Thanks Guys 🙂 I'll look at cleaning up the board as mentioned, although it is much cleaner this time than before.... I reduced my soldering iron temp a bit and I think it is not burning the flux now (set it to 350 deg C except for places with lots of copper where I had to bump it up to 400).
Clem_o I was actually thinking about removing the 18pf caps altogether (I noticed that this starts the HF rolling off at about 75Khz and reduces the slew rate quite dramatically), after looking at the datasheet they shouldn't be necessary, as the gain of the ne is supposedly set at 5.7, and the datasheet says they are stable with gains greater than 3......
Originally there were no caps across the fb resistor, I added these (and the 1K resistors on the input, after EA used the exact same preamp in a later amp with these mods.... The preamp section was picking up rf and this fixed it at the time. I'll try 47pf 🙂
I actually resoldered everything in this section (except the IC sockets) when I did the left and right channels of the power amps, but looks like I need to try harder!!
Tony.
Clem_o I was actually thinking about removing the 18pf caps altogether (I noticed that this starts the HF rolling off at about 75Khz and reduces the slew rate quite dramatically), after looking at the datasheet they shouldn't be necessary, as the gain of the ne is supposedly set at 5.7, and the datasheet says they are stable with gains greater than 3......
Originally there were no caps across the fb resistor, I added these (and the 1K resistors on the input, after EA used the exact same preamp in a later amp with these mods.... The preamp section was picking up rf and this fixed it at the time. I'll try 47pf 🙂
I actually resoldered everything in this section (except the IC sockets) when I did the left and right channels of the power amps, but looks like I need to try harder!!
Tony.
Suggest then to pull out both capacitors and giving it an IMD test, see if that clears it all up anyway (or if the thing suddenly becomes an AM receiver!). Manufacturers tend to be a bit optimistic when it comes to claims of stability, IMHO, but since its being used only a straight amplifier (without complex feedback components) it should be ok. I once tried to use an LF357 in a baxandal-configured tone control and went crazy trying to stabilize the thing...
Cheers!!
Cheers!!
Try to filter the signal at the input (bandwidth limit). Your IC sockets may add extra capacitance, direct solder the devices once you are happy with the preamplifier.
Try and use as small a cap across the feedback resistor as reasonably possible.
-Chris
Try and use as small a cap across the feedback resistor as reasonably possible.
-Chris
OK I'm just about ready to throw this amp out the window 😉 I changed the feedback resistors (they were some really old carbon types not sure why since I had 1% metal film for everything else) I incorrectly bought 4.3K instead of 4.7 but used them anyway...... S/N ratio improved quite a bit, the intermodulation distortion was still there but changed a bit in that it didn't start to rise until 9K... thought I was on the right track. Desoldered and resoldered the IC sockets (because I hadn't done it before)... IM problem went away. THD and noise increased by 20db on the right channel 
Have taken them out again and cleaned perfectly, put back in still 20db higher on right. This pcb is starting to suffer 🙁
I checked with the pc scope, and I have a 100Hz square wave coming out of the right channels opamp
Since it is 100Hz I assume it is power supply related, but square wave!!!! Before I saw this with the scope I took out the 18pf comp caps (no difference) and removed the 470pf caps across the fb resistors (no difference). The blue trace on the attached image is the input to the opamp, the pink trace is the output.....
This is with no signal... also the 0.9mv is really .09mv as I have the SC preamp in X10 mode.
despite the low level, the amp is amplifying it nicely to get a very loud buzz in the right channel speaker.
I'll keep going.... I can't believe such a simple circuit with so few components is giving me so much grief!!
one other thing I did was add in 100nf bypass caps for each opamp's + and - 15V lines, since there were no bypass caps at all... I might take them out again and see what happens....
ripple on the -ve supply rail is around 0.6 mV and on the +ve supply rail is around .2mv
edit: oh and one other thing, touching the feedback resistor lead (on the input side) makes the noises almost completely go away....scope trace changes to high freq hash
Tony.
Have taken them out again and cleaned perfectly, put back in still 20db higher on right. This pcb is starting to suffer 🙁
I checked with the pc scope, and I have a 100Hz square wave coming out of the right channels opamp
Since it is 100Hz I assume it is power supply related, but square wave!!!! Before I saw this with the scope I took out the 18pf comp caps (no difference) and removed the 470pf caps across the fb resistors (no difference). The blue trace on the attached image is the input to the opamp, the pink trace is the output.....
