Author Topic: HP 1740A power supply burn up (Read 7009 times)

bd139 · « **on:** August 23, 2019, 02:11:54 pm »

Just nabbed myself a fairly clean looking HP 1740A off ebay. Unfortunately it was dead. Quick poke and prod session showed that the -15V and 8V rails were off.

Pulled off pass transistor for that rail and fired it up and the thing sprang into action. Seems ok, at least after the power supply!

Investigating the power supply lead to an interesting discovery. Transformer secondary connector appears to have melted and taken half the board with it.

This is what I discovered..

Decided to pull the connector out and it fell to bits in my hand unfortunately.

After desoldering the burned connector and rectifiers I am left with this:

I've just tested the bridge rectifiers and caps and they are all fine. This suggests that it was the connector or the soldering on the board that caused this.

Anyone have any repair tips for this one? I did professional rework years ago and this was "nah chuck it" territory.

I am considering:

1. Clean and grind out any charring.
2. New connectors on both ends.
3. Lift any burned up traces and jump with thick copper wire.
4. Apply copious amount of Kapton tape.

CDaniel · « **Reply #1 on:** August 23, 2019, 02:32:57 pm »

The cause of this failures is usually the connector itself , not good enough to sustain that current for the life of the equipment . It develops contact resistance that produce heat , then more contact resistance until is cooked and burned ... Or insulation failure between pins . Maybe the rest is good , a short should have blown a fuse , not the connector .

bd139 · « **Reply #2 on:** August 23, 2019, 02:34:55 pm »

Yeah transformer is good and secondary comes up. Agree it's the connector that burned up. Looks like a Molex KK 394. Likely to replace it with something else.

Sooooo off to find some 3.94mm pitch connectors that aren't shite

Edit: have checked for conduction between traces and we're good there. Looks like it is only the surface that is carbonised and the plated holes gone. Shouldn't be too painful to repair actually even though it looks like crap now.

BU508A · « **Reply #3 on:** August 23, 2019, 02:51:12 pm »

I don't like these Molex connectors.

What I would do:

- cutting a drill board (Vero board) so it has enough space for this:

A Metz AKL 310-12 terminal spacing
https://www.reichelt.de/12-pin-connection-terminal-5-0-mm-spacing-akl-310-12-p153958.html?

and two of these spacers:
https://www.reichelt.de/spacers-metal-6-edge-m3-12-mm-vt-da-12mm-p231459.html?

- solder the terminal on the veroboard.
- drill two 3mm holes in the PCB for the spacers
- solder wires from the PCB to the veroboard
- fix the veroboard to the PCB
- attach the wires to the connector of the connection terminal
- have fun

Specmaster · « **Reply #4 on:** August 23, 2019, 02:58:02 pm »

Quote from: BU508A on August 23, 2019, 02:51:12 pm

I don't like these Molex connectors.

What I would do:

- cutting a drill board (Vero board) so it has enough space for this:

A Metz AKL 310-12 terminal spacing
https://www.reichelt.de/12-pin-connection-terminal-5-0-mm-spacing-akl-310-12-p153958.html?

and two of these spacers:
https://www.reichelt.de/spacers-metal-6-edge-m3-12-mm-vt-da-12mm-p231459.html?

- solder the terminal on the veroboard.
- drill two 3mm holes in the PCB for the spacers
- solder wires from the PCB to the veroboard
- fix the veroboard to the PCB
- attach the wires to the connector of the connection terminal
- have fun

Yep that sounds like a good plan.

Sent from my POT-LX1 using Tapatalk

bd139 · « **Reply #5 on:** August 23, 2019, 02:58:20 pm »

I don't like Molex connectors now either. Bloody things. The usual suspects used to burn up in PCs as well years ago.

Good idea but alas I've opted to buy 5 pairs of 6-way Molex ones from a UK supplier who does packs of the crimps, the headers and the shells for now as it appears you can only get alternative connectors in packs of 500 million parts etc or delivery nearly as much as I paid for the scope

Specmaster · « **Reply #6 on:** August 23, 2019, 03:02:32 pm »

Quote from: bd139 on August 23, 2019, 02:11:54 pm

Just nabbed myself a fairly clean looking HP 1740A off ebay. Unfortunately it was dead. Quick poke and prod session showed that the -15V and 8V rails were off.

