Maker Pro
Maker Pro

PCB needs repairs

CocaCola

Apr 7, 2012
3,635
Joined
Apr 7, 2012
Messages
3,635
No remove all the ICs from the board, not just the 555 and see if you have 5V coming out of the regulator... This test as I stated will help narrow down the short that is killing the 7805s output... If it's not an IC shorting directly (or enabling a short) this will narrow it down to one of the components that goes from the 5V rail to GND... If when the ICs are removed you have a 5V rail then you can replace them one at a time and see what one creates the short... If this is the case you can focus on parts that are 'connected' when that IC is put in circuit...
 

partyanimallighting

Oct 22, 2012
330
Joined
Oct 22, 2012
Messages
330
That's what I did with the above results. All the IC's were removed and the pcb was tested with the 555 in, then retested with the 555 out and the other IC's remained out of the board. And I'm not getting 5V anywhere near the 7805. I'm getting 18V up to one of the large caps but not on the other. But I'm getting 18V on both on the working pcb.
 

CocaCola

Apr 7, 2012
3,635
Joined
Apr 7, 2012
Messages
3,635
From your post I assumed you were only removing the 555, not that all the others were out as well...

No need to replace any of the ICs (including the 555) at this time until you get 5v out of the 7805 so you can put them away in storage right now and leave the sockets unpopulated...

Finding the fault killing the 7805 is the concern now...

Next I would personally remove the relay and test the 7805 again... Also double check the polarity of the 7805 and make sure you have +18 to the right pin and verify that you have ground to the center pin...

From that point with all the ICs removed and the relay removed you only have a handful of caps and resistors that could be shorting out and would need testing... For a poor mans test, remove the parts one at a time, test and replace... When the 7805 comes to life, the removed part is likely the culprit...

You could also check continuity (board off) from ground to each side of each resistor and cap, in most cases only one side should be common, two sides common and a likely flag is tossed up...

Also of concern might be a hidden solder short under a socket, but if this was working at one time and you have not touched up any of those points this is unlikely...
 

KrisBlueNZ

Sadly passed away in 2015
Nov 28, 2011
8,393
Joined
Nov 28, 2011
Messages
8,393
Good advice there CocaCola.

It's very odd that the 5V rail is slightly negative. If there was a short on the 5V rail, I would expect exactly 0V or a slight positive voltage on the left pin of the 7805 relative to the centre pin, and I would expect the 7805 to be getting hot. I suggest you replace the 7805 again with a new one.

When you power up the board with the new 7805 in it, check to see if the new 7805 is getting hot, and as soon as you feel any heat on it, remove the power. I guess it's possible there is a fault that could be damaging the 7805.

Don't worry about the second big electrolytic capacitor not having 18V on it. The first problem, as CocaCola said, is the lack of a 5V rail.

If there is a short, it would most likely be one of those two mustard-coloured decoupling capacitors I mentioned earlier.

Also check for continuity between the 7805 tab and pin 1 of the 555. These are both on the 0V rail of the circuit and you should measure continuity, or 0 ohms on resistance range, between them.
 

CocaCola

Apr 7, 2012
3,635
Joined
Apr 7, 2012
Messages
3,635
Don't worry about the second big electrolytic capacitor not having 18V on it.

Is this the one that from the bottom board picture appears to have had the solder touched up? If so there might be a solder short under that cap...

Anyway as Kris said the focus now if you have 18V at the 7805 is finding out why you don't have 5V coming out...
 

partyanimallighting

Oct 22, 2012
330
Joined
Oct 22, 2012
Messages
330
OK. I removed the relay and posted pix so that the traces are visible:

http://www.4shared.com/photo/hbPdQEG_/Follow_Spot_250_040.html?

Cocacola, I mentioned before I'm not getting 18V or 5V at the 7805 so the pcb is non-functional, same as before. I am getting 18V at one cap and 0V at the other but Kris says this is unimportant. You guys are saying the issue lies around the 7805 and it's lack of 18V in or 5V out and the important thing is to get this device powered up. I'll make all the checks you recommend and post results in a while.
 

