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    Very low bias and hum on Dynakit ST-70

    lagwil
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    Post by lagwil on Sun Dec 01, 2019 2:09 pm

    Say the happiness that lasts forever. Just when I was so pleased with my set up, the amp gave up on me...

    Last night, after many hours of use, the amp began to hum, not through the speakers, but the thing itself. Today I started troubleshooting by measuring Bias. It measures only 0.45 volts instead of 1.56! When I adjust on the pots, the value does not increase above 1 volt. The hum varies with the adjustment. Tried to replace the rectifier tube but no difference.

    Any help on troubleshooting is very much appreciated!
    Bob Latino
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    Post by Bob Latino on Sun Dec 01, 2019 2:17 pm

    lagwil wrote:Say the happiness that lasts forever. Just when I was so pleased with my set up, the amp gave up on me...

    Last night, after many hours of use, the amp began to hum, not through the speakers, but the thing itself. Today I started troubleshooting by measuring Bias. It measures only 0.45 volts instead of 1.56! When I adjust on the pots, the value does not increase above 1 volt. The hum varies with the adjustment. Tried to replace the rectifier tube but no difference.

    Any help on troubleshooting is very much appreciated!

    If this is an original Dynakit from the 1960's ..

    1. Try replacing the quad cap. On older Dynaco amps, these caps start to leak. When the do leak, the amp won't bias properly.

    2. If you still have the selenium rectifier for the bias system, replace it with a diode.

    3. Replace the two 50 uF caps on the 7 lug terminal strip.

    Bob
    lagwil
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    Post by lagwil on Sun Dec 01, 2019 2:35 pm

    Bob Latino wrote:
    lagwil wrote:Say the happiness that lasts forever. Just when I was so pleased with my set up, the amp gave up on me...

    Last night, after many hours of use, the amp began to hum, not through the speakers, but the thing itself. Today I started troubleshooting by measuring Bias. It measures only 0.45 volts instead of 1.56! When I adjust on the pots, the value does not increase above 1 volt. The hum varies with the adjustment. Tried to replace the rectifier tube but no difference.

    Any help on troubleshooting is very much appreciated!

    If this is an original Dynakit from the 1960's ..

    1. Try replacing the quad cap. On older Dynaco amps, these caps start to leak. When the do leak, the amp won't bias properly.

    2. If you still have the selenium rectifier for the bias system, replace it with a diode.

    3. Replace the two 50 uF caps on the 7 lug terminal strip.

    Bob

    Thanks Bob but all of the above is already done. Replaced the selenium rectifier and caps three month ago. The quad cap is about one year old.

    /Wilhelm
    Bob Latino
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    Post by Bob Latino on Sun Dec 01, 2019 2:44 pm

    Maybe a bad choke ? That choke should measure maybe 55 to 70 ohms across the two terminals.

    Bob
    lagwil
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    Post by lagwil on Sun Dec 01, 2019 4:39 pm

    Bob Latino wrote:Maybe a bad choke ? That choke should measure maybe 55 to 70 ohms across the two terminals.

    Bob

    Ok, do I understand correctly that the choke is connected to terminal 1 and 2 on the quad cap and that I can measure resistance there with the amp turned off?

    Sorry for my basic questions but my knowledge in electronics are limited... Very Happy

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    Bob Latino
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    Post by Bob Latino on Sun Dec 01, 2019 5:03 pm

    Yes - because the quad cap terminals have a very high resistance, you can just measure across those two terminals with the amp OFF and still be fairly close to the resistance of the choke.

    Bob
    MechEngVic
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    Post by MechEngVic on Sun Dec 01, 2019 5:51 pm

    Any luck? Let us know what happens, we can all learn form your issue. And don't worry, this is how you will slowly but surely take command of of that amp.
    peterh
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    Post by peterh on Mon Dec 02, 2019 11:23 am

    lagwil wrote:Say the happiness that lasts forever. Just when I was so pleased with my set up, the amp gave up on me...

    Last night, after many hours of use, the amp began to hum, not through the speakers, but the thing itself. Today I started troubleshooting by measuring Bias. It measures only 0.45 volts instead of 1.56! When I adjust on the pots, the value does not increase above 1 volt. The hum varies with the adjustment. Tried to replace the rectifier tube but no difference.

    Any help on troubleshooting is very much appreciated!
    I'd start with examining the DC voltages as per the manual.
    lagwil
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    Post by lagwil on Mon Dec 02, 2019 2:28 pm

    Bob Latino wrote:Maybe a bad choke ? That choke should measure maybe 55 to 70 ohms across the two terminals.

    Bob

    Very low bias and hum on Dynakit ST-70 20191211


    It measures 61.8 Ohm. I also measured the 15.6 ohms resistor and they measures 15.7 wich I guess is good enough?
    MechEngVic
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    Post by MechEngVic on Mon Dec 02, 2019 4:50 pm

    lagwil wrote:
    Bob Latino wrote:Maybe a bad choke ? That choke should measure maybe 55 to 70 ohms across the two terminals.