Pulled off pass transistor for that rail and fired it up and the thing sprang into action. Seems ok, at least after the power supply!

Investigating the power supply lead to an interesting discovery. Transformer secondary connector appears to have melted and taken half the board with it.

This is what I discovered..

(Attachment Link)

Decided to pull the connector out and it fell to bits in my hand unfortunately.

(Attachment Link)

After desoldering the burned connector and rectifiers I am left with this:

(Attachment Link)

(Attachment Link)

I've just tested the bridge rectifiers and caps and they are all fine. This suggests that it was the connector or the soldering on the board that caused this.

Anyone have any repair tips for this one? I did professional rework years ago and this was "nah chuck it" territory.

I am considering:

1. Clean and grind out any charring.
2. New connectors on both ends.
3. Lift any burned up traces and jump with thick copper wire.
4. Apply copious amount of Kapton tape.

Totally disagree, it's not "nah chuck it territory", you could always solder the secondary direct to the power board if you can't find suitable connectors. You rework guys, I don't know [emoji849]

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bd139 · « **Reply #7 on:** August 23, 2019, 03:05:08 pm »

If someone else was paying the bill it would be chuck it

. This one is for personal interest though as I'm sure you know

ogden · « **Reply #8 on:** August 23, 2019, 03:10:21 pm »

Quote from: BU508A on August 23, 2019, 02:51:12 pm

- solder the terminal on the veroboard.
- drill two 3mm holes in the PCB for the spacers
- solder wires from the PCB to the veroboard
- fix the veroboard to the PCB
- attach the wires to the connector of the connection terminal
- have fun

IMHO way too complex. I would simply introduce short pigtail for cable-cable connector (2x6).

james_s · « **Reply #9 on:** August 23, 2019, 03:15:03 pm »

That looks like a Molex KK, the same problem is very common in pinball machines. There are high current versions available that hold up much better, any number of sites that sell pinball parts will list recommended types.

BU508A · « **Reply #10 on:** August 23, 2019, 03:19:12 pm »

Quote from: ogden on August 23, 2019, 03:10:21 pm

IMHO way too complex. I would simply introduce short pigtail for cable-cable connector (2x6).

Hmm, not sure what you mean.
Can you show me an example? Just curious...

ogden · « **Reply #11 on:** August 23, 2019, 03:28:06 pm »

Quote from: BU508A on August 23, 2019, 03:19:12 pm

Quote from: ogden on August 23, 2019, 03:10:21 pm
IMHO way too complex. I would simply introduce short pigtail for cable-cable connector (2x6).

Hmm, not sure what you mean.
Can you show me an example? Just curious...

Hopefully pigtail does not need illustration. Connector could be automotive or similar

https://www.te.com/usa-en/product-794198-1.html
https://www.te.com/usa-en/product-794199-1.html

bd139 · « **Reply #12 on:** August 23, 2019, 03:41:54 pm »

Ok looks like this isn't the connectors' fault. Looks like cascade failure. Going to bring the entire supply up on a pile of bench supplies and see where I get to.

Specmaster · « **Reply #13 on:** August 23, 2019, 06:00:42 pm »

Quote from: bd139 on August 23, 2019, 03:05:08 pm

If someone else was paying the bill it would be chuck it . This one is for personal interest though as I'm sure you know

Yep I know OK, but I'd still repair it if it was a repair for someone else, the repair it self need not that expensive. If push came to shove, I'd get some single header pins, solder to power board and then sold the secondary wires to the headers directly. [emoji16]

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bd139 · « **Reply #14 on:** August 23, 2019, 06:13:29 pm »

Makes sense.

Ok further debugging. Got the board up and 15V rail up on bench supply. Now there is zero regulation on the 5V rail which suggests that is knackered or something is cocking it. Further debugging continuing.

Found a source of TO100 LM723s which doesn’t hurt too bad as well. I have some DIP ones but that’s fugly hack territory

Specmaster · « **Reply #15 on:** August 23, 2019, 07:01:28 pm »

I would say that the scope has been left switched on for extended time, possibly been in an environment where the owner switched everything on in the morning and off at the end of the day, regardless of if the kit was going to be used or not

Total waste of power and not to mention premature failure of the kit self.

bd139 · « **Reply #16 on:** August 23, 2019, 07:06:07 pm »

Yeah could be right there!

