Re: Tektronix 2235 Scope High Voltage Problem
From: TekMan (and7_at_bigfoot.com)
Date: 09/29/04
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Date: 28 Sep 2004 22:50:39 -0700
bobd426@yahoo.com (Scibuff) wrote in message news:<6fa04d1c.0409281345.22492b7e@posting.google.com>...
..snippety..
> > > >>
> > > >> Are your low voltages proper when you disconnect the HV mult.?
> > > >
> > > > The HV multiplier is a x6 type. Do as Jim proposed and try to get a
> > > > parts scope.
> > > >
> > > > It is possible to build the the HV multiplier from discrete parts
> > > > (diodes, caps, etc), but is really a tedious task and requires HV
> > > > expereince. I did it a couple of times, but would recommend this only
> > > > to someone with a lot of repair experience. The HV is nasty, so
> > > > careful soldering and isolation technique is a must.
> > >
> > > And you have to build it to fit into the available space and not arc to
> > > something else.
> > > >
> > > > you're better of with a commercial build multiplier from a parts
> > > > scope.
> > > >
> > > > hth,
> > > > Andreas
> > > >
> > >
> > > I believe the multiplier is a X-FOUR,not six. My 2215 schematic shows X4.
> > > I also believe he could adapt a HV mult from a 1700 series TEK waveform
> > > monitor,they're about the same size,but don't have the internal cathode
> > > supply diode,no big deal. 152-0900-00,and probably still available from
> > > TEK.
> >
> > Special case was not necessary to prevent arcing. "Rounded" solder
> > blobs are much more important to prevent HV arcing due to corona
> > effects etc.
> > Insulation: A couple of spray paint named Plastik70 (a special brand
> > forHV, a german suppier manufactered this. Don't know if it is still
> > for sale).
> >
> > Multiplier: It's a sixtupler ( x6 ) in the 2235, but a quadrupler ( x4
> > ) in the 2215 - trust me :)
> >
> >
> > hth,
> > Andreas
>
>
> As requested yesterday, here is the results of further tests after the
> top input wire to the X6 (says so on the schematic) U975 voltage
> multiplier is disconnected:
>
> The other voltages are within parameters, e.g. +100, +30, +8.6, -8.8,
> +5.2 volts are all OK. Even the calibrator is working and the Uncal
> lamps come on when knobs are turned.
>
> I reconnected the top input to X6 after disconnecting one end of 5.1K
> R976 (to isolate the circuit after that) and the power lamp will no
> longer come on. It sounds like the HV oscillator is trying to come on,
> as I hear it raising in pitch, then it fails. This repeats about every
> second. Also I know that at least some HV was generated, as when I
> went to reconnect R976, I got "bit" by it!
>
> With an ohmmeter, I also verified all the resistors and diodes in the
> X6/pin 4 circuit that supplies voltage (how much is this - not in my
> manual) to the cathode, control and focus grids, etc. They are OK.
>
> It is looking more and more like the X6 has bit the dust. And no parts
> machines are on eBay now.:-( Maybe that is what mine will become, as
> it was operating great until this HV problem happened.
>
> BobD
To be more precise: The x6 HV multiplier IS DEFINETELY defectctive.
Sorry to say this so clear.
pin4 is approx. -2850V, as Jim already mentioned. But of course, with
a defective x6 multiplier, the HV is overloaded. So that's why you
will not measure the -2850V now.
If it is such urgent: Besides ebay you might try the usual Tektronix
suppliers for these older scopes, e.g. google for "tektronix parts". I
don't know if Tom Gootee is still selling parts, but if I would be in
your shoes: I'd google or wait on ebay.
Good luck!
hth,
Andreas
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