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Tektronix 221 , hand held oscilloscope from the 1970s

Works if you charge up the nicad packs out of the machine, then introduce them, but the charge circuit is not working. The full service manual is out there , but wondered if anyone had worked on one. At the moment I'm working on the power supply board in isolation from the other boards, anyone know if this is valid. The schematic does not seem to show any control lines back to the mains-hot oscillator. Seems it would be operational (if it was working) all the time the scope is connected to the mains, regardless of the on/off switch , which seems to be for the sake of batteries only. Otherwise a matter of making up extender connections for the interboard connectors, to work on the whole scope un-wrapped. Everything, so far tested, in that oscillator in the way of transistors and coils etc seem to be ok testing cold and plenty of high voltage DC from the mains on the main supply cap, but no oscillation

Reply to
N_Cook
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With about half mains voltage , for a bit of personal safety. About 20V over C-E of one driver Q and 120V over C-E of the other, whenever powered up. Plan of campaign will be to try biasing on the off one and see what happens. Could a leaky driver transistor be the most likely cause of non-oscillation? one sliver-mica cap could have metal migration , others are polyester and ceramic. Otherwise loads of diodes and coils.

Reply to
N_Cook

Just scanning the info on Yahoo Tek scopes, the batteries need to be in situ as part of the regulation on these, bad things may happen otherwise? (so they say, no experience with them myself) JC

Reply to
Archon

over

happens.

non-oscillation?

and

yes, by the "power supply board in isolation " I meant with also the 2 battery packs, but minus the other 2 boards and CRT. I've decided to desolder the pair of TO220 2SC2333 500V/2A and try a pair of TO220 BUV46

850V/8A switching Qs, will try later today
Reply to
N_Cook

The batteries do need to be in the circuit and they can't be completely dead, or the scope won't function, even when hooked up to AC. Chuck

Reply to
chuck

over

up.

happens.

non-oscillation?

and

The batteries are modern replacements and good order, the scope works well on battery power but cuts out , as it should, when the batteries run down. Although this range of scopes started in the mid70s , the one I have here seems to be made in 1989 with the later protection against self-imolatation. Try-out replacement Qs in there , but will be tomorrow before I power up again

Reply to
N_Cook

Same static set up with changed Qs. Same ones high and low. Taking the base of the off one high will switch it on, then returns on release, doing that a few thousand times would put some charge in the batteries. Changing the silver mica? cap made no difference. There are a few probably multilayer ceramic caps that are the next things to change for the same reason, especially C622 near the base of that off Q. I wish I knew what all those

+5V taps are doing on the hot side of the charger unit, the +5V only comes up with oscillation of the main power supply off the batteries
Reply to
N_Cook

Some sort of problem with the Motorola "3-layer trigger" CR622 , in the manual as 32V SPT32R and marked around the glass barrel , 1N4148 sized with a blue central band, and neat printing around the barrel as letters ST2PR- so how to read ? could be PR-ST2. Anyway replaced with a 32V diac robbed from a Philips compact flourescent lamp, diac marked BLDB3 , DB3 on the overlay. All a very strange circuit. Voltage over the battery now racing away upwards while I let it, will have to tidy everything up to get it back into its shield and TO220s heatsinked and take some readings as not much useful technical info out there on these compact scopes. I'll have to try the original on a >32V bench ps as well.

Reply to
N_Cook

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