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    ST 70 Quad Cap (Long Post)

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    Bugs

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    Re: ST 70 Quad Cap (Long Post)

    Post by Bugs on Wed Nov 18, 2009 9:20 am

    Got it, thanks Bob. I studied the picture of the bottom of one of the amps you put together off your web site and could figure out the pin-out from that late last night.

    Do different quad caps have different pin-outs?

    If you look at the old Dynaco instruction manual the half circle is shown facing the bias pots with the rectifier pin 8 lead coming off of it.

    Bob Latino
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    Re: ST 70 Quad Cap (Long Post)

    Post by Bob Latino on Wed Nov 18, 2009 9:43 am

    Bugs wrote:Got it, thanks Bob. I studied the picture of the bottom of one of the amps you put together off your web site and could figure out the pin-out from that late last night.

    Do different quad caps have different pin-outs?

    If you look at the old Dynaco instruction manual the half circle is shown facing the bias pots with the rectifier pin 8 lead coming off of it.

    Hi Doug,

    Yes - different quad caps have different lug symbols ...

    The stock Dynaco quad cap had the same SQUARE, HALF CIRCLE, TRIANGLE and NO symbol as on the CE quad cap but on DIFFERENT lugs. Don't go by the symbol location on the original Dynaco quad cap when you install the CE replacement quad cap on an ST-70.

    Bob

    Bugs

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    Re: ST 70 Quad Cap (Long Post)

    Post by Bugs on Wed Nov 18, 2009 9:54 am

    That probably explains a lot of the problem I was having. Thanks again Bob.

    Luddite

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    Re: ST 70 Quad Cap (Long Post)

    Post by Luddite on Wed Nov 18, 2009 11:39 am

    Perhaps I'm stating the obvious, but wouldn't the simplest and most nearly foolproof way to confirm correct installation be as follows: First, check the label on the can, which lists the values for each section (along with their symbol), and second, wire it according to the circuit schematic.

    Charlie

    Bugs

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    Re: ST 70 Quad Cap (Long Post)

    Post by Bugs on Wed Nov 18, 2009 11:43 am

    bnc31 wrote:Perhaps I'm stating the obvious, but wouldn't the simplest and most nearly foolproof way to confirm correct installation be as follows: First, check the label on the can, which lists the values for each section (along with their symbol), and second, wire it according to the circuit schematic.

    Charlie

    Sure it might, but for someone who is new to this hobby (like me) it may not be so obvious.

    Luddite

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    Re: ST 70 Quad Cap (Long Post)

    Post by Luddite on Wed Nov 18, 2009 3:10 pm

    Sure it might, but for someone who is new to this hobby (like me) it may not be so obvious.[/quote]

    Welcome aboard, mate. I know you will enjoy this hobby tremendously. I encourage you to become comfortable with schematic diagrams. That skill will serve you well both in construction and the inevitable troubleshooting you will encounter. Also, continue to make use of this and perhaps other forums. There are folks accessible here with a wealth of knowledge and experience who are more than happy to assist you!

    Best Regards,
    Charlie

    Bugs

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    Re: ST 70 Quad Cap (Long Post)

    Post by Bugs on Wed Nov 18, 2009 3:45 pm

    bnc31 wrote:Sure it might, but for someone who is new to this hobby (like me) it may not be so obvious.

    Welcome aboard, mate. I know you will enjoy this hobby tremendously. I encourage you to become comfortable with schematic diagrams. That skill will serve you well both in construction and the inevitable troubleshooting you will encounter. Also, continue to make use of this and perhaps other forums. There are folks accessible here with a wealth of knowledge and experience who are more than happy to assist you!

    Best Regards,
    Charlie[/quote]

    Thanks Charlie, I look forward to it.

