Blown Fuse Troubleshooting

[Ambugaton]

I know this is probably the most common problem encountered but I wanted to keep track of things I have ruled out and know where to go from here.

Fuse blows with or without power tubes

Light bulb limiter burns bright and continues getting brighter the longer the amp is on

The filter caps and resistors all measure correct. Resistance on terminals are 35K+ from each positive to negative

PT primary resistance between each lead is 3ohm.

I cannot get a reading from primary to chassis, my multimeter only goes to 2M so I don't  know if it is over that or not

PT Secondary HT leads to chassis is at 25ohm.

I was using the following guidance but having a brain fart and can't understand the difference between these two steps.

1. Using an ohmmeter, measure the resistance of the primary (usually Black - Black wires) and the resistance of both leads to the chassis. The primary should be under 1K ohms. If it is not, the transformer is dead.
2. Measure the resistance to the chassis from both ends of the primary winding. It should be more than 1M ohm

[Ambugaton]

Each heater lead to chassis = 19ohm

[Ritchie200]

Number one mentions both steps but only describes the first. Number two describes step two.  Maybe this will make more sense.

1. Using an ohmmeter, measure the resistance of the primary (usually Black - Black wires). The primary should be under 1K ohms. If it is not, the transformer is dead.
2. Measure the resistance to the chassis from both ends of the primary winding. It should be more than 1M ohm.

What amp is this? Pics? Any additional detail would be good.


Jim

[Ambugaton]

My mistake, amp info: Deluxe Lite with PT 041316 (330-0-330, HT 120ma), Mojo768sp OT

Used the attached schematic

[Ritchie200]

Is this a start up problem after your build?  Or is it a repair?


Jim

[Ambugaton]

The amp worked perfectly fine for a few months with daily use until yesterday. Was playing and just went dead, no loud pop or smoke. Just quiet.

[whoops]

Well you clearly have a short.

Check all the wires, any of them become loose?
Any loose component?
Any shorted pin on the tube sockets? measure all of them

Measure resistance in all secondaries and primaries of the Power Transformer and Output transformer

post the results here

[Ambugaton]

Well you clearly have a short.

Check all the wires, any of them become loose?
Any loose component?
Any shorted pin on the tube sockets? measure all of them

Measure resistance in all secondaries and primaries of the Power Transformer and Output transformer

post the results here

These readings are taken from both ends of the leads, not from each lead to chassis.

Resistance of PT Primaries (3.2ohm)

Resistance of PT Secondaries HT (2.8ohm)

Resistance of PT Secondaries Heaters (<2ohm)

SS rectifier so I did not test the 5V windings

Tube sockets show no signs of shorts, grid/screen resistors are of proper value. Went through and tugged on each connection and everything looks good.

[sluckey]

Resistance of PT Secondaries HT (2.8ohm)

That's the culprit. Should be several hundred ohms.

[Ambugaton]

These readings are of resistance from each lead to chassis.

Resistance of PT Primaries to chassis (Can't get a reading)

HT to chassis (26ohm)

Heaters to chassis (20ohm)

[Ambugaton]

Resistance of PT Secondaries HT (2.8ohm)

That's the culprit. Should be several hundred ohms.

I am assuming my next step would be to remove the HT from the ss rectifier circuit and measure them again?

[sluckey]

Resistance of PT Primaries to chassis (Can't get a reading)

That's good.

I am assuming my next step would be to remove the HT from the ss rectifier circuit and measure them again?

Yes. Cross your fingers. Maybe the new reading will be OK.

[Ambugaton]

Of course I get the same reading. So now I have a new road to go down and try to learn. What would cause this issue? This is called the windings being wide open? So something shorted internally of the PT and it now needs to be replaced... but would it just happen again? Did I choose the wrong PT/OT for this build?

[sluckey]

Before you get too sad... While the HT leads are disconnected, plug the amp into your lamp limiter and turn it on. Does the lamp still light up brightly?

