Test Equipment Anonymous

tggzzz wrote: ↑Sat Sep 14, 2024 6:59 am

mnementh wrote: ↑Fri Sep 13, 2024 11:24 pm

tggzzz wrote: ↑Thu Sep 12, 2024 9:33 pm I hope he won't use it to measure the mains.

Well, I did take the precaution of giving him only a pair of 10X probes with it.

Sorry, I have to be boring and I know I'm teaching you how to suck eggs.

Posting a yootoob vid. Oh, the indignity!

Of course you are!

I don't need Dave to show me how to blow up a scope, gorramm it; I can do that just fine all by myself!

mnem

mnementh wrote: ↑Mon Sep 16, 2024 1:28 am

tggzzz wrote: ↑Sat Sep 14, 2024 6:59 am

mnementh wrote: ↑Fri Sep 13, 2024 11:24 pm
Well, I did take the precaution of giving him only a pair of 10X probes with it.

Sorry, I have to be boring and I know I'm teaching you how to suck eggs.

Posting a yootoob vid. Oh, the indignity!

Of course you are!

I don't need Dave to show me how to blow up a scope, gorramm it; I can do that just fine all by myself!

mnem

We know you know.

Does the inexperienced kid know?

Obvious when you bear in mind that that the grounding lead on the probe is a direct connection to mains earth, but it could easily be done.

Another thing which has caught a few out is "live chassis" TVs and radios. No mains transformer, just direct to the rectifier for the valve HT and the heaters were in series with a dropper resistor. Some of the 1950s plugs weren't polarised and anyway, a polarised plug could be wired incorrectly, or the connections reversed. You could end up with the chassis at 240V AC. Normally it would be at mains Neutral, which could still be many volts above earth. The sensible way to work with them is through a 1:1 isolating transformer to give the mains supply. Contrary to what a lot believe, most variacs are auto transformers, not isolating transformers.

The last Tek 475 I picked up didn't power up. I checked its fuse, which was blown. I also checked the mains plug. The fuse in that was OK, but the earth lead was deliberately not connected (cut short with no bare wire left). So I wired it properly. I wondered if it had been done so they would have a floating scope - dodgy for a bench scope. It still blows its own fuse after a couple of seconds. I suspect a bad reservoir cap, or bridge rectifier. I suppose it might be one of the marginally voltage rated tants across the supply rails. I haven't looked at it properly. I'm adding a load of caps to the next order for Digikey.

Zenith wrote: ↑Mon Sep 16, 2024 9:01 am Obvious when you bear in mind that that the grounding lead on the probe is a direct connection to mains earth, but it could easily be done

As a kid I thought earth and neutral were at the same potential.

Now I know better, including that I don't understand all the various types of mains electricity supplies.

Another thing which has caught a few out is "live chassis" TVs and radios. No mains transformer, just direct to the rectifier for the valve HT and the heaters were in series with a dropper resistor. Some of the 1950s plugs weren't polarised and anyway, a polarised plug could be wired incorrectly, or the connections reversed. You could end up with the chassis at 240V AC. Normally it would be at mains Neutral, which could still be many volts above earth. The sensible way to work with them is through a 1:1 isolating transformer to give the mains supply. Contrary to what a lot believe, most variacs are auto transformers, not isolating transformers.

The last Tek 475 I picked up didn't power up. I checked its fuse, which was blown. I also checked the mains plug. The fuse in that was OK, but the earth lead was deliberately not connected (cut short with no bare wire left). So I wired it properly. I wondered if it had been done so they would have a floating scope - dodgy for a bench scope. It still blows its own fuse after a couple of seconds. I suspect a bad reservoir cap, or bridge rectifier. I suppose it might be one of the marginally voltage rated tants across the supply rails. I haven't looked at it properly. I'm adding a load of caps to the next order for Digikey.

I doubt a tant would take out a fuse; I expect it would cause the SMPS to shutdown. IIRC there's a specific node in the PSU that indicates whether the LV rails are sufficiently "balanced". Imbalance prevents startup.

