I was given a Trio CS-1562 for next to nothing, along with an old TO-3 valve-based CRO and some other stuff and figured - why not, right?
I decided to start with the Trio CRO. I was told it was working fine then the display quit with the waveform shrinking down to nothing and never coming back. I was also told the HV was tested as OK, which didn't seem to make a huge amount of sense.
Well, I plugged it in and flipped the switch. The power LED came on but for sure the display was dead. I was getting a squarewave from the CAL output though so I figured that was a good sign.
With the case open, I verified the heater comes on. Then I noticed some cracked joints on the J310\P109 board interconnect, one of which was on the 180v rail. I reflowed the lot, but it made no difference. I also noticed the CAL signal comes straight from the main transformer and simply squared up, so that working doesn't prove much. Time to dig deeper...
I unscrewed the shield from the HV circuitry and had a look. Nothing seemed untoward at first, but diodes D109\D110 do look a bit funny on top. They tested OK in-circuit though, maybe it's just the red paint(?) has gone over time with heat or something? Any ideas?
The HV is made from a transistor based oscillator and transformer and nothing was visibly blown or overheated. Without an actual HV probe\meter, I tried waving my scope probe over transformer T101 and got nothing, and probing the primary winding showed only a stable 15v DC. At one point I also waved around a neon globe on a stick, to no avail. So it looks like the HV oscillator is well and truly dead. So much for "The shop I took it to said the HV is working"
A quick look at the underside showed a lifted and (not too well) repaired trace on Q142. A good clue! Though how someone trashes a big pad on a single layer board is beyond me.
I pulled Q142 and tested it. The B-E junction seems OK but the B-C junction appears to be open. That would explain the no oscillation and no HV. Perhaps the tiny amount of dried up heatsink paste explains the dead transistor. Maybe there is a bad component elsewhere which killed Q142...
Anyway, now for the fun part.
As expected, the 2SD401 isn't manufactured anymore (except by some Wun Hung Lo outfit) and naturally there is nothing on a 2SD401M. I did find in a Mospec 2SD401A datasheet that 'M' corresponds to the lowest (40-80) hFE, and that's it. But a 2SD401A datasheet from NEC has nothing about hFE grades at all.
The only new source was NTE Components who claimed to have a 2SD401M replacement - the NTE375. On paper it looked OK. It has the same voltage (or slightly higher) ratings, a little bit faster, and can handle more current. I figured that sounded great, and they're available at RS-Components.
However, in a post by rhomanski I recently read, he explained NTE components just relabel parts they get from elsewhere. Google, and NTE's own website seems to confirm this. OK, well that's not awesome, but if they work and are in spec, well... that's not too bad right?
Then I looked at the NTE375 datasheet again, and noticed the spec for cutoff currents are much higher than any of the old datasheets I could find. Particularly the IEBO is maximum 5mA for NTE375, and in the NEC 2SD401A datasheet it's a maximum 50 microamps - that's up to 100 times worse!
Even the Chinese 2SD401 only has 0.2mA maximum for both IEBO and ICBO.
Of course, this may not matter. I have no idea how much or if this difference in maximum cutoff current will make in the circuit here. But it does make the NTE375 seem less appealing.
I see some various N.O.S. 2SD401s and 2SD401As on eBay, which are more tempting, but none are 2SD401M. I can't find out what the M stands for.
The 2SD401A at least seems to just be one with higher power handling.
There are some Fairchild 2SD401-Y but no datasheet to explain what the 'Y' means.
Any ideas?
I'm happy with NTE as long as the higher cutoff current isn't going to be a problem, but I have no experience or knowledge to tell me if it will be OK or not.
I decided to start with the Trio CRO. I was told it was working fine then the display quit with the waveform shrinking down to nothing and never coming back. I was also told the HV was tested as OK, which didn't seem to make a huge amount of sense.
Well, I plugged it in and flipped the switch. The power LED came on but for sure the display was dead. I was getting a squarewave from the CAL output though so I figured that was a good sign.
With the case open, I verified the heater comes on. Then I noticed some cracked joints on the J310\P109 board interconnect, one of which was on the 180v rail. I reflowed the lot, but it made no difference. I also noticed the CAL signal comes straight from the main transformer and simply squared up, so that working doesn't prove much. Time to dig deeper...
I unscrewed the shield from the HV circuitry and had a look. Nothing seemed untoward at first, but diodes D109\D110 do look a bit funny on top. They tested OK in-circuit though, maybe it's just the red paint(?) has gone over time with heat or something? Any ideas?
The HV is made from a transistor based oscillator and transformer and nothing was visibly blown or overheated. Without an actual HV probe\meter, I tried waving my scope probe over transformer T101 and got nothing, and probing the primary winding showed only a stable 15v DC. At one point I also waved around a neon globe on a stick, to no avail. So it looks like the HV oscillator is well and truly dead. So much for "The shop I took it to said the HV is working"
A quick look at the underside showed a lifted and (not too well) repaired trace on Q142. A good clue! Though how someone trashes a big pad on a single layer board is beyond me.
I pulled Q142 and tested it. The B-E junction seems OK but the B-C junction appears to be open. That would explain the no oscillation and no HV. Perhaps the tiny amount of dried up heatsink paste explains the dead transistor. Maybe there is a bad component elsewhere which killed Q142...
Anyway, now for the fun part.
As expected, the 2SD401 isn't manufactured anymore (except by some Wun Hung Lo outfit) and naturally there is nothing on a 2SD401M. I did find in a Mospec 2SD401A datasheet that 'M' corresponds to the lowest (40-80) hFE, and that's it. But a 2SD401A datasheet from NEC has nothing about hFE grades at all.
The only new source was NTE Components who claimed to have a 2SD401M replacement - the NTE375. On paper it looked OK. It has the same voltage (or slightly higher) ratings, a little bit faster, and can handle more current. I figured that sounded great, and they're available at RS-Components.
However, in a post by rhomanski I recently read, he explained NTE components just relabel parts they get from elsewhere. Google, and NTE's own website seems to confirm this. OK, well that's not awesome, but if they work and are in spec, well... that's not too bad right?
Then I looked at the NTE375 datasheet again, and noticed the spec for cutoff currents are much higher than any of the old datasheets I could find. Particularly the IEBO is maximum 5mA for NTE375, and in the NEC 2SD401A datasheet it's a maximum 50 microamps - that's up to 100 times worse!
Even the Chinese 2SD401 only has 0.2mA maximum for both IEBO and ICBO.
Of course, this may not matter. I have no idea how much or if this difference in maximum cutoff current will make in the circuit here. But it does make the NTE375 seem less appealing.
I see some various N.O.S. 2SD401s and 2SD401As on eBay, which are more tempting, but none are 2SD401M. I can't find out what the M stands for.
The 2SD401A at least seems to just be one with higher power handling.
There are some Fairchild 2SD401-Y but no datasheet to explain what the 'Y' means.
Any ideas?
I'm happy with NTE as long as the higher cutoff current isn't going to be a problem, but I have no experience or knowledge to tell me if it will be OK or not.
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