Dynaco ST-70 Tube stereo audio amplifier Bad AC hum in both Channels What is causing the fault?

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A local fellow brought to the shop last weekend. He has owned the amp for a few years, used to play beautifully. Then recently it developed a bad AC hum in both channels. He had tried a few things to correct the problem, but then decided it best to bring it to me. The amp had a replacement driver board (VTA) new tubes and filter cap replacement. I do not know the age of the filter capacitor. Code printed on the label is 1116-01. After a bit of investigation I found that the installed CE manufacturing capacitor had 3 open sections. I replaced it with a F&T Authenicap, which eliminated the output hum. Then I thought it would be interesting to cut open the CE Filter & inspect the innerworkings to determine the failure. It appears that the terminals corroded and the sections failed. This is a strange failure, however if the cap is 20 years old, that could be caused by age. I routinely use CE manufacturing caps in many vintage tube amp repairs. Never had a bad one and no reports of failures from customers. This is the first I have seen fail, however with no idea of its age or possible misapplication in the past, I just consider it, another repair. I will continue to use their products.
NOTE: Follow up video showing the power supply power up sequence. • Dynaco ST-70 Tube ster...

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6 окт 2024

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Комментарии : 59

@dwightbehm2886 3 месяца назад

Hi terry. You cut the capacitor apart to see what was wrong with the cirquits good job with that. It reminds me of when I used to work on automatic transmisions back in the 1990s I would remove the oil pan then remove the filter and brake the filter apart to see the contents looking for clutch materials and Metal grindings m four parts of thrust washers between internal components or reaction carrier pinion washers for Diagnostic purposes. Nice terry. Dwight.

@leiferickson3183 Год назад

That corrosion might be due to galvanic problems between different metals. I needed to replace some caps in a 1966 Champ and a 1963 Princeton and ended up ordering Hayseed Hamfest caps (First time on these - I ordered custom caps and they were delivered 7 days from when I ordered!) because I have read too many stories about the recent failures with the CE cans. I am going to be installing the Hayseed cans tonight or tomorrow - They look good but they are very shiny - The can sides are magnetic but the non business end is not magnetic. They seem very solid. They can be ordered with 105C rated caps inside. Even if the CE Cans didnt have these premature failures I would worry in many applications about the temperature ratings which are very low. That cap in the dynaco was rated at 55C !!! The dynacos I have seen run pretty warm - especially with the cage on. Great tip on the authenticaps - I havent seen those before.

@leiferickson3183 Год назад

By the way, back in the 90's and early 2000's when I was repairing amps for a living I used a lot of these CE Dist cans and I was very thankfull that they were available. Nowadays I only work on amps as a hobby for myself and friends so I havent been in the thick of it for a while.

@eugenepohjola258 Год назад

Howdy. I like Your persistence of vision. I also appreciate Your way to explain and describe stuff. Chipping off towards the point sentence by sentence. No motormouthing in circles which only produces concentration fatigue. High regards.

@d-labelectronics Год назад

Thank you my friend. I learned that approach while instructing and using tech data in the Air Force.

@acoustic61 Год назад

Not the first time I've heard of CE Dist. cans failing. Brings into question, the quality and value of those caps because they're expensive. I don't use them anymore unless customer insists.

@johnbravo7542 Год назад

Agree,seen another RU-vidr have same experience although it was ages ago,it may have bee Uncle Doug but not sure.

@rickg8015 Год назад

@@johnbravo7542Blue Glow

@johnparichuk8367 3 месяца назад

I have a VTA ST-120 I built back in 2017. The CE Manufacturing capacitor has held up fine. However, when I first powered it up, the 5AR4 destroyed itself in a flash of white light. I tried replacing it with a solid-state unit but was unhappy with the sound. it turns out that 5AR4 rectifier isn't rated to handle the current of four 6550 output tubes. I ended up installing two 5AR4 tubes per the old H.H. LK-150 schematic. Gone was any arcing. And the low frequency response was much improved. VTA now offers a dual rectifier upgrade. I had another problem, this being the driver board. The center 12AU7 failed twice, introducing hum into the audio on both channels. I ended up building and installing the VTA 6SN7 driver board, which has performed flawlessly. To my ear, it sounds better, too.

@b4467 Месяц назад

Great video! Thank you. I have same amp with same mod, same hum. Finally it blew a fuse, I checked that cap and it was bad. Cut it open like you did and it was corroded just like yours.

@bobblum5973 Год назад

Terry, I always apprecitate how you talk through the troubleshooting steps. Like in this case, where you used past experience to try other things first before checking the capacitor, but then we're methodical when you did finally check it out. It teaches others to learn good troubleshooting techniques. 🙂👍

@David-ym2gb 8 месяцев назад

Terry, I have rebuilt a ST-70 and it had some hum in it, I added another capacitor and a resistor and reduced the hum. After watching your video I did use this same capacitor can as you found bad. I checked it and it measures ok but I suspect under load it is not working as it should. Too bad the new parts are not to be trusted. I would probably have been better off to leave the original capacitor. Thanks for the video. I will probably replace the can with the one you suggested.