Hence I'd start at the big electrolytics near the power cable.

There's a 50V tant across the 50V line. One of the tants has been replaced with a neat butt soldered in job. I'm inclined to replace all the ones in the PSU, but it's a lot of work

When I've seen tants fail spectacularly, they've gone red hot and smoked. I've seen a few which have exploded. I wouldn't put it past them to fail with a dead short. I suspect it's one of the reservoir caps which has failed S/C or fails S/C after a few seconds. I've had problems with them in the past. It might be a bridge rectifier acting strangely, although they rarely fail.

I hope it's just a PSU problem, rather than something more obscure and harder to fix. The second last 475 had a cap in the PSU which had gone O/C and caused terrible problems with ripple. When that was fixed with a bodge, (two 200V caps in parallel), the scope worked and the supply lines were as they should be.

Zenith wrote: ↑Mon Sep 16, 2024 10:11 am There's a 50V tant across the 50V line. One of the tants has been replaced with a neat butt soldered in job. I'm inclined to replace all the ones in the PSU, but it's a lot of work

Sprague C1428? Spec is indeed 50V on a 50V line. Oops.

When I've seen tants fail spectacularly, they've gone red hot and smoked. I've seen a few which have exploded. I wouldn't put it past them to fail with a dead short. I suspect it's one of the reservoir caps which has failed S/C or fails S/C after a few seconds. I've had problems with them in the past. It might be a bridge rectifier acting strangely, although they rarely fail.

I hope it's just a PSU problem, rather than something more obscure and harder to fix. The second last 475 had a cap in the PSU which had gone O/C and caused terrible problems with ripple. When that was fixed with a bodge, (two 200V caps in parallel), the scope worked and the supply lines were as they should be.

I've seen them take out a series resistor (485, tant bead) and spew acid across tracks (1502, milspec axial tant).

The possibility of spontaneous rapid disassembly is one of the better reasons for wearing any form of glasses.

I had a couple go up in smoke and destroy a couple of PSU transistors in a CRC Schlumberger sweeper. I replaced them with modern electrolytics. Racal counters are often full of them. They don't fail that often. If there's anything like high frequency, an electrolytic in parallel with a ceramic works. Mass replacement would be a pain and they seldom cause problems. My last experience of them was a couple of weeks back with a Fluke 8800A with PSU problems which seemed to be down to the tants. There were six and I changed the lot, as it was easy - unlike the Racal counters - and the power supply problems disappeared.

On another forum years ago tants were being discussed, and someone said he'd had one explode and bits of it were embedded in the wall. There are similar, rare, stories about big electrolytics in valve gear, which have been known to blow apart violently. In that case I believe you have electrolysis and then a hydrogen-oxygen explosion. Another problem with electrolytics is a failure where the electrolyte leaks out rapidly and destroys circuit tracks and component leads. When that happened to me I was told it was because of a manufacturing fault in the L.C.R brand capacitors.

I had one go short in a Racal 9915. The thing was an LC network on the main analogue ASIC and took out the L bit totally. Smoked it.

In most cases it's not practical to change all the tants, just the failed ones or those known to cause problems. In particular those Racal counters can be a pig to replace components on. The parts are packed close together, the hole is only slightly larger than the lead and they bent the leads over at right angles. For the most part, the tants sit there working.

You should always snip them flush on the top and remove the leads from the bottom when doing rework. It's never worth fucking the board up.

When I did the mods on the 9915 I had to run it off a 10MHz OCXO I did that which can be seen here:

I did cut them off flush and removed them from the bottom. It was still a mission. Getting at them to cut them off flush wasn't easy, and I have a pair of nice Lindstrom side cutters.

mnementh wrote: ↑Wed Sep 11, 2024 11:49 pm

Zenith wrote: ↑Sun Sep 08, 2024 10:16 am

mnementh wrote: ↑Sat Sep 07, 2024 4:14 pm Calling Cubdriver... calling Cubdriver...

