King Of Tone / Queen of Bone Build Nightmare (Rullywow PCB)

[pringe]

Hi guys,

I've been steadily working on Rullywow's Queen of Bone PCB (based on Analogman's King of Tone...basically a dual Bluesbreaker circuit) for the past week...I arrived at the stage today where everything was ready to go, ICs socketed...however I made a massive newbie error and wired the DC jack as centre positive (my only excuse is that I did it at the same time as the in/out jack and stupidly followed the logic of centre being red wire without remembering I was plugging in a centre negative DC supply...doh).

On powering up I got LEDs working when switching each channel in and out, but (obviously) no signal when switching the channels in. I only realised my mistake after having power on for a couple of minutes so I'm sure damage was done

Once I'd rewired the DC jack correctly I powered on again, and this time I got nothing...the LEDs didnt even turn on any more. I've seen RG Keen's guide for reverse polarity damage and know that it's likely the ICs and electrolytics are fried so I began by replacing all ICs (the charge pump and 2 op amps) but to no avail. I then got the DMM out and recorded the following voltages:

Supply voltage: 9.38v


ICs

IC1: RC4558 (Channel 1):

1.   8.83
2.   8.85
3.   4.39
4.   0
5.   8.50 (starts at 8.60 and slowly falls)
6.   9.10 (8.80 and rises slowly)
7.   8.66
8.   17.40

IC2: RC4558 (Channel 2):

1.   8.84
2.   8.85
3.   4.39
4.   0
5.   8.50 (starts at 8.62 and slowly falls)
6.   9.07 (starts at 8.79 and rises slowly)
7.   8.66
8.   17.40

IC1: TC1044:

1.   9.54
2.   5.95
3.   0
4.   -3.06
5.   -3.10
6.   4.85
7.   4.29
8.   9.54

DIODES:

D15:      in = 9.38      output = 9.54. 1n5817   diode measures:   212
D13:      in = 9.54       output = 14.65         diode measures:   209
D14:      in = 14.65    output = 17.41         diode measures:   209


-

From the voltages above I could see that the charge pump was giving more or less the +18v, but the inverted voltage being -3.1V lead me to suspect blown electrolytic caps on the charge pump circuit. Also the bias coming into pin 3 of each opamp seemed off at 4.39V? Anyway I changed C20 and C21 (I couldn't get to C22 without removing a pot so wanted to see if this cured the problem first), powered up again and this time got different voltages on the charge pump (pin 1 was approx 8.9v and pin 5 was approx -0.3v), but the IC got VERY hot VERY quickly....I double checked the new cap joints and they seemed OK but resoldered anyway...no improvement.

So now I am stumped as the problem seems worse than before, and aside from kicking myself at my schoolboy error I initially made I am not sure what to check for next? Any help would be greatly appreciated!

Thanks,

Steve

PS. Schematic is here:

http://www.rullywow.com/wp-content/uploads/2016/07/QoB2-rev2A-Build-Doc-REVISED.pdf

[GGBB]

C22 will almost surely need replacing, as will C1. I wouldn't bother going any further until you've taken care of those two.

[pringe]

Thanks for the reply - I'll change all the electrolytics out first then! I'm also going to build the charge pump part of the circuit on a breadboard to check if I've fried the TC1044.

[pringe]

I just set up the charge pump part of the circuit on a breadboard with fresh caps and the original IC that was socketed when I applied reverse polarity DC. The only change I had to make was to use 1N1004 as the first diode from the 9v supply as I only has 2 1N5817s left...Interestingly the voltages I got are around about the same as those that that I recorded with the fresh IC in the PCB after I'd sorted out the polarity supply:

1.   8.83
2.   5.70
3.   0
4.   -3.12
5.   -3.21
6.   4.12
7.   4.49
8.   8.83

So therefore it doesn't look like this IC got damaged?

I also realised that the reason I was getting -3.2V from the inverted voltage pin is because the circuit had left off the second capacitor to ground. Looking at the schematic I can see now that the inverted voltage output isn't used, only the voltage doubling part of the circuit, and the op amps are biased to 9v via a 47k voltage divider. Steep learning curve for me but enjoying the journey!

Will replace all the electrolytics on the PCB now and see where that takes me...

[duck_arse]

I can't see any leds on the circuit diagram, so I have no idea how they are wired - but how come you had leds working when the polarity was reversed? surely they would work when the supply was re-right connected, unless they went dark emitting.

and those output caps in parallel, 1uF poly with 1uF electro - what's that about?

