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    Original ST 70 PC3 board 557003 layout?

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    stupidhead

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    Original ST 70 PC3 board 557003 layout?

    Post by stupidhead on Sat Sep 16, 2017 9:16 pm

    Hi folks. I am new to the forum and am on a bit of a journey, to say the least.

    I have very little electronics background, but do have some general understanding of components, soldering and generally a quick study for hands on things. I do a good deal of vintage AR speaker rehab work and have gained a bit of hands on experience with a soldering iron and capacitors/resistors being replaced etc. A little over a month ago I fell into a ST 70 unmolested, and started the research.

    To set the stage a bit here, I am very much an oem minded guy with all things vintage, and generally not a fan of modding unless time has proven that doing so makes sense. My research has led me all over the place with mods and speculations and it all circling to the original design being just a solid design and why mess with a good thing? When I got the amp it was pretty ugly from being stored for who knows how long and no real back history known or to be had.




    I have since completely dis assembled the unit, cleaned and polished the chassis and visual assessment of hardware has revealed a unit that does not appear to have been abused. No evidence of overheating anywhere. I am going to change selenium rectifier and cap can for sure and any other caps and resistors that are either known problems or test bad. I do understand the 7199 tubes are sort of problematic, but there are adapters to be able to roll other values in. My question is I am having trouble finding a layout map for the board, or at least one I can make sense of and if it exists where can I find it? I have a Fluke 115 and have started testing the resistors on the board, but am not sure about the readings being that they are still on the board and in circuit. I have a legend for determining values using the color bands on the resistors and they don't seem to be close to the id value?

    Am i going about this properly or do I need to extract the resistor before testing accurately.

    Not sure about the pic I posted above, possibly a size issue.

    Geoff

    I have had some preliminary banter with Bob and others, but found this forum and figured why not.
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    Bob Latino
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    Re: Original ST 70 PC3 board 557003 layout?

    Post by Bob Latino on Sat Sep 16, 2017 11:09 pm

    If you want to use the original driver board to keep your amp original, below is the parts list for the Dynaco PC-3 board. The PC-3 board parts were never listed in the original Dynaco manual because you were given with your ST-70 amp kit an assembled board. This is the later fiberglass board that dates from the late '60's to about 1976 when Dynaco stopped making the ST-70 amp kit. You have the older brown board that dates from 1959 to sometime in the late 1960's when they switched over to this newer board. The parts list and layout for this board is identical to the board on your amp.

    The .10 uF @ 400 volt caps can be replaced with .22 uF caps. The .05 uF @ 400 volt caps can be replaced with .10 uF caps. The smaller resistors are 1/2 watt. The larger resistors are 1 (or maybe 2 ? watts). Instead of replacing the parts on your original board, it might be wiser to just get a new original type board from Kevin at Dynakitparts. The original brown PC-3 board was made of a really inexpensive phenolic material with open solder traces on the bottom that have been known to lift away from the board on some boards. That material was also not very heat resistant. If you look under the bottom of the board, you will probably see that the board has turned a dark brown/black color under the 7199 tube sockets. The later board shown in the photo is a much better board.

    Bob


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    PeterCapo

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    Re: Original ST 70 PC3 board 557003 layout?

    Post by PeterCapo on Sat Sep 16, 2017 11:39 pm

    A broad discussion, to be sure.  If you have not seen it already, and given your apparent inclinations, have a read here: http://www.audioregenesis.com/documents/ST-70%20Base%20Line%20Testing.pdf

    If it is important to you to keep your Stereo 70 as original as possible, you can try that.  Be aware, however, that the original brown PC board can be difficult to work with.  It has a coating of wax on the foil side.  I suggest leaving the wax alone.  Do not use alcohol, other flux removers or any kind of chemicals on the original PC board.

    With regard to the resistors, they may be fine, meaning still within tolerance.  In-circuit, the values you measure across each one may not match the color code because they are connected to other resistors.  As you have pointed out, to check each one you may have to desolder at least one lead and then measure.  

