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    Chasing Hum with a "new" old ST-120 - solved !

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    vizsla23

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    Post by vizsla23 on Mon Feb 11, 2019 6:58 pm

    Ok, so I pulled some readings from the power supply section:

    Rectifier:
    pins 4&6 are normal.
    Pins 2 and 8 reading low at 391 V DC.

    Quad Cap (all low, some worse than others):
    square 389V DC
    Half circle 384V DC
    (No symbol) 384V DC
    Triangle 323V DC

    Also, after reassembly and powering back on, the power switch started to arc and in able to make an arc when touching the ground wire to the chassis. I dont think it was doing that before. I cant measure anything off the chassis (AC or DC), so something else to figure out now
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    vizsla23

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    Post by vizsla23 on Mon Feb 11, 2019 7:35 pm

    Thanks for jumping in to help Bob!

    So plugging in GZ34 I have and the voltages went up by 10-15V which is counter intuitive seeing as the rectifier for the original measurements was a Weber SS. Two bad rectifers? It's possible, but the guy I bought it from swears that the tube only has a few hours of use on it while he was waiting for the weber.
    Bob Latino
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    Post by Bob Latino on Mon Feb 11, 2019 7:45 pm

    vizsla23 wrote:Thanks for jumping in to help Bob!

    So plugging in GZ34 I have and the voltages went up by 10-15V which is counter intuitive seeing as the rectifier for the original measurements was a Weber SS.  Two bad rectifers?  It's possible, but the guy I bought it from swears that the tube only has a few hours of use on it while he was waiting for the weber.

    What is the AC voltage from pins 4 and 6 to chassis ground ? You should have about 350 - 370 volts AC to chassis ground on both pins. If you do get the 350 - 370 VAC and still get low DC voltages on the quad cap, then maybe the quad cap has a problem and is leaking ?

    Bob
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    vizsla23

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    Post by vizsla23 on Mon Feb 11, 2019 7:55 pm

    I've got 418 VAC at 4 and 6. Your checklist here says 400 to 425 for the ST-120. Assuming same principles apply, it sounds like a bad quad cap?
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    Post by Bob Latino on Mon Feb 11, 2019 8:23 pm

    vizsla23 wrote:I've got 418 VAC at 4 and 6.  Your checklist here says 400 to 425 for the ST-120.  Assuming same principles apply, it sounds like a bad quad cap?

    OK - Sorry. Yes > It is the VTA ST-70 that will give you about 360 VAC from pins 4 and 6 to chassis ground. The ST-120 will have about 410 VAC on pins 4 and 6 to chassis ground. If you check the capacitance on the four sections of the quad cap, make sure you do it with the amp OFF. If you check the quad cap in circuit you will get higher readings then the 80, 40, 30, 20 uF the quad cap should be. On the ST-120, it could also be a miswired SCM or the ESL.

    What I would do first is remove from the quad cap the wires from the SCM and ESL capacitor and see if the DC voltages on the quad cap go up ?

    Bob
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    vizsla23

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    Post by vizsla23 on Mon Feb 11, 2019 8:44 pm

    I'll have to tackle this one tomorrow. The wife wanted the living room back :-)

    How bad would it be to continue to run as-is? If it weren't for the light hum from the transformer, I never would have found this.

    Thanks again for your help!
    Peter W.
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    Post by Peter W. on Tue Feb 12, 2019 8:34 am

    vizsla23 wrote:I'll have to tackle this one tomorrow. The wife wanted the living room back :-)  

    How bad would it be to continue to run as-is?  If it weren't for the light hum from the transformer, I never would have found this.

    Thanks again for your help!

    My concern would be that were a spectacular failure of either the rectifier or quad-cap to take place - something or multiple something else might become collateral damage.
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    Post by vizsla23 on Tue Feb 12, 2019 1:24 pm

    Bob Latino wrote:
    vizsla23 wrote:I've got 418 VAC at 4 and 6.  Your checklist here says 400 to 425 for the ST-120.  Assuming same principles apply, it sounds like a bad quad cap?

