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    ST-70 issue

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    daveshel

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    Join date : 2011-11-06
    Location : Tucson AZ USA

    ST-70 issue

    Post by daveshel on Wed Oct 07, 2015 12:10 am

    I picked up a pretty clean factory wired ST-70 last spring - I kind of wanted to get to know the stock unit for a while before restoring it. I believe the unit is stock, but that it has seen some work this century. The output tubes are the Russian Mullards, and presumably there was some work done under the chassis at that time as well. I bought it from the owner's father, who was fuzzy on the details. I say presumably because I haven't been able to look inside. The chassis screws are that double dot screw that they used on some of the factory wired units, and I don't have that tool. I know I can easily buy a tool or drill the screw out but I didn't have any real reason to do so during this evaluation period. Up until now.

    This evening I was listening and happened to notice that one of my output tubes was glowing much more brightly than the others. Upon closer inspection, I saw that the plates of the tube were glowing bright orange as if molten. There was no issue with the sound on that channel. I turned down the volume for a few seconds, saw that the glow did not diminish, and switched off the power. I've heard of tubes 'red plating' before, and I believe I've just witnessed the phenomenon.

    My quick scan of some search results leads me to believe that this is likely caused by power supply capacitor failure - and I may well be dealing with an original quad cap. Just thought I'd get some thoughts from you guys before I figure out how to get past those pesky screws...

    PeterCapo

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    Re: ST-70 issue

    Post by PeterCapo on Wed Oct 07, 2015 12:21 am

    My suggestion would be to painstakingly clean (interdental brush, 99% isopropyl, blow out with compressed air) and re-tension tube socket contact #5 for all four of the EL34 sockets.

    Bear in mind that if the tube red plated enough, the tube may now be gassy.

    After cleaning and re-tensioning, shuffle the tubes around to see if the problem follows the tube or stays in the same socket - or hopefully goes away.  Other things could be wrong, but IMO the socket contacts are the place to start.

    GP49

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    Re: ST-70 issue

    Post by GP49 on Wed Oct 07, 2015 1:17 am

    It is unlikely that a power supply problem would cause ONE tube to red-plate, since the power supply feeds all four output tubes. If you do as PeterCapo says, and the problem stays with the same socket, the socket contact could still be bad, or you could have an open 1000Ω resistor to pin 5...or a leaky coupling capacitor on the printed circuit board, feeding that tube; or a bad solder connection anywhere along that part of the circuit.

    daveshel

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    Re: ST-70 issue

    Post by daveshel on Wed Oct 07, 2015 2:10 am

    I tried swapping in a tube from a backup set, and shut it down when it started lighting up funny. Looks like I gotta bust those screws and see what it looks like below decks, examining resistors and any other components on that tube socket.

    daveshel

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    Re: ST-70 issue

    Post by daveshel on Thu Oct 15, 2015 10:02 pm

    Finally got past those screws. Below decks looks very clean. Some of the caps look newer than I'd think they would after 50 years - particularly the green 50 MDSs on the terminal strip and the 5.6 MFDs on tube sockets V2 and V7.

    But I see some things I didn't expect:

    1. The 1K resistors that should be connected between pins 5 and 6 of the output tube sockets is instead connected to the circuit board directly at points 1, 2, 22 and 23. This rings a bell - I've read about that mod somewhere. The odd thing is that the #6 lugs on the tube sockets are clean as in they've never had anything soldered there. I find this strange for a factory wired unit.

    2. The sockets on the front panel that are used for bias set and PAM preamp power are not wired, with the exception of a single wire to each socket on the pin used for bias set. Maybe this is normal on later models?

    PeterCapo

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    Re: ST-70 issue

    Post by PeterCapo on Thu Oct 15, 2015 10:51 pm

    I believe you are referring to the 15.6 ohm "Biaset" resistors on tube sockets V2 and V7.

    Most of these look pretty clean underneath, especially factory assembled units, unless they’ve been opened up and hacked around with over the years.

    The 1K grid stoppers were alternately wired as you see – same circuit, nothing terribly unusual about it, and it does not really represent a "mod," at least not the way we may typically think of mods.  From what you have described so far, it seems pretty original - but photos would help, if possible.

