Fixing a laboratory waterbath

[eccerr0r]

Re: Fixing a laboratory waterbath

At this point you can only make a guess... Hope that they are in adjacent pairs and in the sequence the voltages are listed on the transformer. Likely the 8V winding is connected to the fuse and bridge on board to the 5VDC output (but no guarantees, just likely). Where the other wires go to will determine which one is the 15 and 20V, but likely the center ones are 20, and the other to the side is 15V.

Again this is custom so it's only an educated guess without more information.

I'm still disappointed you damaged the transformer while removing it from the board. It would have been really funny to get it working by using a single 120-240V step-up transformer and using the 240VAC winding on that transformer if it really is... Not the most efficient way but you'd only need one transformer that you could swipe from one of those travel adapters...

[vrasp]

Re: Fixing a laboratory waterbath

Thanks for your input! If I were to get three transformers with those voltages. Could I figure out the proper voltages required on each line but wiring them and checking if it works properly? Like if I wire the 8v on the 15v then i ll see it it works and if it doesnt then i try 15v. As long as I dont put a high voltage with a low one I wont fry anything right? What do you think?

[eccerr0r]

Re: Fixing a laboratory waterbath

You *may* be able to depending on where the circuit is connected. But putting 20V on the input that wants 8V may required in more snap crackle pop crispies....

Even the 20V on 15V there's no guarantee it will survive, you'll have to trace where it goes. That 5V difference could fry things... Also you may get nondeterministic behavior if the voltage is not quite high enough. Best to look carefully where they go.

[vrasp]

Re: Fixing a laboratory waterbath

I see. I guess the easiest way to find out right now is by teying to reach those output pins on the transformer. When I measure the resistance, how do I find out the voltage from that?

[petehall347]

Re: Fixing a laboratory waterbath

first check is if they all connect to each other or not . if all in pairs it makes it easier to do a bodge repair . if you can measure resistances you could power it up and see which voltages are which .

[eccerr0r]

Re: Fixing a laboratory waterbath

Based on the board layout an educated guess is that they are isolated from each other, but which voltage to what is the trouble. Since the pins on the transformer is damaged it would be hard to collect data at this point.

But you still know where each wire goes... this is still critical information that's lacking by as far as I know, all who's reading these posts...

[vrasp]

Re: Fixing a laboratory waterbath

I was able to dig through the resin, found what looked like copper, and shoved some quick braid wire in there to make it easier to measure. The resistance between primary and secondary is infinite and between pins of the secondary is 0. There is continuity between the seconday pins. Is that normal?

What can I do to guess the voltages? Also, there were 6 pins that originally came out of the secondary of the transformer. As you can see I only found 4 "holes". What do you guys think I should do now?
https://ibb.co/HTF08G0

EDIT: Would picture of the board this PCB connects to help?

[vrasp]

Re: Fixing a laboratory waterbath

Based on the board layout an educated guess is that they are isolated from each other, but which voltage to what is the trouble. Since the pins on the transformer is damaged it would be hard to collect data at this point.

But you still know where each wire goes... this is still critical information that's lacking by as far as I know, all who's reading these posts...

Here are pictures of the board that this PCB is connected to.

First pic is the PCB. I added it here to make it easier
https://ibb.co/1MGnLvk

This is the board below it. On the right you see 8 holes in which the 8 pins of the above PCB go into.
https://ibb.co/SVxqk04

This is the other side of the board above.
https://ibb.co/n3KXN5h

Hopefully this is clear. Let me know if you need anything else.

Thank you!

[eccerr0r]

Re: Fixing a laboratory waterbath

You may have lucked out, if there's nothing else connected to those 8 pins, one set of the 15 or 20V isn't even connected to anything. The other set is rectified and brought through a regulator and thus, to an extent, doesn't matter what voltage it is.

That regulator would be interesting to know - what's the part number of the TO-220 (metal+plastic) regulator next to the bridge rectifier in the corner? It's the 3-legged device next to the 4-pin DIP which is the bridge - both of these are in the corner farthest from the display board connector on the blue green board (that has the grounding)?

If it's 7812 or LM340-12 then 15V would do, else if it's 7815/7818 or LM340-15/LM340-18 you will need 20V there.

BTW they need to be isolated and not a multitap.

