Need help on installing the temperature knob on Jobo CPA2

Hi,

I recently bought a CPA2 and encounterred some problems that may need you help.

Problem-1:
The temperature in water bath is not the same as what I set by turning the temperature knob. They have about 3℃ difference. E.g. if I set tempeture knob as 35℃, then the actual water bath tempeature is about 38℃.

Problem-2:
The temperature indicator doesn't light up.

After watching some related CPA2 operation videos on Youtube, I found that it seems my temperature knob was installed incorrectly, then I pulled off the knob and found it has a rusty meta around the core of the knob, then it seems the rust causes the indicator not lighting up correctly. see the photo below.

And my questions are:

1. Is the gap between the head and tail of the metal piece supposed to be there?
2. May I know how to install this knob properly so that I won't have temperature shift?

Thanks!

-) At least in theory the tension within the shaft could be adjusted by changing the angular position of that gap in the clamp. N
Now it is sert at lowest tension.

-) I would not bother with the original knob but substitute with a decent knob with clamping shaft, that can be screwed tight. Just buy such knob designed for that diameter of the potentiometer axle.

The OP should call the Darkroom Doctor https://darkroomdoctor.com/ or email him and set up an appointment to drop by his place.

AgX said:

-) At least in theory the tension within the shaft could be adjusted by changing the angular position of that gap in the clamp. N
Now it is sert at lowest tension.

-) I would not bother with the original knob but substitute with a decent knob with clamping shaft, that can be screwed tight. Just buy such knob designed for that diameter of the potentiometer axle.

Click to expand...

Thanks for your reply!

I am stupied in this area, so have some more dumb questions.
You mentioned "by changing the angular position of that gap in the clamp", does this mean I can increase the tension by reduce the gap between the head and the tail of the metail piece?

If you break that knob, finding a replacement will be very difficult. I'd put it back.

Taken directly from CPA-2 user manual:

...which is why the CPA-2 comes with the Jobo color thermometer...to set temp based upon measurement, not control position!
...the light no coming on is likely a burned out neon indicator.

I checked the manual, but my indicator lamp won't light in either heating or maintaining stage.

And I know there might be a gap between the temperature set on the knob and the actual water bath temperature. But just as mentioned in the manual: "it is not unusual for the dial itself to be off by ± 2.0º C"

Indicator lamp is Jobo part 26003, no description unfortunately is provided by Jobo

This place might have one! https://bowanabobsdarkroom.webs.com/jobomanualprocparts.htm

Given that Jobo sealed things for protection from water entry, I have no idea how you replace the neon indicator!

Jobo parts and service
http://www.jobo-usa.com/spare-parts-and-service

wiltw said:

Indicator lamp is Jobo part 26003, no description unfortunately is provided by Jobo

This place might have one! https://bowanabobsdarkroom.webs.com/jobomanualprocparts.htm

Given that Jobo sealed things for protection from water entry, I have no idea how you replace the neon indicator!

Jobo parts and service
http://www.jobo-usa.com/spare-parts-and-service

Click to expand...

Thanks! will take a look at these two websites.
Replacing the neon indicator seems too hard for me, so I think I'll just leave it there.

And I just reinstalled the knob back, there still is a 3℃ diff between the knob and the water bath....

yexiaoxiao2102 said:

And I just reinstalled the knob back, there still is a 3℃ diff between the knob and the water bath....

Click to expand...

My CPE2+ temperature knob wasn't too accurate either, and I had to determine correct temperature with a thermometer. And since I nearly always used the same temperature, I made a tiny mark on the dial indicating the correct setting. I no longer own it, but the indicator light is neon, likely a standard NE2 bulb will work but soldering is probably required.

yexiaoxiao2102 said:

You mentioned "by changing the angular position of that gap in the clamp", does this mean I can increase the tension by reduce the gap between the head and the tail of the metail piece?

Click to expand...

No, the gap within that clamp will remain the same, but by twisting the clamp by 90° you might heighten the tension inside the shaft.
But mounting instead a knob as I described is the better solutuion as you not only can tighten it stronger, but also mount and demount it without any axial force on the potentiometer and thus on the circuit board.