KrisBlueNZ

Sadly passed away in 2015
Nov 28, 2011
8,393
Joined
Nov 28, 2011
Messages
8,393
Yes, the problem is that there's no 5V coming out of the 7805. If there is 18V going into the 7805, this means that either the 7805 is faulty/damaged, or its output is shorted out, i.e. there's a short across the 5V rail.

Also in an earlier post, you said that the voltage on the 18V leg of the 7805 was less than 1V. Are you measuring with your multimeter black lead connected to the board's ground rail? If there is no voltage (or less than a few volts) on BOTH end legs of the 7805, measured with the multimeter's black lead connected to board ground which is the 7805's metal tab, then there is no voltage going IN to the 7805, which is a different fault.

The connection from the left electrolytic to the input of the 7805 is on the top side of the board. Is it possible that you've pulled out the through-hole plating in the positive hole for the electrolytic? In that case there would be no voltage going into the 7805. If the through-hole is missing, this would explain your symptoms (which are hard to explain any other way).

Check for continuity between the positive terminal of the electrolytic nearest the relay, and the 7805 lead that's closest to the micro. There should be continuity. If not, you may have broken the through-hole plating.

To check this, take out the electrolytic nearest the relay (the one with the nasty marks on the underside in your photos) and suck the solder out of the holes. Have a good look at the hole for the positive lead on both sides. There should be a metallic connection like a collar inside the hole that connects the track on the underside to the track on the top side. If it's missing, you'll have to repair it. Let us know if that's the case, and I or someone else will tell you how to fix it.
 
Last edited:

partyanimallighting

Oct 22, 2012
330
Joined
Oct 22, 2012
Messages
330
Hi Kris, I really believe the problem lies with no power going into the 7805 and not with the 7805 itself. Although a novice at this, I understand what you're saying about the through hole. This carries the signal from the bottom of the pcb to the top to supply the traces located on the top of the pcb. I just tested it and I'm not getting continuity so it has to be repaired. Can this be a simple process such as buying a new cap and raising it slightly off the pcb and ensuring that it's soldered to both top and bottom?
 

KrisBlueNZ

Sadly passed away in 2015
Nov 28, 2011
8,393
Joined
Nov 28, 2011
Messages
8,393
Yes, you've got the idea.

You can't really just mount the capacitor above the PCB because you would need to get the soldering iron and solder in underneath it. Probably the simplest fix is this: scratch an area of the solder-resist coating off the track on the top side near the hole, tin the exposed area on the track, and solder a thin wire onto the exposed track and poke it down through the hole. Then you refit the capacitor, and solder the thin wire and the capacitor lead to the track on the underside.

You can use a single strand from a stranded wire, and/or you can drill the hole out slightly so both the wire and the capacitor lead will fit through it.

The rework should be covered completely by the capacitor itself.

You can clean up the PCB with isopropyl alcohol (or any solvent really) and a cotton swab. Push hard and it will remove all the crusty stuff.
 

partyanimallighting

Oct 22, 2012
330
Joined
Oct 22, 2012
Messages
330
Hi Kris, here's my plan. I removed the electrolytics and re-tested the through-holes and they are fine. There are three of them and I tested them for continuity from one side of the pcb, along the trace on the other side to the next device in line, which was the ground trace, a motor lead and the rectifier respectively. So I think that's not the problem. I then tested from the 7805 0V to pin 1 on the 555, continuity is fine. Then I tested each resistor and other device on the pcb for continuity from the 0V pin on the 7805 and got continuity on all devices (O/L at the other end) but got a reading of 0 and 502 on the current limiting resistor that you mentioned before and a reading of 659 and 957 on the second resistor to the right of the diac below the relay. I also pulled the two mustard colored caps from the pcb and tested these also. I'm getting continuity on one (102) and the other one (H74, I think. Can you confirm this number?) is open, probably bad until I get your feedback. So...my plan is to replace the 2 35V caps, the 2 mustard colored caps (once you can confirm that second number) and the 7805. Do you think I should replace the current limiting resistor and the resistor by the diac also?
 

CocaCola

Apr 7, 2012
3,635
Joined
Apr 7, 2012
Messages
3,635
But I'm getting 18V on both on the working pcb.