[pringe]

I'm not sure why there are parallel output caps - I'm trying to learn the circuit as I go and haven't figured that part out yet...though I did come across this article with a circuit for a buffer and they talk about the parallel caps "ensuring that the response remains flat at high frequencies where the rising impedance of electrolytics could affect the sound."  http://www.muzique.com/lab/superbuff.htm Is it being used for this purpose I wonder?

Regarding the other issues: I did a bit of tracing tonight and found that for some reason the LED square pad is negative on the PCB (for the caps it's the opposite)...resoldered and LEDs now work!

I also removed all the electrolytic caps, cleaned up all the pads and resoldered fresh ones in and that seems to have done the trick! The pedal is now working (even the charge pump which got very very hot!) apart from a couple of issues:

1. There is a big pop when turning on either channel...there are 1 Meg pulldown resistors on the input and output so that's not the issue (I don't think?!). I have done a bit of searching and people have been saying the LED itself can cause the pop - I'm wondering if this could be the case with mine? I put a fairly low value CLR in the circuit (~400 Ohms) to keep the LED bright (probably too bright  )....does this sound like the culprit?

2. When engaging the 2nd DIP switch for either channel (which switches in the 2nd set of diodes) I get an awful oscillating sound like a bass synth...lowering the gain pot lowers the pitch until you can hear the oscillations. Pretty interesting but not what I'm after! Could anyone shed any light on why this might be? I've tried swapping the diodes out but the problem remains..

Thanks for any advice, I'm finding all the posts on this forum really helpful for learning (steeply!).

Steve

[pringe]

PS. for future reference is there any way to test if capacitors are healthy in a circuit without either (a) pulling them out and testing in a dedicated meter or (b) swapping them out to see if that changes anything? Thanks!

[GGBB]

"ensuring that the response remains flat at high frequencies where the rising impedance of electrolytics could affect the sound."

That makes sense in cases where the cap value required for linear response is greater than what's readily available in film - you would have been more likely to see that sort of thing in days past when large film caps like 1uF weren't available and cheap and small. Don't know that it applies here - the original bluesbreaker value was a mere 100n. I once saw a post from Brian of MadBean that the lone 1uF film was sufficient (context was the Aristocrat KOT clone project ).

found that for some reason the LED square pad is negative on the PCB (for the caps it's the opposite)

IMO that's correct, but it's only opinion. Cap anodes are typically square. For diodes it is the cathode pad that's typically square. An LED is a diode. Opinions seems to vary widely - MadBean boards make the diode anodes square.

people have been saying the LED itself can cause the pop - I'm wondering if this could be the case with mine? I put a fairly low value CLR in the circuit (~400 Ohms) to keep the LED bright (probably too bright  )....does this sound like the culprit?

A low CLR exacerbates that problem, so it could be your issue. Disconnect the LED entirely and check. If that's not possible and nothing else is connected to it, jumper the LED always on and test. I use the first solution here and find it effective.

No clue about the oscillation - sorry - but it sounds like a build fault (or possibly layout) as I've never come across it before with this circuit.

[pringe]

Thanks for the reply! Much appreciated. I'll do further investigation on the LED issue and see what I xan uncover.

For the oscillation  - having looked at the schematic I see that DIPs 2 and 4 (the ones causing the problem) are both fed from VB. as the problem exists the same on both channels regardless of the the type of diodes used would it be a safe bet to assume there is a problem with the part of the circuit that generates VB? Or would there be other obvious things to check that I have missed?!

[steelie]

HI all wondering if anyone can help. I am building a QoB.
I stupidly put in C18 the wrong way round (reversed polarity). Now the channel won't work at all.
Any ideas on upstream / downstream damage from that?
Thanks in advance for your time.

[idy]

If you are using the Rullywow PCB then c18 is an output cap. It would be exposed to 9v (the voltage B for this pumped up circuit.) There is not much it could hurt by failing.

I would replace it and inspect for solder bridges or other fails.

Is the charge pump still providing normal voltages?

[steelie]

First of all thanks so much for replying.
I will check the CP voltages but channel 1 seems to be working fine. Will check them now.
I am using a Rollywow detours as switch PCBs that are downstream to C18, do you think it could have affected those?
Best wishes and thanks again,

Steelie

[steelie]

HI there,
Thought I would give an update.
Swapped out the capacitor next to C18 and removed a wire clipping that had fallen and crossed 2 terminals on the fDPDT board and hey presto all works. I wanted to thank you as I spent a few hours learning to properly use a multimeter to measure voltages and learned how to use an audio probe. In the end I think it was that wire clipping but I learned a great deal. Wanted to say thanks.
Best

Steelie