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Transfomer Testing?

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Posted on November 28, 2016 at 11:41:54
jk
Audiophile

Posts: 366
Joined: October 4, 1999



Hi all, Wondering if I could bounce something off you.

Background: VTL MB150 amp, intermittent distortion

Progress: Found someone had replaced high voltage fuse holder's spring with non stock spring, which over the years has corroded extensively. Replaced fuse and bias current restored.

At the request of Mr Manely (VTL) I replaced a few sand resistors with proper wire would resistors of same value.

Finding: Distortion noise gone but now just a hum. No music until I power off, then when I listen closely music comes through speaker as capacitors discharge.

Well as you can imagine I am hunting high and low for the cause of this issue. I did come across this burned transformer, as seen in the photo. Wow, the new WW resistor fits awfully nicely into the burn mark.

The truth is I think I have proof that the burn was there before. Gosh the high resistance fuse connection couldn't have caused the resistor to heat up right?

Well since I've looked everywhere else, I bought 2 of these transformer (thank God they are still available). Unfortunately the soldering job is HUGE to get access to the pins.

Today I had a thought. If the transformer was damaged by heat, it would affect conductivity right? I measured resistance pin by pin, and compared it with the brand new transformer. All matches perfectly.

Is this in your opinion enough for me to say the transformer is good and look elsewhere?

If not, has anyone ever disassembled a VTL amp? This is worse than a rubics cube.

Well I can't wait to get back to tube amps, but I've got to sort this one out first. Thank you for any input you can provide. I sure do appreciate it.

John

 

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RE: Transfomer Testing? , posted on November 28, 2016 at 12:21:35
pmaneri@samsontech.com
Manufacturer

Posts: 152
Location: Long Island,NY
Joined: May 22, 2015
jk- Seeing as the burn is on the outside of the transformer, I wouldn't worry about it. Looks like it just burned the coil insulation wrap. I would clean the burned area off with alcohol and carefully inspect the coil windings in that area just to be sure the windings didn't short together.
As far as "No music until I power off" goes, the amp may be biased way too low in current, causing it to be in cutoff mode. Once you turn the power off, if the bias voltage falls faster than the main B+, it may bias itself into the correct region momentarily. Set your bias for 70% Plate Dissipation and try it again.

 

RE: Transfomer Testing? , posted on November 28, 2016 at 13:21:09
jk
Audiophile

Posts: 366
Joined: October 4, 1999
Thank you I'll try that.

 

RE: Transfomer Testing? , posted on November 28, 2016 at 13:24:20
jk
Audiophile

Posts: 366
Joined: October 4, 1999
Dumb question but I don't know the answer
Can I put a 4ohm power resistor across the speaker terminals or should it be a speaker for bench testing ?

John

 

RE: Transfomer Testing? , posted on November 28, 2016 at 14:12:18
pmaneri@samsontech.com
Manufacturer

Posts: 152
Location: Long Island,NY
Joined: May 22, 2015
John - I always test first through a dummy load. Then, once the amp is up and stable, I test at 1 Watt for 30 min, then at 1/3 power for another 30 mins. Only then will I connect the amp to my main speakers.

 

RE: Transfomer Testing? , posted on November 28, 2016 at 14:18:07
jk
Audiophile

Posts: 366
Joined: October 4, 1999
Thanks again mate. Really appreciate your fast response, I can't wait to get these amps back in the system. John

 

RE: Transfomer Testing? , posted on November 28, 2016 at 14:19:28
Chip647
Audiophile

Posts: 2633
Location: The South
Joined: December 24, 2012
Measure some voltages. And make sure you put in the correct value of resistors. Your amp is in cut-off as the other poster mentioned, do you know how to adjust the bias?

 

RE: Transfomer Testing? , posted on November 28, 2016 at 17:45:38
fredtr
Audiophile

Posts: 1973
Location: Phoenix
Joined: January 4, 2005
You're getting very good advice. One additional comment regarding testing, you will need a high wattage load resistor if you are cranking it up during testing. And it is helpful to be able to hear what is going on. Pick up some cheap speakers at goodwill or yard sale, cheaper than high wattage resistors and better for understanding what is going on.

 

Those solder joints on the resistors and cap need to be redone....., posted on November 28, 2016 at 20:34:08
Cougar
Audiophile

Posts: 4562
Location: SoCal
Joined: June 25, 2001
They did not flow properly and will be problems down the road if not already. Just desolder and resolder but use flux this time. They will come out nice and shinny and will have better contact than they do now.

 

RE: Transfomer Testing? , posted on November 29, 2016 at 08:15:20
jk
Audiophile

Posts: 366
Joined: October 4, 1999
Chip, yes I can adjust and set bias thanks. Embarrassed to admit when I put non functioning mono block next to working one, just like you said I found a resistor of 10 ohms soldered where a 120 ohm should be. You called that thanks. I'll power her up on a few

 

RE: Those solder joints on the resistors and cap need to be redone....., posted on November 29, 2016 at 08:16:02
jk
Audiophile

Posts: 366
Joined: October 4, 1999
Thanks Cougar I sure will.

 

RE: Those solder joints on the resistors and cap need to be redone....., posted on November 29, 2016 at 11:05:53
I recommend using a solder wick or sucker to draw off the old blobs. Clean & use small amount flux for new solder job.

You need a small iron point for doing pads.

Good luck!

 

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