No 5 volts


Jim Painter
 

I have completed construction of my QCX+. Lcd screen is solid white (back light only). I find I have no 5 volts out of the 7805 (ic11). Checked connections, soldering etc. all looks ok. Could it just be a bad 7805? Input to 7805 is 13.8 volts.

Thanks. Jim...kq3s


James Daldry W4JED
 

Hi Jim

No 5 volts out? 3 possibilities - (1) 7805 in backwards (2) short from +5 to ground (3) bad 7805, in that order. Actually I would check for the short first. Loose wire clippings and solder strings are very common in new construction. Since the 7805 has a foldback circuit it doesn't get as hot feeding a dead short as it does delivering 1 amp.

73

Jim W4JED

On 8/1/20 9:04 AM, Jim Painter via groups.io wrote:
I have completed construction of my QCX+. Lcd screen is solid white (back light only). I find I have no 5 volts out of the 7805 (ic11). Checked connections, soldering etc. all looks ok. Could it just be a bad 7805? Input to 7805 is 13.8 volts.

Thanks. Jim...kq3s


Alan G4ZFQ
 

I find I have no 5 volts out of the 7805 (ic11). Checked connections, soldering etc. all looks ok. Could it just be a bad 7805? Input to 7805 is 13.8 volts.
Jim

If you measure 13.8 on the actual regulator input wire and 0 volts on the actual ground wire and output wire then it would look like a faulty regulator.

73 Alan G4ZFQ


N3MNT
 

What is the voltage on Pin 2 of the display


Jim Painter
 

Thanks for the quick reply. I don't think it can be in backwards because the heat sync plate is facing to the rear and connects to the rear plate of the cabinet. When I measure from 5 volts to ground, I get a capacitive charge (as I would expect). Checked for solder bridges. Didn't find any, although I must say, the solder pads are very close to the ground plane. I don't remember them being that close on the QCX. I don't think I will tear my QCX apart to see. I guess i’ll start looking for a 7805.

Thanks again to all. Please let me know If you have any further suggestion.

73 Jim...kq3s

On Aug 1, 2020, at 9:13 AM, James Daldry W4JED <jim@...> wrote:

Hi Jim

No 5 volts out? 3 possibilities - (1) 7805 in backwards (2) short from +5 to ground (3) bad 7805, in that order. Actually I would check for the short first. Loose wire clippings and solder strings are very common in new construction. Since the 7805 has a foldback circuit it doesn't get as hot feeding a dead short as it does delivering 1 amp.

73

Jim W4JED

On 8/1/20 9:04 AM, Jim Painter via groups.io wrote:
I have completed construction of my QCX+. Lcd screen is solid white (back light only). I find I have no 5 volts out of the 7805 (ic11). Checked connections, soldering etc. all looks ok. Could it just be a bad 7805? Input to 7805 is 13.8 volts.
Thanks. Jim...kq3s


Jim Painter
 

Pin 2 of the display is about 300 millivolts.




On Aug 1, 2020, at 9:23 AM, N3MNT <bob@...> wrote:

What is the voltage on Pin 2 of the display


Jim Painter
 

Actually, there is 300 millivolts at the output of the 7805.

I am considering cutting the foil at the output of the 7805 to see if its a short causing the problem. The only question is, would I get 5 volts with no load on the 7805?

Jim kq3s


On Aug 1, 2020, at 9:39 AM, Jim Painter via groups.io <Jfpainter@...> wrote:

Pin 2 of the display is about 300 millivolts.




On Aug 1, 2020, at 9:23 AM, N3MNT <bob@...> wrote:

What is the voltage on Pin 2 of the display


N3MNT
 

Can you post a good closeup of the bottom of the board?


Jim Painter
 

Great idea!!! Much easier to scan the board than using a loop. You will notice two places that look like splashes. One on the left side and one on the upper right. They both turned out to be fuzz from cleaning. I must apologies for the soldering. I ran out of my good solder and started using some Chinese solder. Absolutely terrible!!!

  


On Aug 1, 2020, at 10:36 AM, N3MNT <bob@...> wrote:

Can you post a good closeup of the bottom of the board?


N3MNT
 

After a quick check two possible issues.  Solder bridge?  and missing solder connection.


Bob M.
 

Yes, you should get 5V out of the regulator with no load.

There are often small (less than 1uF) capacitors on the output of the 7805 regulators, sometimes tantalum. These have a nasty habit of shorting. Also they could be polarity-sensitive and will do strange things if put in backwards.

Cutting the foil at the output of the 7805 should tell you whether it's the regulator that's bad or a problem on the board. If you can connect a small load to the 7805 output (50 ohms would do) and still measure 5V, then the regulator is most likely good.

Bob M.


ajparent1/kb1gmx
 

Since 5V is used in more than a few places on the board Id assume short
maybe at the regulator but anywhere is possible.