This is with no signal... also the 0.9mv is really .09mv as I have the SC preamp in X10 mode.
despite the low level, the amp is amplifying it nicely to get a very loud buzz in the right channel speaker.
I'll keep going.... I can't believe such a simple circuit with so few components is giving me so much grief!!
one other thing I did was add in 100nf bypass caps for each opamp's + and - 15V lines, since there were no bypass caps at all... I might take them out again and see what happens....
ripple on the -ve supply rail is around 0.6 mV and on the +ve supply rail is around .2mv
edit: oh and one other thing, touching the feedback resistor lead (on the input side) makes the noises almost completely go away....scope trace changes to high freq hash
Tony.
Attachments
OK took out the bypass caps... no difference.... however I have discovered that the problem appears to be with the coupling caps....
If I have the volume at minimum, so effectively shorted input I don't get the square wave on output... if I touch the coupling cap (MKT type and I have tried a new one today with no change) then the square wave gets bigger.... if I touch the left channels coupling cap then the left channel starts to produce a fuzzy square wave too.... when I say touch, I'm only touching the plastic housing with my finger.
Tony.
If I have the volume at minimum, so effectively shorted input I don't get the square wave on output... if I touch the coupling cap (MKT type and I have tried a new one today with no change) then the square wave gets bigger.... if I touch the left channels coupling cap then the left channel starts to produce a fuzzy square wave too.... when I say touch, I'm only touching the plastic housing with my finger.
Tony.
I'm now convinced that the right channel of this amplifier is possesed!! 😉
I suddenly had the thought, what if this square wave is coming back through the coupling cap from the power amp, so I disconnected the preamp from the power amp, and nothing changed....
Then because I'm starting to clutch at straws, I thought I wonder what would happen if I pulled the rail fuses on the right channel of the power amp (pre still disconnected from power amp). Guess what the problem went away completely
So I'm guessing that the right channel (which has always suffered from noise) is emmiting some sort of 100Hz square wave which is being picked up by only the right channel preamp opamp (the left and right opamps are within 3cm of each other on the board, and ironically the left channel one is closer to the right channel of the poweramp) Both opamps are sharing the same regulated supply (however the 15V AC is from an additional winding on the main power transformer that feeds both poweramp channels.
hmmm I just noticed something else, the coupling caps for the preamp are mounted on the board at 90deg to each other.... the left channel is parallel to the right channel power amp, the right channel one is perpendicular to the right channel power amp...
during my testing I got lazy and didn't screw the chassis back together, this resulted in smoke being emmitted from the right channels 6.8 ohm zobel resistor, I haven't changed it (since I was working on the preamp I thought I'd worry about it later), I bet it is the *#%#%$# culprit .... off to do some more desoldering!!
Oh here are the complete results for the preamp with rmaa http://home.people.net.au/~tonywww/series200_preamp_rail_fuses_pulled.htm
I can't beleive I have been desoldering and resoldering all day and it had nothing to do with that part of the circuit!!
Tony.
I suddenly had the thought, what if this square wave is coming back through the coupling cap from the power amp, so I disconnected the preamp from the power amp, and nothing changed....
Then because I'm starting to clutch at straws, I thought I wonder what would happen if I pulled the rail fuses on the right channel of the power amp (pre still disconnected from power amp). Guess what the problem went away completely
So I'm guessing that the right channel (which has always suffered from noise) is emmiting some sort of 100Hz square wave which is being picked up by only the right channel preamp opamp (the left and right opamps are within 3cm of each other on the board, and ironically the left channel one is closer to the right channel of the poweramp) Both opamps are sharing the same regulated supply (however the 15V AC is from an additional winding on the main power transformer that feeds both poweramp channels.
hmmm I just noticed something else, the coupling caps for the preamp are mounted on the board at 90deg to each other.... the left channel is parallel to the right channel power amp, the right channel one is perpendicular to the right channel power amp...
during my testing I got lazy and didn't screw the chassis back together, this resulted in smoke being emmitted from the right channels 6.8 ohm zobel resistor, I haven't changed it (since I was working on the preamp I thought I'd worry about it later), I bet it is the *#%#%$# culprit .... off to do some more desoldering!!
