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All in a day's.... work?

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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OK, so I'm finally able to stand for reasonable periods and move about without a lot of pain, so I decide it's time to calibrate my HP428B clip on milliammeter. As an aside, it's got real live vacuum tubes in it, and is the first piece of test equipment I have purchased which is older than I am.

To cut a short story long, I got this for a reasonable price (almost nothing) because it had no probe. I managed to find a probe on ebay for a reasonable amount (more than the HP438B) and My sister in law delivered it to me, saving me the cost of freight.

The thing is, these devices were calibrated to the specific probe they were delivered with and thus require recalibration if you change probe. Well, as luck would have it, the probe I got was not the specific one that was lost by the person who sold me the meter.

It's not too bad. It reads about 10% low on all ranges. I set my power supply to 5A, 3A, 1A, 300mA, 100mA, 30mA, 10mA, and 3mA (in series with a 4 1/2 digit meter) to verify this. However, my power supply's current limiting supplies a little in excess of 1mA when set to a 1mA current limit, so I turn to my Keithley 228A current/voltage source/sink to get an accurate 1mA current.

Aaaagh! it reports Cal Err 1 and 3

So I pull it down and open it up (after checking the fuses) and do as the service manual suggests. + and - 5V rails are around 5.0x volts, but the 15V rails and the 115V rails are not there at all :-(

I also pull out my other meter just to double-check that it's not the meter, but it's not.

A bit of playing about reveals that I'm getting some sort of low frequency AC on the 15V rails.

The 15V rails are generated under a metal box covering the switchmode power supply. So now I have to remove the bottom, back, one side, the "logic" board, before finally getting to the power supply board. The manual helpfully tells me that this is how to do it without a complete dissasembly, but honestly, there's not much I haven't undone.

Pulling it out it looks unfortunately clean with the only discolouring on the board being a very slight amount under a NTC resistor designed to reduce switch on transient currents.

Having taken the board out, there is now no way to power it up. But I notice that the board uses a TL494 at it's heart and a pair of 555s(!!!!!) to drive the transformer coupled driver to the mosfet gates. Ooh, I remember that someone said they'd never use a 555 is a real design. Gotta take a photo of this.

While getting my phone to take a photo I come across another eBay purchase, a 50/500mV meter I picked up for $1. Hey, it works and it's almost accurate!

And guess what I found with it? It's the innards of a DC blocking BNC-BNC adapter that I opened up. I purchased these for my HP8590 spectrum analyser because it tolerates a maximum of 0V DC on the input!!! I was planning to replace the capacitor inside one of them with a higher value one to allow me to use it on my HP35665A DSA.

So with a few scraps of PCB I adapt it to fit the new cap and decide to compare the difference of the two of them into a 50 ohm load on the DSA. And it works just as expected. However the centre pin pulls out of the 1MHz to 3GHz one. Aaagh! Something else to fix.

Anyway, I've been soldering so I jumper the switch connections on the 228's power supply so it will be "on" and go looking for my "special" power cable.

While I'm doing that I notice that it's 4:30 in the afternoon and the 428B is no closer to being calibrated.

But I do have a photo of a pair of 555s used in a real piece of equipment and a piece of equipment in pieces.

Keithley228.JPG
 

Arouse1973

Adam
Dec 18, 2013
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You can't blame the designer for this, when the 555 came out in 1971 it was the bees knees, and probably designed into lots of equipment of that era. I bet the designer is kicking him self now, if he's still alive :)
Adam
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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OK, my bet is the TL494. The output to the 555's is not doing anything -- it just looks like noise. (I typed this 5 hours ago...)

As it turns out, I do have some TL494's. The only problem is that they're surface mount. No problems! I'll use a breakout board. Oops, breakout boards for SO-16 are much wider than the original chip. OK, so I'll just wire it to a socket and plug it in!

All_I_have.JPG

Add a bit of acrylic and some glue...

