Moderators: Dave Moll, Steve Anderson
Steve Anderson wrote:Harry, as you now have a trace/traces, forget the voltage measurements I mentioned earlier. Instead carefully measure the voltage on the wiper of the H.POS pot as you vary it from one end to the other, it should go smoothly from +8V to -8V (or the other way around). If the pot isn't easy to reach measure at C317/R353. Circuit is 'HOR AMP', Document Number TI2003, bottom left.
If it jumps suddenly from one voltage to another, (which may not be +8/-8) either the internal pot track is broken and/or the wiper isn't in good contact with the track. If this is so, the pot will need replacing. But check for dry joints or cracked tracks on the board if the pot pins are PCB mounted. The problem wll be finding a replacement as it also has the push-pull 'x10' switch, but you could opt to do without it by wiring it in the 'x1' position permanently. I find I very rarely use the 'x10' function, if ever.
There's no harm in going around the whole instrument tapping (gently) with the insulated handle of a screwdriver to see if there are any more gremlins that might otherwise surface later. It's better to do this now while you still have the covers off. When you can soak-test it for a full day with the covers on so it gets to normal temperatures.
With any luck you have a 25MHz dual-trace scope for five bucks and a bit of time and effort.
Steve A.
We still don't know what that extra transformer does, but if the thing's working - leave it!
Later on you may want to check the calibration, both the timebase (time/cm) and CH1/2 (V/cm). But that can be left for now.
Steve Anderson wrote:You're welcome Harry. I quite enjoy doing things like this by 'remote control'. If anything else comes up in the future don't hesitate to ask, if I can help I will - and that applies to everyone.
If you're happy to live with that faulty H.POS pot, OK. I did suspect it was going to be tricky to replace, as you say it may be worn out rather than broken as a result of the impact.
The +8/-8 supplies are a bit low for the output of 7808/7908s but unless this causes problems, again I would leave it alone. You're not going to use it as a laboratory instrument so it should be fine.
I had a quick look at the Z-axis modulation arrangement the other day, you may be in luck, it appears to be DC-coupled, but I need to study it further.
Steve A.
A little later...nope, the Z-modulation isn't DC-coupled, but it is DC-restored with the timebase acting as an internal sync pulse (what they call 'unblanking'). Thankfully the horizontal timebase frequencies for both NBTV and SSTV are very similar, 12.5Hz (NBTV frame) and 15Hz or 16.7Hz (SSTV line). So with a following wind it shouldn't need any modification. However, the Z-input voltage is going to be a bit strange, it needs to be negative, but that's something to look at later...not impossible, just odd.
Unfortunately for NBTV the internal horizontal timebase goes from left-to-right resulting in a left-right reversed image. But it does appear to have an X-Y mode on the main timebase control, fully clockwise. CH2 then becomes the input for an external timebase. But in doing this you lose those DC-restoration pulses, so they would need to be mixed in the the Z-mod signal. Or for NBTV just view the picture in a mirror!
Harry Dalek wrote:...and you get to knock off work early today for this ! ....
Steve Anderson wrote:It's seems to be much the same as I mentioned in another thread recently - it's usually the electro-mechanical things that fail.
Switches, pots, connectors, relays and so on...anything with moving parts. The problem is getting replacements. Often they're 'specials' made to the manufacturers mechanical specification, electrically they're generally the same as any other 'industry standard' part.
The good news with getting a digital scope is that the front-panel switches and pots are often opto-sensors (rotary encoders) with no sliding contacts (pots/switches). The gain/time settings are all done in the software which controls the the gain of the channel and the sampling rate of the A-Ds. The scope I bought over 10 years ago (new) has never had any issue with this problem, I hope it never will.
The other advantage, no CRT. So the whole thing is much more compact - mine is the size of a ladies handbag and takes up little room on the work-bench. No high-voltage power and a lot less power consumption, runs cool, improved reliability... plus many have colour screens which makes each trace easier to identify....also a USB port for saving the whole screen to a flash drive as a GIF/JPG/TIFF, so easy to publish here.
Some also have a network connection (Ethernet) so it can be controlled/automated via a PC. Though I do have a network here I've not found a need to connect the 'scope to it...yet.
It is a significant investment, but you won't want to go back to a CRT scope after you've used a digital one for just a short while.
Steve A.
...and that's me saying that! Someone who has a keen interest in CRTs! Would you want to go back to an analogue multi-meter?
Steve Anderson wrote:Hmm...I'm not so keen on the wide screen display, OK here it's only 2 traces, but when you have four (or even five with trigger view) a more conventional aspect ratio is to be preferred. But I guess you'd get used to it. Horses for courses...interesting to note that Tek haven't done a wide-screen version, but I haven't checked in a few years...so again, I'm probably wrong.
Steve A.
Here's a promising contender, around US$500, up to 4 channels, 100MHz, 8", yes, eight inch colour display, battery portable, guess where its made? It's almost disposable, if it lasts two years, throw it away and buy a new one...60 cents a day....when your income depends on it, 60 cents a day is a bargain!
Steve Anderson wrote:Yep, I'm still bombing around in my 30-year old Nissan with 550,000km on the clock, original diesel engine. As much as I'd love a Jag, this will have to do...
Steve A.
Steve Anderson wrote:Can you move the trace(s) up and down with the position pot(s)? If you can that means at least the vertical amplifier is working, just not getting a signal. Can you get two traces?
Steve A.
Quite an unusual vertical amp, not that it matters right now...
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