Inherited this antique as a DOA, so thought I'd try to fix it for some fun.
Immediate obvious problem was that the electrolytics in the PSU had failed, so on general principle I replaced every electrolytic in the machine, and now it powers on and I've got the vertical amp calibrated against local references (no they aren't certified, but have been checked against kit that
*is* in calibration), and the sweep timings appear to be pretty darn close anyway, so I've not dinked with the sweep unit. +24V, -24V and +105V are rock stable within seconds of power up.
One remaining problem though, when I power on the scope, the scan line is excessively bright even with intensity turned right down and this takes about 20 minutes to settle down to a reasonable level (at which point I need to turn intensity up to about 50%).
Where should I be looking for the culprit - my initial thoughts are pointing me to the HT/EHT section or possibly the "bright-up" circuit. Any suggestions on the likely causes of the problem would be helpful. Could it be the ceramic caps in the HT supply, or possibly the switch on Thermistor (TH301)? How best to check?
Thanks
--
Dave Partridge (remove xwy from replyto email address)
Try to get a schmetic, or better a complete service manual. According to your description, this scope is a Telequipment D75 (Telequipment was buyied once by Tektronix and then spit out again). Schmeatics might be found on the web (bama.sbc.edu, etc.) or for sale at Nigel cooks, UK.
"David C. Partridge" wrote in news:42e5f7b6$0$19754$ snipped-for-privacy@news.dial.pipex.com:
To control display intensity,the CRT grid gets biased more negatively than the cathode,and to 'unblank' the CRT,the grid is moved in a positive(WRT the cathode)direction.These are high voltages,perhaps -2KV or more.
So,trace back from the CRT grid to see what sort of supply provides the grid bias,and to see what circuitry changes the grid bias supply to unblank the CRT.Between the sweep circuits and the CRT grid bias supply will be a "Z-axis amplifier" to provide unblanking drive for the grid supply.It uses the sweep gate (the time the sweep ramps)to unblank the CRT,and the "intensity control" is another input to the Z-axis amp if not to the grid bias supply.Often there s a internal coarse grid bias adj. pot,and an intensity control for fine or front panel adjustment.
It was a combination of items - all in the area you were pointing to! I was looking round trying to think what was wrong - when I spotted item 1 below (and cursed myself for not spotting it last night).
The previous owner had fiddled with the mains transformer jumpering (presumably to try to compensate for the dying e-caps) and the UN-regulated
+30V line was *rather* high. Setting the jumpers to local supply voltage brought that back into line (and the regulated lines were still spot on). The +30V was driving the oscillator in the HT ... which was affecting cathode AND grid voltages.
Dug out HV probes, and HT was now rather low (thinks - someone tweaked this after fiddling with the trafo jumpers). Re-set cathode voltage to -2.5kV.
Adjusted grid cut-off (the coarse adjustment you refer to below) according to calibration procedure in manual.
And NOW everything seems to behave properly. Or almost - there's *slight* drop in trace intensity over the first 20 minutes of use - but I can live with that.
PS Oh yes. I did re-check the calibration afterwards, and (not surprisingly) did need to make some minor adjustments on the main X and Y amps (the ones directly driving the plates on the CRT).
"N Cook" wrote in news:dc7sm4$86k$ snipped-for-privacy@inews.gazeta.pl:
Which also may mean that the CRT filament voltage is higher and thus shortening the CRT's life.(if the fil V is derived from the HV power supply) TEK discovered that 0.1v higher fil voltage drastically shortened their model 2213/15 CRT lifetime.
"David C. Partridge" wrote in news:42e7ead3$0$12893$ snipped-for-privacy@news.dial.pipex.com:
Good,as long as the xfmr insulation holds up. It has to be insulated for more than 2-3 KV cathode voltage. TEK departed from that after experience with the old 500-series tube-type scopes.
No,that is not critical. TEK used a 3 or 4 percent tolerance on it's scopes HV supplies,IIRC.
--
Jim Yanik
jyanik
at
kua.net
>
> David
> "Jim Yanik" wrote in message
> news:Xns96A066AAE917Fjyanikkuanet@129.250.170.84...
>> Which also may mean that the CRT filament voltage is higher and thus
>> shortening the CRT's life.(if the fil V is derived from the HV power
>> supply)
>> TEK discovered that 0.1v higher fil voltage drastically shortened
>> their model 2213/15 CRT lifetime.
>>
>> --
>> Jim Yanik
>> jyanik
>> at
>> kua.net
>
>
>
Well that'll teach me - I was judging this based on a too short "cool down" period.
