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Heidenhain Positip 850 Help Required

adrian

Cast Iron
Joined
Jan 11, 2005
Location
Kent, UK
If anyone on the forum has a Heidenhain Positip 850 I have a favour to ask.

My unit has died and it appears the power supply is at fault. If someone has a similar unit I would be very grateful if they could take the lid off and measure the DC voltages out of the power supply. My plan is to replace the existing with other power supplies, there's plenty of space within to do so.

There are four wires on the output connector, brown, red, black and yellow. As they are laid out it may be black is the common 0V and the other colour wires are the positive outputs, or they are in pairs.

It's really easy to get the lid off, it's seven screws as shown below and the lid slides back and off.

Even though I have a later unit I like this old CRT type because the display is large and bright.
In the meantime I will ask Heidenhain if they can supply a wiring diagram so I can get the power supply repaired, but I am not holding too much faith in them doing so.

Many thanks Adrian.
 

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DeadMahoDude

Aluminum
Joined
Oct 30, 2016
Doesn't seem to be the original HH power supply, at least mine (that I already repaired) looks different. All that matters are the voltages, that go to the connector on the main board, which are labeled on my PCB:
grey: -15V
green: +15V
yellow: -12V
red: +5V
black: GND
brown: +12V
green/brown: RES (no idea, what that is, perhaps for SRAM backup battery)
pink: +UB (most probably for SRAM backup battery)

Looks like your PCB on the left only makes a few voltages and the one on the right has switchmode converters for the remaining ones. Sorry for the shitty webcam pics, my good camera is currently burried in my shed.
 

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ballen

Titanium
Joined
Sep 25, 2011
Location
Garbsen, Germany
Hi Adrian,

My plan is to replace the existing with other power supplies, there's plenty of space within to do so.

I've fixed four different Heidenhain displays, dating in age from ~50 to 20 years old. All four of them had power supply problems, and all four power supplies were repairable with a handful of components costing less than 5 Euros. So if you have the skills, I would suggest simply repairing the power supply rather than replacing it.

Three of the power supplies were linear ones (tranformer, diode bridge, capacitor, 3-pin regulator) and one was switching. For the linear supplies the main culprit in a two cases was dried out thermal grease on 3-pin voltage regulators which cooked themselves to death. As I recall these were 7805 or 7905 or 15 in TO-220 and TO-3 cases. In the other cases the problem was ancient failed electrolytic capacitors. I think there might have also been a shorted or open diode in one of the bridge rectifiers.

Anyway, it took very little time and money to fix these things, and in my opinion that's a better solution than grafting in a new power supply. If you use modern low ESR high-temp electrolytics and modern thermal grease the DRO will probably function for at least another 50 years without problems.

Cheers,
Bruce
 

adrian

Cast Iron
Joined
Jan 11, 2005
Location
Kent, UK
Thank you for the replies.

I have no reason to think this power supply is non-standard, however it's very different to the one in your photo, but the colours match. It's probable just a different model may be older or newer.

Bruce, I would like to repair the existing power supply and replacements were just a temporary fix.

I hope to get the circuit details from Heidenhain as there are two PCB's involved with the power supply.
The 240V mains goes right hand board and then links to the linear power supply on the left, it then loops back to the first where it is sent to the main control PCB together with the 4.5V backup battery.

I am sure it's the one on the left of my photo because it has 240V going in and no DC on any of the four output wires, this would seem to indicate the fault is upstream of the voltage regulators.
There is a small bridge rectifier near the 240V transformer and that would be the first item to check.

Interestingly on the first PCB there is a black box shown below outlined in blue which I can't identify and was covered in a white powdery type dust. When I wiped off the dust there were no markings on it and I have no idea what it is.


Ciao Adrian.
 

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ballen

Titanium
Joined
Sep 25, 2011
Location
Garbsen, Germany
I think HH is unlikely to give you schematics or be of any help. But I'd be happy to be wrong about that.

Does the DRO take 100-240 V 50/60 Hz on a single input? In that case it is a switching supply. Or do you have to manually switch between 100-120V and 200-240V? In that case it is a classic linear supply.

