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    Mark III rebuild troubleshooting...no output!!!

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    steve's garage

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    Mark III rebuild troubleshooting...no output!!!

    Post by steve's garage on Tue Nov 03, 2015 12:03 am

    I have basically rebuilt a Mark III. I changed the output transformer, all of the caps, any resistors that were a little out of spec, new tubes etc.
    But it has come to a pretty frustrating puzzle.

    I have tested the voltages and I think everything seems right. 515v on the B+, 5.2v on the GZ-34 pins 8 and 2, 6.64V on the green filament heaters, and it is biasing fine to 1.56v. But still I am getting no output.

    Going through with an audio probe I find that the signal is good all the way until the 6550's coming from board points 1 and 3. The audio signal on pins 3 and 4 of each 6550 is a hum/buzz though and not anything close to the incoming audio signal. Should it be? Is this evidence of bad tubes? I have gone as far as buying a second set of tubes kt88's and still get the same result(they aren't matched though). Even swapping the rectifier tube has no result.

    The only things I have left to suspect seem unpromising. The power transformer has a blue and white wire instead of a red/black one going to the diode. Also the diode was updated sometime previous to my ownership. But if the amp is biasing right and voltages seem good I can't see how either of those could be a problem.

    Edit: I neglected to say that the output jacks are something I'm suspicious of. This Mark III is wired with 1/4" jacks. Should the ground from the OT be connected to each jack, to another ground, and then go to the speaker?

    Any help would be huge.
    Steve


    Last edited by steve's garage on Tue Nov 03, 2015 12:30 am; edited 1 time in total (Reason for editing : Small mistakes.)

    MontanaWay

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    Re: Mark III rebuild troubleshooting...no output!!!

    Post by MontanaWay on Tue Nov 03, 2015 10:29 am

    the ground speaker post and the negative, usually black, of the OPT, must be connected to chassis star ground, (and ONLY the ground speaker post) so should the B+ ground and driver pcb ground, all should go to the common chassis star ground. And if you're using a 3 wire mains cord, that ground too should be connected to the same star ground!
    So, connect the negative of the OPT to the ground/black speaker post, then run a heavy gauge wire, usually the same gauge as the speaker wire, to chassis star ground.

    steve's garage

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    Re: Mark III rebuild troubleshooting...no output!!!

    Post by steve's garage on Tue Nov 03, 2015 2:32 pm

    Hey thanks Montanaway...everything you've described is what I assumed and how the amp is. I only doubted on the output ground because the output leads are already so close to grounded anyway and in my experience jacks are often the problem is you get no output but everything tests ok.

    I'm going to go over everything for the millionth time i guess, hopefully I'll find the culprit. Still it's odd that everything tested good and has been triple checked yet im getting ZERO output.

    corndog71

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    Re: Mark III rebuild troubleshooting...no output!!!

    Post by corndog71 on Tue Nov 03, 2015 4:57 pm

    Sometimes it's also the little things like checking your speaker connections, interconnects, source (is on and works with other devices), preamp (is on and works with other devices), etc.

    PeterCapo

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    Re: Mark III rebuild troubleshooting...no output!!!

    Post by PeterCapo on Tue Nov 03, 2015 9:36 pm

    steve's garage wrote:I'm going to go over everything for the millionth time i guess, hopefully I'll find the  culprit. Still it's odd that everything tested good and has been triple checked yet im getting ZERO output.

    Can you post a series of photos?

    Why did you change the output transformer and what exactly did you change it with?

    Is the power transformer original or an exact replacement like the ones dynakitparts and Triode USA sell? What are the voltage readings on both sides of the choke and at eyelets 4, 5 and 6?

    What is the AC voltage reading on the bias diode's cathode and what is the DC voltage reading on its anode? What is the value and voltage rating of the two electrolytics in the bias supply?

    steve's garage

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    Re: Mark III rebuild troubleshooting...no output!!!