[Ambugaton]

Before you get too sad... While the HT leads are disconnected, plug the amp into your lamp limiter and turn it on. Does the lamp still light up brightly?

Yes it does. No change

[sluckey]

OK, one more check... Measure the voltage across the HT leads while disconnected.

[Ambugaton]

I am getting nothing.

[sluckey]

OK. Time for a new PT. 

[Ambugaton]

OK. Time for a new PT. 

Out of desperation... maybe I tested wrong?

Clip from each HT lead to probe, still hooked up to limiter, meter on 600VAC. No reading.

If it is a bad PT, how can I keep this from happening again? Or narrow down the cause? Hmm.

[sluckey]

Hard to say. Sometimes stuff just fails. Even PTs. But it's usually because they've been played hard. Sometimes other components fail (shorted filter cap, shorted rectifier diodes, etc) and will take a PT out with them. Proper fusing can help minimize the chances of expensive component failure.

The PT you have should be a good match for a 2x6V6 amp.

[whoops]

Like Sluckey said the HT secondary is shot.
You can have that transformer rewind, try to find a rewinder locally.

I don't know in the US, but in Portugal the rewing costs around 35 dollars.


You should figure out what happened, because theres a big risk that if you put a new transformer in the amp that it will bet burned also. Try to be sure.
Rectifier, Filter Caps....

[Ambugaton]

I live in Alaska so I don't think that will be likely. Thanks for the suggestion though.

I think I have found the problem. A while back while I was doing start up, I was draining the cap charge via clipping from the chassis to one of the filter caps. I realize this is not the best method for doing so? But... forgot to remove the clips and turned the amp on. The 10K resistor between B and C terminal began to heat up and smoke. At the time...(purely ignorance) I decided to keep the resistor in the circuit after testing it and it was still measuring 10K. Looking back on this decision ...I feel dumb. Would this cause a problem that would lead to a failed PT? Can a resistor measure correct resistance but still be a problem?

[sluckey]

Would this cause a problem that would lead to a failed PT?

No. If your B+ was 400v then shorting that 10K to ground would only draw 40mA. That's plenty to fry a 5 watt resistor, but not enough to hurt that PT. Now if you were to put a ground clip on the first or second filter cap and leave it there very long you could take out the choke, rectifier diodes and PT PDQ.  It would all fail right then, not later.

[Ritchie200]

I just saw your sag post/thread.  I guess if there is an upside, you can now source a tranny to give you voltages closer to what you were wanting?


Jim

[Ambugaton]

Cup is half full! Just have to justify it with the big boss and find out what caused it initially

[Ritchie200]

Haha!  We have all been there!


Jim

[Ambugaton]

So good news is that while the Deluxe Lite is down, I have been forced to get more comfortable with an 18w that I previously built (initially stopped playing it because I didn't think el84s were for me). But man... after spending some time with it I am beginning to really enjoy it.

But... bad news is I still don't feel confident with just replacing the PT from the Deluxe and hoping for the best... and I am really trying to find information on the web as to how to approach this situation. If anyone has any recommendations please let me know.

Thanks in advance.

[sluckey]

There is no doubt that the PT is bad. Just replace it. Plug the amp into your lamp limiter and turn it on. The lamp limiter will protect burning up the new PT if there is still another problem inside the chassis.

[Ambugaton]

Rgr. Thanks

[Willabe]

Plug the amp into your lamp limiter and turn it on. The lamp limiter will protect burning up the new PT if there is still another problem inside the chassis.

Don't use larger than a 100w bulb, 60w would be better for this small amp.

[Ed_Chambley]

I have never smoked one, by have has a few fail.  It just happens sometimes.  I have also been successful at repairing some.  Take the covers off and see if you have a obvious burn.  Once I had on actually burn right at where the winding connected to one of the HT wires.  All I has to do was cut a small portion, about 1/4 inch and reconnect and isolate with clear enamel, fingernail polish will work too.

Still running fine 20 something years later.

[Ritchie200]

Ed, I've done the same with voice coils and horns back in the day.


Jim