Op-amp problems? #20m #calibration #opamp


Dave Nelson W6NEL
 

I had a great time building my 20m QCX-mini, probably my best soldering ever, but I am unable to Adjust the I-Q Balance or Audio Phase Shift. When powered up the screen works great, I am able to navigate the settings, have a high 8 BPF, and have a hiss coming out of the headphones that is adjustable with the volume control knob, but adjusting R27, R24, and R17 do not make any difference.
 
I am right in thinking that IC8 and/or IC9 are most likely my issue(s)?


Larry Acklin
 

Inspect the SI 5351 for stray solder. Mine on a QCX+ had clk 0 and 1 shorted together. I could not get a good BPF peak or any nulls until I cleaned it up with extra flux and solder wick. Took several tries. 

73
Larry
KB3CUF 


Zdenek OK2BQN
 

Hi Dave,
Measure IC8 and IC9 at rest and compare with Hans' manual. I measured on IC8 pin 6 and 7 10.34V DC. I replaced it with NE5532 and everything was OK. I wish you success. Write how it turned out.
73
Zdenek


Zdenek OK2BQN
 

Hi Dave,
if you have an oscilloscope. Turn on BPF peak step 8.7.
You have OK. Then measure the IC8 pin7 sidedone. You should have 0.8V peak to peak there. At the output of R27 should be 0.3V peak to peak.
73
Zdenek


PA1DMG
 

Hi Dave,
I just finished building my QCX mini and ran into the same issue. IC6 turned out to be DOA so there was no output on pin 1.
If you don't own a signal tracer you can check the dc voltages on the outputs of the opamps; these are listed in the manual and should all be around 2.4 Volts.

HTH,
Marc.


Steve Mook
 

I am experiencing similar issues: everything works and sounds like a radio, but it is mostly deaf, and there is no effect from the alignment pots. Established continuity and correct component values through the filters/transformer from pin8 of IC3, but the only way to get a little bit of an alignment signal into the headphones is to short R43 and carefully peak the BPF. I went ahead and replaced IC7 because pin voltages seemed a little off and replacing it was easy. Now I suspect IC6, but the voltages look plausible on the DVM (2.4-2.5V ballpark, I don't have a scope) and I want to be a little careful with my remaining new op-amp chip. Any suggestions welcome.

Steve - AD5TF


Daniel Conklin
 

I'm having the same problem and all the voltages check out. I'm going to try some other trouble shooting steps this weekend and see if I can't get it going.
--
73, Dan - W2DLC


Rod Smith
 

Hi
I'm another with this opportunity (I just hope it isn't insurmountable!).  All voltages seem ok, I've checked T1, any number of soldered connections - no joy (and no dual sweep 'scope).  The BPF peaks nicely in the 07 range (08 without a dummy load).  There's some kind of a ~700Hz signal at ICs 6 and 7 ... and audible tones in the earphones.  At low settings of the gain control there's hysteresis - the volume reduces gradually to nothing as I turn the pot anticlockwise, and then doesn't reappear for a few degrees when I turn it clockwise again. 

Thanks very much to those flagging that e.g. they managed to balance I&Q using a scope to see the phases move - I'm intrigued as to why that worked and the procedure in the manual didn't - did one or more of the multi-turn pots end up (when balanced) at the end of their travel perhaps?  I've tried putting each pot to mid range (ohms-wise) and starting adjustments at low and high phase before "balance" - that didn't help.

Thanks Larry for the Si5351 solder-whisker steer - I was hopeful there for a little while but mine looks well shaved :o).   I've been wondering whether my C10 is way-off off-value but that seems very unlikely.

So I can't help but put op-amp failure at the top of my list

Is there a view that the op amps sometimes fail (or arrive failed?) without the dc voltages then being obviously wrong?  If so does anyone know whether they for example fail with low gain rather than no gain at all?

Thanks everyone posting their experiences in solving their particular cases - and especially those with optimistic "can do" approaches, they make a big difference :o)

Fingers crossed I can find a something simple that sorts things out here.  I've ordered a few op amps in readiness for "desperate measures" and it'd be great not to need them!

Cheers

Rod
G0VKX

"When you understand the inside the outside will be just fine"
(Thelonious Monk)


Ronald Taylor
 

Hi Rod. If you’re getting that good of a signal level on the BPF adjustment then you have audio through IC5,6&7. It’s disappearing somewhere through IC8,9 or 10 for some reason. 

Since you don’t have a scope you can use an audio amplifier with 40 or so dB of gain and a DC isolation capacitor on the input and probe the inputs and outputs of each op amp stage starting at IC8B. IC8&9 form the narrow CW audio filter. They are unity gain so you should hear roughly the same level going into and out of each stage. Then check the level at IC9A (pin 1) vs the input of IC10B (pin 6) with the volume at full level. Should be roughly similar. You can also just check in and out of the volume pot for similar levels. Then check the output of IC10B (pin 7). This is where the signal is taken off to go to the point that measures levels for the IQ and phase hi and LO and will be very loud through your test amp if working up to that point. But could also be weak if not getting enough from a previous stage. If you find a point that looks like a problem make sure you have proper resistances and voltages around it before replacing an IC as a last resort. 

I’ve only found one bad op amp in 35+ QCX and QCX+ Repairs. I’ve found one bad one in each of the two minis I’ve seen so far but I don’t want to speculate on what that means. Remember the vast majority of builders are getting good results. Only the problems show up here. 