    Bob

    Very low bias and hum on Dynakit ST-70 20191211


    It measures 61.8 Ohm. I also measured the 15.6 ohms resistor and they measures 15.7 wich I guess is good enough?

    Start following that low bias voltage backwards towards its source, you might have a bad component/connection back in the chain. I don't see how the buzz is explained by this but it does need to be resolved. I know it's probably the first thing you checked but you haven't mentioned them: Have you tried swapping around your tubes? A bad tube can cause buzz and skew voltages.
    lagwil
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    Post by lagwil on Mon Dec 02, 2019 5:10 pm

    MechEngVic wrote:
    lagwil wrote:
    Bob Latino wrote:Maybe a bad choke ? That choke should measure maybe 55 to 70 ohms across the two terminals.

    Bob

    Very low bias and hum on Dynakit ST-70 20191211


    It measures 61.8 Ohm. I also measured the 15.6 ohms resistor and they measures 15.7 wich I guess is good enough?

    Start following that low bias voltage backwards towards its source, you might have a bad component/connection back in the chain. I don't see how the buzz is explained by this but it does need to be resolved. I know it's probably the first thing you checked but you haven't mentioned them: Have you tried swapping around your tubes? A bad tube can cause buzz and skew voltages.

    Actually I havn't since I thought low voltage in both channels indicated something else. Also they are only eight months old. Is it possible to take all tubes out and still measure bias?
    Bob Latino
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    Post by Bob Latino on Mon Dec 02, 2019 5:25 pm

    1. Take out the tubes and measure the AC voltage to chassis ground from pin 4 and then pin 6 of the rectifier tube socket. See if you get about 350 to 375 AC at both measuring points. If you don't then the transformer is bad.

    2. Also measure the AC voltage to chassis ground where the RED/BLACK wire connects to the diode that replaced the selenium rectifier. You should get 50 to 60 volts AC to chassis ground. If you don't then the transformer is bad.

    Bob
    PeterCapo
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    Post by PeterCapo on Tue Dec 03, 2019 3:46 pm

    There's a voltage chart in the manual appearing on the same page as the power supply schematic https://www.dynakitparts.com/wp-content/uploads/Dyna-ST70.pdf  If you can safely take readings at the test points indicated in the voltage chart, it could help to point in the direction of the problem source.  Note that it also gives reference voltage values for the quad cap, the selenium (bias) rectifier and the PCB.  It would also be good to take a reading of your AC mains (wall socket), too, when taking the other readings.

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    mijohn

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    Post by mijohn on Tue Dec 03, 2019 11:17 pm

    lagwil wrote:Last night, after many hours of use, the amp began to hum, not through the speakers, but the thing itself. Today I started troubleshooting by measuring Bias. It measures only 0.45 volts instead of 1.56! When I adjust on the pots, the value does not increase above 1 volt. The hum varies with the adjustment. Tried to replace the rectifier tube but no difference.

    Any help on troubleshooting is very much appreciated!
    Just thought I would remind people of the OP's original description of the problem, which seems to me that he is describing a mechanical issue rather than an electronic one. Is the variable hum he hears when he adjusts the bias heard through the speakers or the amp itself? The mechanical hum and the bias problem could be separate issues.
     
    Mechanical hum has been discussed a number of times on this forum concerning the ST-70 and SP-14 amps. See links below:

    http://dynacotubeaudio.forumotion.com/t3524-mechanical-hum-power-transformer?highlight=mechanical+hum

    http://dynacotubeaudio.forumotion.com/search?mode=searchbox&search_keywords=mechanical+hum&show_results=topics
    PeterCapo
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    Post by PeterCapo on Wed Dec 04, 2019 2:14 am

    Some mechanical humming from power transformers is not unusual even under normal circumstances and results from flowing current. So, electricity and mechanical humming are definitely associated. Sounds like he described the mechanical hum as varying with the setting of his bias pots. The varying mechanical hum sounds like a symptom of another problem, probably whatever is causing his biasing difficulty.

    If the amplifier was working fine for some time and then this problem suddenly came up, it means some part or parts, including possibly tubes, in the amp are failing. Questions:

    - were there any changes to the amp shortly preceding the onset of the current symptoms?
    - is a 3A fuse in the fuse holder, and not more than 3A?

    If the problem is in one channel only, I'd try swapping the tubes from one channel to the other and see if the problem follows the tubes.

    I'd also still refer back to Post n°13.
    lagwil
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    Post by lagwil on Wed Dec 04, 2019 7:02 am

    Bob Latino wrote:1. Take out the tubes and measure the AC voltage to chassis ground from pin 4 and then pin 6 of the rectifier tube socket. See if you get about 350 to 375 AC at both measuring points. If you don't then the transformer is bad.

    2. Also measure the AC voltage to chassis ground where the RED/BLACK wire connects to the diode that replaced the selenium rectifier. You should get 50 to 60 volts AC to chassis ground. If you don't then the transformer is bad.