    What I would do first is remove from the quad cap the wires from the SCM and ESL capacitor and see if the DC voltages on the quad cap go up ?

    Bob

    Ok, so while I was unable to work on the amp last night I did continue to search through the forums and found this very helpful thread (not sure how I missed it): http://dynacotubeaudio.forumotion.com/t171-st-70-quad-cap-long-post

    After cracking open the case today and wanting to take the less invasive route first, I checked resistance of each cap can lead after discharging them and couldn't get any readings above 1M ohms after a good minute of letting the charge build up. I think that pretty much confirms a bad quad cap.

    If I have time to pull stuff apart later to check voltage, I will. The wiring job on this amp is a little tight. The leads to the SCM and ESL were apparently the first connections to each point and were underneath other leads. Selectively removing them and reinstall would require disconnecting the nest above to get to the bottom or getting the iron into some tight spaces. I realize i'll be doing all of this anyway to install a new quad cap, but no sense in doing it twice.

    I will admit, I bought this amp used to avoid the need to build myself (and cost), but there is some fun and satisfaction to be had in the troubleshooting and install of the attenuator. Once I have it running as it should be, I may consider selling it in the future to pay for a low-gain kit.
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    Post by Peter W. on Tue Feb 12, 2019 3:19 pm

    After cracking open the case today and wanting to take the less invasive route first, I checked resistance of each cap can lead after discharging them and couldn't get any readings above 1M ohms after a good minute of letting the charge build up. I think that pretty much confirms a bad quad cap.


    Yes, it does. Do not waste any your time further on this cap. In cap-speak, less than one meg is tantamount to a dead short.
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    Post by Dale Stevens on Wed Feb 13, 2019 7:29 pm

    V, how did u check the resistance on the cap? Ohms or ESR?

    Peter, explain more on the 1M reading: I would have thought that was ok. Maybe i'm thinking on the wrong scale ! Thx Dale
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    Post by vizsla23 on Wed Feb 13, 2019 7:49 pm

    Ohms, using a multimeter. It never actually got to 1M ohms. I quit letting the charge build up after a minute or so when it slowed down. Highest I got was probably .7 M ohms.

    I also walked back through the wiring instructions and confirmed that the SCM and ESL were wired I correctly. With the rectifier checking out, I'm down to the transformer and the cap can.
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    Post by Peter W. on Thu Feb 14, 2019 9:03 am

    Dale Stevens wrote:V, how did u check the resistance on the cap?  Ohms or ESR?

    Peter, explain more on the 1M reading: I would have thought that was ok.  Maybe i'm thinking on the wrong scale !   Thx  Dale

    I keep a pretty good Fluke meter, and on Ohms and an electrolytic cap of say.... 40+/- uF, it will charge to --- on the meter in about 2 minutes, often less.
    It will read (accurately) up to 40 megs.

    1 meg is 2.5% of the range (in megs). With an electrolytic cap, that is pretty close to short.
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    Post by vizsla23 on Fri Feb 15, 2019 9:40 am

    Ok, so by next week I should have a definitive answer, or at least a properly running amp. I just ordered the dynakitparts 600V quad cap for extra security and temp. rating, a new Weber WZ68 since the current one is nearly 10 years old anyway, AND the delay board kit for a little extra life and safety.
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    Post by bbqjoe on Fri Feb 15, 2019 10:18 am

    vizsla23 wrote:Ok, so by next week I should have a definitive answer, or at least a properly running amp.  I just ordered the dynakitparts 600V quad cap for extra security and temp. rating, a new Weber WZ68 since the current one is nearly 10 years old anyway, AND the delay board kit for a little extra life and safety.  

    If I'm not mistaken (and I often am) I was under the understanding that the W1 was the latest suggestion for audio, including a thermistor?
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    Post by Peter W. on Fri Feb 15, 2019 10:32 am

    bbqjoe wrote:

    If I'm not mistaken (and I often am) I was under the understanding that the W1 was the latest suggestion for audio, including a thermistor?

