Modified Heathkit TA-28: Help me understand this

[Frukost]

For those who were wondering, the transistors are properly placed/oriented.

I appreciate all your feedback, but at the same time i feel a bit overwhelmed of all the directions this thread has taken. I think I need to reread all your advice to get an overview of how to fix this.

[duck_arse]

OK, so the other day I finished building a modded version of the TA-28 according to http://tagboardeffects.blogspot.com/2012/09/heathkit-ta28.html (The one with the added trimmers).

....

I don't get it. Did I wire something backwards, or is this expected?

I haven't checked yet, but it struck me today that I may accidentally have switched the transistors place while soldering. Would it be feasible that it could have caused the symptoms described?

(It's hard to see since the transistors printed surfaces are soldered face to face. Here's the veroboard again:

did you wire something backwards? we don't know, you haven't shown us what you've built. please, PLEASE, post photos of your build, with the offboard wirings included.

also, looking at the vero layout, the transistors don't face flat-to each other, yet you tell (please, show) us the transistor labels can't be read because they are face to face.

also, you have not posted any transistor voltages from your built circuit - which is where we are supposed to start the debug process.

[Rob Strand]

I appreciate all your feedback, but at the same time i feel a bit overwhelmed of all the directions this thread has taken. I think I need to reread all your advice to get an overview of how to fix this.

I guess it boils down to if you have a build problem or the circuit needs a tweak to improve the biasing of the transistors.   Those 1.5V circuits can be more sensitive to the specific parts in the build.

You could measure the voltages in the circuit but I was never convinced the voltages marked on the schematic sounded any good.  Maybe someone else can add something here.

Don't worry about the stuff regarding the 1.5V supply pinkjimiphoton and I discussed (not for now anyway).

[Frukost]

did you wire something backwards? we don't know, you haven't shown us what you've built. please, PLEASE, post photos of your build, with the offboard wirings included.

Thanks for your reply.

Besides regular use of the pedal, I've quadruple checked wirings and continuity on everything: The offboard wirings are all good. The LED, button (bypass on/off), jacks and controls works as expected. It's the guitar tone control - pedal interaction that had me wondering.

Regarding photo I need to get back on that. I'm using an ancient phone with an optical lense full of pocket lint, unfortunately.

also, looking at the vero layout, the transistors don't face flat-to each other, yet you tell (please, show) us the transistor labels can't be read because they are face to face.

You're absolutely right. Sometimes my memory plays tricks on me. Sorry for leading you on.

also, you have not posted any transistor voltages from your built circuit - which is where we are supposed to start the debug process.

A concrete advice. Very nice! Here's what I've got with 1,5V (trimmed, measured and confirmed) going into the circuit:

BC550C
C: 0,94V
B: 0,79V
E: 0,30V

BC560C
E: 1,50V
B: 0,94V
C: 1,05V

[pinkjimiphoton]

voltages look good on q1.
q2 is funky

e shouldn't be higher voltage than c. any chance you got that reverse beta'd? bc560c shows pin 3 as collector in the
data sheet, which would make a lot more sense.

i believe your second transistor is installed backwards.

[Frukost]

For what it's worth, here are the photos. Please note that the board is rotated 180 degrees in relation to the casing. It made more sense to me to maintain tone/level wiring as close as possible to their associated pot instead of crossing the wires. I'm aware that this may throw you off, so I've put effort into wiring it right.

Regarding transistor orientations, you can see them in my photo.

Also, during an initial troubleshoot I found a false connection in the soldering which I fixed. There might be others as well.

[pinkjimiphoton]

did you check the pinout for your transistors before installing? that second one looks way off to me. i can't tell from looking at the vero, not all transistors have expected pinouts. in some cases, a wrong pinout will still pass signal thru diode junctions n stuff.
looks pretty clean to me otherwise

[Frukost]

did you check the pinout for your transistors before installing?

I didn't. However, I ordered double components. You know, in case something goes wrong with the circuit  .

So before I start to unsolder things I measured the pins on the spare BC560, assuming it's the same configuration as the one I already soldered. At least the spare one matches these specifications: https://www.componentsinfo.com/bc560-pinout-equivalent/. Unless I'm mistaken, that's the type I need?

[Rob Strand]

I actually think the transistors are in correctly and it is working as expected.

Schematic voltage:
NPN CBE:   1.0V, 0.87V, 0.36V     => VBE=0.51V
PNP EBC:   1.5V, 1.0V, 1.3V         => VBE = 0.50V

Measured with BC550C (NPN) andf BC560C (PNP), hFE ~ 400
Vcc=1.5V
NPN CBE:   0.94V, 0.79V, 0.3V; 
                    0.79V with 1M multimeter load means base voltage is really  0.82V => VBE=0.52V
PNP EBC:   1.5V, 0.94V, 1.05V => VBE = 0.56V


The measured voltages and the schematic voltages aren't far off.

If you look at the VBE voltages they are quite low on the schematic and measurements.
That's why this thing biases with a Q2 collector voltage around 1V.

If I set-up LTspice to simulate the circuit and tweak the transistors models to have the same
VBE it actually comes quite close to the measured values.

