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Hi all, I am in the process of rebuilding a Dynaco SCA-35 and have a question. I am using Dave's EFB board and am using the original output boards, they look good and solid so I am in the process of stripping them and putting on new components, so far so good. The question is regarding the 1 mfd cap on the output board, I can't seem to find it on the schematic or the parts list I have, wanted to know what the DC is across that cap as I have some 160vdc film caps I could use. And certainly, any suggestions are welcomed, thanks!
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Original Dyna boards are known to delaminate quite easily when being soldered/desoldered. The substrate is unstable and the traces are substandard. Why not just buy one of the many sets of replacement boards. They are very cheap in relation to this type of project.
There is also a Dynaco forum on the Asylum that can be accessed from the drop down box on the top of the screen.
Best of luck with your project.
Good question. I was looking into those replacement boards, but the boards on this Dynaco are thick, heavy-duty and the traces are just fine as I have already stripped one of them and started to replace the parts back on, no delamination of the traces are occurring. I do have prograde solder equipment, both Hakko solder & desoldering equipment as well as a lot of experience, so that helps.
When I worked on my Mark IV's, there were no boards available. I had to jump one trace with a wire. On my SCA-35 I think it is a newer version. The boards are not that bad. (YMMV!) I am a very gentle solderer. I have not rebuilt the SCA-35. My plan is to start with the original board, and if disaster happens, buy a new one.
Dave
Agreed, Dave, each repair should be treated on its own merits, if those boards are/were bad, I would replace them with the many available, pre-stuffed boards on ebay, however, these are fine, have been stripped of the old parts and new, higher-quality parts are being sourced and installed, including new Beltron micalex sockets for the output tubes. Those sockets were obviously in bad shape, however, the board mounted sockets (7199 & 12AX7 on phono board) where just as obviously in great shape and retained. Having done numerous restorations and having excellent Hakko solder/desolder gear, I am confident that these will be better than ever with quality parts and construction.
by that logic if you checked a cap with a cap checker and it still met spec would you leave it?
The problem is often not today but, with 40-50 year old parts it can often be the near future IMHO. Take my advice for what it's worth, just what you paid for it! And have a happy New Year.
See the schematic in the link. Looks like C18, 250V rated 1 uF. Use a decent film cap. A solens or something like that.....
Thanks, this schematic does have the proper value for C18 which you noted as 1 mfd @ 250 vdc, my schematic did not have that value, so unless there are multiple versions of the SCA-35... I have several 1 mfd @ 160vdc I would use IF the voltage across C18 is around 100 vdc or less, that is what I was hoping to learn. I see that C18 is across the cathode to screen grid of the 7199, so it could be higher than 100 vdc, time to ask Dave!
Hi
If you can believe the voltage chart in that same document then pin 3 of the 7199 is 32 volts. The cathode is listed at .7 volts as far as I can tell from the bad copy. Makes sense since it ties to the 16 ohm tap and ground through that cathode resistor. So my guess is that even as the tubes are warming up, the screen of the 7199 will never see over 100 volts. You could always tack a cap on there and fire it up and watch. Even a 100V cap could take that turn on surge the first time. My guess is it never anywhere near the 160V rating of the caps you have on hand.
Thanks, dls, that is an excellent suggestion!
Yes, the plate of that section is listed at 55 volts. Even at warmup before the tubes draw the supply down I bet that plate never goes over 100-120 volts. The screen will be below that, and the cathode will be around ground... No way that cap sees over 100-120 volts at turn on if that schematic is correct. Of course it sees 30-40 volts tops at steady state.
Excellent, thanks so much!
Let's just hope the schematic is correct! It should be, it is the Dynaco original kit instructions....
Well, what is the rating on the 1uf cap in there? As long as you are using an equal or higher voltage rating you're OK.
Thanks, but that does not answer the question. The voltage value does not always correspond to the voltages in the set, but is often chosen as the most or cheapest available part. So I need to know the voltage around that cap to make an informed decision.
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