Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: mesa3077boogie on December 24, 2018, 12:36:23 am

Title: Fired amp with el34 in rectifier slot, now amp won't start up
Post by: mesa3077boogie on December 24, 2018, 12:36:23 am
Hey guys. I made a dumb mistake with my mesa stiletto ace combo that is causing it not turn on.

My question for you is where should I start on trying to determine what killed the amp? What are some practical tests I can do with a multimeter?

I fired it up with one el34 and the tube rectifier switched. The fuse blew. No smoke or smell but nothing powers and I'm worried its the power transformer.

I had the chassis out and noticed some burnt looking diodes and one somewhat burnt looking 3 watt resistor.

I have basic understanding of draining caps and safety, etc.

This was a dumb mistake due to me rushing to put things back together.
I can upload some pics or answer anything else to try and fix this.

Thanks,

Mesa3077boogie
Title: Re: Fired amp with el34 in rectifier slot, now amp won't start up
Post by: sluckey on December 24, 2018, 04:08:25 am
Measure all voltages on the PT.
Title: Re: Fired amp with el34 in rectifier slot, now amp won't start up
Post by: mesa3077boogie on December 24, 2018, 09:29:40 am
Quote from: sluckey on December 24, 2018, 04:08:25 am
Measure all voltages on the PT.

I measured 43.4 volts..? I measured directly after the fuse and switch where the wires go into the board. Shouldn't it be seeing my wall AC which is 123?
Title: Re: Fired amp with el34 in rectifier slot, now amp won't start up
Post by: mesa3077boogie on December 24, 2018, 02:11:49 pm
Hey guys I figured it out!

2 things.

Thing 1)
There was a broken connection from a high voltage resistor going to a set of 8 diodes. If I am reading my schematic correctly, this section is directly after the rectifier stage. I suspect that some high AC voltages that would have normally been converted to DC at the tube rectifier we're allowed through due to having the el34 in the tube rectifier slot.

Since I'm not a trained technician this is only a guess.

I was able to clean the char from the area and noticed that the resistor literally drew solder from the PC board upwards towards the resistor itself! I never seen this before. It must have gotten really hot!  I was able to alligator clip the area together for testing.

Thing 2)
As stated in the previous message I was still only measuring 43.3 volts @ the power transformer taps..?

I measured the same 43.3 volts directly after the mains input. This led me to believe that the problem was in the fuse housing and it was somehow eating up near 77 volts . I cut the shielding off the fuse housing and alligator clipped the connections together bypassing the fuse all together. I fired the amp up and sure as shit its back to normal with no issues!

I feel so much better now. The lesson I learned was to take your time and not try to rush through a project when dealing with a precious amp.
Title: Re: Fired amp with el34 in rectifier slot, now amp won't start up
Post by: tubenit on December 24, 2018, 03:58:34 pm
Please let us know what it reads after you have a new fuse and fuse holder installed.  I am curious how this turns out?

With respect, Tubenit
Title: Re: Fired amp with el34 in rectifier slot, now amp won't start up
Post by: mesa3077boogie on December 24, 2018, 04:00:48 pm
Quote from: tubenit on December 24, 2018, 03:58:34 pm
Please let us know what it reads after you have a new fuse and fuse holder installed.  I am curious how this turns out?

With respect, Tubenit

Sure thing, will do! Electronics shop opens day after Christmas.