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Forum section: electronics / repair

author: cleaningOut
author_rank: Contributor
timestamp: February 04, 2015, 04:03:16 am
content: ...

author: tautech
author_rank: Super Contributor
timestamp: February 04, 2015, 04:41:39 am
content: Years since I played with valves.A few observations of your post.Remove 6X5GT and check the heater pins with an ohmmeter for continuity.The old valves if operative should be fine.The 0A2 DON'T have a heater, read the datasheet again.There's not a lot of resistors to check for value drift, pull valves and isolate those you can.Old E-caps could be re-formed maybe, but if you can source them replace them.Good luck, let us know how it goes.

author: SeanB
author_rank: Super Contributor
timestamp: February 04, 2015, 04:51:44 am
content: 0A2 tubes do not have heaters, they are voltage regulators. They are basically large neon lamps used as a voltage reference. It is easy to check a tube for an open heater, it will not have a lowish resistance ( under 100R on the heater out of circuit it likely is fine), and yopu need to check all resistors that they are not open circuit or high in value, and you probably will have to change every electrolytic capacitor and very likely all film capacitors as well.With the main filter capacitors (20uF 350V) replace with 47uF 450V ones, and place a bleed resistor across each of 2M2 1W to keep the voltage on each one about equal. You can use the same value for all 4 capacitors.The 6X5GT can be replaced with a series connected pair of 1N4007 diodes with a series 100R 5W resistor. this is only used as a single diode.

author: Rory
author_rank: Frequent Contributor
timestamp: February 04, 2015, 06:04:29 am
content: Quote from: cleaningOut on February 04, 2015, 05:22:39 amhttp://en.wikipedia.org/wiki/Voltage-regulator_tubeVacuum tube regulators were used much like zener diodes as a shunt regulator. The gas mixtures were optimized for the desired characteristics, neon was only one of the noble gases used.

author: tautech
author_rank: Super Contributor
timestamp: February 04, 2015, 06:30:00 am
content: Quote from: cleaningOut on February 04, 2015, 06:16:55 amHow was that done?Maybe it was un-smoothed DC?Confirm with a scope.You said the heater was OC so it shouldn't pass current.Likely to be a different current path than an operative circuit would provide.

author: tautech
author_rank: Super Contributor
timestamp: February 04, 2015, 06:56:53 am
content: Quote from: cleaningOut on February 04, 2015, 06:49:44 amThat's why I sell Siglent's.Better get a 100:1 probe for your HV work.

author: Rory
author_rank: Frequent Contributor
timestamp: February 04, 2015, 12:33:30 pm
content: Quote from: cleaningOut on February 04, 2015, 06:49:44 amDo you know to build a voltage divider?

author: PA0PBZ
author_rank: Super Contributor
timestamp: February 04, 2015, 12:59:41 pm
content: Quote from: cleaningOut on February 04, 2015, 12:51:44 pmAh yes, the famous 7 Amps voltage divider

author: PA0PBZ
author_rank: Super Contributor
timestamp: February 04, 2015, 01:28:47 pm
content: Quote from: cleaningOut on February 04, 2015, 01:07:38 pm700, but that's ok.

author: N2IXK
author_rank: Frequent Contributor
timestamp: February 04, 2015, 01:40:51 pm
content: In any piece of gear of this vintage, the first thing to do is to just go ahead and replace all the electrolytics and paper dielectric caps. Looks like a total of 5 caps in this thing, so not a big deal in terms of time or cost. Materials science has come a LONG way since this thing was made, and those old caps just don't age very well.Check all the resistors and replace any that have drifted outside their marked tolerance. Vintage carbon composition resistors have a tendency to absorb moisture from the air and shift value, sometimes dramatically.A check of tube heaters with an ohmmeter is a good start, but if you can find somebody with a tube tester, so much the better. None of the types in this unit are still in production, but they were all pretty common types, and a complete new set could be put together on eBay for a few bucks. Highly recommended if you intend to keep an oldie like this in service, as they are only going to get harder to find in the future. Luckily, none of them are really attractive to the tube audio crowd, which keeps the prices down.If a recapping and resistor/ tube checking doesn't restore proper operation, then you will want to go through the assembly portion of the manual and trace each connection that is called out. Being a kit, there is always the possibility that the thing NEVER actually worked properly due to improper assembly by the original builder.

author: PaulAm
author_rank: Frequent Contributor
timestamp: February 04, 2015, 01:55:08 pm
content: You'll also want to carefully check the power supply against the schematic.A big problem with Heathkits is that they were kits and they weren't always assembled correctly.

author: SeanB
author_rank: Super Contributor
timestamp: February 04, 2015, 04:55:43 pm
content: Put the diodes in series, as the peak reverse voltage is a little above what the diodes are rated for. Otherwise you can use microwave oven rectifier diodes, as they will handle 2kV and close to 1A no problem, well above the draw of this power supply. You might even have some lying around. Just use as is then, with the series resistor to reduce the current peaks to something the old transformer windings will handle. The resistor is there for that only, as the tube internal resistance was high so it did this current limiting by itself.

author: N2IXK
author_rank: Frequent Contributor
timestamp: February 04, 2015, 05:20:04 pm
content: A nice way to do this is to install the diodes/resistor in an octal base salvaged from an old tube. That way, the unit remains unmodified, and can be restored to original by just putting tubes back in.

author: N2IXK
author_rank: Frequent Contributor
timestamp: February 05, 2015, 03:23:34 am
content: Don't try powering it up again until you get those caps replaced. This is why vintage gear always needs to be brought up slowly on a variac or similar to try to "reform" the existing caps. You're lucky you only smoked a resistor, and didn't take out the power transformer or the 5V4 tube.

author: N8AUM
author_rank: Regular Contributor
timestamp: February 26, 2015, 06:17:01 am
content: So, did you ever get it going ?I also have one of these and the only problem so far where dried up filter caps.

author: cosmicray
author_rank: Frequent Contributor
timestamp: February 26, 2015, 09:54:53 pm
content: Quote from: N2IXK on February 04, 2015, 05:20:04 pmIn theory, you could take a dead octal relay unit, replace the relay with your choice of solid state. DigiKey still lists empty octal relay cases, but non-stock. They were made by Keystone, but no longer appear on Keystone's site.

author: Andy Watson
author_rank: Super Contributor
timestamp: February 27, 2015, 01:14:22 am
content: Quote from: cleaningOut on February 06, 2015, 05:18:11 amAnd what about the tube in the corner - the rectifier - is that a purple glow between the plates or is it a trick of the camera? If it is glowing it's a sign that it has become gassy.

author: N2IXK
author_rank: Frequent Contributor
timestamp: February 27, 2015, 01:43:47 am
content: A nitpick, but the 5V4 uses indirectly heated cathodes, not bare filaments.The glowing things are upper ends of the cathode sleeves which stick up above the top mica support washer. The filaments (more properly called heaters in this case) are INSIDE those cathode sleeves to heat them up.