Well I got a little sloppy doing late night work and it looks like its going to cost me. I just built and installed the vta board in an original st70, not from the kit but using my own parts. Failed to notice how many leads stuck up beyond the cutout in the original chassis, and gave it a smaller spacer when installing. Well quite a few leads were touching the chassis and I'm probably lucky I didn't get shocked. First power on released quite a bit of smoke from the power transformer, I think it was the insulation melting, powered off very quickly and realized what had happened. It also fried r39, haven't tested everything else yet and id like to avoid that if it's the tranny. replaced the resistor, spaced everything out properly reflowed some solder and fired it back up. Biases good everything seems good temperature wise, no real hum yet but there is an endless and erratic crackling through the test speakers. No flash over in rectifier or any tubes all seem to work well. initial burn up was with a different set of tubes as well, changing them makes no difference. Haven't swapped the 12au7a's yet. How do I test if the transformer has failed before I rebuild and test every component looking for a fault?
+3
Bob Latino
peterh
evangerardo
7 posters
Think I toasted my Power Transformer, how do I test it?
evangerardo- Posts : 12
Join date : 2023-05-12
- Post n°1
Think I toasted my Power Transformer, how do I test it?
by evangerardo Thu Jun 01, 2023 5:43 pm
peterh- Posts : 1833
Join date : 2012-12-25
Location : gothenburg, sweden
- Post n°2
Re: Think I toasted my Power Transformer, how do I test it?
by peterh Fri Jun 02, 2023 3:09 am
The original PA-060 won't stand up to the additional load anyway, get a new one from tubes4hifi.com
that has more capacity . It's the filament that needs beefing up.
that has more capacity . It's the filament that needs beefing up.
Bob Latino- Admin
- Posts : 3263
Join date : 2008-11-26
Location : Massachusetts
- Post n°3
Re: Think I toasted my Power Transformer, how do I test it?
by Bob Latino Fri Jun 02, 2023 10:03 am
Look at the chart below and see if your AC and DC measurements are all OK? They may be a little high due to voltage differences around the USA. Note > with your VTA driver board the voltages on pins 1 and 8 of all the output tubes should be .40 VDC instead the 1.56 VDC as shown in the photo.
Bob
Bob
evangerardo- Posts : 12
Join date : 2023-05-12
- Post n°4
Re: Think I toasted my Power Transformer, how do I test it?
by evangerardo Fri Jun 02, 2023 2:55 pm
Thank you for your quick response Bob. All measurements taken with power on correct?
Bob Latino- Admin
- Posts : 3263
Join date : 2008-11-26
Location : Massachusetts
- Post n°5
Re: Think I toasted my Power Transformer, how do I test it?
by Bob Latino Fri Jun 02, 2023 3:11 pm
evangerardo wrote:Thank you for your quick response Bob. All measurements taken with power on correct?
Yes - with the power on and all tubes installed.
Bob
Dan W- Admin
- Posts : 16
Join date : 2021-12-18
Location : Owner - Tubes4Hifi
- Post n°6
Re: Think I toasted my Power Transformer, how do I test it?
by Dan W Sat Jun 03, 2023 11:08 am
evangerardo wrote:Well I got a little sloppy doing late night work and it looks like its going to cost me. I just built and installed the vta board in an original st70, not from the kit but using my own parts. Failed to notice how many leads stuck up beyond the cutout in the original chassis, and gave it a smaller spacer when installing. Well quite a few leads were touching the chassis and I'm probably lucky I didn't get shocked. First power on released quite a bit of smoke from the power transformer, I think it was the insulation melting, powered off very quickly and realized what had happened. It also fried r39, haven't tested everything else yet and id like to avoid that if it's the tranny. replaced the resistor, spaced everything out properly reflowed some solder and fired it back up. Biases good everything seems good temperature wise, no real hum yet but there is an endless and erratic crackling through the test speakers. No flash over in rectifier or any tubes all seem to work well. initial burn up was with a different set of tubes as well, changing them makes no difference. Haven't swapped the 12au7a's yet. How do I test if the transformer has failed before I rebuild and test every component looking for a fault?
If you end up needing a new power transformer for you ST-70, I have them in stock and ready to go.
itrfguy- Posts : 22
Join date : 2020-10-26
Location : Lehigh Acres, FL
- Post n°7
Re: Think I toasted my Power Transformer, how do I test it?
by itrfguy Sat Jun 10, 2023 10:20 am
I had a crackling sound in my transformer and speakers also. It was a bad rectifier (yellow jacket SSR) and has now been replaced. No more crackling.
Njrob likes this post
Njrob- Posts : 50
Join date : 2022-05-14
Location : Nj
Hi Bob!, Do you have the voltage chart for the vta120 transformers? Thanks!
defec- Posts : 9
Join date : 2010-12-19
- Post n°9
Re: Think I toasted my Power Transformer, how do I test it?
by defec Tue Jun 13, 2023 1:00 am
Ouch, we all feel your pain. I smoked a Mullard GZ34 by having a bad solder joint on one of the rectifier pins. Not near your pain. Please measure the AC coming from the transformer outputs like Bob posted with the rectifier pulled at pins 4 and 6 to see if you get around 360vac. I'd also measure at a power tube socket's pins 2 and 7 to see if you get 6.3ac. it will probably be 6.7 at modern wall voltage.. I'm hopping that you smoked a resistor or two and not the mains transformer.
Bob Latino- Admin
- Posts : 3263
Join date : 2008-11-26
Location : Massachusetts
- Post n°10
Re: Think I toasted my Power Transformer, how do I test it?
by Bob Latino Tue Jun 13, 2023 9:23 am
Below are the VTA ST-120's power transformer normal AC voltages .. NOTE - The VTA ST-70's voltages are the same except the AC voltage across the two RED wires should be about 720 to 750 VAC.
Bob
Bob
Njrob likes this post
evangerardo- Posts : 12
Join date : 2023-05-12
- Post n°11
Re: Think I toasted my Power Transformer, how do I test it?
by evangerardo Wed Oct 04, 2023 2:13 am
Thank you all for the help, especially to Bob for the voltages. It was a combination of a few resistors and the transistorized voltage regulators, got it back together and listening to it right now. Dead quiet no hum hiss or crackle, thanks all!!!
I've been listening mostly in triode mode, the soundstage feels much flatter than I expected, I can't get much in the way of high highs even with the treble maxed out compared to the original circuit or other amplifiers in my system. Using Altec flamenco's and rca clear tops, tried a fisher and Kenwood pre, going to test more. any suggestions?
I've been listening mostly in triode mode, the soundstage feels much flatter than I expected, I can't get much in the way of high highs even with the treble maxed out compared to the original circuit or other amplifiers in my system. Using Altec flamenco's and rca clear tops, tried a fisher and Kenwood pre, going to test more. any suggestions?
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