Elaborate, the one on the left isn't getting 18V looking at your first pic? That is the one that appears to be feeding the 7805...

In this pick test for 18V at the paired circles, give me polarity of the red pair if you can...
attachment.php
 

Attachments

  • 18v.jpg
    18v.jpg
    55.3 KB · Views: 177

KrisBlueNZ

Sadly passed away in 2015
Nov 28, 2011
8,393
Joined
Nov 28, 2011
Messages
8,393
You say you're getting "continuity on one" of the mustard-coloured capacitors? You mean continuity from one side to the other? In that case it's faulty. Capacitors should measure open circuit (except for larger electrolytics, which take a while to charge up, but eventually should read open circuit as well). If you're getting continuity from one end of the capacitor to the other, it's faulty.

You can get a replacement from any electronic component store. "102" is code for 1 nF (one nanofarad). It's a multi-layer ceramic capacitor, but any type of ceramic capacitor will do. In fact you could also use 10 nF or 0.1 uF (point one microfarad); the value is not critical either.

But this doesn't fit with your earlier description of having 0V on the input of the 7805.

I'm really not sure what's happening, and I'm not confident you're doing everything properly. This is what I think you should do.

Solder-suck the positive hole for the left hand electrolytic. (All left-right references relate to the photo in your first post.) Have a good look and try to see whether there is any metallic collar inside the hole, forming a circuit between the track on the top side and the track on the underside. If so, great. If not, do the fix I described earlier.

If the 7805 is not fitted, do the same with the rightmost hole of the 7805. It also needs to have through-hole plating, to connect to the track on the top side. If the 7805 is fitted, don't worry.

Reinstall the two big electrolytics. Don't worry about reinstalling the relay or any ICs. If the 7805 is on the board, leave it there. If it isn't, leave it off.

Connect your 12VAC to the board. Connect your black meter probe to pin 1 of the 555, and with the red meter lead, measure the voltage on the right hand pin of the 7805. If the 7805 is in circuit, measure the leg. Otherwise, measure the pad.

Let me know what you found out about the through-hole plating, and what voltage you measure on the right hand pin of the 7805.

Everything else you've described is fine. Just do that step and let me know, OK?
 
Last edited:

partyanimallighting

Oct 22, 2012
330
Joined
Oct 22, 2012
Messages
330
Hi Cocacola, I'm getting 18V at the blue circles and 0V at the green. Can't give you the red reading as the caps are out the pcb now.
 

partyanimallighting

Oct 22, 2012
330
Joined
Oct 22, 2012
Messages
330
Hi Kris, I tested both mustard colored caps, 102 and 474, one side to the other and one measures open circuit and I get a climbing resistance reading on the other then it goes open circuit. Is this faulty? I'll get the parts tomorrow beforehand, do exactly as you suggested and give you the feedback. Thanks for all the help so far. Talk to you tomorrow.
 

CocaCola

Apr 7, 2012
3,635
Joined
Apr 7, 2012
Messages
3,635
Hi Cocacola, I'm getting 18V at the blue circles and 0V at the green.

The zero at the green is a potential issue...

Can't give you the red reading as the caps are out the pcb now.
The red is a direct measurement off the rectifier bridge, nothing to do with the caps... I need to polarity of the red circles to continue...
 
Last edited:

KrisBlueNZ

Sadly passed away in 2015
Nov 28, 2011
8,393
Joined
Nov 28, 2011
Messages
8,393
Hi Kris, I tested both mustard colored caps, 102 and 474, one side to the other and one measures open circuit and I get a climbing resistance reading on the other then it goes open circuit. Is this faulty? I'll get the parts tomorrow beforehand, do exactly as you suggested and give you the feedback. Thanks for all the help so far. Talk to you tomorrow.
OK. Those readings are normal; the mustard-coloured capacitors are OK.
 

CocaCola

Apr 7, 2012
3,635
Joined
Apr 7, 2012
Messages
3,635
Now check continuity between the red star and the rest of the stars...

Also if you by chance have those big caps off the board a picture of the top of the board so I can see the traces would be helpful...
 

Attachments

  • 18vv.jpg
    18vv.jpg
    56.1 KB · Views: 102
Top