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The Dynaco Tube Audio Forum

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


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    Help with new to me ST-120 Stereo Amplifier - Scratchy noise in right channel

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    Peacemaker


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    Post by Peacemaker Fri Dec 10, 2021 7:38 am

    Hi all,

    I'm new here and to the tube world and recently decided to try one that came up for sale. I purchased a VTA ST-120 that looks amazing and it has been working great. I knew there was a possible issue with it as disclosed by the seller in our discussions and thought this could be a great way for me to learn about tube amplifiers by jumping right in. Not sure that's always a wise course of action but I decided to do it.  Help with new to me ST-120 Stereo Amplifier - Scratchy noise in right channel 1f601

    Here's my issue. The amplifier was working great until a few days ago when I began hearing a scratchy sound coming from the right channel that increases and decreases in volume somewhat randomly. It subsides sometimes but then comes back. I've tried swapping power tubes and interconnects which didn't make a difference. This is VTA ST-120 kit that the seller bought from VTA earlier this year built himself.

    Any thoughts or help from the members here would be greatly appreciated. Thanks!
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    Post by Peacemaker Fri Dec 10, 2021 8:35 am

    The other observation I've made is that the output volume on the right channel is very low compared to the left channel.
    Brap
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    Post by Brap Fri Dec 10, 2021 10:20 am

    Did you switch speaker cables? If it comes out of the left channel then, it is isolated to the right channel circuitry. Also try swapping out the two end driver tubes (small ones) to see if there is a difference.
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    Peacemaker


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    Post by Peacemaker Fri Dec 10, 2021 11:11 am

    Just swapped the driver tubes, power tubes and speaker cables with no change - still coming from the right channel. I also just tested the bias on all four power tube sockets and three read .500 and the one on the right rear fluctuates between 14.00 and 17.00. I then tried swapping tubes and get the same reading from that socket, which tells me that it's not a tube issue, right? What could it be then?

    Another thing I've noticed is that when I place a tube in any of the sockets there is a gap of about an 1/8" between the base of the tubes and the sockets, where the tube aren't seating all the way in allowing them to rock back and forth in the socket. Is this how the tubes are supposed to fit?
    Brap
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    Post by Brap Fri Dec 10, 2021 11:52 am

    If tubes are not making proper contact with the socket, that is an issue. Over time, the sockets need to be tightened up by opening up the bottom and using a fine pair of needle nose pliers, tighten them up, squeezing gently. If the tube just "drop in" to the sockets, tightening is required. Initially appears the socket itself if you have swapped tubes and they work in different sockets.
    ttocs
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    Post by ttocs Fri Dec 10, 2021 11:55 am

    Does the scratchy sound happen with no audio from a preamp, with preamp volume turned down to zero?

    My guess is capacitor(s).

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    Brap
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    Post by Brap Fri Dec 10, 2021 12:19 pm

    Good call, was next on my list but was not sure about his experience with replacing.

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    Post by ttocs Fri Dec 10, 2021 12:39 pm

    Good point. It's always good to repeat the safety tips for checking high voltage circuits.

    Test the volts in capacitors so you know how much is stored. Drain the voltage from capacitors before working on these circuits. Plenty of info on the net for this.

    I bought this great little meter for testing caps in-circuit. I bought mine when I began tinkering with very old guitar amps.
    Help with new to me ST-120 Stereo Amplifier - Scratchy noise in right channel Besr_large_8fa3ab56-90a7-4605-8906-7583a2d7f1e5_1100x
    I bought it HERE .
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    Peacemaker


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    Post by Peacemaker Fri Dec 10, 2021 12:51 pm

    ttocs wrote:Does the scratchy sound happen with no audio from a preamp, with preamp volume turned down to zero?

    My guess is capacitor(s).

    Yes, the sound is there with no audio from the preamp and turned down to zero and when the preamp is powered off.
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    Peacemaker


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    Post by Peacemaker Fri Dec 10, 2021 12:54 pm

    Brap wrote:Good call, was next on my list but was not sure about his experience with replacing.

    I've never done this before and am new to electronics repair, but researching online I'd be comfortable doing it. How do I identify which one is bad?
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    Peacemaker


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    Post by Peacemaker Fri Dec 10, 2021 1:17 pm

    [quote="ttocs"]Good point. It's always good to repeat the safety tips for checking high voltage circuits.

    Test the volts in capacitors so you know how much is stored. Drain the voltage from capacitors before working on these circuits. Plenty of info on the net for this.

    I bought this great little meter for testing caps in-circuit. I bought mine when I began tinkering with very old guitar amps...



    Thank you for the link. Just ordered the meter.
    Brap
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    Post by Brap Fri Dec 10, 2021 2:24 pm

    It's all a process of elimination to find the issue. Takes time but comes apparent eventually. To close out my original suggestion, can you feel resistance when placing the tubes in the sockets and do all of them light up the same?

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    Peacemaker


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    Post by Peacemaker Fri Dec 10, 2021 2:31 pm

    Yes, I can feel resistance when installing the tubes and they all appear to light up the same.
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    Post by ericoto Fri Dec 10, 2021 5:47 pm

    Sounds like a loose connection, that is the most common cause of problems in a project like this. Isolate which channel, open it up and look for a loose or cold solder joints in that channel reheat with a soldering iron any suspect solder joints. I would also look and see if any components are burned or broken. Do you know who built it? If it was working a capacitor burned out unless defective you should see changes. If this doesn't work, get the schematic and test voltages.
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    Post by ttocs Sat Dec 11, 2021 1:10 pm

    First, check the voltage in all the caps to verify they are drained.
    I use clip type probes for most things, these don't work for all conditions but keep things safe and consistent for those they do work well for.
    AnaTek has good instructions, and Uncle Doug (on YouTube) has a very good instructional on the tester's usage. On YouTube just search "uncle doug blue esr" and the video pops up. Uncle Doug is great!

