Re: New guy trying to fix an old Tek scope (442)


Robert Simpson
 

 Hi, I once had a 442 and fixed a problem similar to yours. This was also reported by me on another forum a few years ago: (from that posting)

 

". I have experience with the 442 including fixing the problem you have. I bought one not working about a year ago and completely refurbished it. I then used it for a year when I got a deal on a newer model, then sold my 442.

Here is the short version
That fuse feeds the high voltage supply. I bought a CD manual from QService but gave it away with the new owner. However I still have a printout of the HV CRT section. What I found was the same blown fuse you have. I temporarily put in a 1/10 fast blow until I could order the 1/16 fuse. The scope then showed a trace after about a minute warm up. It blew after a couple of hours of use. When I finally got the service manual, I found that Q458 (? 2SC2333? NTE379?), a power regulating transistor, had been replaced before. So I ordered a replacement as well as the Electrolytic cap C458 (10uf 150V). E-caps are notorious for going bad. After replacing the Xter and cap, the turn on time went down from a minute to 10secs! And it would run for hours. (is there a place to post the schematic .jpg?)

Later I also cleaned the vertical attenuator contacts (caution, be very careful!) and some potentiometers. At one point I was having a strange problem which I traced to the polarity trigger push button switch. I replaced that but after dissecting the old switch found I might have fixed it by just pushing it in and out a hundred or so times. The internal contacts had tarnished from lack of use!

I also found some of the internal wires were breaking loose from the plastic contact carriers. So I re-soldered those. I then gently tugged on all wires and re-soldered any that came loose.
I did several other things including replacing all the large ecaps near the main power supply (Turn on time now down to 3 secs) also added a thin fan for cooling to the back.

Take lots of pictures!!!! It really helps. Also, observed all safety precautions especially those relating to the HV area. I could go into a lot more detail, but it would make a very long post.

 

Bob


--- In tekscopes@..., <tmiller11147@...> wrote:

My first guess would be that C458 is shorted. Is it a tantalum capacitor? If R458 glowed, it should be replaced. It is a 150 ohm, 3 watt 5% wire wound unit.
 
Regards,
Tom
 
 
 
----- Original Message -----
From: pr0phetex
Sent: Saturday, September 14, 2013 5:44 PM
Subject: [TekScopes] New guy trying to fix an old Tek scope (442)

 

Hi everybody! I'm an electronics hobbyist looking for some advice. I found an old Tektronix 442 scope near me for $25 and thought it would be a good choice for a first scope, provided I can get it to work again. I have a Fluke 87 III to measure with, but I don't have a hv probe right now (I could get one if needed).

So here's the problem: I'm getting a pilot light but no trace on the screen. All the outputs on the power supply board are good except the 100v unregulated output which measured around 77v. The 1/16th amp fuse (F722) is blown but I'm not sure whats causing such a high current draw. When I measure the current through across the fuse terminals I'm seeing 170ma and rising. When I left this on for a min several of the transistors got hot but I saw resistor 458 get extremely hot (glowing). I couldn't find any of the transformer coils shorted. Any suggestions on what could be wrong? I am not afraid of replacing parts and I'm not too bad at soldering, but I don't want to start throwing parts at it without understanding what's wrong. Is the resistor bad? It looks like the resistance has lowered.

Here's the manual: http://www.box.net/shared/0jn2bd5njp

Thanks!

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