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Author Topic: Teisco Checkmate 21 Solid State amp schem + help  (Read 35239 times)

galaxiex

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Re: Teisco Checkmate 21 Solid State amp schem + help
« Reply #15 on: July 31, 2014, 12:39:25 AM »
Ok I got the buffer to work. Turns out I had an open 10uF cap. (old stock in my junk) No signal getting thru.

But now the trem doesn't work! Rats! That is the one feature I like about this amp.

Seems like that goofy diode driven trem circuit wants to see some sort of load? on the input side.
As a test I removed the buffer and just plugged the guitar in.
Trem works.
Leave vol up and unplug guitar.
Should still "hear" the trem throb with all the background hiss.
No dice, the trem quits when you unplug the guitar. Weird.

Anyone got ideas how to make the trem work with the buffer installed?

BTW with the buffer installed the amp does sound a LOT better and slightly louder too.
Now to get the trem working again...
Fear leads to Anger, Anger leads to Hate, Hate leads to Suffering.

Roly

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Re: Teisco Checkmate 21 Solid State amp schem + help
« Reply #16 on: July 31, 2014, 09:52:30 AM »
Quote from: galaxiex
I work as a automatic transmission re-builder and the hardest thing to do is trouble shoot something that someone else rebuilt/messed up.

We had a rule that whoever took it apart had to put it back together.  This could get complex if a couple of other techs had been called in to investigate something.


Well your signal grounds go to ground,  The fact that this happens to be positive and the supply rail is -20V is immaterial.

I'll explain.

Signal is an AC entity, and as such it sails blissfully straight through the DC barrier of your input DC-isolating/AC-coupling cap.  So it doesn't actually matter what DC voltage your source is sitting at because it is blocked by C1.

Consider a simple preamp stage.  As the signal goes up and down so the current through the stage goes up and down.  This same changing current must also be being drawn through the power supply lines, and if there is any series resistance (and there always is) then this will cause the voltage on the supply line to have the stage signal imposed on it - it will flap up and down too.

Other stages that are on the same supply will experience this as either negative, or positive, feedback (and low frequency "motorboating" often results from a supply line bypass cap failing low or open).

So we 'by-pass" the supply lines to each other, generally using a large value electro cap.  This effectively short-circuits the two DC supply rails together for AC signals, so for AC signal considerations both actual ground and the supply rail(s) are effectively "ground".

Now, while we are here, take a 0.01uF to 0.1uF disk ceramic or poly and connect it "VHF style" with the shortest practicable leads directly to the +ve and -ve supply pins of the TL071.  Op-amps in general and multi's in particular should have a good HF bypass right on their +/-supply pins for best results.

Personally I'd bump the bias resistors up to 2.2Megs each to get 1Meg Zin, and I'd stick a couple of 1N914 or similar diodes reversed biased across each resistor, and a 10k somewhere in series with C1 for some protection.

Coming together.   :tu:


P.S.
Quote from: galaxiex
Anyone got ideas how to make the trem work with the buffer installed?

I'm on the case.   :trouble


edit: typo
« Last Edit: August 03, 2014, 06:23:27 AM by Roly »
If you say theory and practice don't agree you haven't applied enough theory.

galaxiex

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Re: Teisco Checkmate 21 Solid State amp schem + help
« Reply #17 on: July 31, 2014, 12:03:52 PM »
Quote from: galaxiex
I work as a automatic transmission re-builder and the hardest thing to do is trouble shoot something that someone else rebuilt/messed up.

We had a rule that whoever took it apart had to put it back together.  This could get complex if a couple of other techs had been called in to investigate something.


Well your signal grounds go to ground,  The fact that this happens to be positive and the supply rail is -20V is immaterial.

I'll explain.

Signal is an AC entity, and as such it sails blissfully straight through the DC barrier of your input DC-isolating/AC-coupling cap.  So it doesn't actually matter what DC voltage your source is sitting at because it is blocked by C1.

Consider a simple preamp stage.  As the signal goes up and down so the current through the stage goes up and down.  This same changing current must also be being drawn through the power supply lines, and if there is any series resistance (and there always is) then this will cause the voltage on the supply line to have the stage signal imposed on it - it will flap up and down too.