Regarding the whitish black box, look at the other side of the circuit board underneath. How many terminals does it have and how are they arranged? It might be a potted transformer, in which case it would likely be connected to the blue/black leads that bring in power from the back of the chassis.

Start by tracing those blue/black input power leads.
 

adrian

Cast Iron
Joined
Jan 11, 2005
Location
Kent, UK
That's a good point Bruce, because the unit is auto switching and a second look does show it to be a switch mode supply.

This point did not register when I first looked at it. I assumed it was a standard linear supply being an old 1980's device (assumption being the mother of most F'ups )

I plan to take the two pcb's off next week and will investigate and as you say the chances of getting some information from Heidenhain is slim, but you never know I may catch someone on a good day.

As none of the dc outputs work so I think the fault is either the bridge rectifier, the MOSFET, or the chip driving it upstream of the high frequency transformer. Fingers crossed.

All the best Adrian.
 

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ballen

Titanium
Joined
Sep 25, 2011
Location
Garbsen, Germany
Adrian, did you check the fuse which is on the circuit board? Don't rely on appearances, pull it out and use an ohm meter or continuity tester.

Assuming that is OK, take a close look at the TO-220 components which are heatsinked with thermal grease. Have any of them gotten so hot that they burned the circuit board? Similarly, look at the electrolytic capacitors? Are any of them bulging? Be a detective. Look closely at the circuit board for signs of something that got too hot. Lifted traces, solder flux around a connection, heat marks on the circuit board, plastic that has shrunk and deformed from heat.

If there is nothing obvious, next step is to unsolder and test the electrolytic capacitors, ideally with an ESR meter, if not with a multimeter that has a capacitance tester.

Next step is identify the TO-220 devices. They are probably transistors, mosfets, triacs, or controlled switches. But let's get to that later.
 

adrian

Cast Iron
Joined
Jan 11, 2005
Location
Kent, UK
The fuse is ok and nothing obviously blown, but the heatsink compound does look very dry
Unfortunately my Fluke meter does not do capacitors.
But it's just as easy to replace when they are de-soldered.

More investigation required this week and will update the outcome.

Ciao Adrian
 

ballen

Titanium
Joined
Sep 25, 2011
Location
Garbsen, Germany
Adrian,

Be careful working on this. If connected to the line or for some time after disconnecting you will have about 330 VDC present after the initial rectification. That can kill you.

If I were working on this I would not start removing/replacing components quite yet. I would first use an oscilliscope to step through the different blocks of the circuit to try and isolate the problem area.

A switching supply works as follows. The input 230 VAC is rectified and filtered to get (approximately) 300 VDC. That is then passed through a MOSFET that switches it on and off at tens of kHz. Depending upon the type of switching supply this might modulate the pulse width. That chopped signal then goes through a small high-frequency transformer where the voltage is reduced and then the AC output is rectified and filtered.

So I would first identify the circuit that produces the DC near the input and see if there is indeed DC present. If so, then identify the switching element and see if there is a chopped signal at (say) 20 kHz present at the output. And so on.

With a scope and a probe it takes just a few minutes to do these checks. Then you have a better idea what to look for. But be careful, as I wrote above, 300 VDC can be deadly.

Cheers, Bruce
 

adrian

Cast Iron
Joined
Jan 11, 2005
Location
Kent, UK
I have to confess Bruce I don't like working on switch mode supplies because of the high voltages and this one is really tightly packed with components.

Unfortunately I don't have a scope only a multi meter so it's going to be trial and error with some patience finding the fault.

I did find out the black block that was covered in white powder on the first PCB is a potted transformer giving 24V. I think the function gives extra voltages required by various circuits, like CRT heater and on board memory devices.

Ciao Adrian.
 

ballen

Titanium
Joined
Sep 25, 2011
Location
Garbsen, Germany
I don't like working on switch mode supplies because of the high voltages and this one is really tightly packed with components.

Neither do I, for the same reasons.

Unfortunately I don't have a scope only a multi meter.

If you want to fix electronics, that's a fundamental tool, like a soldering iron or multimeter. These days many people are upgrading to inexpensive digital storage scopes; in the UK you ought to be able to buy a decent used 20 MHz scope for under 100 quid. IMO that would be a good investment for you. Contact your local ham radio club and ask if anyone has an old scope they want to get rid of. Or Ebay.

