Comments for page: EFB in a Dynaco ST-35

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Posted by Art Grannell October 27, 2018 - 08:24 pm
Just a word about your transformer leads. These amplifiers used two different output transformers, that are thought to be electrically identical. The earlier production transformers with the cloth leads were built in the US. Sometime during production, Hafler switched suppliers and used vinyl-leaded transformers manufactured in Japan. The amplifier in this article is a real "odd-ball", with both cloth and vinyl, and I'd only be guessing as to why.

I have replaced leads on Z-565's at the core, but only on the secondary (output) side. The core windings are insulated with an impregnated paper of some sort, that shows some "brittleness" due to age. After removing the end bells, and working carefully, I was able to unsolder and remove the secondary wires at the attachment point and replace them. I'd intended to also change the primary leads, but this turned out to be far more difficult as they are attached deeper in the core to the very small wire used in the primary winding. I was uncertain that I could do this without destroying these valuable units, so I opted for a different approach.

I cut off the old leads about one to two inches past the exit hole and spliced on new leads using uninsulated butt splices and heat shrink tubing. This is a compromise, but if the wire is otherwise unusable, it will work. These splices cannot be used internally because of lack of space. If you remove the end bells, I suggest that fiber or nylon washers be placed under the screw heads. Some Dynaco transformers had them, some did not, but if the end bells are removed, it's a good idea to use them.

Posted by Dave October 27, 2018 - 07:43 am
Hi Brian -- Thanks for the kind words! However, much of the credit belongs to George Ronnenkamp of Audio Regenesis at:

George was originally selling the beautiful boards I now sell, so credit for their physical design and installation material goes straight to him. He is a wonderful friend and superb Electrical Engineer - as is Steve Lafferty, who owns, operates and maintains the Tronola website here.

George approached me several years ago about incorporating my EFB modification into his boards. A rare thing as otherwise, with little exception George's boards remained utterly true to the original Dynaco design and execution. But the EFB modification represented a solid performance improvement over the original design that George appreciated. An arrangement was struck, and the new EFB power supply boards became a reality. Later, when George decided to move away from selling the boards, he graciously allowed me to continue selling the boards under the D Gillespie Designs banner. To keep it simple and economical, I decided to sell the boards un-stuffed, whereas George had been selling the boards fully assembled. That required an assembly manual for each board in addition to the installation manuals, which again, George spearheaded. The rest as they say, is history. At any rate, you should check out George's site. He is as much of a Dynaco fanatic as anyone around, and a stickler for remaining true to the original Dynaco brand. Ergo, the manuals are written much as if Dynaco had written them themselves!

As to your findings, if the electrical condition of Eyelet #9 was as you say for the left channel amplifier board, then the current draw through V3 would certainly have been quite erratic and unstable -- and changed with any heating or mechanical flexing of the board. Therefore, it's virtually a certainty that you found your problem -- Congratulations!

BTW -- If it helps you any regarding the decision to replace your boards, construction of the original boards was never a part of any Dynaco product. They always came fully assembled and ready for installation in the kit. That is one element of the kit then that your dad did not have to build. This is hardly an effort to promote a sale for the new boards, as I understand as much as any the emotional attachment that can be had with things from our childhood that involve our parents. Actually, the old Dynaco boards are the greatest advertisement of all for replacing themselves! The problem you found is common, and will only continue to spread over the boards with time and use. When the time comes, the new boards will return the unit to worry free operation for another 50+ years and beyond -- this because the new boards (and today's components) are so vastly improved over what was originally available at the time. Good luck with your ST-35!


Posted by Brian October 26, 2018 - 05:42 pm

First, I wanted to compliment you on the thoroughness of your instructions. You took the time to create a very detailed and complete technical documentation package and at every point of troubleshooting when I thought to myself, "I wish I had this information," I referenced your instructions and the answer was there. The red-line schematics were especially helpful.

Second, I want to thank both you and Art for sharing your knowledge with me. I would not have been able to do this on my own.

As a result of your recommendation I took a hard look at the underside. Though there are several loose terminals in the sockets, they seemed to be pretty evenly and randomly located throughout the sockets, so I looked elsewhere first. I noticed what looked to be a slight lifting of the eyelet #9 ring from the board.

I had previously noted a slight convex bend in the board (when viewed from above). I took some resistance measurements from the eyelet to adjoining components (R9 and C5) found it changing from open to short as I pressed on the eyelet, causing the board to flex at the same time.

As a test-fix I carefully soldered a wire to the three points, effectively bypassing the eyelet and the traces. It's preliminary, but so far the cathode voltage appears to be stable. I haven't declared victory yet - the tube socket could still have an impact - but it feels good to have a glimpse at success.

