Total Recall sound and voltage problems around bottom half of IC5 +IC2

[tvas22]

Hi everyone,

Sorry, I realised I post this in the wrong forum before.

I have built a lot of stripboard projects and some other bits, but this was my first PCB.

I have had some trouble with it - first I had the balance and feedback pots swapped and was getting runaway feedback pretty much straight of the bat. I have swapped those but I damaged the middle solder pad in both, so I have a wire running off each on the other side of the board. That seems to be OK now.

The current problem I can't seem to figure out is somewhere around the second half of the ne570 or the second half of the tl072 in IC5, the audio is getting destroyed almost like a bit crusher type effect, and becoming really quiet. The voltages are also all wrong around there too (around double what they should be for some pins). The only issue I have been able to identify is R30, which should be 11k but is only actually around 7.5k. I have attached the voltages as a picture here (everything is -dc, I just wrote down the numbers). The audio coming off the MN3008s is fine, it seems to be around pins 10, 11, 12 of the NE570 that the issue becomes apparent - pin 12 is a little distorted, 11 a little more distorted and quieter, and pin 10 is extremely quiet and extremely distorted.

I was wondering if R30 being too low would be the problem, or if anyone had any other suggestions about what might be causing this issue? I'd rather wait to go back to change R30 when I'm sure there isn't another obvious problem.

Thank you for the help!

Update: I went through and double checked all the resistors with a multimeter, and R9, R10, R11, R13, R30, R47, R49, and R59 are all pretty far from what they should be, although the colour codes are correct. R9 is the most out, measuring ~15k rather than 68k. It seems maybe I got a bad batch of 68ks, as the other two are around 49k. R59 measures around 50k, the rest aren't too far out but I have ordered replacements for each of these to see if they make a difference.

[tvas22]

Ok, so I didn't take the IC chips out when probing the resistors... R30 is actually fine.

From reading around other posts that have distortion on the NE570 (though different projects), I'm thinking that it is an issue with the NE570 bias? As the start of this distortion and volume crushing is pin 11 and 10 of the NE570. I have ordered another NE570 of a different brand, as I bought the cheapest I could find in the UK, in case it is an issue with the chip. They aren't too easy to find over here.

On the other posts, it was suggested that the resistor coming off pin 12 is responsible for what happens at pins 10 and 11, and if the voltages are too high (or... too low, if it's negative DC? I'm not entirely sure which way round it should be) for pins 11 (-12.85 vs -8.48 in the doc) and 10 (-13.37 vs -6.76 in the doc) I should change that. I wasn't sure if I should try lowering or raising the value of R30 though?

Thank you for your help.

[Scruffie]

Trying to read resistors in circuit is not a good idea, you wont get an accurate reading half the time due to their place in the circuit. If you need to do it you either have to know what the specific resistor is doing or lift one of its legs to read it.

I haven't got the 570 datasheet in front of me but from your voltages either the chip is dead or you have a short somewhere, several of the pins should be fed from the internal voltage reference and they're all over the place on that second half so it's not a simple resistor error.

I can probably pop you a 571 in the post if your new 570 doesn't work out.

[madbean]

R32 seems to be a problem. Colors are hard to see but that does not look like 7k5.

[Scruffie]

R32 seems to be a problem. Colors are hard to see but that does not look like 7k5.

Looks like one to me.

Shouldn't affect all those other voltages anyway, it's just in parallel with the internal op amps gain resistor.

[tvas22]

Thanks for the replies.

R32 is right, I double checked. The colours are a bit off on the picture I attached, makes it hard to see the violet.

I tried a different TL072 in IC5 as I had one to spare and thought why not, and got initially excited as the voltages that were wrong on both the NE570 and IC5 were now right - however, by the time I got my audio probe set up the voltages were too high again.

From playing around, it seems that the voltages on pins 10 and 11 of the NE570 and pins 5,6 and 7 on IC5 creep up from the correct voltages to around -13, while pins 14, 15 and 16 of the NE570 drift to about -10 from -13. So starting at roughly the correct voltages immediately when powering on but also immediately beginning to change.

Probing around pins 10, 11 (NE570) and 5, 6, 7 (TL072) it seems that they also sound as they should immediately after turning on, but then the sound slowly becomes more distorted and crushed as the voltages change. It seems to stabilise (but as the wrong voltages and sound) between 5-10 seconds after turning on.

I have no idea what I've managed to do... I can't seem to find anything wrong, but the replacement NE570 has dispatched and should arrive in a few days, I'll double check everything over for shorts or other problems while I wait on that.

[tvas22]

Hi,

It took forever but the replacement NE570 came - unfortunately it does not fix the problem. The voltages are still pretty much the same.

While I was waiting for it to arrive I took it apart, made sure there were no shorts that I could see, checked over the orientation of everything and the cap values. I also reflowed any solder joints I thought looked a little rough.

The problem is still the same as it was before - voltage on IC5B and IC2B just creep up after plugging it in. It starts off roughly the right values, then over about 20-30 seconds it creeps up to the wrong voltages before stabilising at roughly the voltages I posted.

I've never come across anything like this before, any ideas I can try would be greatly appreciated! It has me stumped.

[Scruffie]

What are IC2's voltages like if you leave out IC5.

[tvas22]

Thanks for the reply - if I leave out IC5, then IC2's voltages are completely perfect.

[tvas22]

OK. So... I have this almost figured out. First off - some of the 1uf caps and some others were tantalum, which I hadn't realised were polarised (!! pretty stupid, I thought that they were ceramic when they came initially).

Now, everything is almost, almost working. If I probe the top of C32, I get some nice sounding delay. However... only clean guitar from the output still. If I probe blend pin 3 - I just get dry guitar.

I'm thinking I potentially fried the blend pot when I swapped it with the feedback pot. Does this sound probable... or is it more likely to be an issue with C32?