Oh here are the complete results for the preamp with rmaa http://home.people.net.au/~tonywww/series200_preamp_rail_fuses_pulled.htm
I can't beleive I have been desoldering and resoldering all day and it had nothing to do with that part of the circuit!!
Tony.
wintermute - that square wave and that zobel burning is due to high-frequency oscillation that your PC-scope can't see. Its probably in the MHz region. You can't expect that your left and right channel ccomponents and layout are exactly the same, so it may be that there's just a tad more instability on one of the channels than the other.
I thought that the 5534 would be a bit tamer, but seems it isnt. Put a 3.3pF or 4.7pF back onto compensation pins of the op-amp and see if that fixes it.
Cheers
I thought that the 5534 would be a bit tamer, but seems it isnt. Put a 3.3pF or 4.7pF back onto compensation pins of the op-amp and see if that fixes it.
Cheers
Hi Clem_o,
I just pulled the zobel resistor out and it was open circuit... I replaced it, and put the rail fuses back in and guess what It's clean 🙂
I think that my next step should be the mods that you and amp guru suggested 🙂 as it looks like as you say the right channel is oscillating, and that is probably the root of all my problems.
But first one more thing I have to sort out and that is to get the right value in for the resistors which I put in instead of the CCS...
after that, I assume that amp_guru is suggesting an emmiter resistor each for Q1 and Q2 so that they have some isolation, and aren't simply sharing the 22K one connected to the ccs.
Anyway I think it is time for a listening test, to see if my distortion problem has gone away 🙂 (and to make sure that no more magic blue smoke is leaking out 😉 )
Tony.
I just pulled the zobel resistor out and it was open circuit... I replaced it, and put the rail fuses back in and guess what It's clean 🙂
I think that my next step should be the mods that you and amp guru suggested 🙂 as it looks like as you say the right channel is oscillating, and that is probably the root of all my problems.
But first one more thing I have to sort out and that is to get the right value in for the resistors which I put in instead of the CCS...
after that, I assume that amp_guru is suggesting an emmiter resistor each for Q1 and Q2 so that they have some isolation, and aren't simply sharing the 22K one connected to the ccs.
Anyway I think it is time for a listening test, to see if my distortion problem has gone away 🙂 (and to make sure that no more magic blue smoke is leaking out 😉 )
Tony.
I got lost there, re CCS - when was it pulled out? Wouldnt that degrade the performance of the power amp?
You can easily estimate the resistor value by determining how much current the CCS put out originally (I), and then sub a resistor in its place using the rail supply voltage to ground (V). R= V/I
That's going to drop the PSRR a lot...
You can easily estimate the resistor value by determining how much current the CCS put out originally (I), and then sub a resistor in its place using the rail supply voltage to ground (V). R= V/I
That's going to drop the PSRR a lot...
yeah I figured it would reduce the psrr a bit but it was suggested by Hugh Dean (AKSA) ages ago, but I hadn't tryed, so I figured it was worth giving it a shot 🙂 Interestingly it hasn't seemed to affect the noise levels, though thd might be a bit higher.
I worked out the value ok, problem was that I needed a non standard value (about 32K), so I need to do some playing around with the resistors to get it right. I'm going to replace the 22K with a 51K put a 3.3K in where the transistor was and adjust with a small resistor where the 680 Ohm one is. I suppose I could match up some diodes and put the transistors back in too.
The voltage on the collector of Q4 is supposed to be around 28V (it is currently sitting at about 31V) One of the problems I had with this circuit was that the left and right always showed vastly different volatges on the collector of Q4, replacing the ccs with a resistor seems to even them up, could have been that the diodes on q3 were not very close channel to channel, and q3 was being turned on harder on one channel than on the other.
Oh and the listening test shows the distortion is still there.... Must be some weird interaction between the pre and power amp stages which are causing the oscillation in the right channel to be worse.... the amp doesn't distort when driven directly off the soundcard, but does when driven using the same input through the preamp (or from my DV18)...
Tony.
I worked out the value ok, problem was that I needed a non standard value (about 32K), so I need to do some playing around with the resistors to get it right. I'm going to replace the 22K with a 51K put a 3.3K in where the transistor was and adjust with a small resistor where the 680 Ohm one is. I suppose I could match up some diodes and put the transistors back in too.
