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Old 21st May 2020, 4:39 pm   #118
Pinörkel
Pentode
 
Join Date: Jun 2019
Location: Dortmund, Germany
Posts: 161
Default Re: Telequipment D75 scope.

Further analysis of the D755 PSU revealed additional problems. The +24V line was not as stable as I expected it to be and suffered occasional breakdowns with characteristic alternating 50 Hz positive spikes. Since I suspected a rectifier issue, I had a look at the waveforms at the filter capacitor C405. This revealed a non uniform triangle waveform with alternating 100 Hz high and low triangles that occasionally fused into a single uniform 50 Hz triangle waveform.

So the diodes had to be replaced. Since the triangle waveform at the -24 V line filter capacitor also showed signs of alternating triangle wave heights, I replaced the diodes on all three rectifiers. The original diodes were rated with 500 mA and 450 V, according to the manual. The closest match, I could get my hands on were some 1N4005 standard speed rectifier diodes rated 1 A and 600V. After the replacement all rectifiers finally produced clean 100 Hz triangle waveforms. However, the 30 V rail, which I measured at 30.5 V half a year ago before the PSU issues appeared, now is only at 28.8 V and I am not sure about why this is different. Maybe I chose the wrong diodes?

All PSU output lines now seem to be kind of OK. The +24 V and -24 V rails have about 2.5 mV ripple VPP, the +105 V rail has about 15 mV and the +30 V rail 1.7 V. As a comparison, on my D75, the -24 V rail has about 2.5 mV ripple VPP, the +24 V rail 6.25 mV, the +105 V rail has about 12.5 mV and the +30 V rail 1.25 V. So I think the optimal values are ~2.5 mV on the +24 V and -24 V rails, ~15 mV on the +105 V rail and ~1.5 V on the +30 V rail. This means the +24 V line on my D75 has some optimization potential. The higher value in comparison to my D755 manifests itself in form of a superimposed 100 Hz triangle wave. I am still trying to find out where this is coming from, but I can already say that it is not related to TR405 and TR406 (tested by swapping).

In general, the PSU ripple on my D75 is way cleaner than on my D755, although the D75 still has its original filter capacitors and rectifiers. On my D755, there is a lot of seemingly random stuff on the ripple and some small 100 Hz periodic spikes. So I am still not really satisfied with the output. In addition to that, R413 and the capacitors which were swapped by the previous owner look kind of ugly. I wish I could get my hands on a defective set of original filter capacitors in optically nice condition. Then I could hide some new capacitors inside and preserve the original look. See attached images for a comparison of PC149 of my D7 and D755. Also, I am still searching for a source of metal oxide resistors that look similar to the grey-colored ones in my D75.

An additional thing that bugs me with the D755 is the look of the trace when switching the scope on. When that trace appears, it looks like a dashed line at first and then transitions to a continuous line in about 8 seconds. My D75 does not have this issue. So, maybe this is a first indicator of a problem with the CRT. On the other hand, the CRT of my D755 seems to be in much better condition than the one of my D75, when judging by the achievable intensity.
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