    Post by steve's garage on Wed Nov 04, 2015 2:21 am

    To answer Corndog: I checked and always check that stuff religiously.

    Petercapo: Those are the kind of questions I was hoping for. This is a great forum and it's the only thing keeping my frustration/anxiety from boiling over right now. I've been fixing guitars all night and I'm not feeling fresh enough to do all those tests but tomorrow I will get back to you and add some pics as requested. I must admit its looking pretty sexy with those orange drops.

    Bob Latino
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    Re: Mark III rebuild troubleshooting...no output!!!

    Post by Bob Latino on Wed Nov 04, 2015 9:02 am

    Since "steve's garage" is a new member, he actually has a "trial" membership in the forum, and won't be able to post photos or links for one week. (until Sunday Nov 8.) If the member wants to send me some wiring photos at this Email address > Bob01605(at)aol(dot)com, I will be sure to post them in this forum thread.

    As PeterCapo has mentioned - If you changed out the output transformer, what did you change it out with? If it is a Triode Electronics or Dynakitparts output transformer, they will both have the same wire colors as the original Dynaco transformer. If it is a Hammond, Edcor or some other brand, they will have other wire colors and coding. One thing to check - make sure that you have not swapped the positions of the two plate leads (the BLUE and BLUE/WHITE wires) If you swap these two leads, then the output transformer will be out of phase and the amp won't work.

    Bob

    steve's garage

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    Re: Mark III rebuild troubleshooting...no output!!!

    Post by steve's garage on Thu Nov 05, 2015 5:35 pm

    I used the proper replacement OT from triode electronics. It was the power transformer that has a couple of different colored wires, which is the correct model number as well, but instead of red wires from the PT they're grey. The Red/Yellow is just yellow and the red/black is blue and white or black and white and possibly faded over the years.

    In response to PeterCapo Voltage are as follows:
    Choke to pin 8 of rectifier: 520vdc and 6vac which slowly rises
    Choke to B+: 510 VDC and similar to the previous reading in AC..now at 6.4vac and still climbing slowly
    Eyelet 4 -511vdc
    Eyelet 5 - 477vdc
    eyelet 6 - 436 vdc
    Bias diode cathode - .17vac
    Diode Anode - fluctuating between .250vdc and .365 vdc
    Bias caps -
    #1(the one after the diode) is rated 50uf at 75vdc and its voltage readiing is the same as the bias diode cathode(theyre connected)
    0vdc and 0vdc on the other side to ground.
    #2 after the 1k resistor rated at 68uf 400v has -58vdc and 0vac on the negative side and zeros on the plus side which goes to ground

    Sending pics to Bob Latino now.

    Bob Latino
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    Re: Mark III rebuild troubleshooting...no output!!!

    Post by Bob Latino on Thu Nov 05, 2015 6:07 pm

    Here are three photos of the wiring inside Steve's amp. The wiring has been modified which may make it more difficult to find the issue ..

    Bob







    PeterCapo

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    Re: Mark III rebuild troubleshooting...no output!!!

    Post by PeterCapo on Thu Nov 05, 2015 7:03 pm

    As Bob pointed out, this amp has had considerable changes, so all bets are off.  The bias supply readings aren't making a lot of sense referenced to the original Dynaco Mark III.  For example, I'm not sure how you have zero volts on one of the bias caps and -58 volts on the other one.  The first bias cap should not be connected to the bias diode’s cathode.

    If it needs to remain a guitar amp, and given the extensive modifications, I suggest reverse-engineering a schematic if you don’t have one to work from.  It might help to sort things out.

    If you intend to use it just for a hi-fi system, then I’d tear it down to the ground and rebuild it according to proven documentation.

    Bob Latino
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    Re: Mark III rebuild troubleshooting...no output!!!

    Post by Bob Latino on Thu Nov 05, 2015 7:20 pm

    Yes - that wiring has been altered. It looks they put in separate caps to replace the quad cap. It looks like "Bubba" had his way with the wiring on this amp. Check the pictorial below of what a Mark III's wiring should look like.

