Re: 475 with no Display and no HV

Reed Dickinson

I just re-read your trouble note and I suspect you have identified the problem when you said the trace is only 2/3 of the normal width.  This points immediately to a defective U1321 voltage tripler.  Unsolder the lead from T1320, pin 12, tripler and power on, look for normal waveforms around Q1318.  If your tripler is defective a replacement 3X can be had in an NTE539 or ECG539 (an NTE538 or ECG538 would work too).
By momentarily connecting the base of Q1318 to +5 through a 1K resistor you have bypassed all the regulation that you get from Q1306 through Q1316 and you have proved that the transformer and associated circuit are working.  The shortened trace you saw was probably due to the unregulated HV giving a shorter trace or a defective voltage tripler U1321.  I would still look for problems around C1304.  When you power up the time constant of R1304 and C1304 is about 680ms.  This means that you have under a second for the HV to stabilize since the base of Q1306 is being pulled toward +110V until it is nailed at +0.6 by the conduction of CR1304.  After about 1 second the base of Q1306 should go to about +0.6V and oscillations should occur via Q1318.
To troubleshoot the regulator circuit pull F1318 out and disconnect P1313, the transistor Q1318 is now out of the circuit.  Connect a 25K pot between +15 and - 5  and connect the wiper of the pot through a 10K resistor to the Q1306 base node.  Connect a sensitive voltmeter to the - side of C1316 and power on your scope.  Connect another voltmeter to the Q1306 base node and vary the pot.  When the base of Q1306 is around +0,6V you should see some variation at the - side of C1316 (a 4.7K temporary resistor to ground will be necessary to give a little current through Q1316),  If no variation is seen when rotating the pot then you have a problem in the regulator circuit, troubleshoot it and you will find the problem.  If a variation is seen then the regulator is working and you need to fix problems on the secondaries of the transformer.  Before using the following shotgun replacement procedure unsolder the lead from T1320 pin 12 going to the 3X voltage multiplier.  If the multiplier is bad it would put a load on the oscillator circuit and kill the oscillations.  Next, use the shotgun approach and replace C1320, CR1320, CR1321, C1322, C1323, C1328 and CR1329.  I recommend you use HV caps rated at least 5KV as the surge of HV during the first second is high.
Good luck:
Reed Dickinson

On Sunday, March 3, 2019, 12:01:12 PM PST, SuddenLink <bob.ripley@...> wrote:

I missed identified C1304. Instead it was C1468 with a broken lead that I pulled from the circuit board. A perfect example of a mistake made when one works on a problem to long without stepping away for a while.

C1436 on the -8 volt rail was replaced with a 33uf capacitor.


I checked C1304 and it is fine, measures 2.7uf on the dot. CR1304 was replaced with a IN4007 per your recommendation and I tried your test to the base of Q1318.

The first time I taped the base of Q1318 with a 1K resistor in series with the +5 volt rail the waveform at TP1318 more than doubled in amplitude from 760mv p-p to 2.2 volts p-p and has stayed there. The frequency however, is now down to 14.4Khz

Now when I touch the lead of the 1K resistor from the +5 volt rail to the base of Q1318 the Oscillator waveform at TP1318 jumps up to 7.2 volts p-p still at 14.4 Khz. When I take the 1K resistor away the waveform drops back to 2.2 volts.


While typing this report up I turned back to the scope, taped the 1K resistor from the +5 volt rail to the base of Q1318 again. This time the waveform at TP1318 jumped up to 30.8 volts p-p and now I have a trace on the CRT.  Take the 1K resistor away and the waveform at TP1318 keeps running at 30.8 volts p-p and there is still a trace on the CRT. Note: The trace is only covering about a two-thirds of the screen and the horizontal position is behaving strange. Instead of sliding the whole trace left or right. It erases part of trace moving towards the middle of the screen if the Horizontal position is turn left or right. I suspect that is because the HV oscillator is only running at 13.8Khz.

I shut the scope off then turn it back on. The waveform at TP1318 is down to 2.2 volts p-p. Tap the base of Q1318 with the lead from the 1k resistor going to +5 volt rail and the oscillator waveform is goosed to 30.8 volts p-p again. The trace on the CRT is present as before and stays there until I shut the scope off and turn it back on.

How weird is that!

So now what? Is Q1318 flaky? I kind of doubt that. C1316 was replaced with a new 1uf tantalum capacitor. C1317 was pulled and checked. The LCR meter says it’s a 10uf capacitor. I don’t remember what the ESR was but I don’t remember it raising an eyebrow with me. CR1317 was check with the diode function on my HP 3466 DMM but it could be leaky…maybe?

What the Heck?


Sent from Mail for Windows 10

From: Steph L
Sent: Saturday, March 2, 2019 5:43 PM
Subject: Re: [TekScopes] 475 with no Display and no HV

Page 3-35 of manual has a good functional description including point that ambient current supplied to base (of Q1318) determines frequency (ie not LC arrangement) and as such there isn’t a specific cap component for resonate frequency (a function oscillator still relies upon however to function).

You said “The rest of the voltage measurement compared to the manual are no where near correct” so perhaps a closer look at the whole setup noting Q1308/10 push-pull, gain and phase reversal Q1306 and bias arrangement behind Q1310 (ie the Q1312 and associated components).

I have an old Tek544 where the HT falls dramatically when ambient room temp too hot and was amazed in the frequency shift involved.

It is a different circuit but 475 still has resonance involved and the output voltage is regulated by driving current harder into the base.

Just a thought. Please excuse me/any error as nubie here. Interesting fault.

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