Testing ST-70 Iron

Just scavenged the power and output xformers from a ST-70 carcass. I have a variac and a DMM, but no 'scope. How can I test them before I commit them to a new build?

Thanks, folks.

Mike

are these original built transformers? If yes, you may wish to consider using the more robust current versions, as used in the VTA ST70, but it all depends on what you're looking to get out of the ST70.

Bob Latino wrote:1. For the power transformer ... Connect the two BLACK wires to a 120 volt AC power source ..
  Measure the AC voltage across the two RED wires > 700 - 760 VAC
  Measure the AC volatge across the two GREEN wires > 6 - 7.5 volts AC
  Measure the AC voltage across the two BROWN wires > 6 - 7.5 volts AC
  Measure the AC voltage from the RED/BLACK wire to the RED/YELLOW wire > 55 to 60 volts AC
  Measure the AC voltage from the BROWN/YELLOW to the RED/YELLOW - 3 to 4 volts AC
  Measure the AC voltage from the GREEN/YELLOW to the RED/YELLOW - 3 to 4 volts AC

2. For the OUTPUT transformer ...
   The primary side wires BLUE, BLUE/WHITE, RED, GREEN and GREEN/WHITE should have no continuity to ANY OF THE secondary wires YELLOW, ORANGE, BROWN, BLACK
   On ethe PRIMARY side wires measuring from the RED wire to each of the other 4 wires should give a resistance of 20 ohms to maybe 150 ohms
   On the SECONDARY side measuring
     From BLACK to YELLOW - maybe 1 to 2 ohms
     From BLACK to ORANGE - maybe 1/2 ohm to 1 ohm
     From BLACK to BROWM - maybe .25 ohm to .7 ohm

Bob

Great! Thanks, Bob.

I've got the tubes4hifi 9-pin board kit with the cap upgrade and EH 12AU7's, a Tung-Sol rectifier and a brace of Tung Sol EL-34B's waiting to go into a new, shiny Dynakitparts chassis, etc. After I put this thing together and it runs, God willin' and the crick don't rise, I've got my eye on one of your ST-120's.

Thanks again for your help, sir.

Best regards,

Mike

correction to Bob's voltages - if you measure from RED to RED you are going to get 700-750vac (make sure you have a DMM that can handle 1000vac !!!)
you should have half that much from the RED/YELLOW to each RED lead (350-375vac)

tubes4hifi wrote:correction to Bob's voltages - if you measure from RED to RED you are going to get 700-750vac (make sure you have a DMM that can handle 1000vac !!!)
you should have half that much from the RED/YELLOW to each RED lead (350-375vac)

Thanks, Roy. Darn, my meter tops out at 700 VAC. I'll make sure not to measure all the way across the secondary . . .

Is this a 120/240 universal transformer? The lead colors seem different than the stock unit. Anybody know what 'P10 62' refers to? (Is 62 the year of manufacture?)

Thanks,

Mike

unless you have an unusual ST70 with a 240v transformer, they are all 115-120vac input and are marked PA060

I thought I counted a pair of extra leads. Wrong-o. Time to try and figure out which leads are which, given the fading & discoloration.

Thank you so much for this guide to test Dynaco power transformer. My Dynaco distort a little bit after a few minutes of listen. I checked the power transformer and everything were either on the spot or very close except these 2

Measure the AC voltage from the BROWN/YELLOW to the RED/YELLOW - 3 to 4 volts AC
Measure the AC voltage from the GREEN/YELLOW to the RED/YELLOW - 3 to 4 volts AC

I get 0.34 volts AC on both; does that mean that wiring is shot? Can I or should I supply 3v AC from an external source? Any help/ advice is much appreciated. Thank you

Sichum
are you sure that wasn't 3.4vac instead of 0.34vac ?
The brown yellow is the center tap of two brown wires which should read about 6.5vac between them.
The green yellow is the center tap of two green wires which should read abot 6.5vac between them.
The brown-yellow and green-yellow connect thru a 0.02uF cap to ground, so maybe that dual cap is bad if you get a bad reading there.

tubes4hifi wrote:Sichum
are you sure that wasn't 3.4vac instead of 0.34vac ?
The brown yellow is the center tap of two brown wires which should read about 6.5vac between them.
The green yellow is the center tap of two green wires which should read about 6.5vac between them.
The brown-yellow and green-yellow connect thru a 0.02uF cap to ground, so maybe that dual cap is bad if you get a bad reading there.

I'm pretty sure it was 0.34vac, I tried with 3 different volt meters. I will unsolder both of them from the 0.02uF cap and try it again when i get home. Thank you

Here's the result of the re-read with the brown/yellow and green/yellow not connected to the 0.02uF cap
- across the two RED wires > 752 VAC
- across the two GREEN wires > 6.66 volts AC
- across the two BROWN wires > 6.71 volts AC
- from the RED/BLACK wire to the RED/YELLOW wire > 53.5 volts AC
- from the BROWN/YELLOW to the RED/YELLOW - 0.66 volts AC
- from the GREEN/YELLOW to the RED/YELLOW - 0.71 volts AC