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    Assistance with ST-120 problem?

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    skriefal

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    Location : Utah, USA

    Assistance with ST-120 problem?

    Post by skriefal on Sun May 05, 2013 2:52 pm

    The fuse blew on my ST-120. I replaced it and it blew again. After replacing the fuse again and swapping in a spare rectifier for testing (a 5U4GB) the amp is powering up, but there is no bias voltage (and of course no sound). I also tried a spare Weber WZ68 with the same results.

    I do have a TDR board and can hear it kicking in at 17 seconds or so, so that piece seems to be working. I've seen other threads that suggest that this is usually caused by a bad rectifier -- but that doesn't seem to be the case here. Any suggestions? Thanks!

    kevinmi

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    Re: Assistance with ST-120 problem?

    Post by kevinmi on Sun May 05, 2013 3:48 pm

    I have blown a rectifier tube and a power tube without blowing a fuse, so I think you have a bigger problem than a bad tube.

    skriefal

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    Re: Assistance with ST-120 problem?

    Post by skriefal on Sun May 05, 2013 4:17 pm

    That's what I'm guessing too, but I'm too much of an electronics novice to know where to start. My ST-120 was purchased pre-built, and other than dealing with two bad power tubes (which took out the 10-ohm resistors on two of the power tube sockets) there has been nothing else needed until now. Those 10-ohm resistors still test out fine, and probably wouldn't contribute to what I'm seeing now anyway.

    Bob Latino
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    Re: Assistance with ST-120 problem?

    Post by Bob Latino on Mon May 06, 2013 7:00 am

    Hi Scott,

    When you purchased your wired VTA ST-120 no tube set was supplied with the amp because you wanted to use your own tube set. You did mention that you had two bad power tubes which took out the 10 ohm bias resistors. I did send you out some new 10 ohm 2 watt resistors. What kind of tubes are in the amp now and were they new tubes or tubes that had been used before?

    Would you be willing to do some voltage checks inside the amp ? I can give you some areas to check .. We can do them without the rectifier in there so you don't blow a fuse ...

    Bob

    skriefal

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    Re: Assistance with ST-120 problem?

    Post by skriefal on Mon May 06, 2013 10:00 am

    Bob Latino wrote:Hi Scott,
    When you purchased your wired VTA ST-120 no tube set was supplied with the amp because you wanted to use your own tube set. You did mention that you had two bad power tubes which took out the 10 ohm bias resistors. I did send you out some new 10 ohm 2 watt resistors. What kind of tubes are in the amp now and were they new tubes or tubes that had been used before?

    Would you be willing to do some voltage checks inside the amp ? I can give you some areas to check .. We can do them without the rectifier in there so you don't blow a fuse ...

    Thanks Bob.

    After replacing the two 10-ohm resistors I swapped all of the tubes with spares -- including the rectifier, input, and drivers. Overkill perhaps -- but I had the tubes and decided to use them. The tubes are a quad of SED / Winged-C 6550Cs, a pair of Sylvania 12BH7s, an RCA 12AU7, and a Weber Copper Cap rectifier (WS1). All but the RCA 12AU7 were new. They were holding good bias and working well since installation (~2 months) until yesterday, when it appears that the Weber rectifier failed and perhaps took something else with it.

    Sure, I can check voltage in the amp.

    Bob Latino
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    Re: Assistance with ST-120 problem?

    Post by Bob Latino on Mon May 06, 2013 10:23 am

    Hi Scott,

    With rectifier OUT OF THE AMP but all other tubes in there > turn the amp on and wait for the TDR relay to kick on.

    Set you meter for the highest AC votage range. Now measure the AC voltage from pin #4 on the rectifier tube socket to chassis ground. Also measure the AC voltage from pin #6 to chassis ground. Let me know what you get?

    Bob

    skriefal

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    Join date : 2011-09-20
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    Re: Assistance with ST-120 problem?

    Post by skriefal on Mon May 06, 2013 9:15 pm

    Here's what I get with the 'ole Harbor Freight DMM:

    Pin 4 to chassis ground = 424 volts
    Pin 6 to chassis ground = 4 volts

    UPDATE: Seems that I was reading the wrong pins. See post # 16 below...