On the schematic, the base and emitter voltages look a bit high, yet the VBE difference is
in agreement with the measured.   What I think has happened here is resistor tolerances
on the original measurements have raised the base voltage on Q1.  For example if I set
the 82k to 15% lower 82k/1.15   then the simulated voltages start to match the schematic voltages
and I get a collector voltage on Q2 of 1.21V or so.     In the scheme of things that
means tolerances are likely to have caused the voltages on the schematic to be a little off.

In summary I think the measured voltages are representative of the build  being OK.
I don't really understand why the VBE's from the measurements and the schematic
are low.  This is a fact and not something we can control.

So....   my suggestion is to play with R5 and R7.  Perhaps decrease R7 or raise decrease R5 - best to tweak R5 after playing with R7.
The aim is to drop the collector voltage on Q2 a bit.
It's never a bad thing to play with the values on these 1.5V circuits!

[pinkjimiphoton]

did you check the pinout for your transistors before installing?

I didn't. However, I ordered double components. You know, in case something goes wrong with the circuit  .

So before I start to unsolder things I measured the pins on the spare BC560, assuming it's the same configuration as the one I already soldered. At least the spare one matches these specifications: https://www.componentsinfo.com/bc560-pinout-equivalent/. Unless I'm mistaken, that's the type I need?

they have opposite pinouts







like rob says tho, if its working, it may just be a weirdness to the circuit

[Rob Strand]

like rob says tho, if its working, it may just be a weirdness to the circuit

FWIW, I tried the reversed CE theory on the simulation and I got a low collector voltage on Q2.
Not that I trust the transistor models in reverse!  However, it kind of makes sense.
If the transistors are in reverse the hFE is low and that means the transistors
are "off" more than they should be.  That ends up turning Q2 off which in turn will makes
the collector voltage on Q2 drop.

[pinkjimiphoton]

nothing more unlinear than fuzzes

[Rob Strand]

nothing more unlinear than fuzzes

Indeed!  That's why we need master tweakers like yourself.

[PRR]

they have opposite pinouts

Looks same to me? Or should I go back to bed?

[Rob Strand]

They follow the European pin out.

Motorola/ON
https://www.farnell.com/datasheets/727135.pdf

Fairchild,
https://www.elektronik-kompendium.de/public/schaerer/FILES/bc550.pdf

All agree with jimi's pics.

[pinkjimiphoton]

they have opposite pinouts

Looks same to me? Or should I go back to bed?

damn, this is some good weed

[Rob Strand]

damn, this is some good weed

Vintage Heathkit.

[Rob Strand]

See replies #14 and #16 quite a bit of variation voltages - but note the supply is not held at 1.5V.

https://www.diystompboxes.com/smfforum/index.php?topic=90489.0

[Frukost]

See replies #14 and #16 quite a bit of variation voltages - but note the supply is not held at 1.5V.

https://www.diystompboxes.com/smfforum/index.php?topic=90489.0

Just to make sure I'm not misunderstanding:

I read those posts as the values are close to mine, except for the 9V voltage divider ones. Regarding the 1,5V levels I'm don't know if the deviations of those values from my values are acceptable. Are you saying that those values supports the values in my circuit or not?

Also, it seems to me that this:

So....   my suggestion is to play with R5 and R7.  Perhaps decrease R7 or decrease R5 - best to tweak R5 after playing with R7.
The aim is to drop the collector voltage on Q2 a bit.

could be my best shot?

If so, could you please specify what ranges I need to try out? Is 10% steps too much/too little? This is a learning process for me.

[Rob Strand]

I read those posts as the values are close to mine, except for the 9V voltage divider ones. Regarding the 1,5V levels I'm don't know if the deviations of those values from my values are acceptable. Are you saying that those values supports the values in my circuit or not?

The links highlight there's quite a bit of variation.    The voltages cluster around your measurements, the voltage on the schematic and the voltages I've seen in the circuit simulations.    What that's saying is your build is OK and we are just see some natural variations, which I might add is quite normal for 1.5V pedals.

With many of the popular vintage pedals people have been actively cloning them for some time.   Measurements have been taken from real units, clones build and tweaked, and new voltage measurements taken.    The right sound for each pedal is known and the measurements help create a recipe for success.

For the Heathkit we don't have a baseline for the normal sound or for the voltages.    So we can't say what is normal or acceptable.   So what most tweakers do in cases like this is play around with the design as if you were making your own pedal.    Only you can be the judge of what sounds good.  When you get something close to what you like it's a good idea to measure the voltages.   You might then continue to try other values and tweak those.  When you find another combination of parts which sounds good measure the voltages again.   What often happens is despite the different part values the measured voltages might end up near what you got the first time around.    That means you are finding the good zone for that pedal.

The collector voltage on Q2 is highly likely to be an important voltage for the sound.

If you replace R5 with a 10k resistor in series with a 100k trimpot or pot.  Just play with the value until it sounds good.   Then measure the resistance of the 10k+pot and the voltages.

Next change R7 to 6.8k and repeat.   Try to just if this combination is better or worse than before.   If its better then try R7 at 5.6k and repeat.  (If its worse try R7 at 10k and repeat).

After some playing around you should be able to move the design towards a good zone.     You can also get a case where things sound different but not obviously better or worse.   Only you can choose.   When you get to this stage you might try tweaking some other part.

This type of process will get you something you like, or get rid of something you don't like.    It might not be what the "real" pedal sounds like, which in this case we don't have a good idea what that is.