HP 3437A System Voltmeter - first half of teardown, pending repair

[krivx]

It works!
*I'm hoping I can find new 7-seg displays that fit in the front panel cutouts. I'll just stick them on a PCB to match the one that it has. The old one with tiny LED modules glued and bond-wired to a separate PCB is cool for old-shit nostalgia points, but if it doesn't work...

I think I have some old 7-segs that are marked HP. If you can find a P/N on them I'll check and you can have a few.

edit: haha, I didn't see you already had an offer! Still stands anyway.

[c4757p]

Thanks, guys!

PN is 1990-0589 according to the manual. The only marking on it is 5677708J. Dimensions are attached.

[c4757p]

More pictures and information in a couple days - just got my capacitors and fan

[lowimpedance]

And yes, those ROMs get very hot. No wonder they (and the PCB under them) look scorched. The microprocessor also gets very hot. I would not want to run this for long fanless.

The data sheet for the 'A' revision of the 3624 gives a max. temp of 135 deg 'under bias' , 90nS speed, o/p currents 100mA.
I wonder how fast they are working?. Guess they will get a little toasty. Having another look at the picture of them the one with the odd surface texture still looks like it has had a coating applied rather than any heat damage. do all the ROM's appear to be original?, none replaced?.
Anyway its nice to see its not ready for landfill yet.

[c4757p]

The data sheet for the 'A' revision of the 3624 gives a max. temp of 135 deg 'under bias' , 90nS speed, o/p currents 100mA.
I wonder how fast they are working?

The system clock is 3.333 MHz. Not sure what the memory clock is. Not incredibly fast.

Yeah, like I said I did eventually recognize the coating... It is very thin, but definitely there. Still, the PCB under the ROMs is darkened - pretty sure that's from heat! Anyway, I have the new fan installed now, and everything runs just a bit warm to the touch with the fan blowing over it.

[macboy]

I checked my stock of displays and I don't have anything to match.

[c4757p]

Thanks for looking! After carefully cleaning all the contacts, I've discovered that most of the "bad segments" were actually poor contacts, and there appears to be only one segment dead. It's the "A" segment on a digit which only displays 0 and 1, so now it displays U and 1. I can live with that.

[free_electron]

i think that final coating is some moisture protectant . applied like you would apply hairspray. it's not dipped or painted with a brush but vaporised and then probably dried in a pass-thru oven at 60 degree centigrade to get the solvents out.

[c4757p]

Well, I haven't touched this in quite a while - I needed LEDs to replace the ones it has, and that package got tied up in the mail for a while.

This, my friends, is what happens when you put 2013 LEDs in a device from the 70s. Guess I'll be changing a couple resistors too

Every single thing appears to work. I think the only thing I will have to adjust is that trim cap I messed with while cleaning - compensation for the 10V range attenuator, I believe. Everything else is within specifications still (absolutely astounding considering what it's been through).

By the way - does anybody know where I can source a couple replacement buttons? It doesn't seem to be nearly as easy to find replacement parts for HP equipment as for Tek...

One thing that just occurred to me - if the front panel LEDs were failing, what about the optocouplers between the digital and analog boards? Are the LEDs used in optocouplers are unreliable as the ones used on panels?

[c4757p]

Er... am I reading this right? Are they really giving the LEDs (VCC-VF-VCESAT)/R = (5-2-1)/33 = 60mA? I guess that works out to a lower average, once you consider the multiplexing duty cycle...

[sync]

The LED current will be lower. The annunicator data lines are switched with a transistor to VCC.

[c4757p]

Nope, this is the voltage across one of the 33 ohm resistors. (Looks like it ended up offset, must be I swapped the probes after configuring the scope.) Peak is about 2.2V, so 66 mA, and average, compensating for the accidental probe swap, is about 0.3V, so 9 mA. I guess the average is about what I would have expected. I think I will replace the 33 ohm resistors with 91 or 100 ohm resistors, for an average of about 3mA.

(Yes, I notice my calculation is flawed because that resistor serves multiple LEDs. Oh well, I still want about 1/3 current.)

[c4757p]

Update: I will follow up in more more detail soon. Finally got myself a GPIB interface! So I will try out and demonstrate some of the more useful applications of this thing. Just have to wait for shipping.

Hmm... I was planning on selling it when I finish with it, but now I'm just thinking how useful it would be paired with MATLAB...

[echen1024]

Update: I will follow up in more more detail soon. Finally got myself a GPIB interface! So I will try out and demonstrate some of the more useful applications of this thing. Just have to wait for shipping.

Hmm... I was planning on selling it when I finish with it, but now I'm just thinking how useful it would be paired with MATLAB...

Us on the forum buy test equipment to fix and sell. After we fix it, we fall in love with it and keep it.

[c4757p]

That's how I got almost all of my equipment! I sell most of it, but I do fall in love with the occasional item. That's OK, most of them do eventually end up sold for an upgrade..