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
Test Equipment Anonymous (TEA) : Discussion and Group Therapy Thread
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Use tags for the type of equipment your topic is about. Include the "repairs" tag, too, when appropriate. If a new tag is needed, request one in the TEAdministration forum.
Use tags for the type of equipment your topic is about. Include the "repairs" tag, too, when appropriate. If a new tag is needed, request one in the TEAdministration forum.
Re: Test Equipment Anonymous (TEA) : Discussion and Group Therapy Thread
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
Re: Test Equipment Anonymous (TEA) : Discussion and Group Therapy Thread
We know you know.
Does the inexperienced kid know?
Re: Test Equipment Anonymous (TEA) : Discussion and Group Therapy Thread
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.
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.
Re: Test Equipment Anonymous (TEA) : Discussion and Group Therapy Thread
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.
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.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.
Hence I'd start at the big electrolytics near the power cable.
Re: Test Equipment Anonymous (TEA) : Discussion and Group Therapy Thread
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.
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.
Re: Test Equipment Anonymous (TEA) : Discussion and Group Therapy Thread
Sprague C1428? Spec is indeed 50V on a 50V line. Oops.
I've seen them take out a series resistor (485, tant bead) and spew acid across tracks (1502, milspec axial tant).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.
The possibility of spontaneous rapid disassembly is one of the better reasons for wearing any form of glasses.
Re: Test Equipment Anonymous (TEA) : Discussion and Group Therapy Thread
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.
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.
Re: Test Equipment Anonymous (TEA) : Discussion and Group Therapy Thread
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.
Re: Test Equipment Anonymous (TEA) : Discussion and Group Therapy Thread
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.
Re: Test Equipment Anonymous (TEA) : Discussion and Group Therapy Thread
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:
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:
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Re: Test Equipment Anonymous (TEA) : Discussion and Group Therapy Thread
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.
Re: Test Equipment Anonymous (TEA) : Discussion and Group Therapy Thread
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?
