The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Dynaco VTA tube amp kits, all Tubes4hifi.com products and all Dynakitparts.com products


    St-70 dead channel

    Share

    quietglow

    Posts : 10
    Join date : 2012-09-03

    St-70 dead channel

    Post by quietglow on Mon Sep 03, 2012 11:10 am

    Hello all,

    I'm a long time st-70 owner and first time poster, and I'm looking for some help in diagnosing a problem(s) with my amp.

    Aprox. 6 years ago, I replaced a bad driver board with one of those sold by Triode Electronics. I also opted to run it in triode mode. All was well for a year or so, and then I put the amp in storage and didn't use it until last year. When I took it out of storage and put it back into use, I noticed that it didn't bias correctly -- I couldn't get it *lower* than about 1.95V when it was warmed up. Also, it would eat a rectifier tube every few months or so (which I would know had happened because it would start to blow fuses). Yesterday, I replaced a blown rectifier and after maybe an hour of use, the left channel of the amp went dead.

    I've done basic troubleshooting (switching tubes, cables, etc) and that yielded nothing. I opened it up and tested voltages at 3 & 4 of the EL34s to check for a possible bad output transformer, and they were in the 530 - 540 range (which is high from what I am reading).

    Okay, that's the extent of my knowledge. Undoubtedly, I should have explored the bias problem and the blowing rectifiers before something failed. Any help would be appreciated!

    Bob Latino
    Admin

    Posts : 2377
    Join date : 2008-11-26
    Location : Massachusetts

    Re: St-70 dead channel

    Post by Bob Latino on Mon Sep 03, 2012 1:52 pm

    Hi and welcome to the Dynaco Tube Audio Forum.

    It might be helpful if you post a photo of the inside wiring so that we can see if anything has been replaced on your ST-70 amp. There is a "sticky" post above which explains how to post a photo.

    If you amp is dead stock you probably should ...

    1. Replace the selenium rectifier with a diode. Unsolder the two wire leads, remove the screw and pull the selenium rectifier and throw it away (Selenium is a poison!). Replace the diode with a 2 lug terminal strip and run a diode across the two lug strip. Make sure the banded end of the diode is connected to the red/black wire from the power transformers and the other wire that was on the diode goes to lug #4 on the 7 lug terminal strip.

    2. Replace the two capacitors and the two resistors on lugs 1 - 4 on the 7 lug terminal strip. The two caps are 50 uF @ 100 volts. You can replace them with either 50 or 100 uF @ 100 volts. Make sure that you have the positive and negative ends of those two caps on the correct lug. The two resistors on there now are 10K resistors. You should probably replace them with two 5K resistors (@ 1 watt) because a diode has a higher "forward voltage" than the old selenium rectifier.

    Doing 1 and 2 above will give you (except for the two bias pots) a brand new bias system.

    3. Replace the quad cap with an 80, 40, 30, 20 uF cap from Dynakitparts. He sells two types. The 550 volt is a little more expensive but does have greater voltage handling capacity. On the ST-70 you can use either cap. For correct installation the 80 section should face the back of the amp.

    Bob

    Sal

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

    Re: St-70 dead channel

    Post by Sal on Mon Sep 03, 2012 11:27 pm

    Check the 15.6 ohm resistor going from pins 1 and 8 of the EL34 tube to ground. It may be open. Also I second what Bob mentioned above. Your selenium rectifier is probably going bad and you don't have enough negative voltage to bias the tubes.

    Sal

    quietglow

    Posts : 10
    Join date : 2012-09-03

    Re: St-70 dead channel

    Post by quietglow on Tue Sep 04, 2012 9:21 am

    Thank you both! I'll upload a photo this afternoon when I am working on it.

    I replaced the selenium rectifier when I did the driver board upgrade. I haven't switched the caps in the bias system though.

    So could something with the wonky bias be causing the dead channel? Of course what I'm really worried about is that the output transformer is bad on that side.

