Tek 465 trigger problem.

Bad tunnel diode?

Do you have the service manual?

Can you trigger the B sweep?

The 465 was one of those products sold to government agencies and large corporations in great numbers. If you bring out a new model number, you open yourself up to competitive bidding. If you change what's under the hood and keep the model number...not so much. The 465 underwent many changes over its lifetime. Can't say how many of those were in the trigger circuit. Any service manual can help...you can download one...but one matching the serial number is best.

The trigger is a bunch of switches and amplifiers driving a tunnel diode or two. Pretty easy to follow the signal with another scope. If I had to bet, I'd bet on a TD being bad.

I did have a 453 that had trigger problems on some ranges caused by a leaky timing cap in the trigger holdoff circuit.

You should be able to determine something from the relative behavior of the A and B triggers. My 465 is in the attic, so I can't try it, but there's a trigger view function that's picked up after the switching at the first amplifier.

If you find that it is a bad TD, you'll be tempted to sacrifice the B-trigger to fix the A-trigger. Depending on the TD package, there's significant risk of busting all the TD's. Mess with the TD's as a last resort.

I'm sure this hasn't happened to you, but I've chased my tail many times only to find that some pot switch had been inadvertently switched off the detent.

I'm also assuming that you have personally seen the working trigger and it broke later. That's different from, "I bought this thing on ebay..." where someone may have already harvested the TD's.

John Fields schrieb:

I'm sure you'll find some help

here:

HTH

Reinhard

Yep. I found a bad tunnel diode in a 465B that I was fixing, so I tried to move the B channel diode to the A side and ended up with two dead diodes. Oops. I think I killed it with too much heat from my soldering iron.

I eventually had to cannibalize them from another Tek 465 that had too much damage to be worth repairing. I couldn't find any online at a reasonable price. I was lucky and got two diodes out of the carcass. Tucker has the diodes for $25/ea (ouch):

Info on tunnel diodes:

The TDs are Ge and heat will kill them. Use forceps against the body to solder them. Do you have access to a curve tracer? If so, check the TDs for the A sweep. I usually check them in circuit. I think they are 4.5 ma or 10 ma diodes.

Feed in a test waveform to the external trigger input and see if the trigger view works.

You do have it in norm mode vs auto, right?

Can you line trigger?

I wonder if a NPN transistor used kind of backwards around a diode to take advantage of the negative beta to give you that snap effect would of done the same for you ?

Jamie

Remember that this was designed 40 years ago. There are ways to get negative resistance, but the TD is/was simple and fast.

Probably not. Too many other changes would be required.

The problem will be finding a TD with what I guess(tm) has a 5ma turning point and the correct max capacitance. I would be tempted to substitute a more modern silicon or GaAs device. The problem is that they will probably oscillate. Therefore, I would stay with germanium.

My best guess for a substitute are the GE 1N3712 thru 1N3721 series of germanium TD's. The 1N3712 looks like something close to 5ma (152-0125-01). The 1N3712 Cmax is 11pf while the Tek part is 18pf. Close enough.

I haven't gone through all the specs to make sure they're identical and don't plan to do that until the OP identifies the desired Tek part number. Too much work.

There are various DO-17 packaged germanium diodes in the 1N3712 thru

Clues on TD substitutions: Note that this is for a different scope.

Look what I found in my collection: Sealed from the factory and untouched for who knows how long. The tunnel diodes came from a rather large collection of original Tek repair parts. There are probably more tunnel diodes in the box, but it will take some effort as many of the smaller parts are not in numerical sequence: Make me an offer (via email please).

I too have some TD's, just not sure what numbers they are, however, I remember not too long ago replacing a TD circuit with a fast dual comparator from Analog devices. I was able to get the valley effect to make the following circuit happy.. This was to keep an old piece of equipment operating (not a scope).

Jamie

The transistors and ICs are all in sockets and on the Trigger board that faces to the right. While it is a good bet that the "Fire" TD has failed, it may be worth your time to open the scope up and poke around those parts. U-870, and the transistors around that area. There are many waveforms given in the manual and with another scope they can save much time. Luckily it's all out in the open.

While it is open, you might verify all the power supply voltages and ripple levels per the manual. In fact, do that first.

Good luck, the 465 is a fine instrument.

Regards, tm

Hello,

I have a Tektronix 465 which is perfect in every way except that the horizontal trigger has gone belly up and the trace free-runs no matter what.

Clue, please?

Thanks,

Because repair is better than recycling.

Please note that his is a usenet repair newsgroup, not a recycling newsgroup.

Not everyone is as useless as you are, so why don't you pull _that_ trigger?