    In your travels through Dynacoland across the Internet, you will likely see references to the old CC resistors being out of tolerance.  Well, desoldering one or both leads is a very good way to send them out of tolerance.  My suggestion may sound a bit contrarian, but if you value originality and would like to keep the original CC resistors – then I say leave them alone.  No soldering heat applied to the original resistors on the PC board.

    How do you know if your original resistors are good?  Go ahead do all the other work you are planning to do.  When you are finished, take your multimeter, fire the amp up and take readings at the various nodes of the circuit, comparing your readings to the reference values in the voltage table in the original Stereo 70 manual.  If your voltage readings are within 10% (possible exception of voltages on 7199), and if the amp holds reasonably steady bias, and if the amp sounds good, then you should be all set.

    I’d suggest also replacing the bias supply electrolytics.  Cleaning and retensioning the tube socket contacts is a very good thing to do.

    Another contrarian suggestion:  don’t be in a rush to spray the bias pots: http://audiokarma.org/forums/index.php?threads/dynaco-pas-3x-and-the-tone-control-modification.772264/#post-10575661
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    stupidhead

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    Re: Original ST 70 PC3 board 557003 layout?

    Post by stupidhead on Tue Sep 19, 2017 9:27 am

    Hi Bob and Pete et al,

    Thanx for the input and particularly the pic of the parts list. I tried looking in other threads but it would appear as the images have vanished. I have efforted taking readings on my board and here are the results.



    For the most part it seems as most are a bit high and some exceed the 10% tolerance. The two upper center ones reading 181.6 each are tied together on the back side and readings are questionable but appear to be halfish of the noted spec in Bob's pic. Coming from speaker rebuild mentality, my gut is to change out a few of these to get them closer to spec but not real sure just how important that is. Also I am not sure what the yellow ones (silver in Bob's pic) are nor the rectangular ones with paint dots are either and tried getting readings but came up empty. I am using a Fluke 115 and maybe a bit of guidance here please.

    It is not critical to keep everything absolutely original on this, but I do desire as much original as is reasonable.

    Bob, you suggest different values than original on the caps and this confuses me a bit. Possibly a bit of dialog on that to help me try to understand the value change from original design please.

    Pete, I appreciate the wisdom shared on the sockets, pots being lubed (or not) etc.. I will heed these insights and they are greatly appreciated. Being a newbie at this discipline is humbling and I appreciate you folks coming down to my kindergarten understanding to help me sort all this out.

    Geoff


    Last edited by stupidhead on Tue Sep 19, 2017 12:59 pm; edited 1 time in total
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    PeterCapo

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    Re: Original ST 70 PC3 board 557003 layout?

    Post by PeterCapo on Tue Sep 19, 2017 11:46 am

    stupidhead wrote:Also I am not sure what the yellow ones (silver in Bob's pic) are nor the rectangular ones with paint dots are either and tried getting readings but came up empty. I am using a Fluke 115 and maybe a bit of guidance here please.

    They're capacitors and are likely fine.  The domino is 390pf @1000VDC (original spec).  The other is 82pf but I do not know what the original voltage rating is.  Some multimeters cannot read capacitance values that low.  I'd leave them alone.
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    stupidhead

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    Re: Original ST 70 PC3 board 557003 layout?

    Post by stupidhead on Thu Sep 21, 2017 8:14 am

    thanx Pete, that is helpful. Looks like just the black caps will be replaced then. On that note and after further pondering of this aspect of my rehab, I may have put Bob on the spot with my above question. The last thing I would like to see with this thread is to have it blossom into something other than staying on topic. Bob, if I have put you (inadvertently) on the spot, my apologies. For the record my intentions are to keep this rebuild as original (components and design/build) as possible, within reason. I will replace the caps on the board and I will stay with original design values, for now.

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