    Sal

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

    Re: St-70 dead channel

    Post by Sal on Tue Sep 04, 2012 9:36 am

    The good thing is you have a good working channel so you can compare measurements between the left and right sides. Check the 15.6 ohm resistor, if that resistor opens, there will be no current flowing through the tubes and the channel will be dead.

    Sal

    quietglow

    Posts : 10
    Join date : 2012-09-03

    Re: St-70 dead channel

    Post by quietglow on Tue Sep 04, 2012 7:45 pm

    I was sure hoping the resistor was bad, but I just checked and it's testing right at its rated resistance. Is there anything else which can fail besides the transformer which would totally shut the channel down?

    Joe

    Bob Latino
    Admin

    Posts : 2377
    Join date : 2008-11-26
    Location : Massachusetts

    Re: St-70 dead channel

    Post by Bob Latino on Tue Sep 04, 2012 8:19 pm

    Hi Joe,

    This is maybe a pretty obvious question - BUT - have you ruled out the driver tubes as a source of your dead left channel? Maybe tried substituting new tubes for the center and left driver tube? You should rule out a tube problem before checking anything else.

    Bob

    quietglow

    Posts : 10
    Join date : 2012-09-03

    Re: St-70 dead channel

    Post by quietglow on Tue Sep 04, 2012 8:48 pm

    Yeah, I switched the left set in for the right -- for both the output and driver tubes. Still dead, unfortunately.

    Sal

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

    Re: St-70 dead channel

    Post by Sal on Tue Sep 04, 2012 8:55 pm

    OK, your next step to rule out the transformer is to set you meter to measure at least 500 volts DC and measure from chassis ground to pin 3 and from ground to pin 4 of the output tubes. Let us know what you measure. Be careful, there is lethal voltages there.

    Did you swap the middle tube on the printed circuit board with the tube in the channel that works?

    Sal

    Bob Latino
    Admin

    Posts : 2377
    Join date : 2008-11-26
    Location : Massachusetts

    Re: St-70 dead channel

    Post by Bob Latino on Tue Sep 04, 2012 9:04 pm

    You should do a voltage check on the amp - especially the driver board. If you had a VTA driver board I could give you the voltage readings at various points on the board. If you compare the voltage readings at two similar points for each channel and you find a large difference from one channel to the other, you probably have found the problem. Personally, I don't believe that Triode has voltage check readings for their board but you could try Emailing them and asking them for a schematic of the board with the voltage readings at various points on the board.

    Bob

    quietglow

    Posts : 10
    Join date : 2012-09-03

    Re: St-70 dead channel

    Post by quietglow on Tue Sep 04, 2012 9:23 pm

    Huh, I just realized that the only tube I can't swap out is the middle 12AU7 -- the triode board had EF86s on either side and a 12AU7 in the middle. I just looked at the tube, and it's an RCA made in the US. This, I believe, means it's totally ancient. I don't have a spare 12AU7 on hand to swap in. Can I use one of the EF86s just to test?

    Sal, voltages at 3/4 were 530-540.

    Sal

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

    Re: St-70 dead channel

    Post by Sal on Tue Sep 04, 2012 9:31 pm

    Nope the EF86 tube is totally different than a 12AU7. If you have a 12AT7, 12AX7, 12BH7, 5963 or a 5751 tube and many other dual triodes with the same pinout, you can try them out in a pinch.

    As far as your voltage measurements, looks like your audio transformer primary side is good. The screen and the plate of your output tubes is getting voltage from the transformer. Disconnect your speakers and measure from ground to the other speaker taps, compare them with the other audio transformer. I am not familiar with the Triode board to give you assistance on that.

    Sal

    quietglow

    Posts : 10
    Join date : 2012-09-03

    Re: St-70 dead channel

    Post by quietglow on Tue Sep 04, 2012 9:50 pm

    Okay, I'm using the wonders of Amazon prime to get a 12AU7 here tomorrow to swap in and rule that out.

    The Sony "receiver" I'm using as an amp while trying to get my ST-70 running again is exceptionally good motivation to fix my amp!