    DynaElder

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    Re: ST 70 Quad Cap (Long Post)

    Post by DynaElder on Tue Dec 01, 2009 7:08 pm

    For the info of anyone else who reads this hoping to solve their problem: If the hum is in both channels, chances are good that the problem lies in circuitry common to both channels, notably the power supply. The purpose of the quad cap is to filter the pulsating DC from the rectifier into linear DC. Each stage of the quad cap reduces the ripple. Electrolytic capacitors are notoriously fickle and deteriorate even when not used. An oscilloscope can give a visual indication of a filter capacitor failing. I picked up a Heathkit for a song at a swap meet for a song a long time ago. If you are going to do your own diagnosing, it's indispensable.

    edgobb

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    Resurrection of this Thread

    Post by edgobb on Wed Dec 22, 2010 1:40 am

    Hi all,

    I've been using this thread to diagnose a very similar problem that I am having on my stock ST-70. I just got a PAS-2 and hooked it into the system. Prior to this I only had a Bellari V129 phono preamp for spinning my vinyl. I swapped it out for the PAS and got a CD player into it and tested it using both the vinyl and digital sources. (Using the appropriate inputs, of course.) I pushed the amp harder than I have since I've gotten it. That PAS really gives me a lot of gain compared to the Bellari and for the first time since I've had the Dynaco (about 4 years) I really rattled the windows yesterday and this morning.

    This morning came the hum and vibration. At first i thought it was only a physical hum actually in the chassis. It was amplified and intensified as I shifted the top part of the chassis around on top of the bottom plate. I played with it for a while and discovered a very soft hum, equal in both channels, coming from the speakers as well.

    I am/was afraid that I somehow blew my rectifier tube. I just sprung (about two months ago) for an NOS Mullard GZ34 and have been careful not to short cycle the thing, but crazier things have happened to me. It would just really be a bummer to have that kind of $$$ get wrecked so quickly.

    So anyway, finally to the point. I've followed the suggestions in this thread and here is what I found:

    BTW, my line voltage is 122VAC.

    When I tested the resistance of the quad cap, all four sections leveled off at between 160-175 k ohms. It was weird because the resistance reading started high and decreased, which just ain't right. I thought that the auto-ranging function of my DMM was just making it go crazy, so I disabled the auto and just set it for M ohms. In each instance it would start at anywhere from 6-12 M ohms, and then after a few seconds settle down to .16 or .17 M ohms. So to me, this seems to say that my quad cap has bitten it.

    I next tested all of the voltage test points as published in the original manual and here's what I found:
    (This includes all pins of all valves, the quad cap, the selenium, and eyelets 3 & 18. With the proper AC and DC settings toggled on the DMM as appropriate.)

    - All of the AC voltages were slightly high...probably proportionately to my higher line voltage.
    - All of the DC voltages were low. Not within an acceptable tolerance at all but consistently low, about 60% of what they should be.
    - The AC pole of the selenium sandwich was just fine, but the DC pole read ZERO

    - The output bias suddenly jumped to about half a volt on each side, too.

    So I feel good thinking that my quad cap went south? Any thoughts?

    I've yabbered enough and thank you very much for your help.

    ed

    (ps I've got almost all of the parts stockpiled to do a complete VTA rebuild after Christmas...it looks like just in time.)

    anbitet66

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    Re: ST 70 Quad Cap (Long Post)

    Post by anbitet66 on Wed Dec 22, 2010 8:33 am

    edgobb,

    STOP using your amp right away!

    Your DC bias on the output tubes is gone. The selenium rectifier circuit has a problem. You should have a DC reading here at least in the mid 30volt range, not zero. Either a short, or more likely, your selenium rectifier is shot. A silicon diode is needed; a 1N4004 to a 1N4007 is fine. A trip to Radio Shack is in your future. This is causing a heavy load to be seen by the rectifier tube and the resulting low DC voltages in your amp. You may even see the output tubes "red plating" or glowing brightly. Check for wiring problems in the bias circuit.

    Anthony

    edgobb

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    Re: ST 70 Quad Cap (Long Post)

    Post by edgobb on Wed Dec 22, 2010 10:57 am

    Anthony,

    Thank you for the guidance. Truth be told however, I shorted the selenium rectifier when I was testing it, and am pretty sure that I blew it at that time. Embarassed

    I replaced it with a 1N4007 diode this morning and all of the readings are the same as they were in my above mentioned post. (Except that I get a reading of 71VDC on the (-) end of the diode now.)

    Bias circuit wiring looks good and I don't see any cold solder points.

