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Curious problem
Hi all,
I'm now working on a different Philips scope which 'died' suddenly for no apparent reason. Here's a summary of the key points: The smps section had power in, but nothing coming out. I checked the smps outputs for shorts/low-res. One output marked 45V was trying to feed a board with an input resistance of only 6 ohms. I located the board in question. There were several multi-pin connectors around the edge of it running off to other parts of the scope. Now it turns out the smps runs perfectly fine when *one* of these connectors is pulled from the board. With the suspect connector's plug disconnected, the aforementioned 6 ohms shoots up to about 15k and life is great. *BUT* (and here's the thing) the resistance looking into the suspect plug (which has only two wires) is several megohms. And yet this very high resistance, when plugged into the board causes such a voltage drop that the smps shuts down! How is this even possible? I keep thinking there must be something simple I've overlooked, but can't think what it might be. If anyone has experienced something similar in the past and remembers what the underlying issue was, then that could be very helpful! Thanks. |
Curious problem
On Thu, 4 Feb 2016 13:59:27 -0000 (UTC), Cursitor Doom
wrote: Hi all, I'm now working on a different Philips scope which 'died' suddenly for no apparent reason. Here's a summary of the key points: The smps section had power in, but nothing coming out. I checked the smps outputs for shorts/low-res. One output marked 45V was trying to feed a board with an input resistance of only 6 ohms. I located the board in question. There were several multi-pin connectors around the edge of it running off to other parts of the scope. Now it turns out the smps runs perfectly fine when *one* of these connectors is pulled from the board. With the suspect connector's plug disconnected, the aforementioned 6 ohms shoots up to about 15k and life is great. *BUT* (and here's the thing) the resistance looking into the suspect plug (which has only two wires) is several megohms. And yet this very high resistance, when plugged into the board causes such a voltage drop that the smps shuts down! How is this even possible? I keep thinking there must be something simple I've overlooked, but can't think what it might be. If anyone has experienced something similar in the past and remembers what the underlying issue was, then that could be very helpful! Thanks. Regarding "How is this even possible?", I can think of two things. One is mechanical - plugging that 2 wire connector in causes a short near the socket. Can you unplug the other end of that cable and see if you get the result? The more likely cause is a voltage on that two wire cable turns on a transistor or some other device hard wihich then shorts the incoming line from the power supply due to a bad part somewhere else on the board. Good luck. |
Curious problem
On Thu, 04 Feb 2016 10:26:46 -0500, Pat wrote:
Regarding "How is this even possible?", I can think of two things. One is mechanical - plugging that 2 wire connector in causes a short near the socket. Can you unplug the other end of that cable and see if you get the result? Already tried and eliminated, thanks. The more likely cause is a voltage on that two wire cable turns on a transistor or some other device hard wihich then shorts the incoming line from the power supply due to a bad part somewhere else on the board. Good luck. That's what I was wondering. If my DVM on resistance range doesn't use enough voltage to switch such a transistor or diode into conduction, then that could account for it. I'll try it again with an old-style AVO. |
Curious problem
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Curious problem
On Thu, 04 Feb 2016 16:08:38 +0000, MJC wrote:
Mike. "Current blasting"? It would have worked in this instance, but I've found the problem with my good old trusty AVO which confirmed my previous suspicions. People often go on about how modern DVMs are great because they don't turn on pn junctions, which is all very well, but there are odd times when that is precisely what you *do* want the meter to do! This was one of those. Thanks, guys. |
Curious problem
Cursitor Doom schrieb:
On Thu, 04 Feb 2016 16:08:38 +0000, MJC wrote: Mike. "Current blasting"? It would have worked in this instance, but I've found the problem with my good old trusty AVO which confirmed my previous suspicions. ... Would you be so kind and tell us what was wrong? Thanks in advance. Best regardsw Reinhard |