    Last edited by skriefal on Tue May 07, 2013 9:21 pm; edited 1 time in total

    tubes4hifi
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    Re: Assistance with ST-120 problem?

    Post by tubes4hifi on Mon May 06, 2013 10:08 pm

    you are either missing digits in your post or you are missing voltage !!
    both pins 4 and pins 6 should have between 410-425 vac

    skriefal

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    Re: Assistance with ST-120 problem?

    Post by skriefal on Mon May 06, 2013 10:30 pm

    I'm not missing digits. Smile

    I should mention also that my DMM reads 3 - 4 volts AC even when not connected to anything (it's an el-cheapo). So the amp really has 420 volts on pin 4 and 0 volts on pin 6. What could cause this?

    Bob Latino
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    Re: Assistance with ST-120 problem?

    Post by Bob Latino on Tue May 07, 2013 6:34 am

    Hi Scott,

    If you only get 424 volts AC at pin 4 but not pin 6 then > it could be a problem with the relay on the TDR board or a partial connection on one of the relay solder points on the TDR board.

    OK - Get your meter out again, turn the amp on WITHOUT the rectifier in there but with all other tubes. Check the AC voltage to chassis ground where each of the two RED wires from the power transformer connect to the TDR board. You should get your 424 volts or so at BOTH points. If you do then there is an issue on the TDR board. Bad relay, bad solder connection etc.

    Bob

    skriefal

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    Re: Assistance with ST-120 problem?

    Post by skriefal on Tue May 07, 2013 7:41 pm

    I get ~425 volts at both of the RED wire inputs to the TDR board. I guess that means that the power transformer is okay? I suppose the next step is to check the outputs from the TDR board. Are those the two other red wires that connect near the inputs?

    Bob Latino
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    Re: Assistance with ST-120 problem?

    Post by Bob Latino on Tue May 07, 2013 8:06 pm

    skriefal wrote:I get ~425 volts at both of the RED wire inputs to the TDR board. I guess that means that the power transformer is okay? I suppose the next step is to check the outputs from the TDR board. Are those the two other red wires that connect near the inputs?

    Yes - the power transformer is OK. The outputs from the TDR board to pins 4 and 6 on the rectifier tube socket should each carry 425 volts AC as measured to chassis ground AFTER the relay kicks on. Check the AC voltage on each wire that goes to pins 4 and 6 at the TDR board. If you still get no voltage at pin #4 on the rectifier tube socket then the relay is bad or has a bad solder connection on the rear of the board.

    Bob

    skriefal

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    Re: Assistance with ST-120 problem?

    Post by skriefal on Tue May 07, 2013 8:33 pm

    Color me confused. I used the socket diagram and pin numbers shown on this TDSL link to determine which pins to test:
    http://tdsl.duncanamps.com/show.php?des=5U4GB

    Looking at the bottom of the socket I see that the wires from the TDR board are connected to pin 4 and pin 2. There is nothing connected to pin 6. Needless to say, I get 425 volts on pin 4 and 0 volts on pin 6.

    Is that diagram incorrect?

    skriefal

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    Re: Assistance with ST-120 problem?

    Post by skriefal on Tue May 07, 2013 8:49 pm

    Here's a photo of the bottom of the rectifier socket, with what I believe to be pins 4 and 6 circled.


    Bob Latino
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    Re: Assistance with ST-120 problem?

    Post by Bob Latino on Tue May 07, 2013 9:02 pm

    Scott - The technician that wired your amp did the "diode mod" on the rectifier tube socket. The wires from the TDR go to the unused pins 5 and 7. Pin 7 is then connected to 6 through a diode and then pin 5 is connected to pin 4 through a diode. See if you get your 425 VAC at 5 to chassis ground and then 7 to chassis ground. If you DO then the relay is OK. Next try 4 to chassis ground and 6 to chassis ground. If you get no voltage this time at pins 4 and 6 to chassis ground then one of the diodes is bad.