The voltage on the collector of Q4 is supposed to be around 28V (it is currently sitting at about 31V) One of the problems I had with this circuit was that the left and right always showed vastly different volatges on the collector of Q4, replacing the ccs with a resistor seems to even them up, could have been that the diodes on q3 were not very close channel to channel, and q3 was being turned on harder on one channel than on the other.
Oh and the listening test shows the distortion is still there.... Must be some weird interaction between the pre and power amp stages which are causing the oscillation in the right channel to be worse.... the amp doesn't distort when driven directly off the soundcard, but does when driven using the same input through the preamp (or from my DV18)...
Tony.
clem_o said:
hmmm I think I should have read this more carefully!!!! although I did put back in bc556's, when I matched them I ended up with four that had identical hfe which was roughly 20-30% higher than the originals!! (out of 30 I had a range of 75 to 115, the ones I used were about 106 (this is the reading off my dmm) time for those emitter resistors... I assume that this is the point I'm supposed to be putting them in and not on the emitters of the BF469's.
You can split the resistance between two resistors and add a capacitor to ground in the middle so that you can regain some of the PSRR. Maybe a 12K and 20K in series (total 32K). Filter cap 100uF?
May produce a heck of a turn-on transient, not sure, since it'll take a few ticks to charge up the capacitor. Suggest of course 12K resistor to be the one to the rail voltage instead of the other way around. Another trick is to put a diode in series as well, so that drops in the voltage rail are isolated, keeping the filter cap as charged as possible...
Cheers!
May produce a heck of a turn-on transient, not sure, since it'll take a few ticks to charge up the capacitor. Suggest of course 12K resistor to be the one to the rail voltage instead of the other way around. Another trick is to put a diode in series as well, so that drops in the voltage rail are isolated, keeping the filter cap as charged as possible...
Cheers!
RE: BC556 - yep, that's where the emitter degeneration resistors should go, various amplifier designers seem to favor values in the range of 22ohms to 100 ohms... Expect the overal distortion to go up! 🙂
Cheers
Cheers
yeah the distortion went up 😉 had to drop the quiescent current to get it back to an acceptable level...
I have also tried a few other things today, anatech's suggestion of bandwidth limiting at the input.... I tried restricting to 60Khz no real change, tried restricting to 30Khz, small improvement but drop in levels at 20K decided this wasn't helping.....
tried the 4.7pf cap across the 22K feedback resistor in the main power amp just now, distortion went through the roof! to the point where I couldn't get rmaa to sync to the test signal.
The power amp in isolation appears to be fine
The preamp in isolation appears to be fine
When the preamp is connected to the power amp all hell breaks loose... distortion in the right channel goes up by an order of magnitude (actually it's much more than that, from 0.04% to 1.4% on the right channel), luckily at normal listening levels (probably not much more than a couple of watts max) the distortion isn't particularly noticeable, but as soon as the level is increase a bit its not pleasant.
Since the ultimate plan for this amp is only to be driving a pair of woofers up to 300Hz, and I'm going to have to make a new preamp anyway, I'm thinking that I put up with the distortion for the time being and start work on the preamp 😉
I think I will put the ccs back in too, as I think that the distortion performance before replacing it was a bit better.
what a nightmare this has been, I'm sure there is a simple explanation but I'm not sure that oscillation is the culprit.... yes it has oscillated (burned zobel resistor) but this was when the heat sink wasn't properly earthed... the Zobel components don't currently even get warm.....
I'm actually leaning towards a problem related to the volume control, since this is one of the main departures from the original design.... I changed the law faking resistor from 11K to 29K today and the listening test seemed to suggest that the distortion problem wasn't happening until a higher spl.
The other thing I have noticed is that reducing the output on the sound card, and then having to increase the volume control on the amp (to maintain the same level), seems to also reduce the distortion, but only in certain positions, its as if the distortion increases at certain volume pot locations and decreases at others..... there seems to be a sweet spot. The actual output level from the amp is remaining constant through this process the only thing changing is the voltage level being presented to the volume pot, and the position of said pot (to achieve the same output level).
anyway time to pack up for the day I think.
Tony.
I have also tried a few other things today, anatech's suggestion of bandwidth limiting at the input.... I tried restricting to 60Khz no real change, tried restricting to 30Khz, small improvement but drop in levels at 20K decided this wasn't helping.....
tried the 4.7pf cap across the 22K feedback resistor in the main power amp just now, distortion went through the roof! to the point where I couldn't get rmaa to sync to the test signal.
