JOBO CPP-2 refurbishment, need input from experts

[John Brownlow]

OK, this is really a question for anyone who has full refurbed a CPP-2 or might have one on a bench where they can measure some voltages. For the rest of you this is probably going to read like gobbledygook.

I have an old CPP-2 which is mostly working, except that the temperature display is '---'. (It was working until very recently).

Normally this would be a sign of a failed temperature probe but I don't believe it is in this case (I might be wrong!) as all the voltages seem to read within the specs on the schematic shown in the online service manual. The probe shows a resistance of about 800ohms at 15C.

The issue appears to be with IC45. I am getting around 100-150mV on the input to pin 11 which is within the 0-0.5V limits on the schematic, with pin 10 at 0. However the chip is putting out a signal of negative signal out of range.

The one weird thing on my board is that pins 8 and 9, the zero offset, which are both linked to V+ by identical resistors, are at 4.9V and 4.7V respectively, ie pin 8 appears to be shorted to V+. However there's no sign of a short on the board and all the resistances measure correctly.

So my questions to anyone who might be able to answer them...

-- what would be a sensible voltage across pins 10 and 11 of the IC? From the datasheet it looks like -99mV to 999mV.
-- what should pins 8 and 9 be reading at?
-- what would be a sensible resistance for the temp. probe?

My working assumption is that IC45 has failed but I'm no electrical engineer. I attach a schematic with my measured voltages. All help gratefully received, and no, I'm not in the market for a CPP-3, thanks.

I should add that the outputs appear to be working fine, as do the LEDs.

 

[OrientPoint]

Hi,

IC45 is the driver for the display. Pins 8 and 9 are the zero adjustment. You can put a pot on those pins, tied to +5v and use it to adjust zero. You wouldn't be reading voltages off of 8 & 9. I'm not sure what the expected voltage drop across high and low would be on this unit, nor what the resistance of the probe is, but IC45 should be seeing a voltage between 0 and 999mv on the inputs. I have a functioning CPP-2, but won't have time to crack it open and check the actual values for a few days.

"---" is what the chip should put out to the display when there's a negative over-range (< 99mv). Or the IC might just be fried. You can find these chips for ~$7 online if you want to attempt a replacement

A more recent data sheet (only 24 years old!) is attached here, in case you don't already have it.

 

[John Brownlow]

Thanks. I'm trying to figure out whether the chip is fried. They don't use a pot over those pins because the zero is adjusted elsewhere (before it hits the input), but it's functionally equivalent to a 44k pot centred, which is why the voltage difference bothers me.

The fact that there's no voltage drop over one of the resistors and the pin is tied to V+ made me think it was shorted out on the board somewhere, but there's no short across that resistor or anywhere else that I can find. I'm getting 150mv across 10/11 so that's in spec. It doesn't matter how I adjust the gain pot... it always produces and 'under' display.

I have a replacement chip heading to me but I'm trying to do a sanity check!

Thanks for the data sheet... that's the one I've been using.

 

[OrientPoint]

My bet would be a short somewhere, but it can be appealing to just plunk the chip in and if it works move on.

One question: does this unit actually maintain proper temperature?

 

[John Brownlow]

Not right now. I just bought it from a guy who found it in a warehouse. When it arrive it all powered up and the temperature displayed correctly but the heater didn't work, although the temperature board was signalling properly. I started to pull the boards to investigate, which was when this new problem emerged. Now when I turn it on, after 30s the cold water solenoid clicks, nothing else happens.

 

[John Brownlow]

IC45 also supplies the input to the comparators that compare it to the settings on the temperature wheels. Per the display it's reading HLHL which is binary for 10, so it looks high compared to whatever the knobs are set to. So that would fire the cold water solenoid to try to bring it down I guess.

 

[mshchem]

You do know that the display turns off in the dark. There's a little photo eye that cuts off the LEDS to not fog film

 

[AgX]

My bet would be a short somewhere, but it can be appealing to just plunk the chip in and if it works move on.

What do you mean by this? Just taking out the chip will not make the processor work properly. Do you mean "to substitute"?

 

[John Brownlow]

You do know that the display turns off in the dark. There's a little photo eye that cuts off the LEDS to not fog film

Yes, of course. The problem is not that the display is blank but that it displays '---;

 

[John Brownlow]

What do you mean by this? Just taking out the chip will not make the processor work properly. Do you mean "to substitute"?

He means remove the old one and replace it with a new one.

 

[OrientPoint]

Plunking = substitute. Sorry to use archaic slang.

 

[Sirius Glass]

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