I did find out the black block that was covered in white powder on the first PCB is a potted transformer giving 24V. I think the function gives extra voltages required by various circuits, like CRT heater and on board memory devices.

If the output shows 24VAC, that's one item you can cross off your list.
 

adrian

Cast Iron
Joined
Jan 11, 2005
Location
Kent, UK
Doesn't seem to be the original HH power supply, at least mine (that I already repaired) looks different. All that matters are the voltages/QUOTE]

Hello DeadMahoDude, I don't think this power supply is made by HH. I suspect they buy in generic switch mode units and add the second PCB to make up the extra voltages they need. It is odd the PCB is not labeled with output voltages.

Thank you for the voltage chart, your colors match so I hope do the voltages. The faulty unit appears to supply 0V, +/-12V and +5V so if I can't fix the supply it will be replaced.

Many thanks Adrian.
 

ballen

Titanium
Joined
Sep 25, 2011
Location
Garbsen, Germany
Adrian,

The first step you can do with a multimeter.

The large 400V 100uF capacitor next to the central transformer is probably to filter the 300 VDC at the output of the input bridge rectifier, which you can see on the board next to the capacitor. So check if that bridge rectifier has 230VAC on the AC input terminals, and about 330 VDC on the output terminals. Also check the voltage across the large 400V 100uF capacitor.

Cheers,
Bruce
 

adrian

Cast Iron
Joined
Jan 11, 2005
Location
Kent, UK
Thank you Bruce for the advice, I took the PCB off this morning and had a good look and nothing obvious.

Applied 240V and started measuring from the input and everything checked out, got to the output and it's actually working. I re-assembled the Positip and it's working, I do hate intermittent faults. I will leave it running today and see what happens.

The voltages quoted by DeadMahoDude are slightly different to mine but very close.

Black 0V, Brown +12V, Yellow -12V, Red +5V, Grey -17V,
Pink +4.8V (backup battery), Green +15V and Green/Red +5V

Ciao Adrian.
 

ballen

Titanium
Joined
Sep 25, 2011
Location
Garbsen, Germany
Applied 240V and started measuring from the input and everything checked out, got to the output and it's actually working. I re-assembled the Positip and it's working.

Glad to hear it! It's very possible that the connectors had gotten oxidised, and removing and reconnecting them was all that was needed.

If it turns out to be intermittent, then during one of the phases when it is working, make a set of notes of the operating voltages at various key points. Then compare with when it's not working.

In any case it would probably be smart to renew the thermal grease (you said it was dry) and perhaps put five quid into replacing the electrolytic capacitors on the board. Everything else there has a lifetime measured in tens of decades of full-time operation.
 

adrian

Cast Iron
Joined
Jan 11, 2005
Location
Kent, UK
Thanks everyone for the help. I've documented all the power connections which are listed in my previous thread
so I know what to expect in the future.

Fingers crossed it's ok, I have had this unit for about twenty years and this is the first glitch.
I still prefer using it over later ND type displays I have because of it's large display simplicity.
Only down side is they do take up a lot of space.


Ciao Adrian.
 

adrian

Cast Iron
Joined
Jan 11, 2005
Location
Kent, UK
Yes a very nice device for testing capacitors. However on a unit that is over thirty years old, if you have to take the capacitors off the PCB to test, you might as well replace them, the cost of small circuit electrolytics are cheap.

I don't about other countries but in the UK it's difficult to find people who can fault find and repair at circuit board level. Most of the old school guys who used to fix things are gone and unfortunately not been replaced.

Ciao Adrian.
 

DeadMahoDude

Aluminum
Joined
Oct 30, 2016
In case it helps someone, I just botch-wired an old year 2000 vintage ATX PSU into a HH Positip 850 that I got for repairs.

That original piece of shit PSU wouldn't run reliably for longer periods. Quits with clicking sounds after a while.
Second PSU from trash pile Positip 850 does the same....
Sucks if you have already promised to return working display.

So I needed quick replacement. Output voltages are 5V, 12V and -12V, perfect fit for a trashpile ATX PSU :D

DSC_0594.jpgDSC_0593.jpgDSC_0595.jpg
 








 
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