After leaving it off for a day, the original problem returns - sure I could back off the grid cut off much further so that I needed to crank up the intensity really high once warm up was complete, but it still bothers me that I can't work out what the problem might be.
From memory the measured cold resistance across R310/TH301 (Mullard VA1040)was about 66 ohms (100R for the resistor and 130R +/- 20% nominal for the thermistor) - which suggests that the thermistor is about right when cold.
When all is warmed up the resistance is about 39 Ohms - NTC obviously.
I'll quote the relevant part of the circuit description in full:
====QUOTE==== The EHT is derived from a Class C oscillator operating at about 28kHz. The oscillator transistor TR304 uses transformer, T301, as its load and main frequency determining component. A feedback loop, including a high gain amplifier, TR301, TR302 and TR303, regulates the cathode supply, which is set to the correct potential by adjusting R301, and the grid is set to the correct tube cut-off potential by adjusting R315. Diodes D304 and D303 provide half-wave rectification from T301 for the cathode and grid supplies. A five stage voltage multiplier (quintupler) provides the +12.5 kV PDA. The input to the multiplier is taken from the same tapping on T301 as that used for the cathode supply.
Thermistor TH301 limits the output voltage at switch on and C204 reduces E.H.T feddback into the unregulated +30V line. ====QUOTE====
Here's the key parts the circuit in line art monspace font - View with small font I guess.
I measured the cathode to grid voltage at switch on from cold with = intensity set to minimum.
Immediately on switch on the grid was average -63.9V DC negative wrt. = the cathode as measured by my DMM with 10Meg input impedance. It has been increasing (in a negative = sense) steadily over the last five to seven minutes and was -80.6V DC when I removed the DMM. =20
The intensity had been decreasing over that time slowly, and while the = DMM was clearly affecting things as intensity went down quite a bit more when I removed it, it's = clearly not fully settled to "normal state" which will be another 10 or 15 mins.
So clearly something is causing the grid voltage to be low for the first = while after turn on.
The character mode circuit diagram got mangled by outlook express = wrapping lines, so I'm sending this message in HTML form (sorry) with it included again.
"David C. Partridge" wrote in news:42e95741$0$19746$ snipped-for-privacy@news.dial.pipex.com:
Yes,a more-negative grid cuts off the electron beam.(more neg WRT cathode)
Normally,a scope is run for about 20 min before adjusting CRT bias or any other cal adjustments. And you're supposed to let instruments warm up before making measurements,even 100% solid-state equipment is not accurate "instant-on".
an aging CRT(vacuum tube) will take longer to settle down. It could be that your CRT is near the end of it's life.
Maybe you have a temp-sensitive DC restorer diode? (DC restorer is one method to shift Z-axis amp signals to HV potentials needed by the CRT;it essentially is a diode-cap modulator between Z-axis amp and CRT grid.)Commonly used in TEK scopes.
Follow up to tell folks the current status and probable cause of the problem (I think):
I replaced the old 10nF ceramic smoothing caps in the grid bias supply as they were getting distinctly warm after the scope had been on suggesting that they were getting rather lossy (on a damp day they would steam gently for a minute or two after turn on).
Didn't solve the brightness problem but new polyesters do NOT get warm, so I feel better :-).
Static grid voltage at output of grid cut-off adjustment trim-pot was drifting up during warm up, while voltage on input side was pretty much constant. Suggests it might be the trim pot drifting with temp.
The trimpot (R315) is supposed to be a 2M2 linear Cermet 20% 1W according to the parts list, but looks to me like a standard Piher PTC15 1/2W part. Measuring the resistance as the scope cools after power off seems to confirm the hypothesis (10% increase in value over 15 minutes, probably as much as
15-20% by the time it reaches room temp).
If I could source a replacement readily I'd just ram a new one in, but I suspect that the parts list is right and the original WAS a 1W part, and has been replaced somewhere along the way with a 1/2W part. Finding a 2.2M
0.5W trimmer of any sort (never mind one that will fit) seems a bit hard, and as for a 1W part ...!
Before I go totally insane hunting for a suitable part, I'd just like to get a sanity check on my diagnosis from folks more experienced with this than I.
One posible fix (but a bit clunky) is to find out the values of the resistance is from the ends of the trimpot to the wiper, by measuring the existing pot, and replacing it with 2 fixed resistors of a suitable power rating . At least you could then tell if your diagnosis is correct.
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