mine. I use it like this . If I make a circuit modification, first power up gets the dim bulb test. power it on light goes bright then dims down. If it dims down then its good if it stays bright I still have a short in there somewhere..

nosaj

[Enzo]

yes, a 200w bulb is way to big for this tiny amp.


Do you have a variac?

[Kenrod]

yes, a 200w bulb is way to big for this tiny amp.


Do you have a variac?

Yes i do have a variac but wan't using it, but let me go ahead and give you my observations from the last hour.

I double checked the wiring on my bulb limiter: correct
Checked the bulb, it was already a 60w, but that could have been after I blew the fuse with the 200w still in there from a while back.

I lifted several components (R16, R19, R23, R9), one at a time, and never blew a fuse (details to follow).

Here's the interesting observation: every time I powered up (using the limiter) the fuse would light up, then go dark. the filament looked like a tiny lightening bolt for an instant, but it never blew. Every time (or almost) that i previously powered up without the limiter, i would see a definite flash inside the fuse. It would sometimes be blown, sometimes not.

As far as component trouble shooting, they last test i had performed was with no tubes, OT hooked up, R9 lifted between C9 and R23 (only eliminating 1st half of preamp tube).

done for the evening.

Comments?

[Jazz P Bass]

Disconnect the OT.

[Kenrod]

Disconnect the OT.

I only hooked it up on the very last power up to see if the fuse was going to blow with it in circuit (but no tubes.
Do I risk damaging it with the testing? Not exactly sure what your wanting me to do.

[Chuck H]

It seems to me that the fuse blowing is intermittent with the OT hookup and the power tube in or not. I think there may be an intermittent problem with the wiring? It could be an incorrect connection that also happens to be intermittent or an intermittent short due to proximity. I don't feel like we've had consistency in the testing yet.

[Kenrod]

It seems to me that the fuse blowing is intermittent with the OT hookup and the power tube in or not. I think there may be an intermittent problem with the wiring? It could be an incorrect connection that also happens to be intermittent or an intermittent short due to proximity. I don't feel like we've had consistency in the testing yet.

With the limiter in place and I assume soaking up the start up surge of current, the fuse is holding now. I think before it wa a matter if whether it could or could not hold up to that initial rush. I don't know what the problem is though. With the limiter buffering that start up, I can't tell where my problem lies just yet.

I have been testing pretty methodically this evening. Reintroducing parts of the circuit waiting for the fuse to blow.

Limiter bulb never glowing, only the fuse illuminates briefly in start up.

[Enzo]

It is only a 1/8A fuse, but I still am not happy it lights up, that fatigues the fuse heavily.

If yopu disconnect that main 100uf cap, does the fuse still light up? That would be the most likely surge item.

Can you determine the idle current through that fuse once it is powered up? WWithout looking back, is this a slow blow fuse?