Been here all morning; everybody's pretty much gone. Did I walk right past and not recognize you?

mnem

What prizes did you come away with?

A few bits and bobs...

mnem
continued next post...

That's a nice pickup and a very kind thing to give that scope to that kid as a surprise gift. I hope he enjoys it. Do you know if he had anything to use it on, something he could safely probe to get a couple of interesting traces on the display?

Zenith wrote: ↑Tue Sep 17, 2024 8:16 pm I did cut them off flush and removed them from the bottom. It was still a mission. Getting at them to cut them off flush wasn't easy, and I have a pair of nice Lindstrom side cutters.

I don't bother cutting flush on the first hit. Quite easy after that and I have some crappy Xcelite cutters.

25 CPS wrote: ↑Wed Sep 18, 2024 6:47 pm
That's a nice pickup and a very kind thing to give that scope to that kid as a surprise gift. I hope he enjoys it. Do you know if he had anything to use it on, something he could safely probe to get a couple of interesting traces on the display?

He could tinker together a relaxation oscillator using TTL or a 555 timer.

Zenith wrote: ↑Thu Sep 19, 2024 11:23 am

25 CPS wrote: ↑Wed Sep 18, 2024 6:47 pm
That's a nice pickup and a very kind thing to give that scope to that kid as a surprise gift. I hope he enjoys it. Do you know if he had anything to use it on, something he could safely probe to get a couple of interesting traces on the display?

He could tinker together a relaxation oscillator using TTL or a 555 timer.

The (modern equivalent of the) ICL8038 0-2MHz function generator has lots of twiddleability and the diode-shaped sine output is entertaining.

Or give him a bipolar TTL divide by 100/1000 circuit, and encourage him to understand which bits of the waveform are and aren't important. Not so effective with CMOS, though.

CMOS would have the advantage that it could be powered from a couple of cells, without any mains PSUs.

bd139 wrote: ↑Thu Sep 19, 2024 11:00 am

Zenith wrote: ↑Tue Sep 17, 2024 8:16 pm I did cut them off flush and removed them from the bottom. It was still a mission. Getting at them to cut them off flush wasn't easy, and I have a pair of nice Lindstrom side cutters.

I don't bother cutting flush on the first hit. Quite easy after that and I have some crappy Xcelite cutters.

It depends where on the PCB. Some of it's fairly accessible, other parts, such as around the OM335 UHF hybrid, are very cramped.

tggzzz wrote: ↑Thu Sep 19, 2024 11:26 am The (modern equivalent of the) ICL8038 0-2MHz function generator has lots of twiddleability and the diode-shaped sine output is entertaining.

Or give him a bipolar TTL divide by 100/1000 circuit, and encourage him to understand which bits of the waveform are and aren't important. Not so effective with CMOS, though.

XR2206 was another one. You can find function generators on ebay based on them for about £7. Sometimes they are sold as kits. I recall the sine wave has a pip at the peaks. Also on ebay are 8038 based ones for about the same price. I suspect they are based on Chinese copies of the original chips.

Maxim made their own souped up version of the 8038 which went up to 20MHz. I saw a few designs based on them, but they never pushed them to anything like 20MHz because they reckoned they were twitchy because of parasitics at that frequency. They weren't on the market for long and they were quite dear. Occasionally they turn up on ebay but they are very expensive.

There are loads of things the kid could put together which would be instructive, such as a multivibrator with a couple of transistors and other bits and pieces.

tggzzz wrote: ↑Thu Sep 19, 2024 11:26 am CMOS would have the advantage that it could be powered from a couple of cells, without any mains PSUs.

CMOS doesn't use much power but can have unpleasant surprises for the unwary. Mucking about with batteries wears thin. He'll be looking for a nice little variable linear PSU next.

attached is a picture of asta and my ic-V8 2 meter transceiver.

seems my little radio has been in the news lately.

asta has not.