Curious problem
On Thu, 04 Feb 2016 18:38:58 +0100, Reinhard Zwirner wrote:
Would you be so kind and tell us what was wrong? Thanks in advance. The problem was in the EHT converter section which creates 1600V from a 45V input. A BSW68 transistor in this section had fused low-resistance between C&E and was attempting to dissipate 340 watts all by itself - and it didn't even have a heatsink! :) The fact that its case had turned blue also hinted that it had been very hot. I couldn't see it on inspection initially as it was hidden by HV shielding. Now I just have to find an equivalent for it... |
Curious problem
On Thursday, February 4, 2016 at 1:44:26 PM UTC-5, Cursitor Doom wrote:
On Thu, 04 Feb 2016 18:38:58 +0100, Reinhard Zwirner wrote: Would you be so kind and tell us what was wrong? Thanks in advance. The problem was in the EHT converter section which creates 1600V from a 45V input. A BSW68 transistor in this section had fused low-resistance between C&E and was attempting to dissipate 340 watts all by itself - and it didn't even have a heatsink! :) The fact that its case had turned blue also hinted that it had been very hot. I couldn't see it on inspection initially as it was hidden by HV shielding. Now I just have to find an equivalent for it... The BSW68 crosses to an ECG/NTE 190. It would seem that an MPSU04 would work as well. A BSW-68 seems to cross to an ECG/NTE 198. Here a TIP-47 or TIP-48 would do. Check further. It may be easier to use a sub. Dan |
Curious problem
On Thu, 04 Feb 2016 12:01:02 -0800, dansabrservices wrote:
On Thursday, February 4, 2016 at 1:44:26 PM UTC-5, Cursitor Doom wrote: On Thu, 04 Feb 2016 18:38:58 +0100, Reinhard Zwirner wrote: Would you be so kind and tell us what was wrong? Thanks in advance. The problem was in the EHT converter section which creates 1600V from a 45V input. A BSW68 transistor in this section had fused low-resistance between C&E and was attempting to dissipate 340 watts all by itself - and it didn't even have a heatsink! :) The fact that its case had turned blue also hinted that it had been very hot. I couldn't see it on inspection initially as it was hidden by HV shielding. Now I just have to find an equivalent for it... The BSW68 crosses to an ECG/NTE 190. It would seem that an MPSU04 would work as well. A BSW-68 seems to cross to an ECG/NTE 198. Here a TIP-47 or TIP-48 would do. Check further. It may be easier to use a sub. Dan Thank you for those suggestions, Dan. I've got scores of TIP BJT variants lying around in my junk box. Many years ago I had a copy of Towers' Transistor Selector which listed a good range of equivalents for a vast range of devices. I guess these days there are web sites that perform the same function, but finding one worthy of trust that produces reliable results is another matter from what I've seen so far. |
Curious problem
For starters, I usually use the NTE cross reference. Since I have a rather large collection of NTE parts, I use these for experiments. I don't usually use NTE or ECG parts for a repair though. I try to use "real" parts. It does at least give me a quick try. Dan |
Curious problem
On Thu, 04 Feb 2016 14:05:40 -0800, dansabrservices wrote:
For starters, I usually use the NTE cross reference. Since I have a rather large collection of NTE parts, I use these for experiments. I don't usually use NTE or ECG parts for a repair though. I try to use "real" parts. It does at least give me a quick try. Good idea. Maybe I'll look into subbing that BJT for a power MOSFET. A lot of older gear uses BJTs in situations where a MOSFET would be de rigeur nowadays. This transistor functions as a chopper switching an inductive load in this circuit and it wouldn't require much tweaking to sub it for a better device, I reckon. In fact maybe that's why the original part failed or maybe its heatsink - if it ever had one - just fell off at some point, I can't say. |
Curious problem
On Thu, 4 Feb 2016 18:41:31 -0000 (UTC), Cursitor Doom
wrote: On Thu, 04 Feb 2016 18:38:58 +0100, Reinhard Zwirner wrote: Would you be so kind and tell us what was wrong? Thanks in advance. The problem was in the EHT converter section which creates 1600V from a 45V input. A BSW68 transistor in this section had fused low-resistance between C&E and was attempting to dissipate 340 watts all by itself - and it didn't even have a heatsink! :) The fact that its case had turned blue also hinted that it had been very hot. I couldn't see it on inspection initially as it was hidden by HV shielding. Now I just have to find an equivalent for it... Your Towers would have suggested 2n3439/2N3440 or a BUY60. TO5 body sizes are no longer common, but those types are still listed by digikey. BSW68 is 150V, 1A, 40MHz, 800mW, 30min hfe. A ZTX857, in a silicone E-Line body, could probably do it. RL |
