The Electronics Universe

For the resistors you should keep the resistance value the same. Keep the same number of ohms. It actually depends on which part of the circuit the resistor is in. But, if it functions as a dropping resistor and you change the value of it the voltage drop will also change and could throw the required voltage or the current way off. That could be a disaster in some circuits. The power rating of the resistor is not as critical as long as your replacement is rated for MORE power than the original.

I'm not real sure on the value of the caps. But, I would thing that you want to keep their values close to the original values. Again, it depends on the circuit the cap is used in. If you had to replace a 10 uF cap I would not go over 20 uF and I would feel real uneasy about that too. a 12 uF would be a better choice. But, again, that is just my opinion and I might be full or old wives tales LOL. It wouldn't be the first time either.

Just my 2 cents. Good luck with it.

73 DE K7RMJ  Frank

Greetings all,

Well, I have good news and bad news. The good news is that the parts for my Sprague TEL-OHMIKE TO-3 arrived. I ordered them from Mouser Electronics. I ordered them on Thursday and they arrived on Friday, the next day and that was UPS ground shipping from TX. Kudos to Mouser.

The bad news is that I blew up one of the new filter capacitors.

Here is what happened. I replaced all the electrolytic caps. Then gave the two potentiometers a shot of control cleaner because they were rather old and looked like they needed it. Next I did resistance tests with my trusty VOM and all looked good. So I turned the chassis upright and sat it on the bench and plugged it in. That was mistake #2. Mistake #1 was that I did not give it good enough of a visual inspection, but I'll come back to that in a minute.

I don't have a variac to bring the AC up slowly and just couldn't wait so I plugged it in and turned it on. Everything good so far. No odd noises and no smoke... yet. So I sat back in my chair to let it warm up a bit and noticed the magic-eye tube came to life as everything warmed up and started working. So far so good. I turned one of the knobs and suddenly I heard a loud BANG and a loud hissing noise. WOW! I almost jumped out the shop window. And the smoke was just pouring out from under the chassis. So I yanked the AC power cord our of the outlet and stood there with my mouth hanging open wondering what had happened.

There was a LOT of smoke. It literally filled the shop so I opened the door and window then turned on the fan to vent it outside.

When I went back to the bench and turned the chassis upside down to find the problem, all the parts were there in place and none of the caps, new or old, had blown apart. So I started checking them closer and found that one of the new filter caps was extremely hot to the touch. On closer inspection I could see a small hole in the cap. It was about an eighth of an inch in diameter and there was a lot of yellowish brown liquid around the chassis under that cap. It almost looked like motor oil.

I thought I was being very thorough with this repair and had made sure all the screws and nuts and bolts were tight and had star washers where needed to get good connections to ground. So I poked around looking at all the solder connections with a magnifying glass and that is when I spotted the problem. There was a large solder glob puddled between one of the old solder connections and the chassis under one of the solder lugs. It was an old solder connection. Evidently this unit had been worked on before but it was a LONG time ago.

There was a layer of dust on the solder blob and it was not actually soldered to the chassis. Evidently when I turned the chassis back over to power it up this blob moved just a tiny bit but it was enough to short the circuit to the chassis ground. and when it did, POW. Instant smoke bomb.

After I removed the blob I made a series of tests again with the VOM and everything but that blown cap looked good and tested okay. I was really worried about the transformer because there was so much smoke but it tested good so I am hoping it didn't blow out. There were no charred marks on the transformer and no discoloration on the windings so I think it is still good. Those old iron transformers are tough to find and very expensive.

So, lesson learned. Next time I will check every single solder connection visually because you never know if someone else had already been inside and left a time bomb waiting to scare the bejesus out of you LOL.

Now I need to find another 30 MFd 450 volt capacitor. I think I will replace both of them. There are two in the power supply filter. But, with the minimum $6.95 whipping charge I think I'll wait until I get a larger order together before I order a replacement.

I'll let you know if or when I continue to try repairing this unit. It looks pretty good and cleaned up real nice when I gave it a bath but I am a little gun shy on this one.

I found more evidence that someone else had been inside it. There were two places where caps  had been installed in parallel to get the right values, and they did rather sloppy work. They went by that old rule for soldering, "The bigger the blob the better the soldering job".

73 DE K7RMJ  Frank

Hi Buddy,
Yup. Took it down and sold the tower and hazer system. I wish I still had it but there were space limitations on the truck that moved us so it didn't make it down here to Arkansas. I still have the Mosley TA-33 beam and the rotor and thrust bearing though. I bought a Glenn Martin roof-top tower for here in Arkansas. It is only 8 ft tall but when I get it on the roof it will be about 35 feet from the top of the mast to the ground level. Our house sits on a lot that is very slanted downhill toward the back yard. BUT, the old problem of too many huge trees is back with me again. I have to either remove some of them or get a beam with a smaller footprint, whichever is less expensive and I think the new beam would be less. Those little mini beams work pretty good and have a small footprint. If anyone is interested in the Mosley beam it might be for sale or trade soon.

73 DE K7RMJ  Frank

Back in 2011 I put up a tower at the home QTH in Missouri after many years of wanting one. Since then I have moved, but I will never forget this project. It was a lot of fun. The attached document is how it went. The document is a bit long, but it was a very long project. Overall it took over 2 years to get it done. So, kick back with a cold one and take a look. I hope you enjoy it. Let me know if you do.

73 DE K7RMJ  Frank

Congarats Kinky. Glad to hear you got it sorted out.

I took a peek at your schematic in the manual. They both C124 and C128 are on the line that provides 190 VDC. The voltage coming off the bridge rectifier appears to be 290 VAC. I would suggest 350 to 450 VDC for those two caps just to be on the safe side. It may be easier to find 450 V caps but anything from 350 V to 450 V should be safe, IMHO. Anything over 450 V would be okay too.
Good luck with it and let us know how you make out.
73  DE K7RMJ  Frank

A really good ham radio test exam can be found at www.qrz.com
Just click on the Resources tab and one of the selections will be Practice Amateur Radio Exams. You will find practice exams for all 3 license classes. If you can pass those tests regularly at 80% you will have no trouble passing the test for your license.

73 DE K7RMJ  Frank