    Bob

    Hi Bob and many thanks for all great help!

    I have now made more measurements on my Dynakit. I live in Sweden and the socket should give 220–240 V 50 Hz. I measured 235V. The amp is stamped 220V.

    On pin 4 and 6 on the Rectifier tube I get 386V
    Before the diod I get 57V

    /Wilhelm
    peterh
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    Post by peterh on Wed Dec 04, 2019 9:49 am

    lagwil wrote:
    Bob Latino wrote:1. Take out the tubes and measure the AC voltage to chassis ground from pin 4 and then pin 6 of the rectifier tube socket. See if you get about 350 to 375 AC at both measuring points. If you don't then the transformer is bad.

    2. Also measure the AC voltage to chassis ground where the RED/BLACK wire connects to the diode that replaced the selenium rectifier. You should get 50 to 60 volts AC to chassis ground. If you don't then the transformer is bad.

    Bob

    Hi Bob and many thanks for all great help!

    I have now made more measurements on my Dynakit. I live in Sweden and the socket should give 220–240 V 50 Hz. I measured 235V. The amp is stamped 220V.

    On pin 4 and 6 on the Rectifier tube I get 386V
    Before the diod I get 57V

    /Wilhelm
    Are you by any chance close to gothenburg ? Asking since i can fix it...
    lagwil
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    Post by lagwil on Wed Dec 04, 2019 10:22 am

    peterh wrote:
    lagwil wrote:
    Bob Latino wrote:1. Take out the tubes and measure the AC voltage to chassis ground from pin 4 and then pin 6 of the rectifier tube socket. See if you get about 350 to 375 AC at both measuring points. If you don't then the transformer is bad.

    2. Also measure the AC voltage to chassis ground where the RED/BLACK wire connects to the diode that replaced the selenium rectifier. You should get 50 to 60 volts AC to chassis ground. If you don't then the transformer is bad.

    Bob

    Hi Bob and many thanks for all great help!

    I have now made more measurements on my Dynakit. I live in Sweden and the socket should give 220–240 V 50 Hz. I measured 235V. The amp is stamped 220V.

    On pin 4 and 6 on the Rectifier tube I get 386V
    Before the diod I get 57V

    /Wilhelm
    Are you by any chance close to gothenburg ? Asking since i can fix it...

    Thanks but unfortunatley not. I live in Stockholm. Very Happy
    PeterCapo
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    Post by PeterCapo on Wed Dec 04, 2019 1:15 pm

    I suggest checking the voltages on each section of the quad capacitor, per the manual.
    lagwil
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    Post by lagwil on Wed Dec 04, 2019 4:18 pm

    Now I made a lot more measuring. Seems like capacitor lug A-D measures 30-40V to less then specs. Se all measures in the attached image:

    Very low bias and hum on Dynakit ST-70 St7010
    peterh
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    Post by peterh on Wed Dec 04, 2019 4:47 pm

    Do you have another GZ34 ? You might have half a rectifier, that explains the humming
    lagwil
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    Post by lagwil on Wed Dec 04, 2019 5:06 pm

    Nope but I do have a 5U4GB and exactly the same resultat with that rectifier. Alao swaped all EL-34 Tubes. Same...
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    Post by PeterCapo on Wed Dec 04, 2019 5:26 pm

    I am not quite clear on what to expect for the voltage test points with the 220 volt power transformer in use.  

    However, it does seem like the rectifier tube is getting more than enough voltage on its plates, but something on its other side is dragging the B+ down a bit.
    lagwil
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    Post by lagwil on Wed Dec 04, 2019 5:45 pm

    PeterCapo wrote:Some mechanical humming from power transformers is not unusual even under normal circumstances and results from flowing current.  So, electricity and mechanical humming are definitely associated.  Sounds like he described the mechanical hum as varying with the setting of his bias pots.  The varying mechanical hum sounds like a symptom of another problem, probably whatever is causing his biasing difficulty.

    If the amplifier was working fine for some time and then this problem suddenly came up, it means some part or parts, including possibly tubes, in the amp are failing.  Questions:

    - were there any changes to the amp shortly preceding the onset of the current symptoms?
    - is a 3A fuse in the fuse holder, and not more than 3A?

    If the problem is in one channel only, I'd try swapping the tubes from one channel to the other and see if the problem follows the tubes.

    I'd also still refer back to Post n°13.


    Peter, regarding your questions:
    No changes in my setup prior to the problem. Had played for a very long time though. Maybe 7-8 hours of non stop music.
    I use a slow 1,5A fuse since it is a 220V amp.

    /Wilhelm
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    Post by mijohn on Wed Dec 04, 2019 5:57 pm

    Could an overstressed power transformer be causing the problem and producing the mechanical hum, it's rated for 220V and you're giving at least 235V. A Dynakit PA521 transformer rated at 240V would be better for your area.

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