Tek 335

[synx508]

I bought this early 1970s portable from Sony/Tek on Sunday for £10, because it was looking very sad. I wasn't expecting to get it working but after a lot of cleaning it was apparent that it had two faults, the front panel had taken an impact and was distorted around the trigger control switches and these were both jammed in too far and the timebase and vertical deflection circuits weren't working but the EHT was. There was a dot on the CRT that could be moved left and right with the position control but it was mostly on the right of the screen so I guessed that the negative rail was missing or low.
Opening the 335 took most of a day, though, the scope was originally fitted with internal foam padding that the outer metal sleeve slides over - this had turned into a very sticky and greasy sludge that had effectively glued the sleeve on and only when I'd removed it from the front and back ends could I exert enough force to shift the goo on the inner section to break the seal.
The -8V line was then measured at -4.5V and +15V was around 8.5V. The negative rail is referenced to 15V, so when 15V is halved so is the negative rail. Attaching an external 15V supply (only 66mA needed!) brought the negative rail back to approximately -8V and now the trace was symmetrical and could be centred horizontally and vertically. Triggering isn't quite right but works fine on channel 2, there's a bright sharp trace on both channels, the intensified delayed B timebase works properly as do the coupling settings and the magnify feature. It's an incredible box of tricks for its age and size, apparently it's 35MHz which is quite a useful bandwidth.
I've done the rather extensive dismantling needed to access the low voltage regulator, where I believe the problem to be, it's a very simple 1458 op amp referenced to a zener with an emitter follower pass transistor. The ref diode, resistors, adjustment trimmer and pass transistor all appear to be fine so I'll change the 1458 and hope for the best. I might even test the old 1458 to be slightly more certain. Reassembly is a lot of work with a hint of dodgy industrial design in places, like the screw that you can't really access unless you have the world's slenderest screwdriver.
Still need to figure out exactly what's wrong with the triggering switches apart from them being jammed but that's a job for tomorrow.

[MED6753]

I have a Sony/Tek 314 and luckily it just needed a few tweaks to get it fully functional. I was going to re-cap the PSU until I discovered that it was 3 pages of the service manual to disassemble it. Needless to say I decided to pass.

[Zenith]

Interesting. I don't recall seeing the Tek portable scopes on my travels.

It looks as if some of the push buttons have been in the wars.

I'm sure you'll get it working. If it proves hopeless there has to be more than £10 worth of usable parts in it.

[synx508]

The manual told me to remove the wrong screws, which was annoying because it caused the EHT multiplier to fall out and the captive nuts aren't fully captive - luckily they didn't roll out of sight. I should've spotted the mistake, really.
These are the connectors that need to be reconnected when I put the PSU back. The manual suggested writing the connector numbers on tape so I used a label maker, which does that more legibly.

[MED6753]

Interesting. The PSU in the 314 is not modular like the 335 which is why I didn't re-cap it.

[synx508]

I made a silly error during repair, a zener transposed so it because a 0.65V reference instead of 5.1V. I had to dismantle almost everything to remove the low voltage regulator board so I could not only correct my mistake but test the board independently of the scary 2,000V supply.

I took a moment to straighten the front panel with a mix of panel beating with a small hammer and superglue to get the plastic surround roughly back where it should be.

During reassembly I made a few more errors, such as forgetting to reinsert the plastic PSU insulating covers, trapping a wire and a capacitor behind the tube so it wouldn't sit flush (there's no free space in this thing, get the slightest thing wrong and it won't reassemble at all!) bashing the horizontal module against the scope and knocking one of the plate drive transistors out of its socket which caused half the horizontal scan to be eliminated. I was lucky to get away with that one.
Now it works, but could do with a bit more cleaning, an outer case sleeve respray, a bezel for the channel 1 trigger selection button (ideally a new button too) and the left shroud for the carrying handle swivel.

[MED6753]

Looks good. But as you found out servicing those little Tek/Sony scopes will try your patience. But no where near as bad as these abominations. Try stuffing 5 pounds of crap into a 2 pound bag.

[tggzzz]

But no where near as bad as these abominations. Try stuffing 5 pounds of crap into a 2 pound bag.

Is that better or worse than Sinclair stuffing 4oz of crap into an 8oz bag (SC110)? Discuss

[MED6753]

Not familiar with the Sinclair unit so I looked at the internals. At least once you split the case all the circuit boards remain in place and accessible. Not so with the Tek 200 series. You pull it apart and circuit boards and a CRT literally explode all over the bench. Stuffing it all back together is a fucking nightmare.

[tggzzz]

Not familiar with the Sinclair unit so I looked at the internals. At least once you split the case all the circuit boards remain in place and accessible. Not so with the Tek 200 series. You pull it apart and circuit boards and a CRT literally explode all over the bench. Stuffing it all back together is a fucking nightmare.

Sort of. Some of the circuit boards are soldered.to each other; wouldn't expect anything else from Sinclair. (Sinclair would have done that because it was more reliable cheaper.

Must get back to one of my calculators.

[synx508]

The little 335 waited for me to post it to the facebook VETE group and then expired quietly while I wasn't looking. This time it was a CA3046 doing what old CA3046s do and one of my new old stock parts has brought it thundering back into action. U700 if anyone is playing at home. I did consider changing U800 too as it has the same date code, there's another one inside the mysterious cavern of tunnel diodes deep in the centre of the unit but it's working… at the moment.

Edit: forgot to mention how amazing the triggering is on this. I *am* used to HP scopes, though, which use a conceptually different triggering approach.