HP 9810 A transformer

[calc_you_later]

I have recently acquired an HP 9810A and want to get it working again. When turning it on it blows the 6A fuse and no lights on the console turn on, so I took it apart and I think it's the transformer that's the issue.



In the image there are 7 contacts, from top left to bottom right they are:

7 6 5
4 3 2 1

When I continuity test 7, 6, and 5 they are all connected, could it be a short in the coil somewhere?

Also any ideas of what else I could try / test?

Thanks

[wojtek]

You have taken it out of the 9810 so the checking should be simple, just connect it to the power source without any load (but with the fuze of course) and see what will happen :-) From the picture it looks like two sections' windings each with taps so the 765 should be on secondary section and 1234 on the primary, so the shortcut should be OK

[calc_you_later]

I'll try that tonight thanks. When you say without any load do you mean cut the wires to disconnect it from the rest of the calculator?

[wojtek]

Yes. And just in case measure and write down the voltages on all the taps of the primary and secondary windings (voltmeter set to AC). Be careful, on the primary the voltage can be higher than 240V, even if powered with 240 (autotransformer effect). But if you do not have experience with measuring, better omit measurement. If it won't blow up the fuse I would say it should be OK.

[teenix]

(08-04-2016 12:04 PM)calc_you_later Wrote:  I have recently acquired an HP 9810A and want to get it working again. When turning it on it blows the 6A fuse and no lights on the console turn on, so I took it apart and I think it's the transformer that's the issue.



In the image there are 7 contacts, from top left to bottom right they are:

7 6 5
4 3 2 1

When I continuity test 7, 6, and 5 they are all connected, could it be a short in the coil somewhere?

Also any ideas of what else I could try / test?

Thanks

A few thoughts...

I don't know if service manuals are available online, but if so, it would be a good first step as it would surely offer some testing procedures and possibly circuit diagrams, otherwise testing for unknown values may not prove valuable. There are many wires coming from the transformer, not just the ones that are numbered. The extras (black and green) look to be in a white sheath.

Depending on what you used for a continuity test, the result may not prove much as the transformer windings will (should) have little resistance to DC current and may just appear to be shorted.

Does the unit have multiple mains input voltage settings, say for different countries. It may be switched to a wrong setting.

Look for components that look damaged, corroded, swollen, or blackened and try to detect any smells of previous burning around the circuit boards and power supply area. (Mains power disconnected of course) Look for "gunk" that may have leaked out of components or has gotten into the insides accidently (coffee etc) and is causing a short.

If there are plugs from the transformer to the power supply board, you could try unplugging them and power up again. If the fuse stays in tact, the problem is from the power supply into the circuitry.

If the 9810 construction is modular then you can unplug as many modules as you can and try to power up again. You might be able to narrow down which module is causing the problem. You have to be careful here as some circuitry may be powered up in an unstable state with things disconnected and may give you more grief.

If it powers up ok with modules disconnected, you could measure the AC output from the transformer, but you would need to trace the leads to the power supply circuit and work out what lead does what. Measuring the DC power supply voltages would be a next step. Again a service manual would be invaluable.

If you do go unplugging things, try to mark everything you can, or take lots of photos.

Lastly, while fiddling around beware of high transformer voltages, remember they can be lethal, especially if one hand is resting on the chassis and the other touches the mains wiring.

cheers

Tony

[teenix]

I just noticed pin 5 on the transformer looks a bit "stressed". Maybe worth a closer look.

cheers

[Accutron]

Service manual:

http://www.hpmuseum.net/document.php?hwfile=797

[BartDB]

On http://www.hpmuseum.net you can find the service manual and Tony Duell's repair guide.

From the schematic in the sevice manual, it seems that pins 1 to 7 are all taps of the secondary coil - so with a simple multimeter they will all look shorted (primary are probably black wires going straight into the transformer).