[vrasp]

Re: Fixing a laboratory waterbath

You may have lucked out, if there's nothing else connected to those 8 pins, one set of the 15 or 20V isn't even connected to anything. The other set is rectified and brought through a regulator and thus, to an extent, doesn't matter what voltage it is.

That regulator would be interesting to know - what's the part number of the TO-220 (metal+plastic) regulator next to the bridge rectifier in the corner? It's the 3-legged device next to the 4-pin DIP which is the bridge - both of these are in the corner farthest from the display board connector on the blue green board (that has the grounding)?

If it's 7812 or LM340-12 then 15V would do, else if it's 7815/7818 or LM340-15/LM340-18 you will need 20V there.

BTW they need to be isolated and not a multitap.

It says MC7812. Are you telling me all I need is a 15v transformer and that's it? That would be awesome.

[vrasp]

Re: Fixing a laboratory waterbath

This is it. https://ibb.co/TwcgqYm

[eccerr0r]

Re: Fixing a laboratory waterbath

It looks like 15V should do for the "middle" AC output and the 8VAC for the 5VDC circuit Not sure what's up with the other output (the "green" pair on wFziNau.png), just noticed it goes to the display board directly... what it does with it is now the question.

btw all pictures posted so far here...

[vrasp]

Re: Fixing a laboratory waterbath

Thanks for putting all the pictures together. Very helpful!

[vrasp]

Re: Fixing a laboratory waterbath

It looks like 15V should do for the "middle" AC output and the 8VAC for the 5VDC circuit Not sure what's up with the other output (the "green" pair on wFziNau.png), just noticed it goes to the display board directly... what it does with it is now the question.

btw all pictures posted so far here...

Do you want a picture of the display board as well?

I ve never bought a transformer before. It seems there aren't many 15v or 8v ones. I see lots of 12v and 24v. Do you have a website to recommend? How do I avoid buying a low quality one? Does the amperage matter?

Thank you!

[eccerr0r]

Re: Fixing a laboratory waterbath

To confirm that other voltage, yes the other board pictures would be nice.
Ideally you want the voltages to match. Theoretically you can go 'up' a little and it would be fine but it would stress the regulators. If you attach heatsinks to them it may help with the higher voltages.

You should be able to get 16V transformers which would be OK for 15V. A center tapped 16 would get you 8V. 9VAC would be fine for 8V too. But don't use the same 16V transformer for both the 15 and 8V, you need two separate transformers, and then the last 20v winding, still needs investigation.

The thing I'm not sure about is that the display usually requires some current to keep the segments lit, and I've used 7805's before. Using a 7805 with 4 LED 7-seg displays at car voltages, it gets quite toasty with no heatsink and expect similar behavior with similar circuitry.

In any case that original transformer doesn't seem all that large (measure it?) and 1A per winding may be sufficient. I think the 15V winding may be okay to go less than 1A. The remaining winding ... remains to be determined.

What I'm wondering is why did the transformer primary winding blow, still seems kind of odd to me...

[vrasp]

Re: Fixing a laboratory waterbath

I see. I d like to help figure it out but this out of my league. I ve emailed the manufacturer and asked if they have the specs of the transformer or some kind of schematic.

This is the display board.

https://ibb.co/5BXFDzg

https://ibb.co/r7qptKM

[eccerr0r]

Re: Fixing a laboratory waterbath

btw redwire, again we find crossposting on different sites, do you happen to monitor reddit and see this guy's posts there or did you happen to search and find it?

I don't monitor reddit...

[eccerr0r]

Re: Fixing a laboratory waterbath

I see. I d like to help figure it out but this out of my league. I ve emailed the manufacturer and asked if they have the specs of the transformer or some kind of schematic.

This is the display board.

https://ibb.co/5BXFDzg

https://ibb.co/r7qptKM

Eeek. Looks like another board layered on top of the board. Probably a 7107 hidden underneath, but what I don't get is why this board needs 20V... hmm...

wasting more of Topcat's hard disk space here... hate it when resources go poof...

[vrasp]

Re: Fixing a laboratory waterbath

The display board is made of 2 boards. This is it.

https://ibb.co/yhhkXB9
https://ibb.co/VYw70L4

[vrasp]

Re: Fixing a laboratory waterbath

I dont know if this helps, but the green/blue board is also connected to these things on the image.

https://ibb.co/sv7pSH0