    This is true - but as I understand it, the WS1 has a very low voltage drop (1 VDC).

    For some, this may be problematic. For a circuit designed against a tube rectifier, the designed voltage drop will be, typically, between 5 and 15 volts for a conventional tube, less if a mercury rectifier is used (83). And in these troubled times with often-excessive wallplate voltage, the WS1 may exacerbate an already significant problem.
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    Post by vizsla23 on Fri Feb 15, 2019 10:43 am

    Yes, my wall is pushing out up to 123 VAC. I'd like to avoid a variac or transformer if possible. In addition, I didn't want the immediate high voltage surge if the WS1. As I understand it, while Weber doesn't reccomend the WZ68 for hifi, Bob still does.
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    Post by bbqjoe on Fri Feb 15, 2019 10:47 am

    Peter W. wrote:
    bbqjoe wrote:

    If I'm not mistaken (and I often am) I was under the understanding that the W1 was the latest suggestion for audio, including a thermistor?

    This is true - but as I understand it, the WS1 has a very low voltage drop (1 VDC).

    For some, this may be problematic. For a circuit designed against a tube rectifier, the designed voltage drop will be, typically, between 5 and 15 volts for a conventional tube, less if a mercury rectifier is used (83). And in these troubled times with often-excessive wallplate voltage, the WS1 may exacerbate an already significant problem.

    I understand so little about circuitry...
    I would think one wouldn't want voltage dropping?
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    Post by Peter W. on Fri Feb 15, 2019 11:06 am

    bbqjoe wrote:

    I understand so little about circuitry...
    I would think one wouldn't want voltage dropping?

    You, likely, understand more than you think.

    With that in mind consider the design process for the Electrical Engineer laying out a tube-based amplifier circuit. She will design to a goal of XXX VDC as the B+, with a tolerance of +/- XX volts.

    a) At a given wallplate voltage, probably center-of-tariff.
    b) Using a given AC primary voltage.
    c) Using a given rectifier, tube or solid-state.
    d) If solid-state, silicon, selenium, copper oxide, and then rectifier 'speed'. Then half-wave, full-wave or bridge.
    e) If tube, then the nature of the tube - conventional, mercury, half-wave or full-wave.

    Each choice will require a calculation of net-B+. For example:

    Silicon Half-Wave: (ACV x 0.707) - 1.2 VDC (1N4007)= B+
    Selenium Half-Wave: ACV x 0.707 - 5 VDC per section = B+ With the number of sections required determined by the load.
    Silicon Full-Wave: (ACV x 1.414) - 1 VDC = B+

    And so on.

    Point being that the net results may be calculated to a very specific number. And changing any component in that chain will change the net results - possibly with unhappy consequences. Such as inserting a full-wave or bridge in place of a half-wave. OUCH!
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    Post by vizsla23 on Sun Feb 17, 2019 2:32 pm

    Well, I'm a happy camper today! The amp is now running as smooth as silk. Thanks Bob!

    The new and improved quadcap came in the mail this morning from dynakitparts.com. After taking off the sticker and polishing the aluminum can with some steel wool and metal polish, I got to work. Taking out the old cap can was a bit of work. The connections were a bit of a rats nest, particularly on the 80 pf connection. It took some patience to do without melting insulation. I was able to clean up some ends, but a few wires from the transformers were too short to fix. Cutting them back would have made them too short to reach the connection. Reconnecting was a bit easier with clean ends and making one single connection at each point.

    I fired up the amp and my heart sunk...nothing. I promptly shut off the amp and checked my connections. I quickly realized the speakers were still connected to my receiver. I swapped the cables going into the speakers and let the amp rest. I took a minute to polish up the Weber copper cap since I already had everything out.