That said, I have to make a decision on how to proceed. I'm not crazy about my point-to-point solution as a fix. Art had previously suggested I take this time to replace the PC-13 boards entirely, which would be prudent given the problems discovered based on the age of the boards. My cause for pause is twofold:

1) Personal: I have photos of my dad building the Dyna Kit in 1963, and if I replace the PC-13s entirely there isnt much left of the original work! The instruction books even match the serial number of each unit (PAS-3, ST-35, FM-3).
2) Technical: The cloth-covered wire coming out of the transformers is in varying states of quality. The power transformer leads seems to be OK, however the leads that connect to the output tubes is in various states of crumbling. I had to be careful moving the wire from pin 9 to 8 as part of the EFB modification. One of the leads required some shrink tubing to avoid having any more cloth flake off while handling with the needle nose. I think I could get away with splicing for all but one pair, which is in bad condition going through the through-hole into the transformer itself. Im guessing to do the repair properly Id have to send the two output transformers to someplace to have them taken apart and new leads put on?

Anyway, at least Im entering the weekend with some success on the amp. I cant thank you and Art enough for your time and knowledge sharing. Once I make peace with the fact that I need to replace the PC-13s youll see my order!

Posted by Dave October 24, 2018 - 12:59 pm
Hi Brian -- Steve asked me to look in on your conversation here since there was a comment made about an error in the instruction manual for the ST-35 EFB Power Supply Board. There is, and thank-you for spotting it! For the record, the schematic for the board itself is correct, with the overall schematic containing the error. Again, my thanks for spotting it.

If I may comment on the issue you're having however, it would seem to boil down to just a few possibilities. You've already eliminated the coupling cap possibility by replacing them. And, you've eliminated the tube possibility by trying the tube in the other channel, with the problem always remaining WITH THE ONE SOCKET. This is important, and can only be verified by the fact that ANY tube placed into the identified socket will intermittently red-plate -- the point being that no tube installed into its mates socket (i.e. same channel) will show such offending behavior. So running with that assumption then, the only possibilities that will cause the problem you're having based on the symptoms given include:

1. The 1000Ω; resistor attached to pin 2 of the offending socket is defective, or it's associated 470K grid return resistor is defective. Given your examinations, these possibilities are remote, but none the less would cause the problem you're having.

2. Pin #2 within the offending socket is internally broken, so that sometimes the connection is made, sometimes not. With older equipment that has gone through many heat stress cycles, this can happen -- and does, as I've personally witnessed. If this is the cause, usually the bottom side terminal can be simply pulled out of the socket, where normally it cannot be.

3. By far however, the greatest possibility is that a circuit board connection or trace that looks good, is in fact, not. The old Dynaco phenolic boards are famous for developing a complete circular break in the solder connection where leads and terminals of the board components extend through the hole in the board - and/or developing breaks in circuit traces. These are usually quite small and most often cannot be seen with the naked eye, requiring close inspection, ideally with a magnifying glass to spot. It can be either at a component lead, terminal connection, in the middle of a trace, or at one of the eyelets. In this case, a prime target is any board connection or trace involving the path that pin 2 of the offending socket takes in reaching ground through its associated 1KΩ; or 470KΩ; resistors. Remember too that there can be multiple breaks in the path.

The bottom line here is, that if the tube and associated coupling cap for the offending socket have been positively eliminated, and the problem only occurs in the one socket regardless of tube installed, then the problem can only be either the actual terminal for pin 2 in the offending socket, or the connections/components that ultimately allow pin 2 of that socket to reference ground as discussed.

This is one of those cases where if the facts are true, then the problem will be found in the areas mentioned. If the problem is not in the areas mentioned, then the symptoms taken as fact are in question.

Good luck with your search -- I hope this helps!


Posted by Brian October 23, 2018 - 05:11 pm

One other thing: Are you by chance in touch with Dave at all? There is a discrepancy in the ST-35 EFB assembly instructions. On PDF page 6, in the EFB schematic, it shows resistors R3 and R4 for the left channel. On PDF page 7, for the overall amplifier schematic, it shows R1 and R2 for the left channel. The output tubes for both the LC and RC are labeled V2 and V3, also. One side should be V5 and V6.

I was looking at them closely to determine how voltage at the test point could be causing V6 to run away but not V5, and was trying to determine which resistor I should look at. That's when the noticed the discrepancy.

Posted by Brian October 23, 2018 - 05:04 pm

I didn't mean to go radio silent on this thread. I haven't had the time to dedicate to this project daily, so progress has been slow.

I did get the capacitors in and installed. Bottom line is the tube in the same problematic socket continues to run away on occasion.

The RC is rock solid. I can set the cathode bias through the EFB trim pot and it only fluctuates a few hundredths of a millivolt.

The LC is basically unchanged with the capacitor replacements. The cathode voltage, as measured on the EFB LC test point, will sometimes be stable-ish, but always moving around 20-40 mv. Occasionally it'll bounce from 300 mv to 600, and then up to 1.2 v... sometimes it'll stay there and the tube will run away. Sometimes it'll go back to 600 and then back to 300 mv.

I swapped V6 with V3 and the problem stayed with the socket, so it's not a tube issue. I need to go back and perform the other test measurements again, and will also check for unlikely leakage between socket pins. Due to work constraints I won't be able to try anything until this weekend.

Just wanted to provide you the latest, even if incomplete from a troubleshooting perspective!

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