Re: Tek 2213a high voltage problem
From: Jim Yanik (jyanik_at_abuse.gov.)
Date: 12/12/04
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Date: 12 Dec 2004 06:35:49 GMT
"cmdrdata" <cmdrdata@mail.com> wrote in
news:1102805683.524613.81030@c13g2000cwb.googlegroups.com:
> Ok, I got my eBay schematic and trace out the low voltages: all LV are
> in spec. I can hear a faint but constant high frequency pitch sound,
> but don't have a HV probe to check the cathode and anode voltages.
> DS870 ( a neon bulb?) that bridges a connection to the one of the
> filament connection does not glow.
The neons are for limiting G-K voltages to no more than 120V if the DC
restorer diodes or a HV cap fails. IOW,they should not glow if things are
OK,if there is HV.Maybe just a flash on turn on,but not always on.
You can turn the scope OFF,pull the white CRT anode plug and discharge the
CRT anode lead to chassis and if you get a big fat spark,the HV is probably
good.No spark or weak spark probably means a bad HV multiplier. If so,you
need a parts scope or build your own.Its just a multi-stage ladder-style
diode-cap multiplier.
> This is reminiscent of the same problem BobD described in his scope.
> Does anyone have a suggestion on how I can measure HV with a DVM (maybe
> via hi value resistor divider? if so, what values shoudl I use?) If my
> D6 is bad, where can I get a replacement? Thanks in advance for your
> help.
>
> Jim Yanik wrote:
>> Normal troubleshooting is to first check the LV and HV supplies for
> correct
>> V,then scope the Horizontal CRT plates for a sweep ramp or large DC
>> offset(sweep problem),short the CRT vertical plates to see if the
> trace is
>> off-screen vertically (vertical problem).Then check the Z-axis for
> proper
>> unblanking.The internal CRT grid bias adjustment should be able to
> turn the
>> trace on if the CRT is good and LV/HV is working.
>
>
You can make a 10:1 HV probe for your DMM by using 4 22.5Mohm(total 90 M)
0.5W 1% metal film precision resistors in series *if your DMM has a 10 M
input Z*(most do);solder them in series,mount in a plastic tube,like an old
tuning tool. I used a 6/32 screw and threaded hex spacer at one end of the
string,so I could use a TEK hook-tip from the *old* P6000 series scope
probes,and a DMM lead and bannana plug on the other end for the DMM pos
input.It all fit in a red tubular hex tuning tool(~3/8" OD/1/4" ID) that we
used for the old 500 series tube scopes.
**BE sure to connect the GROUND lead -BEFORE- the hot lead,to the circuit
under test.** (otherwise,you may zap your meter)
Of course,we had all this stuff on bench stock.... 8-)
Also,the reason I suggested a service MANUAL is that TEK used to write
these really excellent CIRCUIT DESCRIPTIONS that tell just how the circuits
work,IMO,essential to good troubleshooting.
One very common failure is the focus string of 1/2w resistors increase in
value until they eventually open,and that throws off your focus.You may
need th check them after you verify if your CRT is good or not.(I suspect
not)
-- Jim Yanik jyanik-at-kua.net
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