Wacky voltage readings


Clint Sharp <cjaysharp@...>
 

Sounds likely, IC4 is a nice large SMD so it’s easy to remove and replace with nothing more than a soldering Iron and care. If you’ve a pin chuck and a sewing needle you can put them to good use in the process 

On Fri, 13 Jul 2018 at 03:30, Charles W. Powell via Groups.Io <doctorcwp=yahoo.com@groups.io> wrote:
Alan (G4ZFQ),

I did more pondering on the QCX I am trying to repair and did some experimenting.  I removed the transformer leads that go to pins 7 and 9, and I now get an expected voltage division between R1 and R2 at 2 volts. Shy of the published voltage but exactly half of what is supplied to R2. Regulator output is 5.01 volts but I’m still getting exactly 4 volts at R2 - not sure why.  The LCD seems to operate just fine.  There is obviously a trace or a bus that spans the board but wow! Trying to find that is going to be tough.

With T1, C45, and R9 fully removed, I no longer get high voltages on the ICs, 5, 6, 7, etc.  Rather the voltage that were ridiculously high drop down to about 0.5 to 1.5 volts in most instances.  I no longer have 11 volts anywhere on the IC pins.  Even with C45 and R9 removed, pins 5 and 12 on IC4 still show nearly a dead short to ground (about 10 ohms in the path). If there is a solder bridge somewhere, it’s beyond my capability to see it.  I am beginning to think strongly that IC4 has at least a bad segment in it.  Some of it must be working or the radio wouldn’t receive anything.

At this point in time, I am supposing that there is very little to lose by trying to replace IC4, surface mount be damned.

Thanks for your help with trying to track things down, Alan.  I’m open for more suggestions (from anyone!) if they are out there.  Since it would be ridiculous to try to remount T1 with the existing wiring, I will re-wind the transformer.  Then I can be certain it is mounted correctly.

73,

Charles - NK8O


--
Clint. M0UAW IO83

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Charles W. Powell
 

Alan (G4ZFQ),

I did more pondering on the QCX I am trying to repair and did some experimenting. I removed the transformer leads that go to pins 7 and 9, and I now get an expected voltage division between R1 and R2 at 2 volts. Shy of the published voltage but exactly half of what is supplied to R2. Regulator output is 5.01 volts but I’m still getting exactly 4 volts at R2 - not sure why. The LCD seems to operate just fine. There is obviously a trace or a bus that spans the board but wow! Trying to find that is going to be tough.

With T1, C45, and R9 fully removed, I no longer get high voltages on the ICs, 5, 6, 7, etc. Rather the voltage that were ridiculously high drop down to about 0.5 to 1.5 volts in most instances. I no longer have 11 volts anywhere on the IC pins. Even with C45 and R9 removed, pins 5 and 12 on IC4 still show nearly a dead short to ground (about 10 ohms in the path). If there is a solder bridge somewhere, it’s beyond my capability to see it. I am beginning to think strongly that IC4 has at least a bad segment in it. Some of it must be working or the radio wouldn’t receive anything.

At this point in time, I am supposing that there is very little to lose by trying to replace IC4, surface mount be damned.

Thanks for your help with trying to track things down, Alan. I’m open for more suggestions (from anyone!) if they are out there. Since it would be ridiculous to try to remount T1 with the existing wiring, I will re-wind the transformer. Then I can be certain it is mounted correctly.

73,

Charles - NK8O


Alan G4ZFQ
 

I will save the voltage issue on R2 for further investigation.
Charles,

This comes directly from IC11, the regulator.
Checking C43-46, I found that C45 faults to ground on both sides.
This goes to pins 5 and 12 of IC4. You may lift those with a pin. It looks as if the problem is a solder bridge or IC4.

>Of course that’s complicated by the fact that C45 is connected to R9, making it hard to isolate R9 for testing. R9 has a resistance to ground of slightly less than 10 ohms on one side, and 110 ohms on the other.

If the end connected to IC5 pin 6 is 110 ohms to ground then the fault is on the other end. C45, now removed, or something along that track.

Also, current drain idling is about 117 ma at 12.8 volts on the bus.
Seems about right, I do not know what is specified. But something is wrong with the 5V line.

73 Alan G4ZFQ


Charles W. Powell
 

First, all ICs are in the correct orientation, so that’s not it. I will save the voltage issue on R2 for further investigation.

Checking C43-46, I found that C45 faults to ground on both sides. I removed it to see if the problem was the capacitor, and it made no difference.I’m assuming that since C43-46 have identical function, that this is related to the problem. I have not yet found where the source of this fault to ground might be. Although the soldering around IC5 and surrounding parts is not as pretty as I like to see it (NOT MY BUILD!) I can’t find anything shorted to ground - yet. Of course that’s complicated by the fact that C45 is connected to R9, making it hard to isolate R9 for testing. R9 has a resistance to ground of slightly less than 10 ohms on one side, and 110 ohms on the other. The other three similar resistors have 500K or more to ground. So, presumably something in the chain from pins 5 & 12 of Q4, or the path between C45 and R9 is grounded. I guess I’ll have to keep looking.

Also, current drain idling is about 117 ma at 12.8 volts on the bus. I’m finished with live testing now, because I will need to replace C45. It didn’t survive surgery.

Again, thanks for your help, Alan.