The power amp in isolation appears to be fine
The preamp in isolation appears to be fine
When the preamp is connected to the power amp all hell breaks loose... distortion in the right channel goes up by an order of magnitude (actually it's much more than that, from 0.04% to 1.4% on the right channel), luckily at normal listening levels (probably not much more than a couple of watts max) the distortion isn't particularly noticeable, but as soon as the level is increase a bit its not pleasant.
Since the ultimate plan for this amp is only to be driving a pair of woofers up to 300Hz, and I'm going to have to make a new preamp anyway, I'm thinking that I put up with the distortion for the time being and start work on the preamp 😉
I think I will put the ccs back in too, as I think that the distortion performance before replacing it was a bit better.
what a nightmare this has been, I'm sure there is a simple explanation but I'm not sure that oscillation is the culprit.... yes it has oscillated (burned zobel resistor) but this was when the heat sink wasn't properly earthed... the Zobel components don't currently even get warm.....
I'm actually leaning towards a problem related to the volume control, since this is one of the main departures from the original design.... I changed the law faking resistor from 11K to 29K today and the listening test seemed to suggest that the distortion problem wasn't happening until a higher spl.
The other thing I have noticed is that reducing the output on the sound card, and then having to increase the volume control on the amp (to maintain the same level), seems to also reduce the distortion, but only in certain positions, its as if the distortion increases at certain volume pot locations and decreases at others..... there seems to be a sweet spot. The actual output level from the amp is remaining constant through this process the only thing changing is the voltage level being presented to the volume pot, and the position of said pot (to achieve the same output level).
anyway time to pack up for the day I think.
Tony.
welcome to integrated amps! 🙂 This is one of the reasons why separates are preferred...
Ok, so the power amp doesn't like a 4.7pF on the feedback. Concentrate on the preamp, since it seems that if you drive your power amp externally, everything is ok. Have you put back a small amount of compensation capacitance on the preamp (5534)? Suggest tack-soldering onto the copper side and giving it a quick test. The 4.7pF you removed from the power-amp fb should be ok to try out on the 5534.
BTW, can you tell me exactly how the thing is configured now (since you removed the balance control etc?) - i.e. is it something like
inputs --> cmos switches --> tonecontrol --> gain stage (5534) - volume control --> power amp
Cheers
Ok, so the power amp doesn't like a 4.7pF on the feedback. Concentrate on the preamp, since it seems that if you drive your power amp externally, everything is ok. Have you put back a small amount of compensation capacitance on the preamp (5534)? Suggest tack-soldering onto the copper side and giving it a quick test. The 4.7pF you removed from the power-amp fb should be ok to try out on the 5534.
BTW, can you tell me exactly how the thing is configured now (since you removed the balance control etc?) - i.e. is it something like
inputs --> cmos switches --> tonecontrol --> gain stage (5534) - volume control --> power amp
Cheers
Hi Clem_o
originally it was cmos ---> buffer ---> cmos ---> buffer ---> volume pot ---> ne5534 (gain) ----> tone controls (implemented with another ne5534) ----> balance pot -----> power amp.
Now it is cmos ----> buffer -----> cmos ---> buffer ----> volume pot ---> ne5534 (gain) ----> power amp.
I thought a few days ago maybe the fact the balance pot was missing might be upsetting something so I soldered in a couple of 12K resistors from the preamp side of the coupling cap to 0V to simulate roughly what the balance pot would be doing in it's neutral position (the pot was 10K but I had 12K resistors lying around).
It didn't make any difference 🙂
I'm currently running with 2.2pF caps as comp caps, I'll try increasing them to 4.7pf....
I'm running out of time though, I have a deadline of this Friday to stop spending so much time on the amp.... the GF is getting concerned that I need to stop diy-ing and start looking for a job 😉 I chucked in my job back in March cause I'd had enough, and have been having a very nice holiday 🙂 (luckily I had 3 months long service leave payout).
part of me doesn't want to be beaten by this amp, the other part says just make a preamp and forget it 😉
I had another thought this morning about the 4.7pf on the fb resistors in the power amp, should I have reset the quiescent current after adding??
Tony.
originally it was cmos ---> buffer ---> cmos ---> buffer ---> volume pot ---> ne5534 (gain) ----> tone controls (implemented with another ne5534) ----> balance pot -----> power amp.