4season said:

My CPE2+ temperature knob wasn't too accurate either, and I had to determine correct temperature with a thermometer. And since I nearly always used the same temperature, I made a tiny mark on the dial indicating the correct setting. I no longer own it, but the indicator light is neon, likely a standard NE2 bulb will work but soldering is probably required.

Click to expand...

I did the same thing. I have two marks in the knob, one for 35° C (RA4) and another for 38° C (C41, E6).

yexiaoxiao2102 said:

Hi,

I recently bought a CPA2 and encounterred some problems that may need you help.

Problem-1:
The temperature in water bath is not the same as what I set by turning the temperature knob. They have about 3℃ difference. E.g. if I set tempeture knob as 35℃, then the actual water bath tempeature is about 38℃.

Problem-2:
The temperature indicator doesn't light up.

After watching some related CPA2 operation videos on Youtube, I found that it seems my temperature knob was installed incorrectly, then I pulled off the knob and found it has a rusty meta around the core of the knob, then it seems the rust causes the indicator not lighting up correctly. see the photo below.
View attachment 278381
And my questions are:

1. Is the gap between the head and tail of the metal piece supposed to be there?
2. May I know how to install this knob properly so that I won't have temperature shift?

Thanks!

Click to expand...

The temperature control is not a precise measure, merely a guide. There are so many variables such as the surfaces outside the water bath which allow the water to cool relatively quickly. The actual ambient temperature of the room where the processor is will to a great extent affect this one way or another. I worked this out a long time ago and I have calibrated the setting over several hours by setting the dial at just below the working temp and increasing or decreasing it by small increments until I can get a stable 40C. I say 40C although the working temp of the chemicals is usually 38C, this allows for fluctuation during the process when the chemical containers are poured in and then emptied. Because it is not a precise instrument, the best results come from experience, certainly not blind following of the settings. I have been using one since 1991 and have found this to be the most consistent.

The rust around the inside of the knob and mounting post will not affect the operation of the thermostat nor the light. Because the equipment may have had an uncertain history the rust is almost certainly caused by splashes of water or condensation remaining after use. You have to expect this. I think you are worrying over what is a minor problem when you expect it to work perfectly the same as when it was manufactured.

The image of the underside of the knob showing the spring clip with the groove, this is perfectly normal. The spring allows for expansion of the knob mounting post and also to remain secure. If it were a solid ring the expansion could not occur and the knob may become loose or it could not be easily removed for cleaning. (Which it does not seem to have been done.

Tip Clean it and before replacing, smear the metal clip with something like Vaseline and the rust will not re-form. Don't try to remove the clip you will risk breaking the mounting post.

If the light does not illuminate then that is probably a device fail either due to a broken wire or a bulb fail. It can be repaired by removing the cover and replacing the light with a LED cutting the wire to the bulb and soldering on the led. It is no a difficult job

No one has asked this, but what is the temperature measured inside the solution bottles? I really don't care what the water bath temp is when I use my Jobo, I only care what the solution temperature is. THAT'S what is important.

Yes what's important is bottle temp not bath. Those knobs are only estimates of temperature. I make a mark with a crayon or lab wax pencil. I have a new CPP3, even that needs time to equilibrate. You need the bottles in the bath. Let it sit for an hour or two. Then measure the bottle temp. The Jobo color thermometer is one of the best I've ever used. Get one from Catlabs

The shaft of the temperature control potentiometer has a flat side, the knob is moulded to fit and the metal spring keeps the tension on the shaft.

Don't try twisting the knob while fitted to the shaft, otherwise you'll break the knob attachment, regardless of the spring.

Try and align the shaft flat and knob before pressing it down gently. Once it's on, you should be able to raise or lower the temp of the water in the bath.

The indicator light should come on when the temp request is higher than the actual water temp. The indicator is a simple neon lamp, so if it's not coming on, then the heating element may have an open circuit.

Mike