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Warning about Tenma benchtop linear power supplies

Over the past couple of years, I'm seeing a lot of dead Tenma linear power supplies coming in to my shop. By a lot, I mean seven that I'm staring at, all unrelated, all coming in from different labs. But they're all the 72- series:

72-7245 72-6905 (x 2) 72-6615 (x 2) 72-8700A (x2)

And I've gotten reports of another six or so dead that haven't come in -- n ot sure if they're all 72-series. The ones in my shop seem to have failed in seven different ways, but three of them failed high -- output went to ma x voltage -- thus destroying the very expensive microwave amplifiers they w ere powering. So it seems the supplies don't have crowbars. And on at lea st two of the supplies that failed high, the culprit wasn't the traditional shorted pass transistor, but something broken in the feedback circuit.

Anyway, be warned. And it's a shame, because these supplies feature a gorg eous, massive toroidal transformer. But don't use them to power anything t hat can't tolerate an overvoltage.

Reply to
Jim MacArthur
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Looks like one needs to add/design a variable crowbar circuit to the outputs.

Thanks for the warning, even though we use different power supplies in our shop the information about the risk is valuable.

Part of my general philosophy of not trusting my test gear to always behave properly...

These days we add 6VDC clamp diodes on older TTL video game logic boards that would be extremely expensive to replace/repair if the supply voltage went off...

John :-#)#

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Reply to
John Robertson

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** Why not add an SCR crowbar to the u-wave amps ?

Seems they need such protection and any DC supply might go over voltage for reasons other than internal failure.

10amp SCR, zener and resistor cost about $3 to protect it from any PSU that might come along.

.... Phil

Reply to
Phil Allison

Mostly because it's hard to convince some people to spend $3 until they've lost $3000. But also because these damn amps are so sensitive that I might need something more precise than an SCR and zener.

This one, for example:

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requires 28V for optimal operation. 32 V kills it dead. And this is one of the more robust amps. For many of these amps, losing power without cutting the input signal will destroy the amp, so a crowbar by itself won't do the trick.

And why exactly does a 15W 4 GHz broadband amp cost $2.3K? If any of you want a _lot_ of business, design and market an amp with replaceable transistors, so that when a user accidentally drives an open circuit, they're not out $5K.

-Jim

Reply to
Jim MacArthur

====================

** I made some nice money in the 70, 80s and 90s installing DC relay based protection inside pro audio amplifiers - after they had destroyed expensive loudspeakers.
** Can be made as precise as you like by selecting zeners to suit the trip voltage. Try 2 x 14V 1W zeners in series and 56ohms across the G-K junction of the SCR. Should trip in a couple of microseconds at 32V.

** Data sheet says it will accept 32V DC.

I think you protesteth too much.

** Maybe just add a relay at the input powered by the same 28V supply -wireds so it shorts the drive when de-energised.

.... Phil

Reply to
Phil Allison

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