    It may very well have been the case that they just discontinued wiring the front sockets to power the PAM preamps, not sure.  Do the unused front socket contact lugs look like they've ever been soldered-to?.

    daveshel

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    Re: ST-70 issue

    Post by daveshel on Thu Oct 15, 2015 11:51 pm

    PeterCapo - I think you had the correct approach. After I looked around under the hood and found no obvious issues there, I cleaned all the tube pin sockets with alcohol and intradental brushes. Tested with the backup tubes, then went back to the matched quad. Sounds good.

    So that was that socket conductivity issue that made such a strong case for solid state back in the day!

    Tube Nube

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    Re: ST-70 issue

    Post by Tube Nube on Fri Oct 16, 2015 1:09 am

    As with so many things in life, taking a technological step back usually involves more burden of effort, but greater satisfaction on performance. Maybe we can generalize that much of modern convenience is about ease at the expense of quality.

    So welcome to the audio world of quality. There's a bit more bother, but you won't be sorry!

    Tube Nube

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    Re: ST-70 issue

    Post by Tube Nube on Fri Oct 16, 2015 1:10 am

    But pardon me...with 73 posts, I see now you're no noobie toobie-file.

    PeterCapo

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    Re: ST-70 issue

    Post by PeterCapo on Fri Oct 16, 2015 9:42 am

    daveshel wrote:PeterCapo - I think you had the correct approach. After I looked around under the hood and found no obvious issues there, I cleaned all the tube pin sockets with alcohol and intradental brushes. Tested with the backup tubes, then went back to the matched quad. Sounds good.

    So that was that socket conductivity issue that made such a strong case for solid state back in the day!

    Glad to hear things have improved.  However, it might only be a temporary fix.  I do recommend, in addition to having now cleaned the contacts, to find a way to tighten contacts #5 from the top of the socket.  More cleaning doesn’t hurt and blowing them out with compressed air while still wet with alcohol might prevent some gunk from resettling back onto the contacts after the alcohol dries.

    I have taken original Dynaco tube sockets apart, that is, I’ve removed the individual contacts from the socket body and examined them under magnification.  Over the years, the inside surface area that contacts the tube pin gets sort of tarnished and otherwise grungy, and the contacts have widened.

    It’s easy to re-tension the contacts when they are removed from the socket body.  But, short of taking the socket apart, it is necessary to find some kind of tool(s) to slip into the socket from the top and press onto the side(s) of the contact to close it up a bit - but not too much or you won't be able to get the tube back in.  I’ve used an awl-like tool for this, but whatever works.  Power off, unplugged, of course.

    PeterCapo

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    Re: ST-70 issue

    Post by PeterCapo on Fri Oct 16, 2015 1:09 pm

    Another thought about tightening the contacts...  The contacts I've thus far seen in original Dynaco sockets look kind of like the letter "C" from the top.  If yours are also like this, then when tightening them, I’d suggest pressing on the "top" and/or "bottom" sides to try and close the split in the "C" a bit, if you see what I am getting at.  I would avoid pressing the one side directly opposite the split, as this might open the split more.


    Last edited by PeterCapo on Fri Oct 16, 2015 3:31 pm; edited 1 time in total (Reason for editing : Clarification.)

    Kentley

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    Re: ST-70 issue

    Post by Kentley on Fri Oct 16, 2015 1:55 pm

    PeterCapo wrote:Another thought about tightening the contacts...  The contacts I've thus far seen in original Dynaco sockets look kind of like the letter "C" from the top.  If yours are also like this, then when tightening them, I’d suggest pressing on the "top" and/or "bottom" to try and close the split in the "C" a bit, if you see what I am getting at.  I would avoid pressing the one side directly opposite the split, as this might open the split more.

    Si si, I see the "C".
    Best thing I've found to accomplish this rather tricky task is the smallest jeweler's screwdriver you can find. It's one of those little jobs that you simply have to DO to get a feel for. Once you get the feel, it seems easy. But first time, take it slow! Good luck.

    PeterCapo

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    Re: ST-70 issue

    Post by PeterCapo on Fri Oct 16, 2015 4:27 pm

    By the way, this kind of thing is not limited to old Dynaco tube sockets.  I had installed new, fairly expensive Japanese "Azuma" sockets in my Stereo 70, and last year I had a red-plating incident.  Never really confirmed the cause, but after cleaning and retensioning #5, the problem has not returned.

    daveshel

    Posts : 148
    Join date : 2011-11-06
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    Re: ST-70 issue

    Post by daveshel on Sat Oct 17, 2015 12:26 am

    Thanks for the input on the issue. I'm still wondering about the unexpected wiring of the front panel sockets (not so much) and the 1k resistors (in particular).