    Joe

    anbitet66

    Posts : 131
    Join date : 2009-12-23
    Location : Valley Stream, NY

    Re: St-70 dead channel

    Post by anbitet66 on Tue Sep 04, 2012 10:25 pm

    You should download the schematic for the board. It is basically the same as the original 7199 board just using more commonly available tubes. I have the 6AU6 version. The voltages on the driver tubes will be different for each board (6AU6, EF86, 7199) but they should be similar from side to side on each board. Take readings from each EF86 and both sides of the 12AU7 and compare them. Post the results.

    Tony

    quietglow

    Posts : 10
    Join date : 2012-09-03

    Re: St-70 dead channel

    Post by quietglow on Thu Sep 06, 2012 7:57 pm

    My 12AU7 came in yesterday but unfortunately swapping it in did nothing -- I've now switched out all tubes. So I emailed Triode and asked for a voltage check list which they sent. Below are the voltages they specify followed by my measurements. There is clearly something not right, but I have no idea what. My measurements done with no tubes in the amp besides the rectifier (as directed by the Triode spec sheet)

    Thanks for any ideas!

    Joe

    Triode's Spec:

    EF86
    1 Screen 55 - ­80V
    2 Shield 0V
    3 Cathode 1.2 –­ 1.4V
    4 Filament N/A
    5 Filament N/A
    6 Plate (Anode) 80 –­ 110V
    7 Shield 0V
    8 Suppressor 0V
    9 Grid 0V

    12AU7
    1 Plate (Anode) #1 235 – 265V
    2 Grid #1 80 ­ 110V
    3 Cathode #1 90 ­ 120V
    4 Filament N/A
    5 Filament N/A
    6 Plate (Anode) #2 235 ­– 265V
    7 Grid #2 80 ­ 110V
    8 Cathode #2 90 ­– 120V
    9 Filament CT N/A

    My measurements

    Right EF86
    1. 159
    2. 0
    3. 1
    4. 0
    5. 0
    6. 331
    7. 0
    8. 0
    9. 0

    Left EF86 (the dead channel)
    1. 0
    2. 0
    3. 1
    4. 0
    5. 0
    6. 335
    7. 0
    8. 0
    9. 0

    12AU7
    1. 471
    2. 336
    3. 0
    4. 0
    5. 0
    6. 473
    7. 336
    8. 0
    9. 0

    quietglow

    Posts : 10
    Join date : 2012-09-03

    Re: St-70 dead channel

    Post by quietglow on Fri Sep 07, 2012 9:24 am

    Thinking about this more and looking at schematics, I see two problems:

    1. The voltages are way high. Is this a bias problem? Bad power transformer?
    2. There is no voltage at EF86's grid on the bad channel. Is this enough to kill the whole channel?

    I didn't have time to diagnose #2 further, but maybe the grid coupling capacitor went bad?

    sailor

    Posts : 269
    Join date : 2011-04-04

    Re: St-70 dead channel

    Post by sailor on Fri Sep 07, 2012 12:28 pm

    I found this procedure which I use for testing a Dynaco 70 output transformers.

    http://www.curcioaudio.com/Stereo%2070%20Restore%20Addendumsl-R0.pdf

    Look down the first page and find section B

    quietglow

    Posts : 10
    Join date : 2012-09-03

    Re: St-70 dead channel

    Post by quietglow on Sat Sep 08, 2012 5:10 pm

    So I solved the dead channel issue: it was an open screen bias resistor. Now to order parts to replace the bias circuit (which is still way high).

    quietglow

    Posts : 10
    Join date : 2012-09-03

    Re: St-70 dead channel

    Post by quietglow on Sat Sep 08, 2012 11:36 pm

    Fry's to the rescue for bias caps and resistors. Now biased to a much more reasonable 1.56v. Thanks to everyone for the help.

    Sponsored content

    Re: St-70 dead channel

    Post by Sponsored content Today at 6:14 am


      Current date/time is Wed Sep 28, 2016 6:14 am