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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


2 posters

    mk3 voltage

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    JohnS


    Posts : 6
    Join date : 2023-01-04
    Age : 73
    Location : Fort Worth,Texas

    mk3 voltage  Empty mk3 voltage

    Post by JohnS Thu Jan 05, 2023 5:38 pm

    Hi I am new to forum and to rebuilding amps. I was testing voltage on my amps to check the transformers. I watched several of the youtube videos one unit checked out fine the other witch had a Z 216 power transformer gave me voltage on pin 3 on both sockets. On pin 4 I got no voltage. I then checked pin 5 and got neg. voltage. Not being smart enough to check out anything else I removed the transformer and check for continuity. I got proper reading on the transformer. I am going to rebuild with this transformer but wondering what I might also need to replace. the amps probable didn't have more than 100 hours since new from 1969 I got them from the company that bought them new they were going to be the sound system for a merry go round the company build two units and had these two amps left over they stayed in storage until I found them 20 years ago. I traded them some tools for them tools cost about 20 bucks good deal.
    WLT
    WLT


    Posts : 178
    Join date : 2013-07-13
    Location : Rochester NY

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    Post by WLT Fri Jan 06, 2023 10:29 am

    Z 216. What is the transformer model/info on the other amp? What model amps are these? No mater what the $20 is well spent. Or a few pictures would help.
    WLT
    WLT


    Posts : 178
    Join date : 2013-07-13
    Location : Rochester NY

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    Post by WLT Fri Jan 06, 2023 10:32 am

    I do see the MK III referrence, But no voltage on pin 4. Is the GZ34 installed?
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    JohnS


    Posts : 6
    Join date : 2023-01-04
    Age : 73
    Location : Fort Worth,Texas

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    Post by JohnS Fri Jan 06, 2023 1:59 pm

    The other amp has a A431. I install the GZ34 I got voltage on the blue and blue-white wires. I also checked for broken wires out of transformer. these two amps were factory wired according to serial numbers they were manufactured about 4 mouths apart in 1969





    WLT
    WLT


    Posts : 178
    Join date : 2013-07-13
    Location : Rochester NY

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    Post by WLT Fri Jan 06, 2023 9:25 pm

    After re reading both of your posts I am trying to make sure I understand what you did and under what conditions. If I am misstating your test results please correct. What I think you said was...

    Amp 1 has the Z 216 output transformer. You put the GZ34 rectifier in and measured the voltages at one output tube. You did not state it was happening on both output tubes the same. You had high voltage (+/-480VDC) on pin 3 (plate connection), 0 VDC on pin 4 (screen grid), negative voltage (-55VDC) on pin 5. If so that is not good as pin 4 should have almost the same voltage as pin 3.

    Amp 2 has the A431 output transformer. GZ34 installed. It has both output tubes pin 3 and pin 4 with about 480 VDC. That is what it should be.

    Back to amp #1. Do both output tubes have 480VDC on pin 3? Check but they should. Then do both tubes have 0VDC on pin 4. Check as that would be odd. If only one tube has 0VDC on pin 4 then you may have an open inside the transformer on that winding. Do the solder connections all look good. Some times they are not making a good connection.
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    JohnS


    Posts : 6
    Join date : 2023-01-04
    Age : 73
    Location : Fort Worth,Texas

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    Post by JohnS Fri Jan 06, 2023 10:44 pm

    I checked the solder connection they were good I could not find anything wrong, so I removed the transformer thinking that maybe the wires were broken inside the end bell. So I took my meter and checked the resistance of the transformer it checked out within a few ohms of the test Bob Latino had got in a post a few years ago. That is when I joined this forum. I thought maybe something was going to ground but I should have got some voltage. I have taken the hold thing apart and cleaned it up they had a lot of steel dust on them because of where they were stored [stored on a shelve in a open box at a train car manufacturing plant]. I guess I will order parts manly some caps put it back together and see what I get then. the boards after cleaning look almost new. Just wondering beside the board caps [.02, the two .25 and the two 50mfd] anything else should be changed.
    WLT
    WLT


    Posts : 178
    Join date : 2013-07-13
    Location : Rochester NY

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    Post by WLT Sat Jan 07, 2023 11:54 am

    Well yes. The quad cap is always the most suspect. If you can test all four sections for uF, ESR and leakage current and they test fine then use it for now to get the bugs worked out. Good news is that you have a good DC resistance on the screen grid winding. I assume you measured both tube connections.

    If you are going to replace those parts then testing all the resistors makes sense as well. Or just replace them along with the small feedback caps (pF) anyway. Once done with all caps and all resistors you should not have any problems with the amps for a long time. Just Radios has a nice supply of pF caps and all the other ones needed except the quad cap. Dynakitparts.com has the quad caps and some other parts. Let us know of your progress.
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    JohnS


    Posts : 6
    Join date : 2023-01-04
    Age : 73
    Location : Fort Worth,Texas

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    Post by JohnS Sat Jan 07, 2023 12:28 pm

    I think I found the voltage problem after cleaning the driver board it dawned on me about the problems that fine dust gave me at work. It would get in my electrical panels stuff would blow up. I checked the other amp #2 I got a little resistance across some of the resistors and caps in the middle of the boards it did clean off with a toothbrush and alcohol. I have checked the board nothing seems to have taking it tuff. I am going to change all tube sockets and quad caps. I guess not turning up the voltage during test was a good thing I just put enough voltage to make sure things worked about 80 volts glad I did go any higher
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    JohnS


    Posts : 6
    Join date : 2023-01-04
    Age : 73
    Location : Fort Worth,Texas

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    Post by JohnS Sat Jan 07, 2023 12:30 pm

    Will send photos when I figure out how the week is about up so I can
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    JohnS


    Posts : 6
    Join date : 2023-01-04
    Age : 73
    Location : Fort Worth,Texas

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    Post by JohnS Thu Jan 19, 2023 12:56 am

    I think I finely think I have figured out how to post photos to this forum. I cleaned the driver board all resistors checked out the 390mmf capacitor would not give me continuity from either direct [ I am using a 117 fluke meter}.before I cleaned the board I was getting about 6 megs through the dust I am assuming that might be my problem. here is photos of boards before and after and a photo of amp before any cleaning.

    https://i.imgur.com/YQPkgu5_d.webp?maxwidth=128&shape=square
    https://i.imgur.com/WyMSrYW_d.webp?maxwidth=128&shape=square
    https://i.imgur.com/qYLlCIl_d.webp?maxwidth=128&shape=square

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