    Bob



    steve's garage

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    Re: Mark III rebuild troubleshooting...no output!!!

    Post by steve's garage on Thu Nov 05, 2015 7:46 pm

    The can cap was replaced with separate caps and is wired up correctly. The voltage ratings are increased. I do have a schematic and a wiring diagram and although the caps change the overall appearance dramatically , the amp has been brought back to stock as faithfully as possible. The only exceptions being those two bias caps. Also the switch is on the bottom of one pot, and only the volume pot has been kept in the tone circuit.

    I'll check on the bias cap not being connected to the diode.

    What voltages should I expect to see on the bias caps?

    steve's garage

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    Re: Mark III rebuild troubleshooting...no output!!!

    Post by steve's garage on Thu Nov 05, 2015 7:53 pm

    Also i was hopinh bob might have some idea whats going on with the blue and white and yellow wires coming from the power transformer. Is this something that he's ever seen? Ive retraced and checked every connection in this amp and I believe it's correct. The only mysteries to me are those two wires.

    PeterCapo

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    Re: Mark III rebuild troubleshooting...no output!!!

    Post by PeterCapo on Thu Nov 05, 2015 8:03 pm

    steve's garage wrote:...the amp has been brought back to stock as faithfully as possible. The only exceptions being those two bias caps. Also the switch is on the bottom of one pot, and only the volume pot has been kept in the tone circuit.

    I'll check on the bias cap not being connected to the diode.

    What voltages should I expect to see on the bias caps?

    With all due respect, this is far from an original Mark III.  Download a copy of the original manual and have a look at the pictorial diagram Bob posted.  The differences should be apparent.

    There was only one pot in the Dynaco Mark III, the bias pot, and it did not have a switch.  There was no volume pot or tone circuit in the Mark III.  Among other things, it also looks like your chassis was drilled and an extra tube added – again, an apparently significant departure from an original Mark III.

    In an original Mark III, the "first" bias cap should be connected to the bias diode’s anode, not the cathode.

    Given the modifications to the amp, I have no idea what is really "correct" for it.  What I was pointing out before is that it does not make sense electrically that the first bias cap has zero volts while the second one downstream does have appreciable voltage across it, if it is wired according to the original Dynaco schematic.  Doesn’t make sense, regardless of what the "correct" voltages should be.  Maybe it's because you wired the first bias cap to the wrong side of the diode?

    The wires you are describing coming from the power transformer makes it sound as if it is neither an original power transformer nor one of the direct substitutes by Dynakitparts or Triode USA.  Without knowing exactly what PT you are using and how it is wound, there is no way to know how it could be affecting your situation.

    I tend to doubt that anyone around here would have much of a frame of reference for this amplifier, but maybe someone has some other suggestions. Maybe post an image of the top side and tell us exactly what kind of power transformer it is?

    Bob Latino
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    Re: Mark III rebuild troubleshooting...no output!!!

    Post by Bob Latino on Thu Nov 05, 2015 8:28 pm

    steve's garage wrote:Also i was hopinh bob might have some idea whats going on with the blue and white and yellow wires coming from the power transformer. Is this something that he's ever seen? Ive retraced and checked every connection in this amp and I believe it's correct. The  only mysteries to me are those two wires.

    The power transformer on a stock Dynaco Mark III did not have any BLUE or WHITE wires? That power transformer must not be a stock or a stock direct replacement power transformer and that might be the problem? The BLUE and WHITE wires must be replacing some of the wires below ..

    On a stock Dynaco Mark III power transformer you should have the following AC voltages ..
    Across the two YELLOW wires > about 5 VAC
    Each RED wire to chassis ground about 410 VAC
    Across the two GREEN wires about 6.3 VAC
    GREEN/YELLOW to chassis ground about 3.15 VAC
    RED/BLACK to chassis ground about 55 VAC
    RED/YELLOW is a ground wire and should 0 VAC to chassis ground

    Bob

    steve's garage

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    Re: Mark III rebuild troubleshooting...no output!!!