Curious problem
On Sat, 06 Feb 2016 02:09:03 -0500, legg wrote:
On Thu, 4 Feb 2016 18:41:31 -0000 (UTC), Cursitor Doom wrote: On Thu, 04 Feb 2016 18:38:58 +0100, Reinhard Zwirner wrote: Would you be so kind and tell us what was wrong? Thanks in advance. The problem was in the EHT converter section which creates 1600V from a 45V input. A BSW68 transistor in this section had fused low-resistance between C&E and was attempting to dissipate 340 watts all by itself - and it didn't even have a heatsink! :) The fact that its case had turned blue also hinted that it had been very hot. I couldn't see it on inspection initially as it was hidden by HV shielding. Now I just have to find an equivalent for it... Your Towers would have suggested 2n3439/2N3440 or a BUY60. TO5 body sizes are no longer common, but those types are still listed by digikey. BSW68 is 150V, 1A, 40MHz, 800mW, 30min hfe. A ZTX857, in a silicone E-Line body, could probably do it. RL Thanks, legg. I'll have a rummage round my copious junk box. I doubt the ft figure has much bearing in this application as whatever the rate this thing switches at it won't be anywhere close to a fraction of that. Likewise the hfe is definitely on the low side, so I shouldn't have any trouble matching or bettering that in a subbed part. Just to reiterate here, this is NOT the main chopper in the smps section; it's a self-standing HT generator card with its chopper being driven by a single transistor oscillator stage. In fact it really wouldn't take much modding to swap the BJT for a MOSFET but I'd prefer to stick with the original circuit if poss. |
Curious problem
On Sat, 6 Feb 2016 12:34:17 -0000 (UTC), Cursitor Doom
wrote: On Sat, 06 Feb 2016 02:09:03 -0500, legg wrote: On Thu, 4 Feb 2016 18:41:31 -0000 (UTC), Cursitor Doom wrote: On Thu, 04 Feb 2016 18:38:58 +0100, Reinhard Zwirner wrote: Would you be so kind and tell us what was wrong? Thanks in advance. The problem was in the EHT converter section which creates 1600V from a 45V input. A BSW68 transistor in this section had fused low-resistance between C&E and was attempting to dissipate 340 watts all by itself - and it didn't even have a heatsink! :) The fact that its case had turned blue also hinted that it had been very hot. I couldn't see it on inspection initially as it was hidden by HV shielding. Now I just have to find an equivalent for it... Your Towers would have suggested 2n3439/2N3440 or a BUY60. TO5 body sizes are no longer common, but those types are still listed by digikey. BSW68 is 150V, 1A, 40MHz, 800mW, 30min hfe. A ZTX857, in a silicone E-Line body, could probably do it. RL Thanks, legg. I'll have a rummage round my copious junk box. I doubt the ft figure has much bearing in this application as whatever the rate this thing switches at it won't be anywhere close to a fraction of that. Likewise the hfe is definitely on the low side, so I shouldn't have any trouble matching or bettering that in a subbed part. Just to reiterate here, this is NOT the main chopper in the smps section; it's a self-standing HT generator card with its chopper being driven by a single transistor oscillator stage. In fact it really wouldn't take much modding to swap the BJT for a MOSFET but I'd prefer to stick with the original circuit if poss. ft must be equal or greater, similarly beta, which puts a modest amount of doubt for the 3439/40 parts. They are slower general purpose parts, with higher voltage ratings that are accompanied by lower beta (possibly 25% lower at 1A). The ZTX part is offered as an example of more recent physical equivalents of higher speed switches in the same voltage range. It's silicone package has the same temperature limts as hermetic and it can benefit just as much by heatsinking methods common to TO5, if present in the current application. It should gunction with whatever base drive and SOA load line tailoring that was applied to the original, with reduced switching and conduction losses. I wouldn't advise a switch technology swap until you become more familiar with the circuit functioning as originally intended. There is seldom any noticeable benefit in low/medium power applications. RL |
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