    The assembly manual says the front panel socket cannot be used for bias set is it is wired for preamp power. So I figure that most buyers in the 60s would be using preamps with their own power supply so the socket was wired for easy bias set instead pf preamp power.

    The wiring of the 1k resistors is still puzzling me. I remember reading something along these lines at some point - but I've been reading everything I could find on the subject for 10 years! So whether this is an official circuit revision is later models or a modification published by Van Alstine or somebody...

    PeterCapo

    Posts : 380
    Join date : 2008-12-05

    Re: ST-70 issue

    Post by PeterCapo on Sat Oct 17, 2015 1:57 am

    daveshel wrote:Thanks for the input on the issue. I'm still wondering about the unexpected wiring of the front panel sockets (not so much) and the 1k resistors (in particular).

    The assembly manual says the front panel socket cannot be used for bias set is it is wired for preamp power. So I figure that most buyers in the 60s would be using preamps with their own power supply so the socket was wired for easy bias set instead pf preamp power.

    The wiring of the 1k resistors is still puzzling me. I remember reading something along these lines at some point - but I've been reading everything I could find on the subject for 10 years! So whether this is an official circuit revision is later models or a modification published by Van Alstine or somebody...

    From the Stereo 70 assembly manual with italics added for emphasis: "Note that the use of pin #8 as directed in the wiring instructions means that this pin cannot be used as a connection point for preamplifiers if the preamplifiers take power from the power takeoff sockets of the front panel.  Most preamplifiers do not require the use of this pin.  However, for those which do, some other pin must be used as a bias check point."

    You could power preamps from the Stereo 70’s front sockets while they were also wired for the Biaset check on pin 8, as long as the preamp power connection does not use pin 8.  In fact, if you look at the pictorial diagram for the Stereo 70, this is exactly how the front panel sockets are wired.  In use, you’d check the bias on pin 8 first, then plug in the umbilicals to provide power to the preamps.  But, if the preamp was wired so that it needed pin 8 as part of its power connection, then you'd just pick a different, unused pin to use for checking the bias while still being able to power the preamp.  The only way you wouldn't be able to use the front sockets to both power a preamp and to check the bias would be if the preamp required all eight contacts for its power needs, which was an unlikely scenario.

    For the 1K resistors, if you have a look at a datasheet for the EL34, you’ll see that pin 6 is not connected to anything inside the tube.  Therefore, socket lug #6 is just hanging in space and does not require that anything be connected to it for the amplifier to operate.  You'd end up with exactly the same circuit whether you 1) connect the 1K resistor directly from the PCB to lug #5, or if you 2) connect a wire from the PCB to lug 6 and then the 1K resistor from lug 6 to lug 5 as instructed in the assembly manual and as shown in the pictorials.  For the latter, socket lug #6 is used as nothing more than a convenient wiring connection point because, again, it has no connection to anything inside the tube.  The lack of solder on lugs #6 is evidence that this portion of your wiring was not modified after it originally left the Dynaco factory.

    Based upon what you have thus far described, it sounds to me like you have an original, unmodified Stereo 70 that is just exhibiting original production run variations of no particular consequence to its sound quality.

    PeterCapo

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    Re: ST-70 issue

    Post by PeterCapo on Sat Oct 17, 2015 3:00 pm

    Found some more information regarding the front panel sockets. I happened to recently acquire an original copy of the manual that came with factory assembled Stereo 70s. It is much shorter than the manual that came with the kits.

    The manual for the factory assembled Stereo 70 makes no mention of using the Stereo 70 to power a preamp. In fact, it specifically says that it is to be used with self-powered preamps. The manual also mentions the spanner screws holding the bottom plate on.

    So, for factory assembled units, they apparently never connected wires to power a preamp, which makes sense because someone might possibly try to power a preamp that did not use the more common pin arrangement. With the kit version, specific instructions were included so that the kit builder could adapt the wiring to accommodate preamps with different power/pin arrangements.

    For the 1k resistors, I suspect that it was just more efficient to assemble them at the factory by connecting the resistor directly from the PCB to lug # 5 vs. the way it was done in the kit built units.