    Post by steve's garage on Thu Nov 05, 2015 8:50 pm

    PeterCapo:

    I don't blame you for seeing what looks quite different from the stock Mark3. You are correct on many things. But let's be clear on this; I have digested and stared at both the original schematic and wiring diagram and completely rebuilt it on that.

    When I received it, it had been modified significantly and had an entire tone circuit which I tore down. The "extra tube" you're seeing is the preamp socket but they tried to rewire it as...well something messy. So I have returned that to stock, its just mounted on the other side because that what they did.

    The only pot that's doing anything here now is the volume pot which is as simple as it gets as a shunt to ground. The other pot which appears connected is only an on/off power switch. A removable fuse holder has been put in place and a 3 prong plug.

    Also the negative of the first cap is connected to the negative of the diode correctly upon checking it.

    The Power transformer says " DYNACO 464003 P782" and should be the P782 inside I assume, but its become the point of doubt now for me with those odd colored wire in place of what should be there. I guess I may have to order a replacement but first I think I'll test it according to bob's info.


    PeterCapo

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    Re: Mark III rebuild troubleshooting...no output!!!

    Post by PeterCapo on Thu Nov 05, 2015 8:55 pm

    That's an original PT.  Not uncommon for the leads to become discolored over time.

    Here's what I think I see in your photos...

    At the "bottom" chassis hole in your photos where the PT leads enter the chassis:

    1) the single blue & white lead may in reality be the faded red/black lead.  The red/black lead should go to the bias diode’s cathode (the end of the diode with the stripe or the tip of the arrow).

    2) the single yellow-looking lead may really be the discolored red/yellow lead.  The red/yellow should go to ground.

    3) the twisted pair of whitish-looking wires are actually faded red wires and should go to rectifier tube socket lugs 4 & 6.


    At the "top" chassis hole in your photos where the PT leads enter the chassis:

    1) the twisted pair of true yellow wires should go to rectifier tube socket lugs 2 & 8

    At some point, Dynaco transitioned from cloth covered to plastic covered x-former wires.  Along the way, some of their x-formers had both cloth and plastic, with the plastic holding their original color better.

    If in doubt about the voltage you measure at the leads in question, try measuring the lead again with it temporarily disconnected from the circuit.

    steve's garage

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    Re: Mark III rebuild troubleshooting...no output!!!

    Post by steve's garage on Fri Nov 06, 2015 1:29 am

    I'd love to believe that the red/black or in this case the blue/white which may be faded should go to the diodes cathode and fix my problems, but doesnt the diagram say otherwise? the diagram says red/black to the anode.

    PeterCapo

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    Re: Mark III rebuild troubleshooting...no output!!!

    Post by PeterCapo on Fri Nov 06, 2015 10:02 am

    On the diode body, the cathode is the end with the stripe and/or the point of the arrow, like the schematic shows.  Determination of anode vs. cathode is not a function of the polarity of the circuit.  It is determined solely by the construction of the diode.  The red/black lead (in your case "blue/white," even though it isn't really blue/white) goes to the bias diode's cathode, which, again, is the end with the stripe and/or point of the arrow.  Look at the schematic again.  It indicates the red/black wire as "R-BLK" and clearly shows it connected to the end of the bias diode having the stripe and arrow-point.

    What makes this confusing is that the bias is set up to provide a negative bias control voltage.  Dynaco accomplished this by flipping the polarity of the two electrolytic capacitors in the bias circuit, which results in a negative DC voltage on the diode's anode while there is an AC voltage (the "+" sign in the older manual's schematic) on the diode's cathode.  The diode is functioning as a half wave rectifier.  When the AC voltage from the red/black lead swings positive, the diode does not conduct.  When it swings negative, the diode conducts because the current is flowing (strange as this might sound) from ground "through" the two bias supply electrolytics and through the bias diode, because ground is actually at a greater potential than the negative swing of the transformer on the diode’s cathode.   This charges the bias supply electrolytics with a negative DC voltage for the bias supply.

