TL Audio Fattrack repair

[benacquier]

Hello Everyone,
I'm quite new to the forum and diy audio. I've built some kits, and try to learn electronics through readings and practice, all regarding audio mixing. I've already found many helpful threads in this forum, and would like to get some help about a subject which is not yet developped in the forum, but I might have missed it.

I have a TL Audio FatTrack for quite a long time, and just went through a big cleaning, trying to find what generate problems in the audio path, pulling off all the cards, looking for some visual evidences of failure, but after reassemble the unit still produces most of the issues:

- some pots are noisy when turned, less than before the cards cleaning with non-residue alcohol, but still. Should I try to change a couple of pots to begin with? Change also the electrolytic capacitors? they look fine, but after 15 years of service... maybe something else?

- the 4 stereo channels labelled A, B, C and D have another issue, none of them is correctly left/right balannced. I ran a 1Khz tone @ +4dBu, and get a 5 to 7 dB difference between each Left/ Right channels, on the digital meters. Quite unuseable for summing. Is there a calibration procedure? Could the components from each channel be faulty?

- the tube stage calibration might need a check, but I can't find any procedure about that. One of the photos shows 4 calibration sockets on the tubes pcb?

That's a lot to ask, I know
Any help would be greatly appreciated.


Thank you

Ben

 

[Script]

Noisy pots, if cleaning doesn't help, better be replaced at some point.

Swapping electrolytics, maybe -- maybe not.

But first of all, you need to find out where the deviant 5 to 7 dB come from. Doesn't sound like calibration.

If you swap L & R cables, does the deviation follow ? If you swap the two tubes, does it follow ?

I have no experience with this unit. Do you have the schematics ? If not, see here:

Post in thread 'TL Audio' TL Audio

 

[benacquier]

I've swapped and changed the cables, also tested with the adat inputs, unfortunatly none of them impacted the deviant 5 to 7 dB. I've also ran some loopback tests with my audio interface inputs and outputs, everything is ok on that side.

I've got the schematics from the forum already, but dont really know how to use it for debugging
Maybe focusing on a pcb supporting one stereo channel and its schematic, could help me to replace components, but I need to finish my readings first.

Thank you very much Script.

 

[scott2000]

The ribbon cables used in some of their products can be problematic. As well as the trimmers in various areas. Can usually chopstick stuff to find out some of this. Those light blue rubycon caps they sometimes used were a problem in some spots after long service too. Never seen one of these units so who knows. Don't give up on it. You'll get it figured out.

 

[Script]

What Scott said : chopstick tapping on trimmers and maybe even op amps (socketed, corroded op amp pins).

If the L/R deviation is pretty much the same on all four channels, then something in the summing stage is faulty.

If the pots are noisy without audio going thru them, it might be DC leakage, pointing to a faulty cap (pointing to [a full] recap).

If an inverting op amp has Rin of 10k and a feedback Rf of 22k, it's a gain factor of 2.2, which equals 6.8 dB, maybe pointing to a faulty op amp.

Could measure AC on outputs of all corresponding op amps with test signal going thru unit, to find out where volume deviates. Then take it from there.

Could measure op amp outputs and inputs for DC with no test signal present, to check for DC leakage and /or faulty op amp.

Looks like quite a task but worth it, I think. Good luck and please report back

 

[benacquier]

The ribbon cables used in some of their products can be problematic. As well as the trimmers in various areas. Can usually chopstick stuff to find out some of this. Those light blue rubycon caps they sometimes used were a problem in some spots after long service too. Never seen one of these units so who knows. Don't give up on it. You'll get it figured out.

Thank you for the help scott.
I took the time to chopstick the trimmers, op amp sockets and resolder some caps, but I guess the unit needs a complete recap as it's still noisy without signal through it. Have to order them.
I measured the ribbon cable to change them too.

The back of some pcbs were partly covered by a brownish liquid, maybe leakage? Cleaned with alcohol, add a bit of solder. Maybe it will improve already, really nice from you

 

[benacquier]

What Scott said : chopstick tapping on trimmers and maybe even op amps (socketed, corroded op amp pins).

If the L/R deviation is pretty much the same on all four channels, then something in the summing stage is faulty.

If the pots are noisy without audio going thru them, it might be DC leakage, pointing to a faulty cap (pointing to [a full] recap).

If an inverting op amp has Rin of 10k and a feedback Rf of 22k, it's a gain factor of 2.2, which equals 6.8 dB, maybe pointing to a faulty op amp.

Could measure AC on outputs of all corresponding op amps with test signal going thru unit, to find out where volume deviates. Then take it from there.

Could measure op amp outputs and inputs for DC with no test signal present, to check for DC leakage and /or faulty op amp.

