Troubleshooting help needed for brand new VTA ST-120 - solved !

I recently bought, as a finished product, a brand new ST-120 with octal board.  The left side amplification is much louder than the right.  I wish to trace down the source of the problem.  To do that, I believe i need some voltages or resistances that I can check to see if I can localize the problem.  If anybody has a recommended triage approach, that would be helpful.  Right now I do not have any voltage or resistance data that I can use to check anything.  i have an oscilloscope on order, but it isn’t here and I don’t know how to use it.   Here is what i have done so far:

Chop stick prodding of all joints and examination for cold joints - re-wet a few
swapped input and output wires, all tubes, verified that its the RHS amplification that is the problem

Any links to tables of voltages or resistances I should expect while measuring the board, either off or on, would be greatly appreciated.  Also, any recommended triage approach: do this then that then the other, etc. would be greatly appreciated.

Thanks,

alanhuth

I would email Bob Latino -- he both knows the VTA-120 and is helpful. I would also swap L to R the cables to the inputs to the VTA-120. If the sound imbalance follows the swap, the problem is upstream of the VTA-120. I would then swap L to R the outputs from the VTA-120--if the imbalance doesn't follow the swap, the problem is downstream of the VTA-120. Finally, I would re- tension the pin receptacles on the tube sockets, and clean them with Deoxit (https://www.amazon.com/CAIG-D5S-6-Contact-Cleaner-Rejuvenator/dp/B00I3G272M).

Thanks for your suggestions, Hops. If you re-read my first post, you can see that I already did much of what you suggest. I need some voltage or resistance tables or something I can use to track down the problem. I like your idea of re-tensioning the sockets, but, as I mentioned, this is a brand new amp, so presumably the sockets are tight. I can tell you the tubes are difficult to press in now. but that doesn’t mean one tube doesn’t have a bad contact. So I’m watching videos on how to re-tension tube sockets. thanks for that idea.

Now I need some triage assistance. Perhaps some voltage tables, something like that so I can begin to track down where in the amp the problem is.

Any suggestions most appreciated.

Thanks,

alanhuth

I would read this post: https://dynacotubeaudio.forumotion.com/t2292-vta-st120-step-by-step-diagnostic-checklist. If you have the assembly instructions, near the end of the assembly instructions are a first powerup procedure that includes voltage checks. I would do those voltage checks also.

I would read this post also: https://dynacotubeaudio.forumotion.com/t274-quick-troubleshooting-guide-for-dynaco-tube-amps

Well this is getting curiouser and curiouser. Checked all the voltages on the voltage table and the VTA-120 build instructions. All voltages good on both sides. RHS still very low. Any ideas what to try next? Bob suggested LM334s might be a problem. Their resistance and voltages on the cathodes match. Out of ideas. Help!

I am a newbie. Other than the two links I gave you, I am at the limit of my knowledge.

As this is lack of ( or mismatch) of amplification some AC measurment is next.
Customer does have different volume of the tvo channels, we do not know if
this is to low volume on one channel or to much in the other.
What is needed is typical AC measurment vales :
If amp is terminated with 8ohm power resistors and 0.5Volt of 1000 hz is injected in
the input ( any volume control fully opened ) , how much AC could be found at the
8 ohm outlet ?

I don't think one needs AC voltages in between, the above is enough for detecting which
channel is at fault. I have no vta-120 thus i cannot assist here. Any takers ?

The alternative is to send the amp to bob.

If Bob suggests LM334s, I would replace the LM334 on the weaker channel and see what happens. Can't be too expensive, pretty easy. If that doesn't work, replace the LM334 on the other channel. Use eutectic solder; much easier to use and more reliable if you are new to soldering. VTA-120 was my first solder project; used Cardas and it worked out fine. I think Kester 63/ 37 is eutectic also.

.

PeterCapo wrote:Triode/pentode switches set the same way?  Could there be a problem with one speaker?

peterh wrote:As this is lack of ( or mismatch) of amplification some AC measurment is next.
Customer does have different volume of the tvo channels, we do not know if
this is to low volume on one channel or to much in the other.
What is needed is typical AC measurment vales :
If amp is terminated with 8ohm power resistors and 0.5Volt of 1000 hz is injected in
the input ( any volume control fully opened ) , how much AC could be found at the
8 ohm outlet ?

I don't think one needs AC voltages in between, the above is enough for detecting which
channel is at fault. I have no vta-120 thus i cannot assist here. Any takers ?

The alternative is to send the amp to bob.

I've seen probe leads are rated for maximum voltage but I've not seen them rated for watts.  If it won't blow up the meter, it could confirm there's a true difference in the channel-to-channel balance in the amplifier.  If it's doable, the OP can trace the signal backwards from the amplifier's outputs all the way back to the inputs.  He'd need a steady test tone, however.  There are test discs that have this and don't cost a lot.  What do you think, peterh?

I think the author said he had an oscilloscope. A test tone ( sine ) might be arranged
by a phone or a CD record. But the big issue is to figure out which channel that is
bad. Lack of feedback will make it louder. Or it might be some resistor that has
accidentally been mistakenly replaced with another with wring impedance.

But first must one identify the correct channel

Thanks for helping out, guys. I did order an oscilloscope, but it is coming from China. ETA October.
I’m very certain the RHS is bad, and getting worse. You can barely hear it at settings on other amps that would bring the house down. The LHS is the same loudness as any other amp I’ve had in the system, including a loaner VTA-70. It seems to be performing perfectly.

