A Tale of two Topward 20MHz scopes - 7021 and 7025

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Zenith
Posts: 811
Joined: Sat Oct 22, 2022 9:06 pm

A Tale of two Topward 20MHz scopes - 7021 and 7025

Post by Zenith »

Topward was a Taiwanese company which started in 1978 and seems to have been dissolved a year or two back. They stated their business was basic elecronic instruments; power supplies, function generators, oscilloscopes etc. They didn't make it into the digital age.

7021
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I bought it new on special offer from Maplins in 1989 or 1990. The Toshiba CRT is dated 1988. It cost £225 and I had the sense to also buy the service manual for an extra £10. It served me well and had considerable use for about ten years. Around 2000 it displayed phantasmogoric effects - the display went into and out of focus every few seconds. I whipped out the service manual and decided the problem was the caps and diodes in the EHT circuit. I replaced them all and everything was well. Well for a time and sort of. It began to have problems on channel 2 switching and obvious problems on the 1ms/Div time base range. In 2009 I bought a Tek 475 and only used the Topward 7021 occasionally, to keep it aired.

A year or so back I decided to revisit this neglected old friend and recap it. I didn't get round to it until recently. When recapped the 1ms/Div problems had disappeared. I applied Deoxit to the switches and those problems disappeared. I followed the set up instructions in the manual. I found the procedure for best square wave response was in some respects vague, and the 3dB point was well off. The results were not convincing. The first step was to adjust a preset capacitor "to give the damped oscillation of the square wave in maximum". It wouldn't produce a waveform much like the diagram in the manual. Then I used a DDS function generator to sweep it from 1KHz to 20MHz, and set the most critical preset to give a better then -3dB response and then fiddled with the rest of the presets, more or less in line with the manual, to give the best looking square wave at 1MHz. Then I checked that the 3dB point was still at 20MHz or higher. Adjusting that preset cap for the proper response at 20MHz seems a better bet than the damped oscillation business. I worked through the rest of the set up and checks. Only slight adjustments were needed.

It works pretty well to the spec. The display is a bit dim at the fastest time base setting with 10X sweep on. It would do for audio, or messing with basic electronics, or fault finding. It will produce a display and trigger up to 45MHz.

It's a three channel scope. The third channel is a permanent external trigger view. It has very limited attenuation options and bandwidth. I suppose it might be useful in odd circumstances. More illustrious makers than Topward make the same dubious claim for extra channels.
DSCN3445.JPG

7025
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This is a strange story. Last year I saw someone walking round with it at a swapmeet. He noticed I was looking at it and said it was free and that it didn't work. Free is always good and it was worth having for the lever switch knobs alone. The weekly recycling wagon has a section for small electricals. So I accepted it with thanks. Service information for the 7025 appears to be unobtainable, but it bears a strong resemblance to the 7021. The CRT is the same Toshiba CRT as in the 7021. There's also a later 7025A, which it looks as if Topward made until the end. There were 40MHz, 60MHz and 100MHz models and they all included a delay sweep version. Rapid Electronics sold Topward scopes, rebadged as Rapid, when analogue scopes were a thing. Rapid is an electronics supplier in the UK along the lines of RS, Farnell etc. I'd guess there were a few other rebadged versions.

http://spencetek.com/osc.htm
DSCN3434.JPG
It's been knocked about a bit. One of the feet at the rear was broken away. It had had a blow to the top denting the case. The bezel had taken a biff and a piece of plastic was cracked and raised. The power switch knob was missing. I was tempted to rob it of the lever switch knobs and other bits and pieces and dispose of it. However, curiosity got the better of me and I powered it up. It produced no display. The Channel 1 amp has an output on the back and that was producing a signal which seemed OK. As far as I could tell, using the 7021 manual, the power lines were all OK. I checked out the CRT heater circuit. The transformer winding was good, as was the heater, at least they had what I thought should be the right resistance. Then I noticed there was a 3R6 resistor in the heater circuit which was open circuit. I assume it was there to stop current surge at power on. The 7021 does not have this resistor. I replaced it with a 4R7. I powered on the scope and it now produced a display.

The problem now was the vertical shift had limited range on both channels and the display clipped. A fault in the final Y amp? Possibly a dead transistor or resistor. I checked what was happening at the Y outputs to the CRT with another scope. The display changed significantly when I connected a probe to one of the outputs. Odd. The probe seemed to move more freely than I thought it should. I looked with jeweller's eyeglass, and the pad to which that output pin was connected had lifted, and there was a broken circuit trace. I resoldered the pad with a generous dose of solder and flux and replaced the trace with a bit of wire. I powered up the scope and found that the vertical shift worked properly and there was no clipping.

I cleaned the switches and went through the set up procedure as for the 7021. The presets have different IDs, but Topward had printed their function on the PCB. The PCBs are also laid out differently. It needed very little adjustment. It seems to be a slightly better performer than the 7021.
DSCN3453.JPG
It's always a satisfying thing to get old scopes to work properly, even if they are not the last word in performance. Why is it I seem to have so much of this old junk in pairs?
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