Modified Heathkit TA-28: Help me understand this

[pinkjimiphoton]

my brother mike confirmed that the one i built years ago sounded identical to his original, i asked him last nite.

[pinkjimiphoton]

so i built it. used a 2n3391a hfe 600 something for q1, and a 300hfe 2n3906.
confirmeaad multiple bias points where it would pass signal, but really only one range that sounds good,
about 1.25- 1.55v. i used the op's layout from tagboard.
i DID add a 1n4001 between earth and b+, band to + and a snubber mcc of 2.2n between the rails to help with any rfi cuz i plan on running it on a battery.

can absolutely confirm frukost is right, the tone controls actually work kinda backwards... well... keyword, is kinda..

all the way down is definitely brighter and fuzzier with the treble control turned all the way off... i mean, standard guitar knob, b500k/.022 . its trippy. thinking as the cap loads the guitar down, eventually the resistance just makes the load big enough that it shorts the tone pot out of the circuit?
beats me.
its cool tho.

video or it never happened, right?
its a @#$%in' insurrection on my bench... checks it out:

https://www.youtube.com/watch?v=JHiAYddx2Ic

nice sounding fuzz, does some weird stuff, will easily run on a 9v supply as shown in the vero




sounds great. kinda blatty and sustainy at the same time. i think it may be a keeper, i got it in a recycled box already.

making the two 1k's on the output as shown brings it up to a useable output level.
very stoked to revisit this all these years later.

so yep.... i'd say normal behaviour is being observed by this cool little circuit.

[Rob Strand]

so i built it. used a 2n3391a hfe 600 something for q1, and a 300hfe 2n3906.
confirmeaad multiple bias points where it would pass signal, but really only one range that sounds good,
about 1.25- 1.55v. i used the op's layout from tagboard.
i DID add a 1n4001 between earth and b+, band to + and a snubber mcc of 2.2n between the rails to help with any rfi cuz i plan on running it on a battery.

Way to go jimi!

can absolutely confirm frukost is right, the tone controls actually work kinda backwards... well... keyword, is kinda..

It's pretty weird - but I see it and believe it!


So just decipher the voltages.   The good bias point is pretty much
biasing the collector of Q2 in the vicinity of Vcc/2.   Move
either side of that for tone variations.

So pretty much like most pedals.


pinkjimiphoton Video adjusting vcc

1) 7:15
Vcc = VE2 = 1.22V   
VC2 = 0.42V

Vcc/2 = 0.66V
VE2 - VC2 = 0.80V
So Q2 collector biased below Vcc/2 (and clearly Q2 not saturating).

2) 11:09
Vcc = VE2 = 1.56V   
VC2 = 1.19V

Vcc/2 = 0.78V
VE2 - VC2 = 0.37V
So Q2 collector biased above Vcc/2 but Q2 just off saturating.


BTW, your commentary cracks me up every time.

[pinkjimiphoton]

'tis a shitty job, but some buffoon gotta do it

[Rob Strand]

I found a video from your younger brother,

[Frukost]

Rob Strand: Thanks for your elaborate answer, it clarifies a lot for me!
Also, thanks to pinkjimiphoton who corroborated my initial observation. It would be cool to compare with an original TA-28, but until then I'll see if I can make time for tweaking the circuit sometime soon!

[Paul Marossy]

yeah, those are a lot hotter than the original transistors!
but i bet it still sounds ace

cool trick doing the voltage supply! i forget what we did years ago, let me see if i can find my old layout from days of yore...

I get a kick out of the $17.99 price tag. I saw one on ebay the other day, they are asking like $600 for it! 

[Rob Strand]

Rob Strand: Thanks for your elaborate answer, it clarifies a lot for me!
Also, thanks to pinkjimiphoton who corroborated my initial observation. It would be cool to compare with an original TA-28, but until then I'll see if I can make time for tweaking the circuit sometime soon!

I think jimi did the more taxing part of it.   The numbers just prove we are dealing with basic biasing issues. (At least  it makes  sense now.)

The reverse tone control behaviour shows we don't understand something.   There's something deeply puzzling about that even now.

Something which perhaps isn't so obvious is the way jimi checked it.  Since we have the luxury of changing the supply voltage with the trimpot jimi's way of playing with the voltage is a sensible way to shift the bias (at least for this circuit)  However, if we wanted to do more or less the same experiment but with the supply voltage fixed then playing with the part values like I outlined before is the way to do it.   If you had a 9V pedal you don't have the luxury to tweak the supply voltage to find where it sounds good.

