JOBO CPP2 power board: Differences in found values vs Schematic

[Juharr]

Happy New Year All!

I'm rebuilding the power board of a JOBO CPP2 for a friend of mine. There is some oxidation, corrosion and some burned traces from it cooking in water that sat in the bottom of the case. I've been working my way through it repairing traces, reflowing solder and replacing any failed components. The circuit seems pretty straight forward but there are differences between the published schematic (version 2 board) and some of the actual component values on the board.

I was wondering if this was there result of a previous repair where someone grabbed off the shelf replacements, or if it was a production change (which there were probably many over the years). The question I have is are the differences simply based off of what was available off the shelf? Or is there an engineering reason for them? For example the primary filter cap on the high power DC side is shown as a 420mF electrolytic on the schematic but the burned one I pulled off the board is 2200mF ( I can speculate on reasons why one value would be better than the other ie: higher inrush current at start up with the larger cap which could stress the rectification components vs loading down the power supply when the motor is running for the smaller value). There are some other small changes like this that don't seem vital to operation but I thought it would be prudent to check in if anyone has experience with this particular circuit since the choice is to restore components to the published spec, or to replace things like for like with the values as found on the board.

Also any ideas about preventing this kind of damage in the future? Considering drilling a weep hole in the bottom of the case so water/chems don't accumulate.

Thoughts?

TIA

Justin Harrison

 

[ic-racer]

I'm not sure what happened to your unit, did it come from a community darkroom?

I got my Jobo CPP2 in 2001 and last time I performed routine maintinance, the electronics and PC board all looked fine.

I do drain the water bath after each use.

 

[Juharr]

Yeah it's been used by a darkroom/lab group. So lots of possible variables on how it got wet in the first place

 

[koraks]

Welcome aboard @Juharr!

It would help enormously if you could share the schematic you've found as well as some clear photos of both sides of the actual PCB, and specify which differences you see.

 

[Juharr]

Thanks! Link to service manual

Dropbox

www.dropbox.com

link to some pics

Dropbox

www.dropbox.com

I list the discrepancies in a few. C1 on the power board is one of the changes. It's a version 2 board per the manual.

 

[Juharr]

Thanks! Link to service manual

Dropbox

www.dropbox.com

link to some pics

Dropbox

www.dropbox.com

I list the discrepancies in a few. C1 on the power board is one of the changes. It's a version 2 board per the manual.

Other changes on the board:
D1, spec'd as an 1n4001, was replaced with a jumper. On the schematic it looks like it's functioning as polarity protection. Someone decided it wasn't needed and they could save the 3cents?
R4 is a 10k high wattage resister instead of the spec'd R2||R3 42k resistors that would give a value of ~21k. R4 is 'Rs' for pin 5 of the CA3079, the data sheet for the chip indicates a 10k 2 watt resistor for 120VAC input and....
Ooooo.... just answered my own question there , the board was probably printed to accommodate 220VAC and 120VAC mains with component changes. So 10k is correct for 120VAC mains, at least by the data sheet. So that problem is solved. Thanks for the query, it made me think

 

[koraks]

Thanks; here's the power supply schematic:

D1, spec'd as an 1n4001, was replaced with a jumper. On the schematic it looks like it's functioning as polarity protection. Someone decided it wasn't needed and they could save the 3cents?

I think D1 probably blew and someone jumpered it. This is NOT a good idea; D1 protects the rest of the circuit from the back-EMF of the motor coil, which can cause a momentary polarity reversal on the power supply rail. While damage as a result of this is not very likely (mostly due to clamping diodes in any IC's on the same power rails), it would be a nightmare to troubleshoot the problem if it would occur. You'd keep swapping out IC's that would work for a while and then give up the ghost for seemingly no reason...

R4 is a 10k high wattage resister instead of the spec'd R2||R3 42k resistors that would give a value of ~21k. R4 is 'Rs' for pin 5 of the CA3079, the data sheet for the chip indicates a 10k 2 watt resistor for 120VAC input and....
Ooooo.... just answered my own question there , the board was probably printed to accommodate 220VAC and 120VAC mains with component changes. So 10k is correct for 120VAC mains, at least by the data sheet. So that problem is solved.

Yep, that sounds good, although it's a little odd that the transformer is indicated to be for a 110V primary winding. Given the different typeface on the lettering, I assume the '110V' was overlaid on the original schematic and they forgot to change the value for R4.

 

[Juharr]

Thanks for the feedback, it's always good to have a second set of eyes on things .

The rectifier for the 24VDC supply has also been previously replaced (judging from the way it's crammed in there and pics of the board I found on the net).

The board is dated 12/1995 so I'm guessing that sometime in the past 30 years the 24VDC rail failed taking the rectifier, C1, D1, and probably T1 (Q1 lol) with it.

The 470mF cap was replaced with the 2200mF during the repair either because it's what someone had, or there was a service bulletin from JOBO that recommended the mod. Why they replaced D1 with a jumper is odd but maybe it was a different 'repair'. I agree it seems like it should be there.

The 10k power resistor for R4 tracks with the data sheet for using the CA3079 with a 120VAC supply. So all is right with the world again.... Or at least this small part of it.

Thanks again.