73,

Charles - NK8O

On Jul 11, 2018, at 4:00 PM, Alan G4ZFQ <alan4alan@gmail.com> wrote:

I show 19.91K from R2 to ground, and 9.96K for R1 plus 9.95K for R2.
C6 shows resistance across it of 9.96K, which it should since it parallels R1, so I do not identify a problem there. I get 3.98V at R2 on one side,

Charles,

I get 3.98V at R2 on one side,
It should be 5V on one end. What current is the CQX taking?
Looks like the centre of R1/2 is going to ground when IC4 is switching.
Check for shorts to ground on C43-C46 and the associated pins of IC4.

and nominally 0.15 volts on the other side. That is also what I show +/- 0.05V at the four points on T1 which is quite low for the published specification of ~2.36V. And I still have the following on IC-5, way out of spec:
Is it possible IC-4 is fried and that’s where the voltages or lack there of originate?
It is possible but more likely is a solder bridge.
Again, it is possible one of the capacitors is faulty.
Are the ICs correctly oriented?

73 Alan G4ZFQ




Alan G4ZFQ
 

I show 19.91K from R2 to ground, and 9.96K for R1 plus 9.95K for R2.
C6 shows resistance across it of 9.96K, which it should since it parallels R1, so I do not identify a problem there. I get 3.98V at R2 on one side,

Charles,

I get 3.98V at R2 on one side,
It should be 5V on one end. What current is the CQX taking?
Looks like the centre of R1/2 is going to ground when IC4 is switching.
Check for shorts to ground on C43-C46 and the associated pins of IC4.

and nominally 0.15 volts on the other side. That is also what I show +/- 0.05V at the four points on T1 which is quite low for the published specification of ~2.36V. And I still have the following on IC-5, way out of spec:
Is it possible IC-4 is fried and that’s where the voltages or lack there of originate?
It is possible but more likely is a solder bridge.
Again, it is possible one of the capacitors is faulty.
Are the ICs correctly oriented?

73 Alan G4ZFQ


Charles W. Powell
 

Thanks for the suggestions Alan. I show 19.91K from R2 to ground, and 9.96K for R1 plus 9.95K for R2. C6 shows resistance across it of 9.96K, which it should since it parallels R1, so I do not identify a problem there. I get 3.98V at R2 on one side, and nominally 0.15 volts on the other side. That is also what I show +/- 0.05V at the four points on T1 which is quite low for the published specification of ~2.36V. And I still have the following on IC-5, way out of spec:

pin 1 10.99v (2.64)
pin 2 0.46v (2.46)
pin 3 0.17v (2.26)
pin 4 0.00v (0.00)
pin 5 0.12v (2.28)
pin 6 0.15v (2.46)
pin 7 12.1v (2.44)
pin 8 12.8v (11.67) (input voltage 13.1, bus voltage is 12.8v after the protective diode.)

IC 6 is worse:

pin 1 11v
pin 2 11v
pin 3 11v
pin 4 0.0v
pin 5 11.4
pin 6 11.4
pin 7 11v
pin 8 128v


Is it possible IC-4 is fried and that’s where the voltages or lack there of originate?

73,

Charles

On Jul 11, 2018, at 2:10 AM, Alan G4ZFQ <alan4alan@gmail.com> wrote:


I get normal voltage readings at Clk0 and Clk1. That's where it stops. Voltage readings are very low on the transformer test points (pins 5, 6, 7, 8)
Charles,

Get this right first. It all starts here.
R1/2 soldered, no bridges?
5V on R2?
No short to ground at C6?

73 Alan G4ZFQ

but then I get to the ICs. Almost all of them, instead of
showing ~2.5 - 5 volts on the various pins, show > 11 volts. The really odd thing is that some of these are in the audio chain, and despite that, I at least have sidetone audio and variable hiss with the volume pot. I can't for the life of me figure out why this is so high, nearly across the board. I also know it receives down to at least -73 dBm. Not great, but it's a start.


Alan G4ZFQ
 

I get normal voltage readings at Clk0 and Clk1.  That's where it stops.  Voltage readings are very low on the transformer test points (pins 5,  6, 7, 8)
Charles,

Get this right first. It all starts here.
R1/2 soldered, no bridges?
5V on R2?
No short to ground at C6?

73 Alan G4ZFQ

but then I get to the ICs.  Almost all of them, instead of
showing ~2.5 - 5 volts on the various pins, show > 11 volts.  The really odd thing is that some of these are in the audio chain, and despite that, I at least have sidetone audio and variable hiss with the volume pot.  I can't for the life of me figure out why this is so high, nearly across the board.  I also know it receives down to at least -73 dBm.  Not great, but it's a start.


Charles W. Powell
 

I get normal voltage readings at Clk0 and Clk1.  That's where it stops.  Voltage readings are very low on the transformer test points (pins 5,  6, 7, 8) but then I get to the ICs.  Almost all of them, instead of showing ~2.5 - 5 volts on the various pins, show > 11 volts.  The really odd thing is that some of these are in the audio chain, and despite that, I at least have sidetone audio and variable hiss with the volume pot.  I can't for the life of me figure out why this is so high, nearly across the board.  I also know it receives down to at least -73 dBm.  Not great, but it's a start.

I'm working on this for my brother-in-law, and either he's cursed, or the board is.  I've found three faults so far, and there is work to be done on the rest!  The strangest one is that the volume pot appeared to be soldered in place perfectly well.  It turns out, the wiper lead was not making contact with the board for some reason.  Even though I re-flowed the joint and did everything I could to fix it, no luck.  So I put a wire from the wiper lead to the outboard pot connection point and that fixed the first issue.  Second was a burned away trace that I needed to jumper (very inelegantly, above the board, and the third at the toroids.   But the voltage thing puzzles me.  I'm surprised it hasn't gone up in magic smoke yet.