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    Troubleshooting ST-70 Hum - now blowing fuses! Bad output transformer?

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    bearwalker01


    Posts : 4
    Join date : 2016-11-04

    Troubleshooting ST-70 Hum - now blowing fuses!  Bad output transformer?   Empty Troubleshooting ST-70 Hum - now blowing fuses! Bad output transformer?

    Post by bearwalker01 Fri Dec 27, 2019 2:23 pm

    Hello all!

    I've been working to get my Dad's old ST-70 back up and running, learning a lot on the way.  I did the VTA board upgrade, added the pentode / triode modification, replaced the quad cap (twice now!), and added the upgraded input / output connectors.  But was getting a good 60 Hz (likely) hum from both channels.  

    I cleaned up the wiring a bit, and then the hum came from one side only.  But then I blew out a KT77 tube (my fault) and in the process took out the rectifier tube.  I replaced both, but then started blowing fuses.  

    Current status:  I turned on system this morning (after getting new 3A fuse) and blew out the fuse again.  I started looking for shorts in my wiring, and found that all 4 quad cap connections were zero ohms to ground.  I removed all connections to the quad cap, and measured resistance to ground, and saw the values gradually move up to >3 Mohms, just as I would expect (based on an older post on this forum from Bob).  

    I then checked each wire to ground, and found that there is an open circuit (ohms > meter maximum, >20Mohms) for all wires to quad cap except the left channel output transfer red wire (center tap), which reads 30 ohms.  I suspect this is the problem, as the other output transformer's red wire to ground reads open circuit (>max resistance on ohm meter).  

    I read the old post (from 2009) about checking the transformer resistances to each lead, and both primary and secondaries look within stated range.  But this was checking between transformer wires, nothing stated as to values with respect to ground.  So between taps on the transformer, things look OK, just that the resistance to ground on the red center tap looks low.  

    Am I describing a failed output transformer?  

    Steve
    avatar
    bearwalker01


    Posts : 4
    Join date : 2016-11-04

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    Post by bearwalker01 Fri Dec 27, 2019 2:34 pm

    Links to images:

    Overall:  https://photos.app.goo.gl/phVWE3Jt4ZRqwaLM9

    Troubleshooting ST-70 Hum - now blowing fuses!  Bad output transformer?   IMG-6446

    Close-up of Quad Cap wiring, without one of the two output transformer center tap wires:  https://photos.app.goo.gl/5qHEEHrxuDY5Wmx8A
    peterh
    peterh


    Posts : 1823
    Join date : 2012-12-25
    Location : gothenburg, sweden

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    Post by peterh Fri Dec 27, 2019 2:48 pm

    bearwalker01 wrote:Hello all!

    I've been working to get my Dad's old ST-70 back up and running, learning a lot on the way.  I did the VTA board upgrade, added the pentode / triode modification, replaced the quad cap (twice now!), and added the upgraded input / output connectors.  But was getting a good 60 Hz (likely) hum from both channels.  

    I cleaned up the wiring a bit, and then the hum came from one side only.  But then I blew out a KT77 tube (my fault) and in the process took out the rectifier tube.  I replaced both, but then started blowing fuses.  

    Current status:  I turned on system this morning (after getting new 3A fuse) and blew out the fuse again.  I started looking for shorts in my wiring, and found that all 4 quad cap connections were zero ohms to ground.  I removed all connections to the quad cap, and measured resistance to ground, and saw the values gradually move up to >3 Mohms, just as I would expect (based on an older post on this forum from Bob).  

    I then checked each wire to ground, and found that there is an open circuit (ohms > meter maximum, >20Mohms) for all wires to quad cap except the left channel output transfer red wire (center tap), which reads 30 ohms.  I suspect this is the problem, as the other output transformer's red wire to ground reads open circuit (>max resistance on ohm meter).  

    I read the old post (from 2009) about checking the transformer resistances to each lead, and both primary and secondaries look within stated range.  But this was checking between transformer wires, nothing stated as to values with respect to ground.  So between taps on the transformer, things look OK, just that the resistance to ground on the red center tap looks low.  

    Am I describing a failed output transformer?  

    Steve
    If the other secondaries on that transformer measures higher values then the center /red
    yes, you might have a bad transformer.
    Remove the transformer ( carefully, unsolder wires instead of clipping) , examine the
    red cable for damages,
    Open the endbells and check that no wires are clamped, and repeat the ohm between red and
    transformer iron.

    If still short, get a new transformer.
    avatar
    bearwalker01


    Posts : 4
    Join date : 2016-11-04

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    Post by bearwalker01 Tue Dec 31, 2019 8:09 pm

    Update:  solved the hum problem (silly me, put NFB wires into wrong terminals).  But now only have the left channel working.   I’ve done the usual to troubleshoot (swapped input cables, output tube swap, input tube swap, checked voltages, etc).  I’ve also rechecked the transformer resistances and get the expected low values.   I do have the rectifier diode mod installed, dropping Vac to ~200Vac from ~390Vac but output DC values are around 480Vdc. Voltages at quad cap are 440/430/430/380 Vdc, last one being B+.

    I also checked interval VTA board voltages, and got consistent results on both sides.  

    I checked speaker output values (Vac, low values).  The working left channel shows variable values with input from preamp (variable Vac values) but there is no Vac on silent right channel.  

    I still suspect bad output transformer, but would like ideas on how to confirm before I spend the $$$ on a replacement.  It is the original output transformer.
    Roy Mottram
    Roy Mottram
    Admin


    Posts : 1840
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    Post by Roy Mottram Wed Jan 01, 2020 8:59 pm

    both channels are identical, so look for any difference you can find. Compare voltages. With amp off, compare resistance readings. Make sure no bad solder connections.
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    rjpjnk


    Posts : 262
    Join date : 2018-07-18

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    Post by rjpjnk Thu Jan 02, 2020 10:37 am

    I would not buy a new transformer before confirming that the current one is defective with certainty. You can do this.

    As mentioned above, you have two identical channels: one that works and one that is broken. Find the differences.

    It does sound like symptoms as pointing to an OT issue. One way to be certain would be to unsolder all the transformer leads and measure the resistances between leads and from each lead to ground (Transformer housing). You mentioned above that you did some of this but I'm not sure if all the leads were disconnected. If they were and you measured anything other than open circuit between a lead and ground there is a short somewhere in the transformer. If this is the case do what peterh said above and open the transformer bells to look for a problem.

    You can also unsolder leads on both transformers and look for any differences in resistance as mentioned above.
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    bearwalker01


    Posts : 4
    Join date : 2016-11-04

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    Post by bearwalker01 Thu Jan 02, 2020 9:17 pm

    Resolved! Followed instructions provided and transformer had good resistance, no shorts. So then I went back to my soldering, touching up the inputs to the non-functioning side. I then plugged it back in, and behold, music from both channels! Looks like my soldering skills need to improve...

    Thanks to those who provided guidance here, it was quite helpful in figuring out what to exclude.

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