Help with strange audio fault

Alan G4ZFQ

Let's throw some logic at the target.
Jim,

Sorry I do not understand what you are trying to prove.
Ted has quoted 5.88 volts at pin 5 0f IC10. Measured with his DVM. This approximately 1/2 the supply voltage which is what one expects from the potential divider.

So - either the +12 volt end of R40 isn't making the trip or the ground end of R39 isn't.
Which would give a big error when measuring pin 5 voltage.

Perhaps you can explain why pin 6 is at 5.36 volts. Where does that come from?

73 Alan G4ZFQ

Alan G4ZFQ

James, I am thinking about just trying small wire bridges to isolate which one is open circuit.
Ted,

I see no evidence to suggest there is a problem with R30, 40.
If you must you can test this with a meter from the resistor ends to their respective +/- points.

I think Jim's test is not relevant, I repeat:-

I'm concerned about pin 6 which should be connected to pin 7 by R38. As there should be no other DC connection to pin 6 it should therefore be close to the pin 7 voltage.
Is R38 soldered? Check it's value, that each end is actually connected to the IC pins. Check the resistance from pin 6 to ground.

73 Alan G4ZFQ

Ted 2E0THH

Thanks Alan

I'll make a start in 30 mins and get to the bottom of it.
Would an incorrect value resistor value have prevented the set from working in the first place?

My sincere thanks to you all

73s Ted
M7ECH

geoff M0ORE

I think in an earlier posting you said that you were running on a battery. Now you are saying that the battery icon is showing empty. Have you tried a fresh battery? A dying battery would allow tthe rig to work for a few seconds then die. A couple of hours later, the battery has recovered enough to give a few more seconds. Some parts of the rig may be more sensitive to lower supply voltages than others.

James Daldry W4JED

Hi, Alan

When you have any of the pins of an op amp against the rail, the readings on the rest of the pins are meaningless. 40+ years of experience. The input impedance of an op amp is essentially infinite, so the voltage drop across R38 can't be happening - pin 6 should be 11+ volts - except for the fact that the output is against the rail, which makes all the other readings bad. With pin 6 higher than pin 5, if it was working correctly, the voltage on pin 7 should be 0, not 11. You can't have a working op amp and have a half volt between the inputs. Not in this universe. So, the symptoms suggest a bad op amp.

However, we have Mr. Ohm to contend with. You can't do the "read the 10K bias resistor" test, opening and shorting the power pins, without a change when the power points are shorted, unless the power is open to one of the resistors. Use the ohmmeter and check from the "not pin 5" ends of R 39 and 40. One line has to be open.

Jim

On 1/15/19 10:12 AM, Alan G4ZFQ wrote:
Let's throw some logic at the target.
Jim,

Sorry I do not understand what you are trying to prove.
Ted has quoted 5.88 volts at pin 5 0f IC10. Measured with his DVM. This approximately 1/2 the supply voltage which is what one expects from the potential divider.

So - either the +12 volt end of R40 isn't making the trip or the ground end of R39 isn't.
Which would give a big error when measuring pin 5 voltage.

Perhaps you can explain why pin 6 is at 5.36 volts. Where does that come from?

73 Alan G4ZFQ

Alan G4ZFQ

Would an incorrect value resistor value have prevented the set from working in the first place?
Ted,

Pin 5 voltage is correct and sets the operating point of IC10B, R38 feeds back to pin 6 and should make it close to pin 7 voltage. But we do not see this.
I'm suspecting that R38 is open circuit or simply not connected, dry joint?
I think IC10B would work while C23 charges, then stop. But I am still guessing. Let's see what you find.

73 Alan G4ZFQ

Ted 2E0THH

HI Geoff

Thank you for helping out.
It is a small (fused!) 12v sealed lead acid and is still constant at 12v.
I trickle charge it every night in any case.

I have also repeated the fault with my bench PSU, the fault actually started while using the PSU.

It is just a curiosity at this stage as to why the battery icon is hollow, I am sure it will be a setting somewhere.
I will look into it once I get this awkward fault found.

73s Ted
M7ECH

Alan G4ZFQ

With pin 6 higher than pin 5, if it was working correctly, the voltage on pin 7 should be 0, not 11.
Jim,

But Ted says pin 6 is lower than pin5...
Let's see what he finds

73 Alan G4ZFQ

James Daldry W4JED

Hi, Ted

Battery Full setting is menu item 7.3 .  7.4 specifies "bar size".

Jim

On 1/15/19 11:49 AM, qrp@... wrote:
HI Geoff

Thank you for helping out.
It is a small (fused!) 12v sealed lead acid and is still constant at 12v.
I trickle charge it every night in any case.

I have also repeated the fault with my bench PSU, the fault actually started while using the PSU.

It is just a curiosity at this stage as to why the battery icon is hollow, I am sure it will be a setting somewhere.
I will look into it once I get this awkward fault found.

73s Ted
M7ECH

Peter GM0EUL

The empty battery icon got me too.  To get a reading in the battery icon you have to set a jumper on the board.  See the manual page 97 item 7.2/7.3/7.4 and page 104 item 9.1

Good luck with fixing the audio fault.  I hope you get to the bottom of it, I'm watching with interest and learning a lot.

73, Peter GM0EUL

James Daldry W4JED

My guess, still, is a loose ground on R39. The audio plays until C24 charges.

