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    Lost left channel on St-70

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    4MarkNY

    Posts: 18
    Join date: 2009-10-13

    Lost left channel on St-70

    Post by 4MarkNY on Wed Oct 14, 2009 12:43 pm

    I was just a few minutes into a listening session, when the left channel faded out, not to return.
    I first tried swapping the interconnects, no change, same side. I next swapped the 7199s, noticing that the left front power tube and the 7199 on the left side were only warm, while the other power tubes (including the back left one) where up to temp.
    The 7199 now came up to bright and temp when moved to the right. The other now, dark and cold....
    The power tubes did the same when swapped, ie; everything on the right, and the back left tube were bright and hot, while the left 7199 and the front left el-34 were only midly warm ( actually didn't notice at the 7199 at all.)
    I don't have a spare rectifier tube (got one on order now, tho.) I do have a Sylvania 5v4ga , and a RCA 5r4xxb in the heathkits.
    Is this likely a rectifier tube gone bad?
    Opinions?
    I'm going to follow the Curcio audio engineering "Dynaco Stereo 70 repair guide" as I delve into this, but not until I change out the rectifier tube, as the possible culprit.
    As you might guess, I'm fairly new to tubes, and without a tube tester, or even a good substitution manual.....

    Thanks,
    Mark

    GP49

    Posts: 467
    Join date: 2009-04-30

    Re: Lost left channel on St-70

    Post by GP49 on Wed Oct 14, 2009 12:48 pm

    The rectifier tube is not likely to be your culprit. Its DC output feeds ALL the tubes. If it went bad, NONE of the tubes would have any plate voltage and you would have NO warm tubes at all.

    There will be no harm done by plugging in a 5V4 to test, but it won't likely make any difference.

    Did you look to see if any of the tube filaments were out? I do not have a Stereo 70 at hand right now to see how the filaments are wired up but there could be a bad solder connection somewhere in the filament wiring that would interrupt the filament feed to those particular sockets.

    Bob Latino
    Admin

    Posts: 1821
    Join date: 2008-11-26
    Location: Massachusetts

    Re: Lost left channel on St-70

    Post by Bob Latino on Wed Oct 14, 2009 3:22 pm

    GP49 is correct - If you lose the rectifier neither channel works. The problem is somewhere on the left channel probably on the left side of the driver board either a capacitor, resistor or a bad solder trace. A few things to try ..

    1. With the amp off measure the resistance across each resistor on the BAD LEFT channel and compare it to its MIRROR RESISTOR on the good right channel. If you find a large descrepancy between the value of a resistor on the good right channel and the same resistor on the left channel you most likely have solved your problem.

    2. If you have a capacitance meter - do the same thing with each capacitor comparing the good right channel's value with value of the same capacitor on the left channel.

    3. Examine carefully with a magnifying glass all the solder traces AND solder connections on the bad left channel. Reflow any suspect solder joint or solder trace.

    4. Measure the value of the 15.6 ohm resitor on the left channel and compare it to the same resistor on the right channel. (this resistor rarely goes bad but could?)

    Understand that some ST-70's are now 50 years old and the brown phenolic board that they were made of was not of the greatest quality. If you intend to play this amp on a daily basis you are probably better off in the long run to just replace the entire driver board and all driver board parts with a new driver board and parts set (about $50). This is not that hard a job. You just solder the parts onto the board, pull the old board, install the new board and reconnect the wires.

    Bob

    4MarkNY

    Posts: 18
    Join date: 2009-10-13

    Lost left channel on St-70

    Post by 4MarkNY on Wed Oct 14, 2009 7:40 pm

    GP49, Bob-
    Thanks for the info, what I don't know would fill a book!
    I saw a $50.00 replacement board on ebay , but it would be taking the slow boat from China. A couple of used ones too, but that would only add to the trouble IMHO.
    Where do you recommend I source a new board for the stock 7199 config? Or should I be looking for the updated Triadelec board, or similiar?

    Thanks
    Mark

    Bob Latino
    Admin

    Posts: 1821
    Join date: 2008-11-26
    Location: Massachusetts

    Re: Lost left channel on St-70

    Post by Bob Latino on Wed Oct 14, 2009 7:58 pm

    Hi Mark,

    If you go with a stock board I recommend the replacement stock driver board and parts set from Dynakitparts at the link below.

    Dynakitparts replacement ST-70 stock board

    If you want an upgraded driver board my favorite is the VTA driver board at the link below.

