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Author Topic: REPAIRING TEN-TEC PWR SUPPLY 961 BUT... NEED VOLTAGE PARAMETERS  (Read 1627 times)
W6PJJ
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Posts: 55




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« on: December 27, 2012, 06:47:52 PM »

My Ten-Tec Paragon Power Supply Model 961 gave up the ghost, so I decided to repair it but, when I broke out the manual for the power supply I was a bit disappointed, there were no reference voltage check points or any mention of voltages at particular junctions of components on the boards.

I am looking for a reference voltage test chart for this supply, if you know where I can find one, it would be most appreciated. I contacted Ten-Tec they do not supply a reference voltage check list.
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KE3WD
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« Reply #1 on: December 28, 2012, 12:33:15 PM »

Here's a tip about checking voltages where silicon solid state transistors and diodes are concerned. 

**A bipolar silicon transistor, whether PNP or NPN, signal or power level, when in an analog circuit where the transistor is always going to be biased ON for proper unit operation, can be quickly checked by placing one of the test probes of your DMM or VTM on the Base, and the other on the Emitter.  Polarity not important at this point, you are looking for the signature of a silicon transistor that is properly biased on, and in any of these circuits, that B-E voltage drop should equal about 0.6 to 0.7VDC in a healthy circuit situation.  ANY other voltage found there, significantly higher or significantly lower will indicate either a bad transistor or a failure of an associated part that is in that transistor's circuit, such as a resistor or cap. **

Diodes, it is much the same, but for a rectifier silicon diode, expect that volt drop to be a bit lower than the signature for the transistor, around 0.5 - 0.6VDC. 

A Schottky diode will have an even lower voltage drop reading when healthy, around the 0.2VDC level. 

Use of a common filament lightbulb wired in series with the AC input of the power supply under test is a very good idea.  A 60 - 100W bulb will suffice.  If you turn the supply on and the bulb lights brightly, that is an indication that there is something in that supply drawing way too much current.  A shorted transformer, or a shorted rectifier, or in some designs, could indicate shorted Pass Transistor(s) on the heatsink. 

Yes, the lightbulb can remain in series while doing the above B-E etc. tests, as well, in maybe 90% of cases, provided that the bulb is not brightly lit, indicating a SHORT condition. 

Lightbulb in series also saves wasting money replacing blown fuses. 

Any common light socket can be used, wired between a plug and a socket.  Placing a switch across the lightbulb contacts allows for switching the bulb in and out of circuit handily. 

I *always* fire up the unknown repair unit under the bulb first, my Variac is wired in before the lightbulb circuit so I can bring up the AC slowly and *watch that bulb*.  After a bit of experience with the bulb, you will find that it serves as a right dandy indication of Amperes drawn, at least as far as "Too Much" goes, and prevents a lot of accidents that make the repair even more costly or difficult. 

"You don't need a schematic to troubleshoot and repair a power supply." 

An old prof used to tell us that in school years ago, and with time and experience the old adage has held true.  Even with the advent of switchers.  Of course, I first had to study the theory, care and feeding of the switching power supply plus experiment on some broken ones before that was possible.  But for your analog TenTec supply, it should be rather smooth sailing. 

Likely the reason that TenTec never took the time to print out voltage testpoints is because of the above quoted statement. 

You can do this. 

73
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W6PJJ
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« Reply #2 on: December 28, 2012, 02:57:03 PM »


Thanks for your reply and the information. This is not just a basic 12VDC power supply it incorporates several levels of safeguards, over current, over voltage, triggers and resets... and I can assure you, if the schematic was not important the Manufacture probably would not have provided one with the user manual. I did some basic measuring last night... we did discover several areas of concern, no Bias on the Pass Transistors which is controlled by a chip regulator and a control transistor and the JFET that supplies power to the PWR on LED is not switching, pointing to the Regulator Board. But my garage was very cold and I gave up quickly, hi hi. I appreciate you taking the time to explain the process and procedures you use, when we get the garage warmed up we will dive into this project again. TNX

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KE3WD
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« Reply #3 on: December 28, 2012, 04:05:32 PM »

Didn't mean to imply that the skat is unimportant, meant to try to tell you how I have to handle the customer pieces where that voltage point information is unabailable, sometimes the schematics can't be found, either, but we like to give it the old college try anyway, and more times than not can come up with the repair, along with the happy customer that will not only return, but likely give us that invaluable referral and reputation via the customer's bragging rights. 