Anyway debug stuff...

Ok managed to step test the power supply. With the board out, I rammed MJE3055T transistors in the sockets as that's all I have around with the same pinout (no heatsink required here as no load!). 15V supply brought up - no problems at all. 5V came up, hmm not good - rammed at the max voltage on the rail. transistor in saturation. Measured voltages around the LM723 on that rail and it's FUBAR. Desoldered it carefully and chucked another supply on the 5V as a reference and brought the (floating) -15V input supply up. So the source of the problem appears to be the LM723 on the 5V rail was toast. Checked pass transistor in the chassis and that is good so it's possible that something murdered the LM723.

HP power supplies debugging HP power supply...

Found some terrible dry joins on the connectors so have cleaned and resoldered these.

PA0PBZ · « **Reply #17 on:** August 23, 2019, 07:17:34 pm »

Quote from: bd139 on August 23, 2019, 07:06:07 pm

Found some terrible dry joins on the connectors so have cleaned and resoldered these.

That just looks terrible! Also, the caps look like they lost their underwear but that could be a trick in the picture?

bd139 · « **Reply #18 on:** August 23, 2019, 07:51:37 pm »

Caps are fine up close - bad angle makes them look nasty.

tautech · « **Reply #19 on:** August 23, 2019, 08:04:54 pm »

Quote from: PA0PBZ on August 23, 2019, 07:17:34 pm

Quote from: bd139 on August 23, 2019, 07:06:07 pm

Found some terrible dry joins on the connectors so have cleaned and resoldered these.

That just looks terrible! Also, the caps look like they lost their underwear but that could be a trick in the picture?

Common in HP1740's.
The connector series used, their long pins, tension on the interconnects, thermal cycling etc cracks many of the pins.
Good idea to reflow all of them in the scope.
Sometimes you find the original solder fillets a bit on the skinny side and they need to be bulked up for physical strength.

These post cracks are generally most prevalent around the interconnect PCB.

bd139 · « **Reply #20 on:** August 23, 2019, 08:09:29 pm »

Yep have reflowed them all now. Disappointing for sure. Project shelved for a few days now as it's a bank holiday weekend and stuff isn't going to make it here until next week. Awaiting:

10x bridge rectifiers.
1x LM723H.
5x 6-pin Molex connectors (shells, headers and pins)

tautech · « **Reply #21 on:** August 23, 2019, 08:23:38 pm »

If you think some gorilla has been in this scope before you take note of how the timebase boards, timebase shaft and shaft lock rings have been installed as it affects the timebase operation and the tensions placed on the selectors which can impact on the wear and life of the contacts.
It's not hard, just keep your eyes open for anything that seems misaligned.

BU508A · « **Reply #22 on:** August 23, 2019, 08:33:40 pm »

Quote from: bd139 on August 23, 2019, 08:09:29 pm

Yep have reflowed them all now. Disappointing for sure. Project shelved for a few days now as it's a bank holiday weekend and stuff isn't going to make it here until next week. Awaiting:

10x bridge rectifiers.
1x LM723H.
5x 6-pin Molex connectors (shells, headers and pins)

Only one LM723H? What, if it explod^W let out the magic smoke again for some reason?

bd139 · « **Reply #23 on:** August 23, 2019, 08:59:54 pm »

I'll just have to wait another 3 days

I'm a bit annoyed because I had a box of the damn things a few years back. Said to myself: "hey I've never used these, I'll chuck them on ebay". Went for a small fortune which I wasted I'm sure. Should have nicked a couple out of the box first.

bd139 · « **Reply #24 on:** August 23, 2019, 09:13:13 pm »

Quote from: tautech on August 23, 2019, 08:23:38 pm

If you think some gorilla has been in this scope before you take note of how the timebase boards, timebase shaft and shaft lock rings have been installed as it affects the timebase operation and the tensions placed on the selectors which can impact on the wear and life of the contacts.
It's not hard, just keep your eyes open for anything that seems misaligned.

Surveying the rest of the instrument suggests it hasn't been used that much and is in perfect condition. There has been another repair on the power supply by the looks evidenced by a lot of flux that shouldn't be there. This might explain why the flood gun illumination isn't working. Will debug that one once I've got the smoke to stay in this part of the power supply


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Author Topic: HP 1740A power supply burn up (Read 7009 times)

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