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    ST-70 VTA left channel driver problem

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    w_barton

    Posts : 4
    Join date : 2014-08-15

    ST-70 VTA left channel driver problem

    Post by w_barton on Sat Aug 16, 2014 7:25 pm

    Hi,
    I am new to this board and I came here to gain your help for a problem that I am having with an ST 70 with a Vta board. The left channel has incorrect nominal voltages. The voltage measured are as follows. (The measurements were made without any tubes installed other then the 5AR4).
    Pin1 = 550Vdc

    Pin 2 = 493

    Pin 3,8 = 534,546

    Pin 6 = 550

    Pin 7= 499

    R17 and R 15 smoked with the application of power. I have an older VTA board with the orange drop capacitors.

    I check for shorts, but I didn't find any. The resistant path from B+ to all the different circuits and their resistance were nominal.
    I unloaded the B+ cable and the voltage was somewhat higher(500 volts).
    I have met a dead end on all the testing I have done. So now I come here seeking help from savvy and professional techs who are well versed with this amplifier. Any help you can provide will be greatly appreciated

    Warren

    Bob Latino
    Admin

    Posts : 2415
    Join date : 2008-11-26
    Location : Massachusetts

    Re: ST-70 VTA left channel driver problem

    Post by Bob Latino on Sat Aug 16, 2014 7:52 pm

    Warren,

    Do not operate any tube amp with JUST the rectifier installed. Unless you have driver and output tubes in the amp to act as a "load" the AC and DC voltages within the amp will be much higher than normal and these voltages will be "out of range" (as you have found). The higher voltages may not only burn out resistors (as you have done) but also put certain capacitors at risk by allowing them to look at higher than normal voltages.

    Replace the burned out resistors and do a full voltage check on the amp with all tubes installed. Use the chart below as a guide ..

                              GZ34/5AR4 – Pin 2 to ground – 420 - 450 volts DC
                                                Pin 8 to ground – 420 - 450 volts DC
                                                Pin 4 to ground – 355 - 380 volts AC
                                                Pin 6 to ground – 355 - 380 volts AC

                Any EL34 > Pin 1 or pin 8 to ground – approx .400 volts DC (depends on bias setting - center the bias pots for this test)
                                              Across pins 2 and 7 – 6.0 – 6.5 volts AC
                                              Pin 3 to ground – 400 - 435 volts DC
                                              Pin 4 to ground – 400 - 435 volts DC (2 or 3 volts higher than pin 3)
                                              Pin 5 to ground – minus 30 to minus 40 volts DC
                                              Pin 6 to ground – minus 30 to minus 40 volts DC

                                     Quad cap –-- Section # 1 (square symbol) –------ 420 – 450 volts DC
                                                        Section # 2 (half circle symbol) --- 420 – 445 volts DC
                                                        Section # 3 (NO symbol) ----------420 – 445 volts DC
                                                        Section # 4 (triangle symbol) ------  380 – 420 volts DC


    Bob

    w_barton

    Posts : 4
    Join date : 2014-08-15

    thank you for the quick response Bob

    Post by w_barton on Sun Aug 17, 2014 1:44 am

    I will implement your answer and let you know the results. The quad cap must be the OEM version (30-20-20-20 uFd). Is the orientation of the cap on the board critical ? Thank you again Bob for your help.

    Warren

    Bob Latino
    Admin

    Posts : 2415
    Join date : 2008-11-26
    Location : Massachusetts

    Re: ST-70 VTA left channel driver problem

    Post by Bob Latino on Sun Aug 17, 2014 8:42 am

    w_barton wrote:I will implement your answer and let you know the results. The quad cap must be the OEM version (30-20-20-20 uFd). Is the orientation of the cap on the board critical ? Thank you again Bob for your help.

    Warren

    If you have a 30, 20, 20, 20 quad cap on your amp, more than likely you have the original Dynaco quad cap or a replacement cap with the same value to the sections as the original cap. An 80, 40, 30, 20 cap from Dynakitparts is a better choice as a replacement cap. Just have the "80" section face the back of the amp and all the other sections of this cap will be in the correct position.