    Turn on the tester, short the probes together, then press the tester's power button again to "Zero" the tester for the probes being used.
    Help with new to me ST-120 Stereo Amplifier - Scratchy noise in right channel Bluees14
    Help with new to me ST-120 Stereo Amplifier - Scratchy noise in right channel Bluees15


    Test the capacitor in question and be sure that the reading is steady for a good measurement.
    Help with new to me ST-120 Stereo Amplifier - Scratchy noise in right channel Bluees17


    The tester has a guide on the face that shows acceptable ESR values with capacitors of varying voltages and values. If the tester has no reading (dashed line), or high reading, the cap is bad.
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    Post by Rbertalotto Sat Dec 11, 2021 2:58 pm

    Does this amp have the optional level control? If it does, I bet dollars to donuts that's your issue. Mine did exactly what you were saying. If I turned the level control up and down a few times it would go away. I finally sprayed contact cleaner on the potentiometer and it has now been fine for months.
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    Post by Peacemaker Sat Dec 11, 2021 3:44 pm

    Rbertalotto wrote:Does this amp have the optional level control? If it does, I bet dollars to donuts that's your issue. Mine did exactly what you were saying. If I turned the level control up and down a few times it would go away. I finally sprayed contact cleaner on the potentiometer and it has now been fine for months.

    This amp does not have the optional level control. I've actually considered retro-fitting that optional level control into it because some really like it. Just not sure that I need it.
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    Post by Peacemaker Sat Dec 11, 2021 3:49 pm

    Thanks for the the detailed information and resource link. My tester is supposed to arrive next week. Looking forward to using it and hoping it helps me identify the problem with my amp. If it is a bad capacitor, where would I source it? VTA?



    ttocs wrote:First, check the voltage in all the caps to verify they are drained.
    I use clip type probes for most things, these don't work for all conditions but keep things safe and consistent for those they do work well for.
    AnaTek has good instructions, and Uncle Doug (on YouTube) has a very good instructional on the tester's usage. On YouTube just search "uncle doug blue esr" and the video pops up. Uncle Doug is great!

    Turn on the tester, short the probes together, then press the tester's power button again to "Zero" the tester for the probes being used.


    Test the capacitor in question and be sure that the reading is steady for a good measurement.


    The tester has a guide on the face that shows acceptable ESR values with capacitors of varying voltages and values. If the tester has no reading (dashed line), or high reading, the cap is bad.
    avatar
    Rbertalotto


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    Post by Rbertalotto Sat Dec 11, 2021 4:29 pm

    Just not sure that I need it. wrote:

    Its nice to have when you are switching inputs around and not damage your speakers.... to not have to shut off the amp and then power it back up. I wish there was a better stepper type control available. Maybe there is?
    Brap
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    Post by Brap Sun Dec 12, 2021 7:31 pm

    I typically use the mute function on my preamp when switching inputs. Much easier and it’s on my remote
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    Post by Dale Stevens Mon Dec 13, 2021 10:20 pm

    Brap, explain " mute when switching inputs on preamp" . Thx, Dale
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    Post by mrnik0 Tue Dec 14, 2021 3:28 am

    Hi Peacemaker,
    I would suspect capacitor leakage as well but as this is a home made amp it could also be a dry solder as we don't know the builders experience I would first have a very good look at all the solder joints to check that they are well done, I have seen projects where the connections have been wrapped around a solder post but no actual solder was used!
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    Post by BCROW Tue Dec 14, 2021 4:45 am

    Could also be a bad carbon resistor. If you are not familiar with electrical/electronic safety, best to ask someone that is knowledgeable to help out. Dangerous high voltages are present. Capacitors can still have high voltage charges long after the unit is turned off or unplugged.
    Brap
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    Post by Brap Tue Dec 14, 2021 9:03 am

    Sorry, I did not explain right. I never unplug the inputs to the amp. All my signal sources go through my preamp. For example when I switch from streamer to TT, I press the mute switch on the preamp or it's remote to zero any signal to the amp. You may be talking about the physical connection to the amp when switching sources without a preamp.
    Agree with the previous post about solder connections. Always good to have a hot iron and go through them to ensure that issue is off the list. Also, yes, be careful, there is hi voltage lingering. Also, don't wear watches or rings when inspecting.
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    Post by Bob Latino Tue Dec 14, 2021 9:21 am

    Peacemaker wrote:I also just tested the bias on all four power tube sockets and three read .500 and the one on the right rear fluctuates between 14.00 and 17.00. I then tried swapping tubes and get the same reading from that socket, which tells me that it's not a tube issue, right? What could it be then?

    Check to see if the 10 ohm 2 watt resistor that goes from pins 1 and 8 on the right rear socket does still measure about 10 ohms. If any tube will not bias in that socket, this is usually a sign that this resistor is bad. You can check the value of this 10 ohm resistor without even taking the bottom cover off the amp. Make sure that the amp is OFF and measure the RESISTANCE to chassis ground at all four bias measuring points. You should get about 10 ohms at all four bias measuring points. See if the right rear socket measures significantly different than the other three sockets.

    Bob

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