Toft ATB Master Module upgrade - Help Please?!?

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morls
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Toft ATB Master Module upgrade - Help Please?!?

Post by morls » Mon Dec 15, 2014 11:42 pm

Hi,

I have an ATB08, and am trying my hand at doing the master strip mods, posted at various places around the internet, recommended by Jim Williams. I'd like to say thanks to Jim for his generosity in sharing his expertise.

I've done a few builds in the past - a couple of Hairball 1176 clones, and a matched pair of Drip Pultecs - but am still very much a beginner. I plan on having the Toft for a long time, so it makes sense to me if I start to get to know how it works, even if I make a few mistakes along the way.

Which leads me to my current situation:

After doing all the mods - removing and substituting caps, swapping opamps, installing bypass caps, I have had some mixed results. I have gone back over my work twice now, each time double-checking all soldering and made sure I have the right opamp pins linked to ground. I've touched up a few bits of sloppy soldering, but the basic, persistent issues have remained the same:

The Master Outputs are crystal clear, and sound GREAT. Before getting the Toft my monitoring setup was Lavry Blue - Passive Attenuator - Power Amp. Compared to this, monitoring through the Toft was difficult, hence my decision to upgrade. Now the Main Outs are just so much better, it's really, really exciting. Thanks Jim.

Unfortunately, the Monitoring outputs (main speakers, alt speakers, headphones) are not so good:
- the left channel is not working, stereo or mono.
- the left LED and left VU meter aren't working, unless I push the mono switch. Then all meters work, but still no sound from left channel;
- the sound in the right channel is attenuated and muffled;
- the headphones have the same issue, except both channels DO work in mono.

I've gone over things a couple of times, but without a schematic (PMI seem very hesitant to send one), and with limited knowledge, I've pretty much reached the limit of what I can do without help.

Can anyone offer advice/help?

Thanks,

Stephen

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Post by Jim Williams » Tue Dec 16, 2014 8:42 am

If you removed caps you probably broke a through hole on the pcb. That will stop the signal there. I short them out with a wire as those pcb's are very fragile and easy to damage.

Trace the signal through with an oscillator or music. Where it stops is where the damage is.
Jim Williams
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Post by Jim Williams » Tue Dec 16, 2014 8:42 am

If you removed caps you probably broke a through hole on the pcb. That will stop the signal there. I short them out with a wire as those pcb's are very fragile and easy to damage.

Trace the signal through with an oscillator or music. Where it stops is where the damage is.
Jim Williams
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morls
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Post by morls » Tue Dec 16, 2014 2:49 pm

Thanks Jim, I really appreciate your help.

I don't have an oscilloscope yet, but I've traced a sine wave through, and might have found at least part of the problem. There is a lot of distortion happening at IC9, which is a TL072 that wasn't replaced. I did, however, install 0.1uf caps from pins 4 and 8 to ground. I'll remove these and see what happens.

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Post by morls » Thu Dec 18, 2014 11:14 pm

Ok, so I've been working my way through the circuit using a -20dBFS 1kHz tone, and have found something that might be the issue. All is well up to IC3, and I have noticed that the signal across R5 and R6, which feed pins 5 and 6 of IC3 (LME49720) is distorted. Where it enters the resistor it's clean, but on the other side it's distorted and attenuated. If my circuit tracing is correct R5 and R6 feed input B of IC3, which is actually the left channel of the monitoring.

I've checked R7 and R8 which feed input A, and the signal is clean both into and out of the resistor.

I removed R5, and it measures at 14.96K ohms, which is within tolerance, so I'm a bit confused.

Is the distortion across R5 and R6 likely to be the cause, or just a symptom of something else? I don't know if a resistor can degrade the signal like this yet maintain it's specified resistance.

At this point, I think I might change R5 and R6, but any advice would be greatly appreciated.

Cheers

Stephen

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Post by Jim Williams » Fri Dec 19, 2014 7:58 am

It's not a resistor or anything the was left alone. The problems are located where you did the work.
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morls
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Post by morls » Fri Dec 19, 2014 11:12 am

Thanks Jim, much appreciated.
Stephen

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Post by morls » Wed Dec 31, 2014 1:29 am

Hi again,

The tracing of the circuit is going well, and I've found quite a few areas that needed tidying up. It's been a great way for me to gain some skills, and it seems the second time around I'm doing much better. It's still not up and running, but I feel I'm getting closer.

I have come across a couple of capacitors that have me scratching my head. C25 and C26 were replaced with 1000uf 35V panasonic FM's. C25 has the negative to ground, which makes sense, but if I follow the polarity marked on the circuit board C26 has the positive to ground. There is another small cap in this area with the same polarity (according to the PCB marking) as C26.

This seems wrong to me - I would have thought that all polarized caps would need the negative terminal grounded, and I can't see connecting positive to ground being correct.

Is it possible the PCB is wrong for these 2 caps, C26 and it's neighbour? Should I reverse them?

Thanks

Stephen

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Post by Jim Williams » Wed Dec 31, 2014 8:02 am

If you reverse those you will let the smoke out. Contact Toft for a schematic.
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morls
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Post by morls » Wed Dec 31, 2014 11:38 am

That's the thing, I've been in contact with PMI repeatedly and they won't send me a schematic. Apparently their policy is not to send schematics to individuals. It's very frustrating!

