the legendary tg12412

[drask]

Hi drask. Unfortunately, I don't have any samples yet. I'm waiting for the last PCBs (Band1) and in the meantime I'm working on the enclosure/front panel. Once it's complete I will post some samples. 

Nice Warpie, I'm waiting for this

 

[Michael Tibes]

...

Yes, the Zeners are regulators.

Thanks abbey road d enfer. Unfortunately, I'm really struggling to find the cause of the heat problem so I might have to leave with it as I can't think of any other possible solution.

Maybe you could replace the voltage dropping resistors / zeners with offboard LM317/337 regulators? That would move the heat source away from the board and supply clean stable voltages - which would probably be good anyway, unless the less stable original version contributes to the special magic of the unit...

Michael

 

[Michael Tibes]

Well, overkill is a relative thing. It might be overkill in mass production, but if it gets the job done (and probably even improves it) and saves me time I'd rather spend a few bucks and an hour of work on some regulators before I twist my brain for too long. Also I'm pretty keen on getting heat sources away from the audio circuit, simply because I'm lazy - I don't want to get back to things a few years later in order to fix burned or dried out components. That is certainly more relevant if the unit gets used a lot like the stuff I like for my work.

I imagine that the original unit would probably behave the same. It is most likely even the best possible aproach at the time and not a real problem back then, considering it was designed for a very exclusive circle where maintenance was not an issue and the expected product lifetime was most likely below ten years. The original designer most certainly wasn't aware that he was designing a future classic and some crazy people would dig up the schematics decades later in order to build such units again.

Anyway, congretulations to your work, I wish I had the time to build me a few of these as well - stunning!

Michael

 

[leadbreath]

Warpie, u said the biggest resistor rating was 2W? Why don't u try a 5 or 10W one and see what happens?

 

[Michael Tibes]

If the problem is intermittend, did you check for oscillation?

Michael

 

[warpie]

You mean oscillation in the audio signal path?
If so, I've checked. The output looks as good as the input, in the scope.

 

[Biasrocks]

Biasrocks, thank you for the info. I haven't built the limiter so it's good to know that this problem doesn't appear on the limiter. I will also try your suggestion about the current flow. Can you please explain me why I need a 10ohm resistor in series with the 2 Watter? Can I not just put my DMM in series straight after the 2Watt resistor?

Some time ago, I measured the current by inserting the DMM in series from the PSU +28V rail to the main board and it measured around 200mA.

You can do it that way, the math is easier at 10ohms.

Perhaps an intermittent solder joint or a bad transistor pulling too much current??

Regards,
Mark

 

[Michael Tibes]

1 is kinda strange, do you have any load? if you just measure dc without any other load than the dmm things might take a while to settle? Otherwise a leaky cap?

2 might be ok at first glance, as long as all 3 points are at he same dc level.

Did you hear the beast with music?

Michael

 

[Michael Tibes]

regarding 2: is there any crackle? If I get it right, then things will be fine when all 3 points are at the same dc level because then there is actually no dc across the pot. It would only be a problem if there was dc between these points, but measuring from 22 to 23 or 25 should show no dc. The voltage between the pot and gnd is irrelevant in terms of crackling in this case. So if I get things right there might be nothing to improve, getting rid of the voltage between the pot and gnd might even affect the transistor bias?

What are you measuring at the output, mA - not mV? Maybe just connect 47k from out to gnd and see what happens?

Michael

 

[warpie]

regarding 2: is there any crackle? If I get it right, then things will be fine when all 3 points are at the same dc level because then there is actually no dc across the pot. It would only be a problem if there was dc between these points, but measuring from 22 to 23 or 25 should show no dc. The voltage between the pot and gnd is irrelevant in terms of crackling in this case. So if I get things right there might be nothing to improve, getting rid of the voltage between the pot and gnd might even affect the transistor bias?

You're right. I was measuring with reference the gnd. There's no DC when measuring between pins. There's no crackle but I was thinking that it might develop after a period of time.

What are you measuring at the output, mA - not mV? Maybe just connect 47k from out to gnd and see what happens?

Sorry, I meant mV and not mA (I edited my post). Thanks for pointing it out. A 47K resistor improves the DC by 5-6mV so I now read around 50-55mV. What amount of DC is acceptable at the output and moreover, is there's any danger of damaging the output transformer?

 

[Michael Tibes]

I'd replace the output cap with the biggest foil cap at hand and measure again, keeping the 47k. That would be in order to make sure you haven't bought a batch of leaky 470uF caps. If I'm not overlooking something then there should be no dc. I am lost as well in terms of 'how much dc is too much', that should also depend on the transformer.

Michael

 

[warpie]

Thanks Michael, I'll try a bigger lytic although the 470uFs I tried are from different batch and different brand.
FWIW, Maybe I should mention that instead of the original 2n4898 and 2n4910 transistors, I'm using NTE218 and NTE175,
which are supposed to be modern replacement.

Also, one more thing if you don't mind. Can you please elaborate a bit about the 47k resistor? I'm experimenting with different values and I noticed that the lower the value, the lower the DC. Is there a limit as to how low the resistor should be?

 

[Michael Tibes]

There isn't much to elaborate, it's just a path to ground to discharge the output cap. 47k is just a rough guess, not too low to present a load but low enough to show some discahrging effect after a little while... And it's mostly available 

Lowering the resistor wouldn't be a cure but just a cover up, if you want to get rid of all the dc you gotta find out where it comes from. I just glanced at the schematic and it seems like nothing else was connected to the output after the cap, so I believe theoretically it could only be leakage. That's why I would try a film cap, afaik they shouldn't have any leakage. I usually have something like 10uF lying around, that should do to see what difference it makes.

Michael

 

[Michael Tibes]

I'm not good at mathematics, but I'm sure the 470uF are on the very safe side not to affect the lowend - with the lowest possible load. Often a lot smaller values are used. 10uF might be ok with a not too high load, that's probably why it mesures ok. If the offset is gone with the film cap it points to leaky caps in my opinion.

Michael

 

[warpie]

Thanks for all you help Michael. Really appreciated! 

 

[leadbreath]

hey warpie, did you finally finish this project?

 

[warpie]

hi leadbreath,

I've put it aside at the moment as I'm very busy with other things.

 

[etheory]

I could be completely wrong here, but here's a hypothesis.

Film caps that I am aware of are non-polarized, but electros (at least most of the ones I've come across recently) are polarized.

So, I noticed recently in one of my projects that if you insert an AC coupling cap in an audio path backwards, such that the difference in voltage across it is opposite to it's polarity, then it maintains a DC offset.

However, simply reversing the cap, fixes this completely. The dc offset falls to 0 with a time constant relative to RC (where R is the load impedance and C is the capacitor value itself).

Is it simply possible that the schematic you are using has the polarity of the AC output cap wrong?

This is conjecture, but I have seen the same symptoms before in different circuits.

 

[thomasdf]

Hi folks

This site given on the first page : http://www.tgmixers.co.uk/Frames/frameset.html appears to be dead.
Anyonce had the chance to gather informations from there (schematics etc) ?

Best,

Thomas

 

[ed rees]

That's Brian Gibson's site. Hope he's ok, I know he had some personal troubles recently.    As far as I can remember there were never any TG schems on the page.