Weird scope pattern artefact - Page 1
Forum section: electronics / general

author: Chris Wilson
author_rank: Super Contributor
timestamp: March 13, 2012, 10:08:55 am
content: I have acquired a late eighties Yaesu Bandscope. It was designed as an add on for Yaesu transceivers of the period, and could also be used as a (very) basic oscilloscope. It works, but when testing it as just a scope with a circa 500KHz input signal to the Y axis, it displays a strange artefact on the display. The higher the frequency the more pronounced it gets, until around 2 MHz it looks like it's displaying two traces and it's hard to tell which is the artefact and which the real Y axis input. Can anyone tell me why it might be doing this please? Maybe it's just because it's a lousy scope, maybe there's something up with it? Thanks for any advice here, it's a unit I want to use as it's part of a period radio station I am assembling. I have the manual with schematics and circuit diagrams for it. I am awaiting electrolytics to re-cap the PS and any other 30 year old electrolytics on the boards. HT and other voltages are in spec, not checked for ripple, was just going to re-cap it anyway, due to age. I will try and attach images, if they are too big they are athttp://www.chriswilson.tv/scope/scope.htmlCheers.

author: wkb
author_rank: Super Contributor
timestamp: March 13, 2012, 10:38:59 am
content: I'd read the manual carefully, somehow my gut feeling tells me this is not the symptom of something broken.If the manual is in electronic form (PDF or so) maybe you can post it (or a reference to it) for people to read up and (hopefully) offer help?As for the caps: if it works leave it alone.. In those days people used quality components that were allowed to cost a couple of $$.

author: Balaur
author_rank: Unknown
timestamp: March 13, 2012, 10:41:26 am
content: While I know nothing about the particularities of this device, it looks like it doesn't blank the trace during the retracing.Themanualsays nothing special about this mode, you can reasonably expect to behave like a normal oscilloscope.Cheers,Dan

author: wkb
author_rank: Super Contributor
timestamp: March 13, 2012, 10:42:55 am
content: Hmm, is it this one?http://www.foxtango.kc9foz.com/FT101ZD-901-902/YO-901.pdf

author: wkb
author_rank: Super Contributor
timestamp: March 13, 2012, 10:44:21 am
content: Quote from: Balaur on March 13, 2012, 10:41:26 amThe retrace even if the retrace supression is fubared would be a straight line in my experience.

author: Chris Wilson
author_rank: Super Contributor
timestamp: March 13, 2012, 10:58:42 am
content: Yes, thanks for the immediate replies, that's the manual for it in your link. I have not tried it for its original purpose connected to my transceiver as it's come without the 11 pin connector lead, so I have to source some connectors to make up a lead.

author: Balaur
author_rank: Unknown
timestamp: March 13, 2012, 12:21:13 pm
content: Quote from: wkb on March 13, 2012, 10:44:21 amDepends on the slew rate of the fast ramp. It looks like the retrace is 4 times faster than the direct trace. It's may still be normal for this device, but this is just an hypothesis, of course.

author: Chris Wilson
author_rank: Super Contributor
timestamp: March 13, 2012, 01:01:59 pm
content: A pal has suggested the scope doesn't have a Z axis function, so no retrace blanking can, or should, occur? Crude, but maybe it's not knackered and should be like this?

author: wkb
author_rank: Super Contributor
timestamp: March 13, 2012, 01:02:11 pm
content: Quote from: Balaur on March 13, 2012, 12:21:13 pmThe 'retrace' (to call it that) appears to be more of a sinewave. Probably only connecting it to its intended transceiver will tell if it is OK or not.

author: w2aew
author_rank: Super Contributor
timestamp: March 13, 2012, 01:13:25 pm
content: Quote from: wkb on March 13, 2012, 01:02:11 pmThe retrace is a sinewave because this scope is a recurrent-sweep type, not a triggered type. Thus, the horizontal drive on this scope is a free-running sawtooth oscillator, and you're seeing the sinusoidal input signal "stretched" during the retrace. The block diagram doesn't show any provision for retrace blanking, but the detailed schematic might show something - I'll have to have a look at that.

author: Chris Wilson
author_rank: Super Contributor
timestamp: March 13, 2012, 03:03:56 pm
content: Thanks very much w2aew! I posted this question on the Fox tango forum, a place dedicated to old valve or valve hybrid Yaesu gear, and someone has come back who owns one of these boat anchors and says he thinks it is the retrace blanking capacitor leaking, or the amp? So it appears this should NOT be happening. As i think it reads IF from the transceiver I can't see how it can give a usable trace at that sort of frequency with no retrace blanking, thinking about it. Cheers. Liked your last video on YT re cable length testing, please keep them coming, I learn from each and every one. Just fixing up a job lot of plugins I found for my new old stock 7633Luckily only issues so far seem to be broken knobs and a broken "quill shaft".

author: saturation
author_rank: Super Contributor
timestamp: March 13, 2012, 07:25:43 pm
content: Does lowering the intensity setting help? It could make the appearance usable without further work.

author: w2aew
author_rank: Super Contributor
timestamp: March 13, 2012, 09:47:51 pm
content: Quote from: Chris Wilson on March 13, 2012, 03:03:56 pmI took a brief look through the schematic in the manual that you linked. The quality of the schematic in that manual isn't great, but I didn't see any evidence of any retrace blanking. It could be that it doesn't have any, since most of the time a station monitor like this is typically used a fairly low sweep speeds, where the retrace is so much faster than the main sweep that the beam is nearly invisible during retrace. It's only when you start cranking up the sweep speed that the retrace can be seen.I have a Kenwood SM-220 station monitor (similar to the Yaesu that you have), and I can observe the same retrace when at the fastest sweep speeds. This unit has retrace blanking through a simple cap from the horizontal oscillator to the intensity grid, which works well until you get to the fastest possible sweep where you get about 1/2 of the retrace visible.