    Any thoughts?

    ed


    anbitet66

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    Re: ST 70 Quad Cap (Long Post)

    Post by anbitet66 on Wed Dec 22, 2010 11:53 am

    My only thought here is the rectifier tube (?). Not sure. The output bias is only 0.5 volts? That would be kind of low, but then the DC voltages you got would be high. How does the rectifier look? Any unusual glowing, or otherwise? Double check that -71VDC on the circuit board at eylets 1 and 2. Or at pin 5 on the EL34's; just don't be using speakers as a load. Use an appropriate load resistor (resistance and wattage). Or junk speakers just in case.

    edgobb

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    Re: ST 70 Quad Cap (Long Post)

    Post by edgobb on Wed Dec 22, 2010 12:14 pm

    Rectifier looks fine. I've got nothing that looks out of place. It is a new tube, but I don't have a tester. I'm probably in denial and hoping that my quad cap went, cause I have another one sitting here. A GZ34, however....not handy.

    I've got it on my bench and have been testing without a speaker load today. I'm gonna have to drag out a junk speaker from the garage, as I don't have time to build a load testing circuit. Smile

    I should say that the 71VDC is actually -71VDC. The voltages at eyelets 1 and 2 are both about -29VDC, but eyelets 22 and 23 are -35VDC. The output tubes pin 5s all correspond, of course.

    I've got to leave the shop for a while, but will test with a load upon my return. Man, I really hope that tube isn't fried. Sad

    Thanks again.

    ed

    anbitet66

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    Re: ST 70 Quad Cap (Long Post)

    Post by anbitet66 on Wed Dec 22, 2010 12:52 pm

    Ok,

    a quick way to test is to replace the rectifier with diodes. Attach a couple of 1N4007's to the rectifier socket. Anodes to pins 4

    and 6 and cathodes joined in pin 8. Doesn't have to be fancy, just bend the leads to fit snugly in the socket. Take your readings

    and see what happens. If the voltages are not much higher, then you have me stumped.

    Anthony

    edgobb

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    Re: ST 70 Quad Cap (Long Post)

    Post by edgobb on Wed Dec 22, 2010 3:35 pm

    Well. No dice. Voltage readings still the same. (except eyelets 3 & 18 now read zero as well.)

    You're stumped. (And so am I.)

    I'm going to replace the quad cap and see if it gets me anywhere, i guess.


    anbitet66

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    Re: ST 70 Quad Cap (Long Post)

    Post by anbitet66 on Wed Dec 22, 2010 4:23 pm

    Good luck.

    It probably feels better that you know the 5AR4 is not to blame. If the capacitor is weak, it may cause the problem, but I would have

    thought it would have much more hum. Lets hope this is it.

    Anthony

    edgobb

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    Re: ST 70 Quad Cap (Long Post)

    Post by edgobb on Wed Dec 22, 2010 4:37 pm

    Anthony,

    Thanks so much. I really appreciate your help.

    Ed

    tubes4hifi
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    rectifier mod

    Post by tubes4hifi on Wed Dec 22, 2010 5:17 pm

    everyone should invest 25 cents in their amp to replace the selenium rectifier!!
    everyone should invest 50 cents in their amp to mod the rectifier as was shown in this other post here:
    http://dynacotubeaudio.forumotion.com/t512-rectifier-modification-for-the-gz34
    also, as to testing the quad cap, it will always discharge while testing, but it should stay above about 1 Meg-ohm, lower than that and it's leaking too much. That's about 1/2 ma of current leakage at 500v.
    Bias voltage on EL34s is typically around -35vdc, which you can safely measure at pin 6
    (where the 1K resistor connects to pin ) on the output tubes, not directly on pin 5

    edgobb

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    Re: ST 70 Quad Cap (Long Post)

    Post by edgobb on Thu Dec 23, 2010 12:50 am

    Ok. I put in the new quad cap and DC voltages are back up. The hum has disappeared and everything seems to be functioning normally. My left channel bias won't come up past 1.2VDC and the right is normal. I'll trade the left and right output tubes tomorrow and see if the low bias follows the tubes. (too hot to mess with right now.)

    Thanks Roy, for the clarification as well.

    Ed

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