    With the speakers plugged in and and the copper cap bright and shiny, I flipped the power switch and gave it another go. First item to tackle was dropping tube bias...a lot. After scrambling to dial back each pot, I was able to take a breath. First thing I noticed was that there was zero transformer hum and dead silence from the speakers. I cranked the attenuator and still, no hum or buzz. Checked B+ which was right at 430V. Popped on a record, and I'm off to the races.

    My last and final project will be to build and install the time delay, which is in the mail. After that, I think I'll spend some time enjoying the amp for a few months before ultimately selling it and buying a new kit. After spending some time under the hood, I realized there is a lot I would do differently in my build, starting with better wire management. I can tell the original owner started off with the best intentions, but got anxious to complete the job. It's by no means horrible, but I would have just done things a little neater. Im also running highly efficient Klipsch speakers, and I really notice change in noise when swapping driver tubes. I think the low gain amp will suit me better. I look forward to the satisfaction of building my own amp. Fixing and improving this one sure has been fun.
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    Post by bbqjoe on Sun Feb 17, 2019 2:59 pm

    vizsla23 wrote:Well, I'm a happy camper today!  The amp is now running as smooth as silk.  Thanks Bob!


    Yay for happy endings!!! Chasing Hum with a "new" old ST-120 - solved ! - Page 2 CTIeH1x
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    Post by Wharfcreek on Thu Feb 28, 2019 10:40 am

    I'm all for 'happy endings' too! In reading through this thread, I kept wondering; was that amp 'bad' in the first place?

    I know you're already WAY too deep into this process for considering this, but perhaps it will come in handy in the future. I chased a hum problem once, spent about 3 days on it before I finally 'fixed' it. In the end, it was my 'high-dollar' interconnects!! I gave up on 2 pre-amps, a CD player, and even rebuilt an amplifier.....all to find that my 'hum' was just a 'good' set of RCA cables gone bad!! I now keep a set of 'grounded' RCA plugs on my bench as 'tools' for testing amp noise. When hooked to speakers, inserting a set of 'grounded' RCA plugs into the 'input' of the amp is very 'telling'. If there is NO noise with them plugged in (and the amp 'on' of course....assuming it's a 'good' amp in the first place as well)...then the amp is most likely 'not' the problem.

    After I had this problem I decided to 'test' a bunch of my RCA interconnect cables. I took a set of double-female RCA adapters and plugged them in to the ends of each set I had, then plugged the 'shorted' or 'grounded' RCA plugs in to the double female connectors. Then I simply plugged the cords into the amp and turned it on. The results of this were very surprising!! I have (had) a number of 'better' sets of cables that turned out to be 'junk'....where as some of the really 'cheap' stuff as supplied with like a new CD player or similar...those turned out to be just fine. As I ran my hand over the cable, the 'inducted' noise in some of the 'better' cables was really disappointing! Clearly they are NOT what they say they are!! Likewise, some of the 'junk' stuff was really very good...completely quiet and no apparent 'leakage' from the shielding. So, my conclusion was that you do NOT always get what you pay for in this area!! "Marketing" is everything!!....dammit!!

    Anyway..... I know this may no longer be relative. But I share it as a consideration for what may be helpful in the future!!

    TSD
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    vizsla23

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    Post by vizsla23 on Thu Feb 28, 2019 9:46 pm

    The cap can was definitely the issue. After replacing it, the hum was gone and my B+ voltages were back to normal.

    I still get some EMI hum when the TV is running which is nearby, but I also had that before with my headphone amp. The solution: keep the TV off when listening to music.
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    Post by Peter W. on Fri Mar 01, 2019 9:01 am

    vizsla23 wrote:The cap can was definitely the issue.  After replacing it, the hum was gone and my B+ voltages were back to normal.  

    I still get some EMI hum when the TV is running which is nearby, but I also had that before with my headphone amp.  The solution: keep the TV off when listening to music.

    http://dynacotubeaudio.forumotion.com/t4005-line-noise-suppression

    Might try this, if something other than TV noise turns up.

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