Check your binding post connections. It does not take too much time/effort. I ran into a similar problem when I built my pair of M-125s. One worked perfect and the second had weak channel. I swapped amps to verify it was not the speaker or cables. I triple tested every joint and every measurement with my multi-meter was within spec. I finally hooked up an oscope and saw inconsistent output at the binding post. I upgraded all my posts and soldered the connections - don't trust the nut connections. This fixed the problem in my case. I hope that yours turns out to be something as easy to fix. I probably could have used the existing posts but I only like to fix my problems once and do not mind spending some $ if it eliminates the problem for good.

Mike

alanhuth wrote:Thanks for helping out, guys.  I did order an oscilloscope, but it is coming from China.  ETA October.  
I’m very certain the RHS is bad, and getting worse.  You can barely hear it at settings on other amps that would bring the house down.  The LHS is the same loudness as any other amp I’ve had in the system, including a loaner VTA-70.  It seems to be performing perfectly.  

You could start comparing all resistors between left and right in case that a mistake has been done.

I like the binding post idea. Wishful thinking I suppose. I’ll check it out. I’ve compared the resistance left to right and all match pretty well. I have not re-melted each solder joint. They look good, but I guess you never know for sure. not sure that’s a good idea. I would think there’s the possibility that you create more problems than you solve. Is that right?

alanhuth wrote:I like the binding post idea.  Wishful thinking I suppose.  I’ll check it out.  I’ve compared the resistance left to right and all match pretty well.  I have not re-melted each solder joint.  They look good, but I guess you never know for sure.  not sure that’s a good idea. I would think there’s the possibility that you create more problems than you solve.  Is that right?

If you just bought it, don't you have warranty and means to send it back for repair ?

Not to be snarky but have you checked your hearing?

What about polarity of your speaker cables?

Did you try multiple tubes in the center socket? (Which affects both channels)

I would resolder everything and check for a good physical connection particularly on the tube sockets.

Replace the LM334 in both channels. (Buy a few of them, they're cheap.)

Thanks for the helpful suggestions, corndog. My hearing is not great, but I can tell the difference between a very loud speaker you can hear outside the building and one which you need to put your head right next to the speaker to hear. This is not subtle.

Is it generally understood to be a good idea to re-solder everything? I was afraid that it might make matters worse - convert a good solder joint to a bad one. Is that not a thing?

I ordered some LM334s. We’ll see if that helps.

Does anybody have a schematic or voltage chart for the OCTAL BOARD? Bob doesn’t have one, and Roy is temporarily unavailable.

My 1st tube amp was an ST70 with 3rd party driver board. It sounded ok but kind of dull. I eventually stripped it down completely. I replaced the tube sockets, driver board, and even added a cap board in lieu of the can cap. It was a lot better sounding after that.

Tube amps are deceptively simple and yet just complex enough to be maddening.

These are always tough and always subjective. I had a friend who felt his big Yamaha monster had a weak channel and we spent many a day trying to run down the problem. To me it was stereo separation in the mix that emphasised certain instruments and colored the sonics in each channel... to him or was an electronic flaw that became an obsession. Inequal output from each channel. We swapped and switched everything we could and took it in to a tech. He was convinced there was a problem that needed to be repaired but we could find nothing. I think some of the early stereo mixes really separated instruments L-R so heavy stuff like bass etc. would appear to create an inbalance in the system. I am not saying you are not hearing correctly or unsophisticated in your listening, but at least run a mono signal with equal L-R output from your preamp for an accurate read. If I got that far down into the weeds and still felt there was a sonic problem I would dump the amp and get something else unless you really enjoy the troubleshooting journey. I am just saying you may never find a specific problem, or a fix. Some things just are what they are.

Hi Alanhuth,
I have a word document that I kludged together from this here forum on diagnosing the ST-120.
I have 2 with Octal's & the AB board. I'd be happy to send it to you. Publish these stats and folks can help you.

Also, I have the original ST-120 Octal Board Documentation, I'll see if there is a schematic.

Send me a PM with email address and I'll send you what I have.

Cheers,

Blue

Aguaazul, any info you have ( other than Bob's or Roy's intellectual property) should be posted on this forum rather than private messaged. I am sure others would like to see it -- I know I would. I think the VTA stuff is a great bargain - much higher quality sound for the price than you can get elsewhere. However, as a result, there is a little more involvement and maintenance required.

I'm pretty sure that they don't want that info posted, but if you own an octal board, you should own the schematic. I am going to send to @alanhuth the schematic with the parts layout, not the complete install file.

Along with a file I created with that helps document what's going inside your amp, taken from a few posts on the forum.

I'll wait for Roy to send it to you then, but you said he was not around. Comment>

Finally... I don't think Bob messes with the octal board. (I'm actually pretty sure!)

Aguaazul

OK, all you troubleshooting wizards, time to put on your thinking caps.  

If you disconnect the LHS and RHS output transformer secondaries, the yellow (NFB), orange (8ohm), and brown (4 ohm) wires on the LHS all measure around 6.7 Kohms to the chassis.  The same wires on the RHS measure around 27 ohms.  Primaries the same 2M ohms to ground.

BUT, if you disconnect the ground on the RHS, the amp works!  Both sides equally loud.  What is going on?