When I used my scope to roughly align the IQ etc it was because for the first time ever I just wasn’t getting it the normal way. It must have been that some controls were near end of travel. I almost always get them aligned using the normal process and even that one I had to use the process to do final alignment once I roughed it in. I hope all this helps and wish you luck. 

73... Ron

On Fri, Jan 15, 2021 at 09:17 Rod Smith via groups.io <rodjsmith=btinternet.com@groups.io> wrote:
Hi
I'm another with this opportunity (I just hope it isn't insurmountable!).  All voltages seem ok, I've checked T1, any number of soldered connections - no joy (and no dual sweep 'scope).  The BPF peaks nicely in the 07 range (08 without a dummy load).  There's some kind of a ~700Hz signal at ICs 6 and 7 ... and audible tones in the earphones.  At low settings of the gain control there's hysteresis - the volume reduces gradually to nothing as I turn the pot anticlockwise, and then doesn't reappear for a few degrees when I turn it clockwise again. 

Thanks very much to those flagging that e.g. they managed to balance I&Q using a scope to see the phases move - I'm intrigued as to why that worked and the procedure in the manual didn't - did one or more of the multi-turn pots end up (when balanced) at the end of their travel perhaps?  I've tried putting each pot to mid range (ohms-wise) and starting adjustments at low and high phase before "balance" - that didn't help.

Thanks Larry for the Si5351 solder-whisker steer - I was hopeful there for a little while but mine looks well shaved :o).   I've been wondering whether my C10 is way-off off-value but that seems very unlikely.

So I can't help but put op-amp failure at the top of my list

Is there a view that the op amps sometimes fail (or arrive failed?) without the dc voltages then being obviously wrong?  If so does anyone know whether they for example fail with low gain rather than no gain at all?

Thanks everyone posting their experiences in solving their particular cases - and especially those with optimistic "can do" approaches, they make a big difference :o)

Fingers crossed I can find a something simple that sorts things out here.  I've ordered a few op amps in readiness for "desperate measures" and it'd be great not to need them!

Cheers

Rod
G0VKX

"When you understand the inside the outside will be just fine"
(Thelonious Monk)


Rod Smith
 

Hi Ron many thanks for the helpful and welcome info and advice.

I'd been working on the assumption that there's a problem somewhere around IC6 and IC7 stages/ circuitry on account of not being able to get I&Q in balanced quadrature.   

I'm absolutely with you regarding bad op amps - at this stage (unless and until more info becomes available) I'm assuming that (as is usual) it's far more likely I've made an error than that a part is faulty.  At a guess some minor fault in the tracks or soldering of SMD parts could be in-between those two probability-wise.

35+ QCXs!  I've only built one ("regular sized") QCX before, and a QRP Labs receiver module, progrock, and VFO; they all worked first time.  As did the only other kit I've ever made, which was and is a Heathkit OS2 oscilloscope I built 50 year ago (that worked first time too!).  My homebrew stuff has never worked first time mind you -  but always there's a way :o)

Your suggestion of probing with some audio gain is good - thank you.  I still do have that OS2 'scope and I can just see audio with it on the IC6 and IC7 op amps - but by putting some audio gain ahead of the Y amp I should be able to get an idea of the absolute levels and understand better what's stopping the I&Q from balancing in quadrature.

Thanks again

Rod
G0VKX

"When you understand the inside the outside will be just fine"
(Thelonious Monk)







 


Rod Smith
 

Hi all

I'm happy to say that my 20m QCX-mini now has a receiver with balanced/ quadrature I&Q!

It turned out to be IC7 and once I managed to replace it all was well.

FWIW it turned out all I needed to do really was use my finger to fault find.  Holding a small metal probe to the inputs of each op amp in the audio chain in turn, working back from IC9, I soon found that I didn't hear any hum from the input to IC7B.  I checked everything around IC7 very thoroughly indeed; I didn't want to assume IC7 was at fault rather than me and I really really really didn't want to try replacing it!  But all connectivity was good, all the resistors fine, the capacitors not s/c; and so I hadn't much choice.  Despite first practising a few times on a "scrap" piece of board, I made a truly awful mess of the replacement (the older I get the better I was) - even managing to melt closed two of the 8 sockets of the header that the control board should plug into..  But, once I sorted that out and a deep seated solder bridge I'd made between pins 7 and 8 I applied power.  All worked a treat - and the IQ balance etc was a piece of cake.

Oh - I replaced IC7 with a TS922ID.  Thanks ohwenzelp 
 
(#60436)

Thank you very much everyone for the positive constructive encouraging help here.

Tomorrow the transmitter ...

72/ 73

Rod
G0VKX

"When you understand the inside the outside will be just fine"
(Thelonious Monk)


Daniel Conklin
 

Congratulations and good DX!
--
73, Dan - W2DLC


Rod Smith
 

Hey ho.  After tweeking L2 I had a good solid tx o/p of a Watt using an 8V supply and was everything behaving as it should.  Except that on touching the frequency control (on receive) there was an intermittent (dis)connection going on somewhere - loud crackles - and in trying to find out where the fault was I must've shorted something around the control board "plug".  Result solid blocks on the display and a sick empty feeling in the stomach.

My mini-brick is put to one side pending more patience, calm, and positivity :o/

72/73
Rod
G0VKX