I_can_make_this_work.JPG

And just connect some tiny flying leads

Just_starting.JPG

OK, that's held in place, now do the rest...

Damn_I_just_spotted_it.JPG

Oh, bugger. I just spotted the obvious mistake :-( AAAAAAAAAAAAAAGH!

Take_2.JPG

I really should have taken my shoes off when I counted those pins the first time. This isn't really elegant, but it's going to work...

All I have to do is remove the original chip.

not_a_bad_job.JPG

Too easy!

Im_not_falling_for_this_twice.JPG

And add a socket so that I don;t have to do it again.

And...

Same fault :-(

Here's the schematic:

Schematic.png

The mosfets in red are getting very warm -- but not so warm that they're going to burn up. My guess is that they're both turning on or the transformer has a shorted turn.

The two devices highlighted in green are the 555's.

The blue line joins two parts of the same transformer.

Interestingly, this schematic shows the two gate resistors with 22R crossed out and 18R substituted. Mine has 22R resistors.

T304 seems to have something to do with current limiting, the voltage generated by this current transformer is used to adjust the dead time of the TL494.

Also interesting is that (unloaded) the +115V and the +15V rails are about a third of their appropriate values, but the -115 and -15V rails are near zero.

I think my next target will be the rectifiers on the secondary side of T303.

CR303 to CR306 are MUR1615CT (600V 16A ultra fast recovery)

CR315 to CR318 only have a Keithley part number, but on inspection are MUR860 (600V 8A ultra fast recovery)

Hmmm.... If the fault lies there, I don't have anything on hand. But that's for tomorrow.
 

Harald Kapp

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That piece of PCB looks super clean. Good luck getting these fine tools back to work again.

Ooh, I remember that someone said they'd never use a 555 is a real design.
If you know their limitations, why not? Just don't expect super accurate timings. But I'm sure we're not talking MHz switch-mode power supplies here, more a few 10 kHz, a range where a 555 can be a fine chip.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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If you know their limitations, why not?

Indeed. Overnight I considered that I was having triggering issues when trying to read the various outputs. Obviously this was exacerbated by the fact I didn't want to leave the power on too long (and because the point I was probing was *very* close to live mains (yeah, sue me, I choose safety over valor -- especially when working alone).

Actually -- to digress -- I was thinking about a mod to my "special" power cable. I'm thinking of a plastic box with a large double pole "push to make" switch. That will require I keep one hand on it, and any shock will almost certainly make me release the button. It certainly won't guarantee safety, but it will at least prevent me using both hands (I prefer using none, but in situations like this I need to use one) and staying "live" with a continuing shock. (grim thoughts, but when working on live equipment, avoiding the possibility of a shock, or at least minimising it's harm is always at the forefront of my thoughts).​

Back to the circuit...

What appears to be a current limit (or at least current control) changing the dead time seems to be an interesting feature. If it turns out to be a shorted rectifier (and what are the common faults in a switchmode power supply?) then it is quite remarkable that it didn't blow the mains fuse (although I expect it was/is stressing some components significantly.

It is interesting that I noted the hot transistors when I felt radiant heat (!) from above the live board. Naturally I'm not going to stick my fingers on them when they're live, but after powering down and checking with the multimeter that they're within millivolts of ground, they were hot, but not so hot I left any flesh on them :D. My calibrated finger measured the case temperature at "6". Later tests done with maybe 10 to 20 seconds of "on" time didn't elevate the temperature more than to about "3". None of the rectifier heatsinks were enough above ambient to be noticeable. Alas, I didn't probe the transformer.

And I guess I should move this thread from off-topic to repairs or similar, since it has drifted rather back on-topic.

Oh, and for anyone about to complain about the quality of my photos, they were taken in a shed, at night, on a mobile phone with less than perfect lighting conditions. Feel free to point me here. In all seriousness, I'm pretty amazed they turned out as well as they did.