The best test for the transformer is to disconnect all the secondary connections and apply power - if the fuse blows again it is likely the transformer is at fault.

However, it would be good to first check the rectifier bridge and capacitors after the transformer as this could also be a likely cause.

[calc_you_later]

Thanks! I hadn't seen the service manual before, I'll read it soon. And thank you teenix, must appreciated. I had a brief inspection of the boards and couldn't see any obvious failures but I'll have a closer look at the rectifier circuit.

[calc_you_later]

So I replaced the fuse and disconnected the secondary from everything and it still blew the fuse... When it happened smoke seemed to come from the two wires in the top left of the photo but I didn't think that could happen. I don't really know what this means but I'll poke around further.

I cut the wires to the fan to take this assembly out and interestingly the two wires test for continuity, but it looks like an AC fan so maybe this is correct?

[teenix]

(08-04-2016 05:36 PM)calc_you_later Wrote:  So I replaced the fuse and disconnected the secondary from everything and it still blew the fuse... When it happened smoke seemed to come from the two wires in the top left of the photo but I didn't think that could happen. I don't really know what this means but I'll poke around further.

I cut the wires to the fan to take this assembly out and interestingly the two wires test for continuity, but it looks like an AC fan so maybe this is correct?

Perhaps you have found the problem, or part of it.

It would be easy to see if the insulation has broken down between these two wires. Just as a quick test, you could split open the white sheath and then separate the wires, making sure everything is isolated from them and try again.

It looks like they connect to a large capacitor, so that might need looking at as well as it may have contributed to the fault.

From what I can see, there is some dirt or dust covering the chassis and fan entry so a general clean up may be in order. Be wary of moving old wiring though. All sorts of problems can happen.

cheers

Tony

[Alejandro Paz(Germany)]

A better way to test a transformer is to use a low voltage 50 (60) Hz signal on the primary and on the secondary (one at a time) and measure the voltages on the primary/secondary respectively. That is what some engineers at HP used to do. The low voltage source was an HP oscillator, don't ask me the model, with appropriate short-circuit protection, of course.

Knowing that something, most probably, on the primary side makes 6 A fuses blow means don't connect that to mains till problem fixed.

[Dwight Sturrock]

(08-05-2016 04:21 AM)Alejandro Paz(Germany) Wrote:  A better way to test a transformer is to use a low voltage 50 (60) Hz signal on the primary and on the secondary (one at a time) and measure the voltages on the primary/secondary respectively. That is what some engineers at HP used to do. The low voltage source was an HP oscillator, don't ask me the model, with appropriate short-circuit protection, of course.

Knowing that something, most probably, on the primary side makes 6 A fuses blow means don't connect that to mains till problem fixed.

A function generator could be used for that purpose.

[calc_you_later]

I've done some further investigation and it seems like the issue is with the large red component in the previous image. It was this component which blew when connected to the mains so I've taken it out of the circuit with the hopes of identifying and replacing it.

These are all the details I have about it:

• Live & Neutral from the mains go into it, and two wires come out of it and go into the transformer.
• The case is grounded.
• Inside the case was filled with epoxy so I can't see any of the internal components.
• It's about 6cm tall, 3cm wide and 2cm deep (with a rounded rectangle profile).
  

I've uploaded some images of the casing here: http://imgur.com/a/tAaLR

The text on casing says:
LC2002 2R50-2 B/4
0.2uF +2x2500pF XY +
2x5mH 2A 250V~ HPF
550-6 / 560-7 5.76

Does anyone recognise the circuit symbol or know what this component is? It looks like a transformer and some weird capacitor to me.

Thanks!

[Accutron]

It's an AC line filter. You should be able to find a modern replacement.

[Dave Shaffer]

(08-18-2016 02:41 PM)Accutron Wrote:  It's an AC line filter. You should be able to find a modern replacement.

For testing purposes, you can probably just skip it. It is supposed to filter out noise spikes on the AC line.