    Bob

    skriefal

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    Re: Assistance with ST-120 problem?

    Post by skriefal on Tue May 07, 2013 9:17 pm

    Aha. So those are diodes.

    I do get 425 volts at both pins that are connected to the TDR (which apparently are pins 5 and 7 -- so I had the pin numbers reversed before). I get 246 volts at both pin 4 and pin 6.

    Sal

    Posts : 223
    Join date : 2009-02-05
    Location : Central New Jersey Dynaco-ST70.com

    Re: Assistance with ST-120 problem?

    Post by Sal on Wed May 08, 2013 2:42 pm

    Hi Scott,
    I am the technician who built your ST-120. Since your ST-120 is not blowing fuses anymore and with the 5U4 tube installed, can you measure the voltage from pin 8 (white wire is soldered to pin Cool of the rectifier tube socket to ground. You should measure around 450-500v B+ so be very careful when you use your meter. If you measure that voltage range, go to the quad capacitor and measure the voltage left most quad capacitor lug to ground and then the bottom lug to ground . With your ST-120 oriented like the picture shown below so you know what lugs I am talking about. Let us know what you measure.

    Regards,
    Sal


    tubes4hifi
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    Re: Assistance with ST-120 problem?

    Post by tubes4hifi on Wed May 08, 2013 4:47 pm

    nice work Sal Very Happy

    ramon68

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    Re: Assistance with ST-120 problem?

    Post by ramon68 on Wed May 08, 2013 6:30 pm

    If I decide to built a 120, I'll refer to your work pictured above as the template for a beautiful job.

    Bob Latino
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    Re: Assistance with ST-120 problem?

    Post by Bob Latino on Wed May 08, 2013 6:36 pm

    If you build the VTA ST-120 from a kit, you get included with the kit's assembly manual that color photo printed out on a piece of photo paper to show you where the wires should go ... After each step you just check what you did against the color photo. If you do make an error, you will see your error immediately ...

    Bob

    skriefal

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    Re: Assistance with ST-120 problem?

    Post by skriefal on Wed May 08, 2013 8:54 pm

    Sal wrote:Hi Scott,
    I am the technician who built your ST-120. Since your ST-120 is not blowing fuses anymore and with the 5U4 tube installed, can you measure the voltage from pin 8 (white wire is soldered to pin Cool of the rectifier tube socket to ground. You should measure around 450-500v B+ so be very careful when you use your meter. If you measure that voltage range, go to the quad capacitor and measure the voltage left most quad capacitor lug to ground and then the bottom lug to ground . With your ST-120 oriented like the picture shown below so you know what lugs I am talking about. Let us know what you measure.

    Thanks Sal!

    The first time I powered up the amp with the 5U4GB installed it blew another fuse. I had two more fuses, so swapped one of them in and tried again.

    This time it fired up and didn't blow the fuse. For a sanity check I re-measured pin 5 to ground and verified that I'm still seeing the same ~425 volts. Yup. So far so good. Then measured from pin 8 to gound. Nothing -- zero volts. From your statement above it would appear that this is incorrect. Because there was no voltage here I did not test the quad cap lugs.

    Sal

    Posts : 223
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    Location : Central New Jersey Dynaco-ST70.com

    Re: Assistance with ST-120 problem?

    Post by Sal on Wed May 08, 2013 9:03 pm

    Thanks for the compliment guys.

    Scott, I just sent you a PM with my cell phone number. Give me a call when you can.

    Sal

    skriefal

    Posts : 116
    Join date : 2011-09-20
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    Re: Assistance with ST-120 problem?

    Post by skriefal on Wed May 08, 2013 9:10 pm

    Just to verify that I'm testing the correct pin Smile...


    Sal

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    Re: Assistance with ST-120 problem?

    Post by Sal on Wed May 08, 2013 9:20 pm

    That's the correct pin.

    nmchiefsfan

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    Re: Assistance with ST-120 problem?

    Post by nmchiefsfan on Thu May 09, 2013 12:45 am

    I am pretty sure Pin 8 is the recifier output and is a DC voltage so make sure your meter set accordingly.

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