AC4TV blowing fuse

[dju]

hi all, I have one of the cheap Vox AC4TV combo's and it starting blowing the 125mA fuse. A little background, I decided to try tube swapping on this amp since it was a single 12ax7/el84 amp and would be cheap to try this on. on one of my swaps the 125mA fuse blew. I figured maybe one of the tubes was bad and the fuse blew. So I put back the original tubes and the replacement fuse blew. 125mA fuses are hard to come by locally so I had to order them online as it would be cheaper than driving all over the place trying to find them. I tried powering up with no tubes installed. pop goes the fuse. So after some time with the amp setting in the closet I decide to take a look at it again, this time I hook up a variac transformer and bring up the voltage to 10 volts, no problem. 30 volts, no problem, all the way to 120v with 20 volt increments and no problems. the amp works. I try poweing on with the variac at 120v and pop goes the fuse. I decide to google the amp as I am wondering if I somehow got the wrong fuse value. I find that I did have the correct fuse value but I also find that some of the ac4tv amps have the same problem and it was somehow determined that the rectifier diodes were the problem. So I swap out the rectifier diodes with some new 1N4007's and after going from 20V to 120V in increments on the variac no problem. power up at 120v and pop goes the fuse. I don't have much if any of a trouble shooting background so I was hoping somebody would have some suggestions on how to find out why there is to much current at power up but no problems once you get powered up when using a variac?

thanks for any help ya'll can give,
dj

[Tillydog]

Are you using the correct *speed* of fuse?

Check the end caps of the fuses, they should have a 'T' or an 'F' as part of the markings. Check and information on the amp or handbook - they should also specify T or F.

I would think that you need a 'T' (time delay?) / slow / anti surge fuse. Fitting a 'normal' 'F' (fast?) fuse would likely give the results you are seeing without any fault being present.

If the correct fuse still blows, then there is another problem...

Edit: Found this-

[img:721:362]http://www.swecheck.com.au/images/conte ... rkings.gif[/img]

[dju]

hi Tillydog, the fuses were supposed to be slow blow fuses but I will check what's marked on the end cap and compare with the chart you listed to verify.

thanks,
dj

[dju]

just to update the question about the fuse, it was labeled T125mA250V.
Still need to try and figure out why to much current is being used at power up.

dj

[Tillydog]

Sounds like you have the correct fuse, then :-/

Did anything odd happen when the fuse first blew? (Sparks? No speaker connected? Bent pin on tube?, etc..) Are the tubes in the correct sockets? (Sorry, but I have to ask!)

What test equipment do you have access to? What's your competence like? It's best not to go rooting around inside with the power on unless you know what you're doing - there are voltages in there that can potentially kill you, which and be present even when the amp is powered down and unplugged - especially with no tubes.

Unless someone comes up with known problems, *I* would attack it by trying to isolate the power transformer secondaries and see if it still blew fuses. If so, I think it would be a bad PT. If not, then there must be a fault in the circuit.

I'm not sure how many components are mounted on the PCB, though, which makes isolating different parts of the circuit more difficult.

Sorry - not much help...

[dju]

hi Tillydog,
there was nothing that stood out when the problem started, no sparks or smoke. just no sound. the tubes were located in the correct sockets.
I have a variac, DMM, an uncalibrated oscope that I still need to learn how to use and a signal generator which I need to learn how to use with the oscope.
My skills are lacking in the troubleshooting department but I have a strong enough background not to get electrocuted. I have my bleedoff resistor with alligator clips to drain the filter caps before working on it. I attach my ground DMM lead to a ground point on the chassis and I actually put my free hand behind my back when probeing around inside with the positive DMM lead. so I don't think I will get electrocuted but who knows, I have done stupid things in my past when I wasn't thinking.
how would you go about troubleshooting the power transformer? what types of things do you look for when testing it?

thanks for you replies,
dj

[Phil_S]

While it isn't full isolation, you can start by pulling all (two) tubes and seeing if it eats the fuse with no tubes. Then put in the 12AX7 only and see if that causes the problem. We know it blows with both tubes in sockets - don't go there but your might also try with only the power tube in it's socket and no preamp.

To really isolate the PT, you will need to disconnect it from the circuit. By pulling the tubes, you are disconnecting the filament secondary and the rectified B+ supply. You will need to disconnect the high voltage secondary before it enters the rectifier and lift the center tap (if it has one) to fully isolate the PT.

[Tillydog]

... how would you go about troubleshooting the power transformer? what types of things do you look for when testing it?

Have a good look for shorted wires, blackened or burnt components, etc. with the power off (check that nothing has dropped onto the circuit board - like a washer, or something).

Are there also fuses on you PCB? Are these OK?

Two approaches to testing the PT:

1) Label, disconnect (& insulate) all the secondaries from the power transformer and see if fuse still blows. If not, leave it powered up a while and see if the transformer gets hot.

2) Build a 'light bulb limiter' (or use your variac to run the amp at reduced voltage) and check voltages coming from transformer with no tubes installed.

Do you have a schematic?

I found this one, but it may not match your amp - proceed with caution.

If that schematic were accurate, would check AC voltages across each transformer secondary and DC voltage to ground at the +ve ends of the 100uF and 33uF capacitors in the bottom right (either end of R16) and the 10uF capacitors C7 & C17.

Install the 12AX7 and repeat (verify that filaments are lit);

Install EL84 & 12AX7 and repeat.

When EL84 is installed, also measure the DC voltage to ground at EL84 pins 7 & 9 (likely 300V+) & pin 8 (should be ~12V, but could be 300V+). The EL84 will need ~30 seconds to warm up.

It might be worth making a 'light bulb limiter' in any case (sticky at the top of the forum) - I would expect the light should go off completely with no tubes installed.

Incidentally, you should be able to test the 1N4007 diodes using the diode test on your multimeter - it should register ~600mV with the probes connected one way and 'over range' with them the other way. If it reads close to zero both ways, then the diode is U/S.

[Cwrikard]

Just wondering if you figured out a solution?

[Stevem]

You have a bad power supply filter!