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Old 29th Oct 2014, 11:01 am   #1
mimmo2010
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Default Oscilloscope HP181T

Hi everyone, I'm struggling with a difficult repair an oscilloscope HP181T, the Party Supply + 100 Volt, does not work well out 89 volts and the transistor A8Q1 heats up a lot and it breaks. I also discovered 'another failure there a lamp shown on the diagram A8V1, 83 volts, it also broken, Replaceable Parts "Electron tube: Voltage 83.0 V +/- 1.0 V Part Number Z83R4A.In this case how can I fix it? Help me.
greetings Mimmo
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Old 29th Oct 2014, 3:02 pm   #2
orbanp1
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Default Re: oscilloscope HP181T

Hi Mimmo,

From the schematics, and from the picture of the board, it looks that A8V1 is a neon lamp, it gives the 83V reference voltage. You need that reference voltage operational to get the correct +100V output.
If you do not find such a replacement, you can use a Zener diode, like an 1N4762A, a 82V 1.3W Zener diode. The current through that stabilizer lamp is about 3mA, and that matches well the operating current of a 82V 1.3W Zener diode.
The exact voltage of the Zener is not important, you can adjust the 100V with the R10 trimmer pot on the board.
The other thing is that if that Q1 pass transistor is warming up, even at 89V output, there must be some short on that line. Find the short! Remove the loads, start with the plug-in modules.
You can also measure the resistance of that line to the ground when it is not powered up.
There is a 0.5A fuse in that line, so the correct load could only be a couple of hundred miliamps. At 100V that means about 300 Ohm to 1 KOhm load (330mA to 100mA).
Probably some filter capacitor is shorted on that line.

Good luck, Peter

Last edited by orbanp1; 29th Oct 2014 at 3:15 pm.
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Old 29th Oct 2014, 9:44 pm   #3
mimmo2010
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Default Re: oscilloscope HP181T

Great, thank you very much Peter.
ok you know how it goes.
regards
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Old 29th Oct 2014, 11:31 pm   #4
WME_bill
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Default Re: oscilloscope HP181T

Neon stabiliser, in HP180 series main frame power supplies. These were also used by Tek, Solartron, Philips around the 1970s. Usually the Z83R4, or V83R4 made by GI, Signetics/Hivac. (Or Mullard ZZ1000 , 8228 or ZA1004). All around 83-84V or 103v, at around 1m/a with a remarkably low temperature coefficient, much better than a similar voltage zener diode. These neon stabilisers are often the first to fail from old age in such power supplies.
All unobtainable now. Most neon indicator lamps are around 60v at 1ma would be useable, but in this circuit, well beyond the range of the adjustment trimmer.
In 1982, Philips published the attached modification for the PM3262 family as a replacement for the V103R2 stabiliser neon, 103V at 0.8ma, using two transistors and a low temperature coefficient zener. I have used this in a HP180, after changing the ratio of the two resistors to give 83V instead of 103V. Much the simplest to do, easily available parts, and with a low temp coefficient.
Later versions of the power supplies for this family of oscilloscopes used a different circuit with a 7V zener diode, and better overload protection than just blowing the fuse on the HP180A series (HP180C & D). wme_bill
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Old 30th Oct 2014, 11:26 pm   #5
mimmo2010
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Default Re: Oscilloscope HP181T

Hello, WME_bill very very good, as you put resistors for 83 volts?
Thank you very much
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Old 31st Oct 2014, 1:03 pm   #6
orbanp1
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Default Re: Oscilloscope HP181T

Hi Mimmo,

Here is the Philips circuit for 83V with a 6.2V Zener.
Note that the transistors used need to be higher voltage than the average, minimum 100V.
Also, simulation shows that the dynamic resistance of this circuit is about 640 Ohm, while the 82V Zener's is only 200 Ohm.
This circuit was a solution when higher voltage Zener diodes were not yet widely available.

Peter
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