What's your most repaired TE

[joeqsmith]

I have a knack for damaging power my bench power supplies.  They get a lot of abuse because they are used for pretty much everything I play with. 

I have three of them the I frequently use.  One is home made and the other two are from HP.  I received the two HPs damaged.  After repairing them, I have since damaged the 6289a 50V supply several more times over the last 10 or so years. 

The first time I blew the output stage.  The second time I fried it, I beefed it up using better transistors then added a TVS to the output and figured that was the end of that.    Time goes by and again I lost the output.   I figured for sure I lost the output stage again but everything was good.   I found a capacitor had shorted.    The next time I was floating the supply and I don't remember what I did but it again lost the output.  Again, the output drivers were all fine.   This time a zener had shorted.    Last week, it once again lost the output.   As I work through it I discover the same zener had failed.   This time it got a much larger part.  Now we just wait for the next failure.   

I've damaged other equipment but this supply by far has been damaged more often than anything else I have.   

What TE do you own that you have needed to repair more than any others?   


     

[don.r]

Mine tend to be the vintage units. One of my old Wavetek sig gens keeps doing something. Voltage regulators, caps drying, resistors blowing open, I even had the cable to the main front BNC break off probably from opening the case so much from previous repairs. I find a repeatedly sick puppy never fully recovers and needs to be attended to indefinitely.

[AndyC_772]

Such a device would fail my basic fitness-for-purpose test, that any piece of test equipment should be at least 10x as reliable as the thing it's being used to develop.

Perhaps you've got a tired unit, which has suffered a lot of abuse and still has latent damage that will result in outright failures. Maybe the way you use it is particularly demanding. Maybe it's just not HP's finest hour.

Either way, if it were mine I'd get rid of it and look for something that won't distract me from the work I actually need to do. Personally I use HP 6632/3/4B supplies, and I've only ever had to replace one op-amp in one unit out of 5 or 6.

[joeqsmith]

Perhaps you've got a tired unit, which has suffered a lot of abuse and still has latent damage that will result in outright failures. Maybe the way you use it is particularly demanding.

Both but more so the later. 

[Tomorokoshi]

My pride.

 

[Kalvin]

If you are using a zener as a TVS, it won't work. The zeners are not designed to absorb large energy transients. Get a proper unidirectional transient voltage supressor diode instead. Adding a fuse to the output should not hurt either.

[bd139]

Tektronix 4xx series scopes are the ones I spent most of my time fixing. 465/475/465B/453 units.

Capacitors, switches, transformers, bridge rectifiers, pots, attenuator modules, just about everything in the HT side of things.

Oh and everything I have bought that was from Philips has caught fire at some point, usually the X2 capacitors or mains filters.

When you drive a classic car, you have to put oil back in it every 50 miles

[joeqsmith]

If you are using a zener as a TVS, it won't work. The zeners are not designed to absorb large energy transients. Get a proper unidirectional transient voltage supressor diode instead. Adding a fuse to the output should not hurt either.

I didn't post to talk about how to improve this particular power supplies robustness but I appreciate your concerns over my equipment's well being.  I know what both TVS and zeners are and believe it or not when I write that I have added a TVS to the output, I actually have added a TVS and not a zener.     

A fuse would be a poor choice without using the feedback to compensate for the error,  making it less flexible.

[TiN]

So far Tektronix TDS7404 scope.

[_Wim_]

I know what both TVS and zeners are...

I also know you know how to kill them 

[joeqsmith]

I know what both TVS and zeners are...

I also know you know how to kill them 

I try my best to make sure no meter survives.  Sometimes the designers do a great job and the meter wins.   It's rare. 

[duak]

I've repaired a few power supplies over the years.  I've been lucky that I've only personally fried one lab supply: an Amrel unit blew an internal diode when I tried to charge an almost dead car battery.  I probably swapped the leads but I wonder if I didn't set the output voltage too low and if it had a down-programming circuit, maybe it tried to discharge the battery.

One of my little Lambda supplies, an LP-412 developed a flakey voltage set pot that caused the output to spike to max regardless of the setting.  Turns out that when the output voltage is greater than the set voltage, the supply's design shunts the reverse current thru said pot.  If there's a large cap or a battery on the output and the pot is set to a low resistance, it draws a large reverse current and burns a spot on it.  I added a PNP power transistor to the external programming terminals that shunts the reverse current away from the pot thus protecting it.

Most recently an HP6002A that I got because the previous user couldn't repair it.  After one of the paralleled pass transistors and the downprogramming transistor were replaced, voltage mode worked but current mode was really unstable with a crazy sawtooth waveform.  Turned out the original LM301A opamp had been replaced with an OP07 precision amp that caused one of internal DC busses to oscillate.  I recall also swapping out an LM358 for the correct LM1458 - both dual op-amps, but quite different characteristics.

I have a question for the OP joeqsmith: what zener diode has died on your supply? I ask because I have an hp6284A which is from the same family as the hp6289A and when I looked at the '89A's manual from Keysight, the last page covers a design issue that appears very much like the down-programming issue above.

I have an hp6012A 60 V, 50 A switching lab supply that has a problem with current limit.  I haven't needed the supply for a while so I haven't spent time on it.  I wasn't too impressed with some of the design details and see that hp revised it to 'B' fairly early on.

I've repaired a few Tek and Wavetek function and pulse generators over the years.  One common user generated failure is that the output transistors, usually 2N3866/2N5160s are bad.  These are getting pricey so I bought a few extra last time.

In general, I've seen electrolytic caps open and tantalum caps short in all sorts of equipment and especially in stuff built more recently.  So far, I haven't seen any power line X & Y caps fizzle out but I suppose it'll happen sooner or later.

Cheers,

[joeqsmith]

I have a question for the OP joeqsmith: what zener diode has died on your supply? I ask because I have an hp6284A which is from the same family as the hp6289A and when I looked at the '89A's manual from Keysight, the last page covers a design issue that appears very much like the down-programming issue above.

VR5 1N3824, 4.3V 1W which is part of the bias supply for one of the transistors that makes up the output regulator.    The manual's print date is 1967.  The errata sheet is from 71.    It looks like I first repaired the supply in 2003, so it's failed twice in 14 years and it sees a fair bit of use.