    While the diode must be wired properly, whether it solves your amplifier's problems or not might be another matter entirely.

    I have looked at your photos and though I can see your "blue wire" (that I call "blue/white" and believe to actually be the faded red/black wire), I cannot see the diode and how it is currently oriented.  You might actually have the diode properly oriented already, I don't know.  Even with the correct transformer lead connected to the correct end of the diode, you may still have other issues.

    My suggestion would be to disconnect your "blue" wire, secure it somehow so that it is physically stable and does not touch anything, and then measure the voltage between the wire and the chassis.  This will tell you if it is indeed the red/black wire and if it is still functioning, provided the appropriate ground references are connected.  As Bob indicated previously, you should get something in the range of around +55 volts AC provided your meter is set to an appropriate AC voltage (not current) scale, your meter leads are connected to the right places on the meter and lead polarity is consistent (red meter lead to "blue" wire, black meter lead to chassis).  With meters, remember always to switch between appropriate AC and DC scales, voltage or current, and, if necessary, accordingly change the lead connections to the meter depending on what you intend to measure.  Easy enough to miss this kind of stuff.


    Last edited by PeterCapo on Fri Nov 06, 2015 11:30 am; edited 4 times in total (Reason for editing : Add comments. Spelling. Clarify.)

    corndog71

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    Re: Mark III rebuild troubleshooting...no output!!!

    Post by corndog71 on Fri Nov 06, 2015 11:18 am

    Maybe it's time for a complete rebuild. Strip it down. Clean up the connections. Maybe get some fresh sockets. Check the PT to make sure the raw raw voltages are what they're supposed to be. Consider replacing that choke. etc.

    It's a lot easier to troubleshoot when you build it back up from scratch. Take your time.

    PeterCapo

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    Re: Mark III rebuild troubleshooting...no output!!!

    Post by PeterCapo on Fri Nov 06, 2015 1:49 pm

    Hold on a minute. Steve, is that bluish, boxy part above and slightly to the left of your "blue" lead supposed to be the diode by any chance? If so, would you please exactly quote all of the markings on it?

    steve's garage

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    Re: Mark III rebuild troubleshooting...no output!!!

    Post by steve's garage on Fri Nov 06, 2015 4:52 pm

    Yeah that's the diode alright. One of the few remaining "inherited" parts in the machine. It's an AB1 or ABI 6637. The positive is going to the blue/white wire and the negative is going to the negative of the 50uf cap.

    PeterCapo

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    Re: Mark III rebuild troubleshooting...no output!!!

    Post by PeterCapo on Fri Nov 06, 2015 5:01 pm

    I think it's in backwards.

    Have you ever seen a datasheet for it?  I don't remember ever seeing one like it and would like to know what it is.  Looking now, but if you have anything on it please post the link.

    PeterCapo

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    Re: Mark III rebuild troubleshooting...no output!!!

    Post by PeterCapo on Fri Nov 06, 2015 5:09 pm

    I can't find anything.  If it is a regulating diode or something else fancy, things may not run properly even when you turn it around.  Without knowing exactly what kind it is, I'd highly recommend changing it out for a known compatible type like a 1N4007 or UF4007 and connect the cathode to your "blue" wire.  You may find additional problems afterward, but if you connect it the way the schematic shows, at least that part will be right.

    steve's garage

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    Re: Mark III rebuild troubleshooting...no output!!!

    Post by steve's garage on Fri Nov 06, 2015 5:11 pm

    I have checked it several times now due to you suspecting it being backwards, and my answer is the same. The end marked with a plus goes to the blue/white or red/black and the negative goes to negative of the bias cap. That is correct isn't it???

    I haven't been able to find anything on it either. It seems to be out of production. I'm ordering some parts soon for another project and was thinking I might as well throw in a new diode.

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