------------------------------ Date: Wed, 1 Sep 1993 12:07:42 -0700 (PDT) From: Ed Faulkner Subject: Exploding Cap You probably made the mistake of wiring the cap in backwards. This is easy to do in the bias circuit as it supplies a negative voltage instead of posative. Remember that in the bias circuit the posative lead of an electrolytic cap goes to ground. You will probably hear from more people that have made this mistake (I have). Its to easy to get in a hurry (wanting to listen instead of solder) and just connect the negative lead of the cap to ground :-) ______________________________ Ed Faulkner \ Everything should be made as simple Computer Maintenance Tech II \ as possible, but not simpler. Washington State University \ --Albert Einstein 100 Sprout Rd. \ Richland, WA 99350 \ faulkner@beta.tricity.wsu.edu \_____________________________________ ------------------------------ From: shoppa@almach.caltech.edu (TIMOTHY D. SHOPPA) Subject: Re: Dynaco ST-70 Capacitor Explodes Date: 2 Sep 1993 16:04 PST In article <2626fjINNb5c@uwm.edu>, torgd@tolstoy.lerc.nasa.gov (Russell DeAnna) writes... >I finished assembling my latest ST-70 last night. I rebuilt the >amplifier with these changes: >... >(4) Replaced the selenium part in the rectifier stage with a silicon diode. >... >before turn-on.) After about 45 seconds one of the electrolytic capacitors >used in the rectifier section exploded (a 50mf 50V I think). >Thank goodness no one was injured. (I normally turn on the amplifier >while it's resting on its side after making modifications, >so that I can monitor the progress of both sides of the chassis. >Next time I'm going to set the amp in its proper orientation.) >... >So does anyone have any clues. I think the capacitors were oriented in the >proper direction. The one that blew is gone, so I can't verify that one, but >the other cap is in the proper orientation. This was my first guess, since >I've installed these caps in the wrong direction once in the past with a >similar conclusion. Could the switch for the B+ be a problem? or the >method of connecting the two output EL-34s to the PC board through the plate? Ah, number (4) is probably the cause of your problem. In the days of my youth, when I would refurbish old ham transmitters, I had this happen to me too. Replace a selenium rectifier with a silicon, turn the unit on, then after a minute or two there was the HORRENDOUS CAPACITOR KABLOOIE. Good silicon rectifiers have a forward voltage drop of less than 1V (maybe a few volts at worst), and a negligible reverse current flow. Selenium rectifiers can have a considerable forward voltage drop (in some old 800V B+ supplies, I measured forward voltage drops as high as 100V), and sometimes a sizable reverse current flow (which drains the capacitors when the rectifier isn't forward biased.) I don't know of any modern books that say anything about selenium rectifiers, but if you go back to an early 70's or earlier Amateur Radio Relay League Handbook, I'm sure there are statistics about common selenium rectifiers. I've also heard that selenium rectifiers deteriorate over time. (i.e. their internal resistance will increase causing an increase the voltage drop across them and a decrease in the voltage actually across the capacitor). So chances are the capacitors had more voltage (say 25V-100V) accross them after you switched to modern rectifiers. If this by itself wouldn't blow the capacitor, then the higher surge current (due to the lower internal resistance of a silicon vs. selenium rectifier) into the capacitor may have done it. Increased B+ voltage may give your amplifier better performance, but require retweaking the bias settings (chances are you are quite familiar with this already). If you want to make your silicon diodes more "selenium-like" and retain the B+ voltage you had with the selenium rectifiers, a power resistor in series with the silicon diode might do the trick. Resistors are nice and linear, whereas selenium diodes were probably rather nonlinear in their forward voltage drop vs. current, but it's a start. Unless you are particularly fond of the Dynaco's original capacitors, replace them with something more modern. You can probably get more capacitance in a smaller package these days. And use caution with the old capacitors, especially the blown one: as I understand it, older capacitors very often had PCB's (nasty chemicals) in them. Tim (shoppa@almach.caltech.edu) ------------------------------ Date: Tue, 7 Sep 93 12:45:20 EDT From: jas@proteon.com (John A. Shriver) Subject: Capacitor Explosions and safety The capacitors that tend to explode in tube audio equipment are aluminum electrolytics. They are not dangerous, unless physically harm you when they explode. There's nothing in there but aluminum and some reasonably innocuous salts. The capacitors with PCB's in them are oil-filled capacitors. They were almost never used in consumer equipment, maybe in a dozen products total through the entire tube era. (A few Marantz power amps.) They are very stable, most unlikely to explode, and hermetically sealed. They were the most expensive and high-quality capacitors of their time. Most have ceramic insulators around the terminals, some cheaper ones have rubber seals. They are typically rectangular. Trademarks for PCB are Chlorinol, Pyranol, etc. Unless a PCB capacitor is leaking, I'd just leave it be. It will be very hard to find a recent non-PCB one to replace it. They don't tend to leak, either.