Jim

On 1/15/19 11:55 AM, Alan G4ZFQ wrote:
With pin 6 higher than pin 5, if it was working correctly, the voltage on pin 7 should be 0, not 11.
Jim,

But Ted says pin 6 is lower than pin5...
Let's see what he finds

73 Alan G4ZFQ

Ted 2E0THH

Gentlemen

I had another solder around r39 but to no avail.
I did socket the OP amps when I built the QCX40 so I just removed IC10

These are the values in ohms from the socket holes to ground
8) 1371
7) 216
6) 95k
5) 5.3K
4) 0
3) 217k
2) 15M (climbing as I read it)
1) 15M connected to 2

I have been called out for a few hours so will jump back in at 2200h.
I am immensely grateful for all help.

73s Ted
M7ECH

Alan G4ZFQ

Ted,
These are the values in ohms from the socket holes to ground
8) 1371
7) 216 *Are you sure?? With no IC10 should be close to infinity.
6) 95k *Ditto. If this is true check again after removing one end of R37
5) 5.3K
4) 0
3) 217k *Should be connected to pin 7. Probably is, you got pin 7 wrong.
2) 15M (climbing as I read it)
1) 15M connected to 2
Check all these again, maybe you counted wrongly or something.
If pin 3 IS connected to pin 7 then R38 +95K=215K confirming a leak to ground at pin 6, possibly through C22.

73 Alan G4ZFQ

James Daldry W4JED

Hi, Ted

Where did I write the word "solder"? My guess is broken foil on the ground end of R39. While you have IC10 out of the socket, check the voltage on pin 5. If the battery is 12 volts, it should be 6. If it is 6, try swapping the IC with one of the others of the same type. Easy way to eliminate the chip as a suspect. After all, if a chip is going to fail without smoke, it will probably happen during the first few months of use.

Jim

On 1/15/19 1:53 PM, qrp@... wrote:
Gentlemen

I had another solder around r39 but to no avail.
I did socket the OP amps when I built the QCX40 so I just removed IC10

These are the values in ohms from the socket holes to ground
8) 1371
7) 216
6) 95k
5) 5.3K
4) 0
3) 217k
2) 15M (climbing as I read it)
1) 15M connected to 2

I have been called out for a few hours so will jump back in at 2200h.
I am immensely grateful for all help.

73s Ted
M7ECH

Ted 2E0THH

Dear James and Alan

I am so sorry. I was on a call out so that last message was rather garbled.
As well as checking for dry joints I also tried bridging in case there was a break in the foil either way off R38.
I only had time to try it with without soldering so that it my next task.
I will also try the same with R39.

I don't have any spares of the op-amp but I already tried swapping IC9 and IC10 with others on the board (I can't remember which) but the fault remained and repeated. I think I will order some in any case.

Alan, I rechecked the resistances on the empty IC10 socket
8) 1371
7) 240K (yes I can't type Alan)
6) 118K (I will check this later with one end of R37 removed)
5) 5.3K
4) 0
3) 240K
2) 27M (rising)
1) 27M

And voltages in the empty IC10 socket
8) 12
7) 0
6) 0
5) 6
4) 0
3) 0
2) 0.9
1) 0.9

Right, I will try and see if any foil bridges work and then get some sleep. I should get stuck in properly again tomorrow afternoon (GMT)

Again, my sincere thanks gentlemen.

73s Ted
M7ECH

Ted 2E0THH

Good morning Alan & James

I have snipped the top end of R37 as you suggested Alan.
I measured both resistance scenarios, the non snip measurements are somewhat at variance to those of yesterday, I have no idea why.
No R37 snip
8) 1371
7) 273k
6) 154K
5) 5.3K
4) 0
3) 273K
2) 30M
1) 30M

R37 snipped
8) 1371
7) infinite
6) infinite
5) 5.3K
4) 0
3) infinite
2) 30M
1) 30M

I haven't had a chance to try any of the foil bridging yet, I will get a chance to try later this afternoon after I fix the snip and reinsert IC10

73s Ted
M7ECH

Ted 2E0THH

Hi Alan

I am out on call right now but was just reading more carefully you last email on my phone.
>> If pin 3 IS connected to pin 7 then R38 +95K=215K confirming a leak to ground at pin 6, possibly through C22

Are you implying that C22 might be faulty? If so I better get one ordered.

Huge thanks

Ted
M7ECH

Alan G4ZFQ

Are you implying that C22 might be faulty? If so I better get one ordered.
Ted,

Assuming the PCB is clean it seems by far the most likely fault.
Funny I thought that before I realised it was not an electrolytic.

73 Alan G4ZFQ

Chris Wood

Hello Alan, Ted,

Alan may remember that I had a similar fault with my QCX 30m, and described in earlier posts.

My kit was version 4 and the capacitors supplied for C21and C22 are indeed 1u instead of 10u.  The manual says they are ceramic, but upon further inspection since building, (and checked just now) I think the ones supplied in my kit are actually tantalum and are polarised.  At least they look, for reference, identical to the part sold by CPC as MCCB1V105M2ACB which is a 1u 35v tantalum.  They do not look like standard 105 ceramics, which are less elongated and marked differently.

I assumed they were ceramics when fitting them because they are the same colour as the others, and I was expecting them to be ceramics, but that was my mistake.

So I now think my problem was that I had one or both of these reversed, and I corrected the error unwittingly when I removed and refitted them, producing a working QCX.

Obviously Ted's kit may have had ceramics supplied, not tantalums, but I do again recommend checking.

Best wishes

Chris
G4CWS

Ted 2E0THH

Hi Chris

I am pretty sure these are ceramics:

Are your tantalum caps actually yellow? I didn't notice any polarisation markings during construction, but I have to confess I wasn't looking for any. Did your set work at all Chris? I just checked the construction manual V4 and Hans lists C21 and C22 as ceramics.

Now I'll have a careful look!