Topic 'Re: Tektronix 485 will not start' in forum 'Electronics' - dslreports.com

Re: Tektronix 485 will not start

Mon, 03 Dec 2012 10:57:19 EDT

neonhomer posted : I'm not entirely sure what is wrong with mine. I think I tracked it down to a bad diode pack.

This thing has been sitting under my bed for a while now.

Re: Tektronix 485 will not start

Tue, 20 Nov 2012 13:34:50 EDT

IllIlIlllIll posted :

said by ChrisMH:

The HV winding is fine. Two of the lower voltage windings are shorted to each other.

that stinks.
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Re: Tektronix 485 will not start

Tue, 20 Nov 2012 12:55:33 EDT

ChrisMH posted : The HV winding is fine. Two of the lower voltage windings are shorted to each other.

Re: Tektronix 485 will not start

Mon, 19 Nov 2012 08:30:21 EDT

IllIlIlllIll posted :

said by ChrisMH:

My transformer has a shorted winding.

as i expected.
CRT scopes are just a "glorified" CRT television set. :D
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Re: Tektronix 485 will not start

Sun, 18 Nov 2012 23:11:22 EDT

ChrisMH posted : My transformer has a shorted winding.

Want to sell me yours? :D

Re: Tektronix 485 will not start

Sun, 18 Nov 2012 11:07:13 EDT

neonhomer posted : FYI - That was my thread.... I never did fix it. I still have it in the closet. I'm thinking about getting rid of it. I believe I had isolated down to the HV diode (if I remember correctly) being bad, but I don't have another scope to fully check this thing out.

Re: Tektronix 485 will not start

Tue, 13 Nov 2012 23:31:53 EDT

ChrisMH posted : I have an HV probe coming in a day or so.

However, I have removed pin 3 of comb U which, according to the manual disconnects the HV. Still does not start.

Chris

Re: Tektronix 485 will not start

Tue, 13 Nov 2012 23:17:48 EDT

IllIlIlllIll posted : do you have a high voltage meter to check for the presence of high voltage to the CRT ?
»www.electronicrepairguide.com/im···eter.jpg
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Tektronix 485 will not start

Tue, 13 Nov 2012 14:03:12 EDT

ChrisMH posted : I found this related thread, but it kind of petered out with no resolution: »Tektronix 485 - no power up

I have a 485 that won't start. It makes a "tick tick tick" sound when powered
on which, I believe, is the inverter trying to start and being shut off due to
the protection circuitry.

DS1668 and DS1669, the neon lights located under the HV plastic cover on A11 are
blinking on and off which, I believe, is the source of the "tick tick tick".

Following the troubleshooting section starting on p.4-7, I have looked at
Balance Node TP1951 and confirmed that it is going more negative than -200mV
during startup, indicating a short on positive supply or high beam current. I
found and replaced shorted capacitor on the +15V supply that was on board A7
(Vert output amp) (I forgot my notes so I am not sure exactly which one I
replaced), but the problem persisted.

I put a scope on TP1951 and the waveform at that node looks nearly identical to
the one on p. 4-9 waveform D, indicating a short on the +9V supply. However, an
ohmmeter on the +9V TP to ground shows a resistance of 105, which is identical
to a working 485 I have. I have checked all of the positive and negative
regulated supply test points and none appear to be shorted.

Following the Balance Node Note on p.4-9, I put a scope on TP1958. The note
states that an HV current fault is present if the voltage at that test point
goes positive during the 20ms sampling period. The waveform I get is identical
to p.4-9 E, where the caption seems to indicate that this is a normal waveform.
The pictured waveform and my waveform, while identical, both go positive during
the sampling period, so I'm a little bit confused here.

I then tested TP1959 (LIM) and it goes negative during the 20ms sampling period,
which, according to the balance node note, indicates a "beam limit fault." I
have traced the beam limit circuitry and have not been able to find a problem
with it. I have tested the capacitors for shorts, the transistors for shorts,
the diodes, and the resistors for correct values. I have found no burned traces
or components or other signs of distress in the beam limit circuit.

I am at a loss on how to proceed. Does anyone have any ideas what to check
next?

Thanks for any help,
Chris