    Bob

    w_barton

    Posts : 4
    Join date : 2014-08-15

    Re: ST-70 VTA left channel driver problem

    Post by w_barton on Tue Aug 19, 2014 4:27 pm

    I installed all the tubes and it brought everything back to its nominal voltages except for the for the right channel -50Vdc. It reads 200 Vdc and keeps wanting to fry R17. Pin 6 of 12AT7 has 204 volts instead of the expected 300. It was the right channel that was originally had the problem rather than the left that I mentioned previously. Where is the -50volts coming from?

    zx

    Posts : 194
    Join date : 2011-08-05

    Re: ST-70 VTA left channel driver problem

    Post by zx on Tue Aug 19, 2014 5:24 pm

    Your Bias??%..............only the bias would be about -50V ...maybe sometimes....funky

    BOB........




    thanks for the site Bob.......

    Bob Latino
    Admin

    Posts : 2415
    Join date : 2008-11-26
    Location : Massachusetts

    Re: ST-70 VTA left channel driver problem

    Post by Bob Latino on Tue Aug 19, 2014 5:48 pm

    w_barton wrote:I installed all the tubes and it brought everything back to its nominal voltages except for the for the right channel -50Vdc. It reads 200 Vdc and keeps wanting to fry R17. Pin 6 of 12AT7 has 204 volts instead of the expected 300. It was the right channel that was originally had the problem rather than the left that I mentioned previously. Where is the -50volts coming from?

    Check that (right channel) R24 and R26 are 1,000,000 (1 meg) ohms. I hope that you have a capacitor tester. Measure the value of C2 and C4 and see if you get about .10 uF. Also check that there is no continuity across C2 and C4.

    If these measurements come up as OK, then measure and compare the value of every driver board resistor on the LEFT channel (odd numbered resistors) with their RIGHT channel counterpart (even numbered resistors). In other words R1 should have the same value as R2, R3 should have the same value as R4. The only thing that won't apply is R39 which is usually a 10K or 7.5K resistor on the VTA ST-70 board.

    Bob

    w_barton

    Posts : 4
    Join date : 2014-08-15

    Left channel issue resolved

    Post by w_barton on Fri Aug 22, 2014 2:40 am

    Just an update to the issue with the Dyanco ST-70. Before I start, I just to thank Bob and ZX for your valuable inputs to the problem.
    I did what Bob recommended and everything was normal. I then decided to go back to step one and disconnected all the Resistors and caps that I thought would be a factor to this problem. I then did check the continuity of the various pins on the tube holders to insure that there were no internal short between cathode and grid or plate. The resulting reading proved to be normal and then I removed one of the 1M voltage divider and found that the voltage had disappeared. I found the problem and the problem was that the R25 (1Meg) resistor laid  over the 300 volt run. Over time, the 300 volts broke down the wire wound resistor and started arcing to the resistor. Thus, transferring the 300 volts to the grid circuit and to R17 which at the voltage value, it was generating close to 8 watts of power. No wonder the 1/2 watt resistor was turning to ashes. I removed the 1 Meg resistor and installed it on the bottom side of the PC board. Now there is ~ 30 volts on the grid and R17 has no heat issues.
    May I make some recommendations to reduce this issue on other kits of this version that I am working on.
    1. Mount R25 on the bottom side of the board and if you  find that you need to mount the resistor on the top part of the board, use a  carbon resistor, rather than a wire l wound resistor and put a spacer between the resistor body and pc board before soldering.
    I had the power amp playing for  a while before it the right channel dropped out. I will address that tomorrow.  
    The amp was warm and detailed and sounded pretty good for what I was using to drive it.
    I was using an Infinity bookshelf and Luxman (TP-117?)preamp. For a media player I was using the Samsung Tab 7.

    And thank you again for your help.

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