    It’s beginning to seem like you might possibly have a museum quality original there.

    stewdan

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    Re: ST-70 issue

    Post by stewdan on Sat Oct 17, 2015 6:54 pm

    Hi --- Peter, for point of information, I have several Dynaco ST70's (factory and kit forms) and the some Heathkit Mono Pre-Amps (originally used with Heathkit W5M and W4 Mono Amps) that have the same power plug wiring as the Dynaco Mono Amp Power Plugs and ST70's front panel power receptacles.

    The front panel tube sockets were fully wired on my factory built ST70 and it looks like the same wiring that was used in the rest of the Amp, so maybe at some point they wired everything and at some point did not??

    If I remember correctly, there were also EICO and Knight Amplifier kits that had the same tube socket power receptacle and my Heath Mono Premaps worked just fine with one of the EICO Amps.


    PeterCapo

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    Re: ST-70 issue

    Post by PeterCapo on Sat Oct 17, 2015 7:52 pm

    I suppose so.  Or, maybe I am incorrectly interpreting what I am reading.  I'm going by what I see in the kit assembly manual as well as the manual for a factory assembled unit.  The kit manual does seem to mention the possibility that the wiring for the preamp power takeoff might need to be changed for some preamps.

    They probably did things somewhat differently over time.  For example, in the late 1970s I started out building their solid state kits and the manuals sometimes included inserts with instructions for changes to the usual assembly procedure.

    The manual I have for the factory wired unit is all of about four or five pages long plus the front and back covers.  I didn't want to take up too much space here, so here is just the cover and the page that mentions using it with a self-powered preamp.  As I mentioned, I did not see anything in this manual about using it to power preamps, but maybe there was some other documentation that came with the amp that provided that information, I don't know.  If anyone wants the whole manual, I have it scanned to a .pdf - PM me.



    daveshel

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    Re: ST-70 issue

    Post by daveshel on Sat Oct 17, 2015 11:06 pm

    It makes sense that they stopped wiring them for use with the old preamps as time went on and their powered preamps became more popular. My unit has a serial number of 6638812 on a tag on the back, so it would have been produced not lone before to the one in the image. I wonder if the 66 is the year.


    PeterCapo wrote:It’s beginning to seem like you might possibly have a museum quality original there.

    If only that meant I could sell it for the big bucks!

    Bob Latino
    Admin

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    Re: ST-70 issue

    Post by Bob Latino on Sun Oct 18, 2015 9:51 am

    In your serial number > 6638812 .... The first 6 indicates that this was Dynaco's 6th product. All Dynaco ST-70's have serial number that begins with a "6". The next "63" indicates that amp was made in 1963. Now I am not totally sure about the rest > the "8812" part. One theory has it that the next "8" meant the 8th week of the year (late February of 1963) and the "812" meant that this was the 812th unit made that week.

    Bob

    daveshel

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    Re: ST-70 issue

    Post by daveshel on Sun Oct 18, 2015 3:07 pm

    Interesting - thanks, Bob. That never would have occurred to me - if you look at the products list on the old Greg Dunn Catalog History page it's the 4th product on his list. But then if you look closer at a link from his page you are correct: http://home.indy.net/~gregdunn/dynaco/components/partno.html

    The interesting thing is that although it was professionally assembled, it has the look of something handmade rather than coming off of an assembly line.

    PeterCapo

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    Re: ST-70 issue

    Post by PeterCapo on Sun Oct 18, 2015 3:16 pm

    daveshel wrote:The interesting thing is that although it was professionally assembled, it has the look of something handmade rather than coming off of an assembly line.

    It likely was handmade.  In the late 1970s, for instance, I had a friend who went on a tour at Dynaco and he commented about the "little old ladies" sitting at workstations building the amps...

    peterh

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    Re: ST-70 issue

    Post by peterh on Sun Oct 18, 2015 3:30 pm

    daveshel wrote:Interesting - thanks, Bob. That never would have occurred to me - if you look at the products list on the old Greg Dunn Catalog History page it's the 4th product on his list. But then if you look closer at a link from his page you are correct: http://home.indy.net/~gregdunn/dynaco/components/partno.html

    The interesting thing is that although it was professionally assembled, it has the look of something handmade rather than coming off of an assembly line.
    Equip of that era WAS hand assembled. Robots was way later and way larger series.

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