Around 2018/2019 2021 there was thread for another 1.5V pedal (it could be 3V).   It was a 1970's kit from Europe.  As I remember that circuit had a few quirks as well.  I can't find the thread.

I get a kick out of the $17.99 price tag. I saw one on ebay the other day, they are asking like $600 for it! 

Once the vintage and collector labels appear common sense goes out the window.    Back then $17.99 wasn't exactly cheap.


FYI,
Here's the old thread.
Josty GU-15 Single Guitar Booster kit 1974.  Sold throughout Scandinavia/Europe and maybe UK, 
https://www.diystompboxes.com/smfforum/index.php?topic=126307.0

[soggybag]

yeah, those are a lot hotter than the original transistors!
but i bet it still sounds ace

cool trick doing the voltage supply! i forget what we did years ago, let me see if i can find my old layout from days of yore...

I get a kick out of the $17.99 price tag. I saw one on ebay the other day, they are asking like $600 for it! 

I feel there should be a merit badge you could earn for building one of these!

I have a friend who owns one. It's in the same enclosure shown in the picture. He complained it never sounded "right." After searching the forum here I found that R10 and R12 on his box were the 1k shown in the original schematic. Where they should have been the 10k shown on the Jack Orman schematic. This must have been a typo in the original, someone dropped a 0. This change really woke it up. It sounded pretty good after that.

[Paul Marossy]

yeah, those are a lot hotter than the original transistors!
but i bet it still sounds ace

cool trick doing the voltage supply! i forget what we did years ago, let me see if i can find my old layout from days of yore...

I get a kick out of the $17.99 price tag. I saw one on ebay the other day, they are asking like $600 for it! 

I feel there should be a merit badge you could earn for building one of these!

I have a friend who owns one. It's in the same enclosure shown in the picture. He complained it never sounded "right." After searching the forum here I found that R10 and R12 on his box were the 1k shown in the original schematic. Where they should have been the 10k shown on the Jack Orman schematic. This must have been a typo in the original, someone dropped a 0. This change really woke it up. It sounded pretty good after that.

Yeah after having recently (mostly) fixed an old Heathkit ocsilloscope, I've discovered that they apparently made a habit of having errors and/or things that don't make sense on their schematics.

[amptramp]

yeah, those are a lot hotter than the original transistors!
but i bet it still sounds ace

cool trick doing the voltage supply! i forget what we did years ago, let me see if i can find my old layout from days of yore...

I get a kick out of the $17.99 price tag. I saw one on ebay the other day, they are asking like $600 for it! 

I feel there should be a merit badge you could earn for building one of these!

I have a friend who owns one. It's in the same enclosure shown in the picture. He complained it never sounded "right." After searching the forum here I found that R10 and R12 on his box were the 1k shown in the original schematic. Where they should have been the 10k shown on the Jack Orman schematic. This must have been a typo in the original, someone dropped a 0. This change really woke it up. It sounded pretty good after that.

Yeah after having recently (mostly) fixed an old Heathkit ocsilloscope, I've discovered that they apparently made a habit of having errors and/or things that don't make sense on their schematics.

Wait 'til you see what Heathkit did on the AJ-1510A FM tuner, the first digital synthesis tuner they made (and one of the first that existed).  Tons of design errors and a last-minute addendum to the power supply to keep it from going unstable.  And nobody seemed to know that bypass capacitors for logic should be capable of rapid response, not the 10 µF electrolytics they used.  One of the plug-in boards on the motherboard had no capacitors at all.  It's a wonder that any of these are still working.  I generated a blurb on diyAudio with a list of necessary and nice-to-have modifications and they are extensive.

[Paul Marossy]

Yeah after having recently (mostly) fixed an old Heathkit ocsilloscope, I've discovered that they apparently made a habit of having errors and/or things that don't make sense on their schematics.

Wait 'til you see what Heathkit did on the AJ-1510A FM tuner, the first digital synthesis tuner they made (and one of the first that existed).  Tons of design errors and a last-minute addendum to the power supply to keep it from going unstable.  And nobody seemed to know that bypass capacitors for logic should be capable of rapid response, not the 10 µF electrolytics they used.  One of the plug-in boards on the motherboard had no capacitors at all.  It's a wonder that any of these are still working.  I generated a blurb on diyAudio with a list of necessary and nice-to-have modifications and they are extensive.

Yikes. I suppose around that time digital stuff was still a new thing to most electronic design people, so it's not too surprising that they had some bugs. Kinda reminds me of when Fender decided to go with a SS amp line and they had no clue what troubles that would cause them.