    VTA ST-70 driver board

    Bob

    4MarkNY

    Posts: 18
    Join date: 2009-10-13

    Lost left channel on St-70

    Post by 4MarkNY on Thu Oct 15, 2009 8:58 am

    Oddly, when I went to verify the situation yesterday after it sat untouched overnight, it started up with both channels, ran for 10 minutes, then blew the fuse, replaced, back to left channel out.
    I will be ordering a VTA driver board. which leads me to more questions: Hi gain vs. lo gain. I don't want to listen to a "over-sensative" amp for lack of a better term. So my presumption would be that the low gain would be for me. what are the trade-offs of one over another.(yes, different tubes, but I come here with no existing bias/knowledge)
    Should I order the stuffed board ? put it together? My point is to learn here, so I suppose I should DIY. I've replaced capacitors in KLH model sixes, through the 4 inch hole in the back. my soldering skills could only improve, I suppose...But how exactly do you guys hold a soldering iron, a roll of solder and the wire against a termination point with only two hands?
    Lastly, I was thinking that I would order/change out the annoyingly small inputs at this time on this chassis,...or is the cutout in the replacement chassis bigger too?

    thanks-
    Mark

    Bob Latino
    Admin

    Posts: 1821
    Join date: 2008-11-26
    Location: Massachusetts

    Re: Lost left channel on St-70

    Post by Bob Latino on Thu Oct 15, 2009 9:52 am

    Hi Mark,

    The VTA "high gain" board is really a medium gain board and uses 12AT7 driver tubes. A true "high gain" version of this board would use 12AX7 tubes. The lower gain VTA driver board uses 12AU7 or 12BH7 tubes. Both versions sound about the same to me.

    The more modern input jacks have a 1/2 inch spacing instead of the old 3/8 inch spacing. The cutout in the original chassis and any replacement chassis's are the same. It is just that the two holes that hold the RCA input jacks are further apart. These input jacks may be used on any ST-70 chassis..

    You should probably Email Roy Mottram directly about the VTA driver boards and the input jacks at > roymottram@yahoo.com or visit his tubes4hifi web page on the VTA ST-70 driver board at the link two messages above.

    Bob

    tubes4hifi
    Admin

    Posts: 915
    Join date: 2008-11-30

    filament power on one side

    Post by tubes4hifi on Fri Oct 16, 2009 1:05 am

    Hi Mark,
    before I read you got it (temporarily?) working again, I would have thought maybe your power transformer had lost all filament output on one side (that would cause both the output tubes on one side and ONE of the 7199 tubes to not light and be cold. It sounds like you've got a cold soldier joint, bad wiring connection, or a filament de-coupling capacitor problem.
    Most likely a bad soldier joint from old age or wires being moved around.
    I've probably worked on over 100 ST70s over the years, and I know Bob has also.
    I'd say 90% of them had 7199 tubes that were worn out. Hard and expensive to find replacements.
    A new VTA board will not only replace 90% of the amplifier parts, but will transform your 50 year old amp into essentially an all new amp except the transformers, chassis, and your output tubes.
    For under $150 you can have an amp that blows away just about anything available under $2000 with an incredible transformation of the sound quality (IMHO). If you spend the same amount of money on a stock replacement PCB and new 7199s (if you can get all that for that price!!) you'll wind up with new parts that sound 50 years old. I do know some people want that very vintage sound, which is why the old PAS3 preamp is still popular, but when the same money can buy something so much better it should be an easy decision to upgrade.
    Roy

    4MarkNY

    Posts: 18
    Join date: 2009-10-13

    Lost left channel on St-70, now returned....

    Post by 4MarkNY on Fri Oct 16, 2009 7:18 am

    Well, last night I was able to attend to the internal exam of the amp. Sure enough, there was a loose wire on V-3, 7. I soldered it back in place and looked for any other candidates, but everything else seemed reasonably secure. I wasn't very impressed with the wiring job in that corner, but I suppose that it's worked like that for a long time.
    It went back in place and fired up nicely.
    I will re-wire that section when I install the new board this winter, but only after I'm able to work the next trouble, which is a hum in the other setup hardware, a pair of Heathkit A-9c amps that has gotten a buzz in the left amp (monoblock setup).
    Thanks for your help, and pleased to find a forum with such a high level of experience, and willingness to help those on the front end of the learning curve.

    Mark

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