From your report, I'd be thinking that JFET not switching is telling me something important...

There is likely only one thing, possibly just one component stopping the show.  Don't get all flustered by the "safeguards, over current, over voltage, triggers and resets" stuff unless and until you eliminate the simple things.  Rectifiers.  Unregulated supply feeding all the rest, sometimes a disconnect is in order to see what that is doing.  Filtering.  Excess ripple can cause all sorts of problems with the rest of the circuits.  'Scope is handy, but not a necessity there.  The common DMM set to AC and used on DC rails can tell you a lot about the filtering/ripple situation.  So can simply shutting it off and connecting a known good electrolytic of proper or better voltage rating right across the connects of an existing one.  If tacking in the extra cap with a couple of insulated jumper leads makes things happen, well, you've found a bad cap. 

When something simple like that JFET switch is inoperative, I consider that clue to be a "gimmie" -- don't convince yourself that it is something complicated yet.  JFETs often don't switch in the presence of ripple. 

And, yes, I've owned and operated TenTecs over the years, because I'm first and foremost a CW freak.  TenTec QSK being the thing of beauty.  But they liked that power amp design that, while being super clean, needs to have a power supply that can cut off in the presence of high SWR before an RF transistor can give it up instead.  But I must say that TenTecs protection circuits are very good, incorporating rather conservative design and haven't YET had to delve into the protection circuits in them, other than Ye Olde Fast Circuit Breaker types found in the C supplies from days gone by.  The electromechanical breaker would sometimes fail.  But then I've yet to own a brand new TenTec out of several. 

You can find it.


73
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W6PJJ
Member

Posts: 55




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« Reply #4 on: December 28, 2012, 05:15:27 PM »

Didn't mean to imply that the skat is unimportant, meant to try to tell you how I have to handle the customer pieces where that voltage point information is unabailable, sometimes the schematics can't be found, either, but we like to give it the old college try anyway, and more times than not can come up with the repair, along with the happy customer that will not only return, but likely give us that invaluable referral and reputation via the customer's bragging rights. 

From your report, I'd be thinking that JFET not switching is telling me something important...

There is likely only one thing, possibly just one component stopping the show.  Don't get all flustered by the "safeguards, over current, over voltage, triggers and resets" stuff unless and until you eliminate the simple things.  Rectifiers.  Unregulated supply feeding all the rest, sometimes a disconnect is in order to see what that is doing.  Filtering.  Excess ripple can cause all sorts of problems with the rest of the circuits.  'Scope is handy, but not a necessity there.  The common DMM set to AC and used on DC rails can tell you a lot about the filtering/ripple situation.  So can simply shutting it off and connecting a known good electrolytic of proper or better voltage rating right across the connects of an existing one.  If tacking in the extra cap with a couple of insulated jumper leads makes things happen, well, you've found a bad cap. 

When something simple like that JFET switch is inoperative, I consider that clue to be a "gimmie" -- don't convince yourself that it is something complicated yet.  JFETs often don't switch in the presence of ripple. 

And, yes, I've owned and operated TenTecs over the years, because I'm first and foremost a CW freak.  TenTec QSK being the thing of beauty.  But they liked that power amp design that, while being super clean, needs to have a power supply that can cut off in the presence of high SWR before an RF transistor can give it up instead.  But I must say that TenTecs protection circuits are very good, incorporating rather conservative design and haven't YET had to delve into the protection circuits in them, other than Ye Olde Fast Circuit Breaker types found in the C supplies from days gone by.  The electromechanical breaker would sometimes fail.  But then I've yet to own a brand new TenTec out of several. 

You can find it.


73

I'm certain I can... we have been working on the ham gear for many years, but about 15 years ago we sold off most of the test equipment and the scope would really come in handy in looking at the DC. When we get the garage warmed up we are going to cure that problem but with hands shaking and a fair mood of disgust due to the cold we have to tackle this down the road or work on it in the Dining Room. HI HI.

73's and TNX for your advice and support Clark.
 Cool
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