HP RPG issues

[anewmanx]

The original post was me asking why a resistor had weird coloring. I was ignorant of the fact that it was an inductor. Many thanks to those who helped!

Now I'm just I'm trying to repair the rotary pulse generator from my HP 6034A power supply. Any advice is appreciated. All resistors are in spec. Both NPN tranistors have no leakage and have a beta of around 15 so they appear healthy. Turning the knob does cause a change in the voltage that is going to the base of each transistor, but it doesn't appear to be enough voltage to turn them on.

[TheSteve]

Any chance that is an inductor and not a resistor?

[paschulke2]

Most likely it is an inductor and not a resistor. If the "yellow" was gold and if you read the bands from right to left in your photo, it would read:

Code: [Select]

wide silver /      gold     / green / blue / silver /
"military"  / decimal point /   5   /  6   /  10%   / = 0.56µH 10%


[anewmanx]

+1 thank you all for the information.

I still haven't quite figured out why this thing isn't working. I replaced the incadescent bulb with a white LED with a series resistor and when I rotate the knob I can definitelly see a swing in the voltage that is going to the base of each transistor from 0-0.30 volts. It doesn't seem to me that the .3 volts is triggering the NPN transistors and opening them to ground as they should. I've manually grounded the logic pins on the IC that float around 1.5v to verify that output goes high and that appears to work correctly.

Odd. I guess I'll figure it out eventually. I'm wondering if it is that the phototransistors on this device behave differently with the light spectrum from the LED.

http://hparchive.com/Bench_Briefs/HP-Bench-Briefs-1987-04-06.pdf

That's the only schematic I could find for this thing.

[paschulke2]

At least now you know the proper value of the inductor: yellow, gold, violet (looks a lot like blue on your photo) = 4.7µH. This inductor is shown in the schematic.

I wouldn't be surprised if the phototransistors were sensitive in the (near) infrared. If this was true, a white LED would probably not work …

[anewmanx]

This is a very good point. I will have to try a different light source then. Do you think an ir emitter would work? Perhaps try a red led first?

[paschulke2]

I'd replace the bulb with a bulb (maybe T1, 5V, 80mA):

The small bulb inside the RPG had failed, this drives the photo-sensitive resistors. A small torch produced an output signal. I tried replacing the bulb with a LED but it needs a bright side-emitting output, so fitted a new bulb. Initially the RPG would only count up, not down. This was traced to only having Channel 1 output, not the 90° phase shifted Channel 2 output. This was due to the bulb being slightly mis-aligned, a small piece of blu-tack solved this! The RPG is now working correctly.

(quote from http://www.azurelectronics.com/Repair%20HP%201980B%20Oscillscope.htm )

[anewmanx]

Definitely the white led was the problem. I stuck the generator in front of an incandescent lamp I had in the house and it definitely responds with different resistance values.

I may still try an ir led or red led first. Putting another incandescent bulb is just perpetuating a design flaw, too unreliable.


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