Oh well... Back out to the shed. If it is the rectifiers I'll have to buy some components (gasp).
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Yep, two of the MUR860s have failed.

Time to order some parts!
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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OK, 10 MUR860's ordered and should arrive in 2 days -- they say dispatch next business day (let's see how good they are).

The original TL494 is allowed to replace the Franken494.

Original back in place (Large).JPG

So, time to clean up and get on with the HP428B! The shed is a mess (actually, it's tidier than it's been forever, but I don't want to admit that)

TheShed (Large).JPG

Back to the HP428B. And I think I've discovered why the calibration is off!

Thats_why_its_out_of_cal (Large).JPG

The seal is broken! Oh well, I was going to break it anyway.

But there's a huge disappointment.

looks_like_im_older_than_it_is (Large).JPG

I'm older than it is :-( On the positive side, it's not as old as me :-D

This is what I expect to find inside (from the manual)

what_I_Expect.png

All that point to point valve wiring goodness.

But look what I find:

oooh_not_what_i_expected (Large).JPG

This is actually the prettiest picture I had. Taken after I had finished, but it's not what I was expecting.

The back side is kinda weird:

a_mix_of_old_and_older (Large).JPG

There's a mix of old and new(?). Point to point wiring from the transformer and for the electrical wiring to the fuses. Is there even a tagstrip there?

As it turns out, calibrating is a long, time consuming process. Since all I'm worried about is the inaccuracy, I just adjusted a trimpot which trims the meter reading. I run out of adjustment and I don't feel like changing the series resistor, but it only reads about 1% low now. I can live with that.

The trimpots are pretty noisy, and the zero adjust pots (coarse and fine) are even more noisy. I think I'll replace them with a 10 turn pot if I can find one that fits and works with at least one of the current knobs. But not today!
 

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Harald Kapp

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Nice workbench you have there. I'm jealous :D
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Nice workbench you have there. I'm jealous :D

It's inside a 3x4 metre garden shed that has been fully lined and insulated. There is also some garden equipment in it, but I try not to let that happen too much :)
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Woo! Hoo!

RS delivered the parts today, I removed all 4 diodes and replaced them. Only one was shorted, but it's good practice to replace them all.

On power on, the 10V rail was a little off, so I adjusted it. Aaaagh! the -15V rail was -30V, and the +/-115V rails were around 150V

Silly me. Only one rail is regulated, the others rely on load. Place a load on the 15V rails (around 1.5A) and both drop to a little under 15V and so are adjusted.

As I disconnect everything and flip the board ove I get a nasty tingle from one of the caps. The +/- 115V rails are now at 200V and I have just brushed the terminals. All the other caps discharge within seconds -- not these. OK, discharge them for my safety, unsolder the "special" power cable and jumpers then reassemble.

It all goes back together easier than it came apart, and without the top or bottom cover on I power it up[ and it passes all of its self tests.

I probably need to recalibrate it, but that can wait for another day. (as can the photos)

Oh, and the failed diode showed no signs of heating, cracking, smoke, etc. It was either a very sudden very massive overload or a "I'm at the end of my life" failure. I guess we'll see when I calibrate it (thus putting it through its paces).
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Nothing too exciting in these pictures.

Firstly the "free" freight option by RS arrives like this. Not bad service for a $10 order (I've had the same for a 40c order!)

RS_delivery.JPG

And the removed diodes:

one_is_not_like_the_others.JPG

Other than the position on my workbench, there's not a lot you can tell by looking that one has failed. I've not been able to get back into the workshop yet to recalibrate it, but I'll report back when I do.
 

shrtrnd

Jan 15, 2010
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If the wife asks you why you keep going back out to the shed, tell her you're working on a defibulator in case she ever needs to use one on you.
(The tingle from the cap when flipping the board over).
Scratch that, if it's unappreciated humor.
Glad you're feeling well enough to get back out into the shed. Get lots better, soon.
 
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