Allison
-------------------------------
Please reply on list so we can share.
No private email, it goes to a bit bucket due address harvesting


sean <kc2lwx@...>
 

Any closer  shots of the solder side of the board

De kc2lwx 
Sean



On Sat, Aug 1, 2020 at 4:21 PM, ajparent1/KB1GMX
<kb1gmx@...> wrote:
Since 5V is used in more than a few places on the board Id assume short
maybe at the regulator but anywhere is possible.

Allison
-------------------------------
Please reply on list so we can share.
No private email, it goes to a bit bucket due address harvesting


N3MNT
 

Two more solder joints to check.  Is there still no 5V with front board disconnected?


Jim Painter
 

Hi Jim,
Well the situation has gone from bad to worse. The first two suspected solder bridges turned out to be glare from rosin. I didn’t get a pic of the second two.

Now for the bad news. I cut the etch for the 5 volt, turned it on and got ready to measure. Pop noise and smoke. D3 blew up. Not sure what other damage. I have a diode and a 7805 on order. I now have no 12 volts either. 

I will let you know what I find.

Thanks...Jim...kq3s


On Aug 1, 2020, at 5:13 PM, N3MNT <bob@...> wrote:

Two more solder joints to check.  Is there still no 5V with front board disconnected?


N3MNT
 

photo with additional two ( red).  If the diode blew after you cut the trace.  you may have a short on the 12V side.


Jim Painter
 

Thanks Jim, apparently I destroyed the S1 switch when I blew D3. That’s why I’m not getting 12 volts now. 12 volts comes in the input connector, follows a foil to JP3 and out to the S1 on the display board. I checked out S1 with an ohm meter and it is bad. Not sure where I can get another one.  Anyone have any ideas?

Thanks for the photo update. I will check it out.

Jim...kq3s 


On Aug 2, 2020, at 8:37 AM, N3MNT <bob@...> wrote:

photo with additional two ( red).  If the diode blew after you cut the trace.  you may have a short on the 12V side.
<board.jpeg>


N3MNT
 

S1 is not critical at this point as you can jump it .  The QCX has no switch.  Based on D3 blowing up and the damaged switch, it points to a 12V side major short.  I would not apply power until you find the short.  Do you have access to a current limited power supply.  If so I would use that for the next power application. 


Jim Painter
 

Jim, I checked out the two areas circled in red. The lower left is the earphone jack. From one angle it looks like there might be a short but changing the angle they are clearly OK. The pins in question are normally shorted in the head phone jack anyway ( normally closed). The four pins in the upper right  are JP3. Pins 7-8 and 9-10 are each joined by a foil. So they’re OK. Though it doesn’t look like in the photo. Good idea about the switch. Unfortunately I don’t have a current limiter but I may be able to borrow one from someone in the club (Skyview).

Thanks again...Jim...kq3s

P.S. I should be able to find a 12 volt short with an ohm meter.


On Aug 2, 2020, at 4:59 PM, N3MNT <bob@...> wrote:

S1 is not critical at this point as you can jump it .  The QCX has no switch.  Based on D3 blowing up and the damaged switch, it points to a 12V side major short.  I would not apply power until you find the short.  Do you have access to a current limited power supply.  If so I would use that for the next power application. 


Jim Painter
 

Hi Jim, I got new 7805 regulators and D3 diodes today. I removed d3 and the 7805.
Checked for shorts to ground on 12 volt and 5 volts. No shorts.

Checked the old 7805 with a meter. Definitely read different from the new ones. Pin 3 to pin 2 (output to ground) reads open both meter directions. Pin 1 to pin( 2 input to ground)reads about 20k. Pretty much the same as the new one. The 7805 may have been the problem to start with.

C37, c38 and c51 are 10uf electrolitics. C37 is the output of the 7805. C38 is the input. C51 is the output of the earphone circuit.

Checking with a digital ohm meter. C37 and c51 read dramatically different from c38. 37 and 51 read a very quick 0 ohms then open. C38 reads reads short then charges as I would expect.

I could see how c37 could have been damaged when d3 shorted but c51 seems far distant from d3.

These are 10uf 16 volt. Can anyone recommend replacements? I’m thinking Digikey but I want to make sure I get the right ones.

73...Thanks...Jim...kq3s


On Aug 2, 2020, at 5:46 PM, Jim Painter via groups.io <Jfpainter@...> wrote:

Jim, I checked out the two areas circled in red. The lower left is the earphone jack. From one angle it looks like there might be a short but changing the angle they are clearly OK. The pins in question are normally shorted in the head phone jack anyway ( normally closed). The four pins in the upper right  are JP3. Pins 7-8 and 9-10 are each joined by a foil. So they’re OK. Though it doesn’t look like in the photo. Good idea about the switch. Unfortunately I don’t have a current limiter but I may be able to borrow one from someone in the club (Skyview).

Thanks again...Jim...kq3s

P.S. I should be able to find a 12 volt short with an ohm meter.


On Aug 2, 2020, at 4:59 PM, N3MNT <bob@...> wrote:

S1 is not critical at this point as you can jump it .  The QCX has no switch.  Based on D3 blowing up and the damaged switch, it points to a 12V side major short.  I would not apply power until you find the short.  Do you have access to a current limited power supply.  If so I would use that for the next power application.