Looks like quite a task but worth it, I think. Good luck and please report back

Hi Script,
I'm really graceful for your answers. It took me quite some time to go through some of your suggested tests, and have still some work to understand and manage the most of them.
After cleaning and reassembling, most of the issues remained. Some improvements for the noisy pots, and for the 2 channels unbalanced preamp stage -I didn't wrote about that in my previous posts, which are now balanced again.
The L/R deviations for the 4 stereo summing channels (ABCD) is still @ 5/7 dB. Two of them are left heavy and the other two right heavy. The PCB holding the ABCD pots is made of a few components. It has 2xTL074CN chips, 8x25V 100uF 85°C electrolytic capacitors, resistors, and the 4xA10K pots.
Testing the 2 chips, referencing to the pin connections pdf, revealed differences between the chips. As I'm quite a big beginner, I still need to learn how to make relevant component testing. While doing so, I can start to change components on a PCB. Is it a good plan?
I did a list of the components per PCB, and looks after the components on the web.

But I don't know how to choose the right upgrade for the caps. I found the same references for the chips - MC4580Ĺ and TL074CN, but could it be useful for the sound to upgrade them as well?

I hope to use the Fat Track for its many features in an hybrid mixing flow, in my already treated room. I do love the stereo widness it brings, the palette of saturation from the tube stages is really noticeable and the tube choice and drive greatly impact the sound. Not so sure about the low- shelf and the hi -pass filters by themselves, but combined with another set of filters it can sound great on everything, to my ears at least,


Any help would be greatly appreciated, thank you!

Ben

 

[Script]

Pan pots ok ?

Swapping components at random is seldom a good idea. First need to find out what is faulty, only then start ordering.

Complex unit. Four identical channels make it easier to compare.

But could be many things, and combinations of things. So need to search & measure main audio path systematically from input to output, starting with:

- balanced input jacks, if hot or cold does not connecting properly, then you get -6dB
- input buffer op amp (measure all pins) compare between channels
- if pan pots are faulty, you get L/R imbalance
... and so forth...

++
"brownish liquid" on pcb could be leakage (although usually found on side where elcos sit) or could be spilled coffee or like

Also could try and measure some elcos...

 

[benacquier]

Thank you very much, will learn to do proper testing and update here.

 

[Gesse]

Not sure, will this help, but Verdeaudio also repairs all kind hardware in Finland, maby you can ask advise. Or is he also here diy forum

verdeaudio.com

 

[ben acquier]

Thanks for the tip Gesse, should have updated earlier about the fattrack but I was waiting for a fully recovered unit...

I'm close to a fully working unit on the analog side, only issue remaining now is a occasional jump of the gain on the left Mic Channel, and a little bit of noise on it too.

More than 10 pcbs inside.
I've replaced capacitors on most of them, and will finish with the channel ones', which should solve this (?). Changed tube sockets and opamps.

No more difference of level between left and right in the 4 stereo summing channels which was the main issue.

I'm still a noob but spending time learning with the schematics provided here was a great opportunity to practice. The number of pcbs and their very closed position in the unit made it quite impossible for me to follow the signal path and test it. Guess some noob luck @...

I use the unit daily, and really like to A/B with rme converters, while waiting for the components order.

Will update, cheers

 

[Gesse]

No worries!
One tubetracker owner here too, lovit! Maby new tubes are next what I have get.

 

[Script]

only issue remaining now is a occasional jump of the gain on the left Mic Channel, and a little bit of noise on it too

A TL Audio EQ unit that I 'fixed' the other month (--not a FatTrack though--) had similar symptoms.

The 'fix' was to clean the 'Instrument In' jack connector, which internally also serves as a 'switch' when nothing is inserted. But the internal metal contacts were corroded and wouldn't conduct properly -- hence mysterious noise and occasional gain issues on one channel.

I had suspected one of the tubes and / or its connectivity, but the tube itself was fine and swapping the tube didn't 'fix' it.

 

[Ike Zimbel]

A TL Audio EQ unit that I 'fixed' the other month (--not a FatTrack though--) had similar symptoms.

The 'fix' was to clean the 'Instrument In' jack connector, which internally also serves as a 'switch' when nothing is inserted. But the internal metal contacts were corroded and wouldn't conduct properly -- hence mysterious noise and occasional gain issues on one channel.

I had suspected one of the tubes and / or its connectivity, but the tube itself was fine and swapping the tube didn't 'fix' it.

The DI jack switching contacts issue is a REALLY common problem across a wide range of pre-amps with DI input option. Probably the first thing to check.

 

[ben acquier]

Thank you for sharing your knowledge, didn't know about the 'instrument in' switch...Really appreciated the help

I'll clean it this weekend and report.