Now it is cmos ----> buffer -----> cmos ---> buffer ----> volume pot ---> ne5534 (gain) ----> power amp.
I thought a few days ago maybe the fact the balance pot was missing might be upsetting something so I soldered in a couple of 12K resistors from the preamp side of the coupling cap to 0V to simulate roughly what the balance pot would be doing in it's neutral position (the pot was 10K but I had 12K resistors lying around).
It didn't make any difference 🙂
I'm currently running with 2.2pF caps as comp caps, I'll try increasing them to 4.7pf....
I'm running out of time though, I have a deadline of this Friday to stop spending so much time on the amp.... the GF is getting concerned that I need to stop diy-ing and start looking for a job 😉 I chucked in my job back in March cause I'd had enough, and have been having a very nice holiday 🙂 (luckily I had 3 months long service leave payout).
part of me doesn't want to be beaten by this amp, the other part says just make a preamp and forget it 😉
I had another thought this morning about the 4.7pf on the fb resistors in the power amp, should I have reset the quiescent current after adding??
Tony.
Hey Tony,
Never give up. But you can take your time and think about it. Eventually you'll win.
-Chris
Never give up. But you can take your time and think about it. Eventually you'll win.
-Chris
Thanks for the encouragement Chris 🙂
OK I just put in the 4.7pf caps.... no difference 🙁 I decided to put up another RMAA output. This is taken at the 8 Ohm dummy load.
the freq response of the right channel is interesting to say the least 😉
also note the distortion in the right channel is pretty much an order of magnitude higher than the left. Left channel THD (at the moment) when directly driven from the sound card is around .05% so it is also a bit higher, but obviously not nearly as bad as the right channel...
I might try putting the 18pf caps back in and see what happens, though I have my suspicions that they won't make any difference....
One other thing, after checking the schematic for the original preamp, the output of the tone controls passes through a 680 Ohm resistor before being fed to the balance pot, I'm wondering if inserting a resistor between the output of the ne5534 and coupling cap will possibly make a difference, might be worth a try 🙂
Tony.
OK I just put in the 4.7pf caps.... no difference 🙁 I decided to put up another RMAA output. This is taken at the 8 Ohm dummy load.
the freq response of the right channel is interesting to say the least 😉
also note the distortion in the right channel is pretty much an order of magnitude higher than the left. Left channel THD (at the moment) when directly driven from the sound card is around .05% so it is also a bit higher, but obviously not nearly as bad as the right channel...
I might try putting the 18pf caps back in and see what happens, though I have my suspicions that they won't make any difference....
One other thing, after checking the schematic for the original preamp, the output of the tone controls passes through a 680 Ohm resistor before being fed to the balance pot, I'm wondering if inserting a resistor between the output of the ne5534 and coupling cap will possibly make a difference, might be worth a try 🙂
Tony.
tony,
I'm pretty confident that the problem is that 5534 patched straight into the amplifier's input. This shouldn't look like a problem, but when circuits are close to each other its really very difficult to get a layout that will keep things stable. One way of "decoupling" the whole mess is to put a resistor in series. That 680ohm probably did two things - allowed the balance control to work as the "bottom" of a resistive divider, as well as being a series resistance to 'decouple' the whole preamp-poweramp loop enough to keep the system stable.
Don't bother with going to 18pF, that's going to slow down the 5534s and detract from the liveliness of the HF response of the system. Just put enough series resistance between the output of the 5534 and the power amp - 680 ohms, maybe 1K...
cheers!
I'm pretty confident that the problem is that 5534 patched straight into the amplifier's input. This shouldn't look like a problem, but when circuits are close to each other its really very difficult to get a layout that will keep things stable. One way of "decoupling" the whole mess is to put a resistor in series. That 680ohm probably did two things - allowed the balance control to work as the "bottom" of a resistive divider, as well as being a series resistance to 'decouple' the whole preamp-poweramp loop enough to keep the system stable.
Don't bother with going to 18pF, that's going to slow down the 5534s and detract from the liveliness of the HF response of the system. Just put enough series resistance between the output of the 5534 and the power amp - 680 ohms, maybe 1K...
cheers!
May sound silly but, have you checked the solder joints on your inputs (connector to wire, and wire to board)?
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