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Post by The Scum » Wed Dec 31, 2014 2:23 pm

I can't see connecting positive to ground being correct.
Even if the negative lead is tied to something that's below ground, like a -15V supply rail?

You're describing a very common configuration for supply decoupling caps...negative is relative. If they were in backwards, they'd have popped as soon as you powered it up, and looked like an exploded mouse nest.

Do you have the chops to begin drawing your own schematic?
"What fer?"
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morls
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Post by morls » Wed Dec 31, 2014 3:03 pm

The Scum wrote:Even if the negative lead is tied to something that's below ground, like a -15V supply rail?

You're describing a very common configuration for supply decoupling caps...negative is relative.
Ah! Thank you! Now I understand what this cap does. Seeing such a wide trace on the PCB made me think it was ground, but it's the -ve supply rail. I'll now go through and relocate the caps from those opamps (pins four and eight) I mistakenly connected to this rail, thinking it was ground.

My mistake was that of assuming a supply rail would not be any wider than necessary so as to avoid any loss due to impedance or similar, hence a wide trace must be ground... :oops:
Do you have the chops to begin drawing your own schematic?
The thought had crossed my mind. I think I could do it, but I want to try one more time with PMI. I've just emailed them back asking them if they will supply a schematic to my music business, as this is set up in my name.

Thanks for all the help.

Stephen

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Post by The Scum » Wed Dec 31, 2014 3:33 pm

My mistake was that of assuming a supply rail would not be any wider than necessary so as to avoid any loss due to impedance or similar, hence a wide trace must be ground...
If they're made to any sort of modern spec, the entire surface of the PCB, everywhere there isn't some other signal, will be covered with copper, and that copper will be grounded. When a component needs to meet ground, it gets little crosshairs to join it to that beefy ground.

http://en.wikipedia.org/wiki/Copper_pour

The supply traces will then usually be the heaviest traces on the board, but for most everything in a mixing console, there's not much current, so .024" might be heavy enough, especially if routed in a star configuration on the board.
"What fer?"
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morls
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Post by morls » Wed Dec 31, 2014 8:27 pm

Thanks for the link, it's an informative article.

I don't think this PCB has a ground plane. It has traces on both sides, and one method I've been using to follow the signal through is to hold the board up to a strong light. This makes it easy to see the trace on the side facing me, and I can also just make out the trace on the other side. I think if there was a ground plane this wouldn't work.

The way I've been determining ground is to use my multimeter to test continuity between the sleeve of the TRS outputs and whatever point I'm testing.

I've found that there is continuity between ground and the tip of the ALT. SPKR output, which is an unbalanced 1/4" and should therefore be +ve. Fixing this, as well as grounding the opamp stabilizing caps properly, might go a long way toward sorting this out.

I might have found another part of the problem. Following Jim's directions for the mod, I bypassed C1 and C2 with a wire. But there is also a second, small, PCB, holding the Right channel output/insert/monitor jacks, and it's own C1 and C2. Thinking they were the equivalent of the main board caps, I removed these and put a wire in place. Was this a mistake?

I've thrown out all the caps I removed. Does anyone know the value of the original caps, C1 and C2 on the little PCB, PC111-03?

Stephen

morls
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Post by morls » Sat Jan 10, 2015 4:35 pm

OK, so I have made some good progress here, thanks to the help offered. Thank you very much, it's one of the great things about the internet when generous people share their expertise.

I decided to set out and draw a circuit diagram of the master module. This involved removing a couple of connector sockets so I could see where all the traces were going, and a lot of head scratching as I tried to understand what was happening in the circuit. It's been a really rewarding thing to do, and has taught me a lot.

I've checked all the work I have done, and I can identify 3 errors I made:

- When I first did the mods, I followed the directions given by Jim to bypass C1 and C2 with a wire. However, I also did this to C1 and C2 on the small PCB holding the right channel TRS jacks, thinking they were the right channel equivalent of C1 and C2 on the main PCB. I was wrong - they are involved in coupling the ring of the right monitor and alt monitor jacks to ground. Assuming their equivalents for the left channel are C63 and C64, I've replaced C1 and C2 on the small PCB with 22uf 16V caps. I think this was the cause of the initial problems I had after doing the mods.

- My lack of knowledge about electronics led me to reverse the polarity of C26, prior to asking about it here. This impatience resulted in my blowing out the -17V power rail in the PSU. I'm hoping this is the full extent of the damage caused. I have replaced both C25 and C26 with low ESR 1000uf 25V caps, with correct polarity.

- some of the small caps i placed on the opamps from the power pins to ground weren't actually going to ground, as I didn't fully understand the circuit. After doing my circuit diagram I checked and corrected all of these caps, making sure there was continuity between where I was connecting them and the sleeve of the TRS output jacks.

I've double and triple-checked every connection I made, and ensured there is continuity through each. I am as confident as I can be that I have now understood the circuit sufficiently to make the right changes.

So, I hope that once the PSU is repaired the console should be up and running.

Thanks again for the help. I've learnt a lot, and overall it's been a really worthwhile thing to do, even though the service folks at PMI must think me a fool.

Cheers

Stephen

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