K8040 Zener Diode overheating

I assembled the kit, and everything appears to look exactly like it should. I’ve checked over the diagram several times. The 20v zener diode in the power supply overheats. What should i do to figure out why this is happening (the diode is oriented correctly)? Everything appears to function other than the diode overheating. There are no shorts with the soldering job. What could cause this to happen? What tests should i do?

How do you know it is overheating ? It is normal for the diode to get quite hot. If the +35V rail measures correct and if the 7805 positive +5V regulator does not heat too much and if the 5V is present and everything works fine, then there is no excess current flowing trough the zener, so everything should be OK. Please check.

Thank you for the reply. I checked the voltages, one voltage was 35.6 and the other was right around 5 like it was supposed to be. To illustrate how hot the diode gets, it melted the solder when i was testing it. The voltage between the regulator and the diode was 10.5. (however i’m not sure if thats significant or not) What else could be at fault? Again, thanks a million.

Apparantly, the +5V circuit is drawing too much current. Cut the zener and check with an A-meter. Idle: +/- 15mA, ON: +/- 26mA
Next, check all +5V powered circuits for wrong resistor values, soldering errors etc…

i checked the current on it, and its obviously drawing way too much like you suggested (92ma and 102ma). I’ve been having trouble locating the issue. I checked my solder job for shorts and i made sure components were all proper polarity, resistors correct values. i’m really not sure what to do at this point.

Does the current drop if you remove either the µcontroller or the op-amp ? This might allow you to better locate the location of the problem.

Is it possible to send the amp to velleman? What is the cost? I havent had the time to look at it in days (and i just want to use it).

Please return it to:

Velleman Kit Tech Dept.
Legen Heirweg 33
9890 Gavere

Sorry, no pricing. Depends on shipping, nature of the fault etc…
Certainly a lot less than the price of the kit…

It would be a lot of money to ship, i just calculated it out.
I’ll have to figure it out. I’m fairly certain a part is faulty because my other amp is working fine. Just pull out the ic and see if it still draws that much current? (and you mean the 12C508A right?). Then do the same for the 7293?
Thanks Again

Exactly. However, we assume that you mean LM258 instead of 7293…

What would happen if pin 8 of the 12C508A was not seated properly? Pin 8 is connected to ground. I bent the pin when i put it in. I corrected it, but the circuit is still drawing too much current. It could still be another problem, but i would imagine i probably damaged something by not having that connected to ground.

If the amp is working properly, as you stated, the ground connection of the PIC was not the problem…

i pulled out both the 258 and the 508 and it still draws too much current.

i think i may have finally located the problem. i measured resistance btween the cathode side of the backlight and ground, and to my surprise it was only one ohm. i’m stumped, r6 is the correct value… any ideas? when i removed the led it doesnt seem to overheat anymore.
Thanks for your patience!

I finally solved the problem. I’m certain there was a short on the resistor 6. I couldnt even see the short with a magnifying glass, but i narrowed it down to that. I cleaned all the possible connections up and it worked out. Thanks for the help.
Problem Solved.

Happy to read you’ve managed to solve the problem. Once again, a soldering error was at the cause of troubles.