@kgsalvage6306 Год назад

I use a tubing cutter all the time and recap my old cans. I cut them right at the seam at the base of the can. Do it carefully and you can epoxy the can back together and you can't tell. Put some electrical tape so not to scuff them up. I drill a small hole right beside the terminals and run the lead from the cap and solder to the terminal. I usually use Nichicon pencil caps. They are thin and long. I previously used CE dis. Or Hayseed Hamfest cans quite a bit also , and never had an issue. Just a little cheaper and keeps the amp somewhat authentic. Nice repair!👍

@brendajanes4139 Год назад

Hi Terry, I have had 2 of these CE caps fail in my Dynaco ST-70 amp in a matter of 6 months, I had read that these caps are made poorly and prone to failure. A friend of mine had one blow up on her with only 300V!

@jerryking2418 Год назад

Informative video. I thought CE Manufacturing had a reputation of quality control. Let's remember the universal truth that nothing is perfect.

@fiddlix Год назад

I am another living testament to the high failure rate of CE Dist. Caps specd as yours.

@jeffberwick Год назад

I have also had 2 failures in CE caps that had a 500V rating for my McIntosh 240 amp. I realized that the line voltage was probably too high, since the McIntosh has a very efficient solid state rectifier and exposes the cap to a higher voltage now (due to 120-125V line) than the 110V that was more common in the good old days. I now run the McIntosh on a variac that reduces the line voltage to 110V, and have not had a failure for 5 years with the CE cap.

@edwardhannigan6324 Год назад

Cool video Terry and an unusual failure..? Trust D-lab to get it sorted though..Nice amp by the way.Thanks for sharing more info..Ed..uk..😃

@ΑργυρηςΚαραπετσας Год назад

Once again Terry you do magic!

@jenniferwhitewolf3784 Год назад

It is unfortunate but true... I have encountered several fails with CE caps in the recent 2 or 3 years.. Among many of the Portland OR tube builders, consensus is no CE any more. F&T and J-J have so far been reliable.

@BobDarlington Год назад

Just in time. I have one on the bench right now.

@hershelshochter4703 Год назад

ive never seen internal corrosion like that, but i did see corossion creep up along one leg into a cap caused by hot glue. it sucked up moisture and killed the cap it was supposed to hold down.

@pedrodepacas4335 Год назад

You mention potential voltages above 500 Volts. It would be interesting to know what the operating voltage is during startup and once the unit is warm.

@d-labelectronics Год назад

Follow up video: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-aYoY12wwKyc.html

@hestheMaster Год назад

Metal fatigue of the electrolytics from regular voltage spikes may be to blame here after looking inside. Heating up the insides could allow moisture from the capacitor material to corrode the inner terminal connections if no protective material was present. May be just a lifespan issue as well. The higher voltage rating should cure the problem with the new replacement can.

@tombaffa9374 Год назад

Awesome job

@jmsbriann Год назад

Good Job Terry!

@michaelspiering7585 Год назад

I think that is a good way to get a estimate of the plate dissipation, but to be more accurate one should measure the voltage drop across the primary windings of the output transformer because the screen current is also part of the cathode current.

@brentnearhood8874 Год назад

Thanks! Terry

@Jah_Rastafari_ORIG Год назад

I don't suppose they'd replace it as 'factory error', would they..?

@johnbravo7542 Год назад

That CE cap seems quite dry inside not too dissimilar to a 50 year old can cap I've seen opened up.

@JasonTHutchinson Год назад

I'm still not sure what caused it to bulge. Interesting how the leads corroded though.

@pstock49 Год назад

Good tutorial. I must do the same with my ST70 and this is very helpful. Went looking for the F&T multi-cap you used in the repair but couldn't find it. Could you show the source? Thanks

@waynegram8907 Год назад

TERRY, The Filter capacitors Inner terminal connectors are Electrodes which I think the Electrodes need to have an oxide coating or coatings? is it Galvanic problem between the filter capacitor "electrode metals"? The filter capacitors Electrode Metal Fatigue?

@patlaw53 Год назад

We use a LOT of CE can caps in rebuilding Hammond Organs and Leslie Speakers. The reports of early failures are increasing. Our community is concerned about their long-term reliability. Terry, where did you get the F&T can?

@d-labelectronics Год назад

I buy them from this ebay seller: www.ebay.com/itm/312203986432?hash=item48b0cec600:g:5Z0AAOSw-6pbYLj-&amdata=enc%3AAQAIAAAA8HrjfK6X%2BYc4s8%2B2VGjrJvwjLTmmIz5wX%2BSSIIjNLqP5W4ZzhSS0bDH1tgRlxpVIlZ%2BHpYqPXvHWeok8%2BxyedgcwuAxb%2FCFuyEpo2FDEmDe9mPonwFKxnrbUGwqCLPepRotenPigqlKg8jIXOj3vPF2SFflNLTATkjteYzMwxyI0PVs83noB%2FDCB2r141h9OfUkIx86cwRuBt9zuvsJZjLP%2Fn9pgYtq0%2FMAhHryyIvmOIpiwU1EDqfES284%2FJDdWUKpEoExjfR%2B6%2FxszE5yxk7wK9CfhWfvOKDFIsHuGDZzPy08VC%2FYGLOJ2xAcVYEAosg%3D%3D%7Ctkp%3ABk9SR4yl5pajYg

@fiddlix Год назад

Is that $122 for one cap can or 2 like in the photo..?

@patlaw53 Год назад

@@fiddlix The title says it is for two.

@fiddlix Год назад

@@patlaw53 That more sense now. Thank you. Must have been an oversight on my part. I’m very sorry.