Other stage that are on the same supply will experience this as either negative, or positive, feedback (and low frequency "motorboating" often results from a supply line bypass cap failing low or open).

So we 'by-pass" the supply lines to each other, generally using a large value electro cap.  This effectively short-circuits the two DC supply rails together for AC signals, so for AC signal considerations both actual ground and the supply rail(s) are effectively "ground".

Now, while we are here, take a 0.01uF to 0.1uF disk ceramic or poly and connect it "VHF style" with the shortest practicable leads directly to the +ve and -ve supply pins of the TL071.  Op-amps in general and multi's in particular should have a good HF bypass right on their +/-supply pins for best results.

Personally I'd bump the bias resistors up to 2.2Megs each to get 1Meg Zin, and I'd stick a couple of 1N914 or similar diodes reversed biased across each resistor, and a 10k somewhere in series with C1 for some protection.

Coming together.   :tu:


P.S.
Quote from: galaxiex
Anyone got ideas how to make the trem work with the buffer installed?

I'm on the case.   :trouble

You Sir, are just AWESOME!  :dbtu:

Thank you so much for the easy to understand explanation! I love it!  <3)

In all the time I have been messing with electronics and reading tech books and whatnot,
I have never really "got" the supply/signal relationship and the need for bypass caps etc. (I just knew circuits needed em cuz the book said so)

I just now did, (Big light bulb emoticon) Thank You!!!!

PS. 10K in series on the opamp side or guitar side? Or does it matter? I'm guessing guitar side.

Also, I got lots of 1N914's, no problem. I even understand reverse bias.  :cheesy:
« Last Edit: July 31, 2014, 12:13:02 PM by galaxiex »
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Roly

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Re: Teisco Checkmate 21 Solid State amp schem + help
« Reply #18 on: July 31, 2014, 03:05:07 PM »
My pleasure.   :)

.
A 10k (or so) goes between the external input socket and the op-amp input pin.  It and the cap are in series so it doesn't matter which is first.

When you have diode clamps you also have to have some series resistance or the diodes could be overwhelmed by what could possibly come up the front end (e.g. accidentally hooking the output of another amp to the input of this one, has been known to happen in the rush of setting up; walk across a carpet and pick up a few kV of static charge, then touch the end of your guitar lead before you plug it in).  The resistor limits the current flow, and it's the current that does the damage.

Since it's a -ve supply rail these clamp diodes go across the input with their cathodes pointing towards the +ve/ground end so they are normally off.  If the input voltage tries to go outside the supply bounds these diode clamp it.

.
Now looking at your op-amp-killed-the-radio-star problem.

Because the input bias resistors on the op-amp bias the input to about -10V, and because it is a 100% feedback voltage-follower, the op-amp output will also be at -10V.

When it is first turned on the output cap C2 10uF will be discharged, so when the op-amp output drives it to -10V the other side, the input/trem/diode side, will also go to -10V and present that to the amp input.  This totally bogs up the critical biasing of the trem diode and kills its action by reverse biasing it by -10V.  Signals go past just fine, they just don't get modulated by the diode which is cut off hard.

Eventually the cap will charge up via the diode leakage (cos there is no other path), the input voltage will fall to something reasonable, maybe minutes to millivolts, and the trem should start working again.  But 10uF charging at microamps will take forever (and maybe never if the cap leakage discharging the cap is larger than the diode leakage charging it.  It may end up floating about at a few volts, depending on the temperature and general mood, and never fully charge back to zero volts).

The simple answer is to provide a (better) charging path.  The impedance at this point is already obnoxious, and the buffer makes it unimportant anyway, so we can't do a lot to offend it.

If you add a ~100k resistor across the (old) input, that is from the end of C2 not connected to the op-amp output, to ground, it should charge C2 and settle to around zero in about 2-3 seconds after switch on.  This cap should have its -ve end to the op-amp output and +ve end towards the trem diode.

.
I would expect it to be slightly louder and a bit more lively with the guitar isolated from the previous loading effect of the low-Z input.

.
This trem BTW only modulates downwards on peaks about 20-30% of the time leaving the signal full and unmodulated for 80-70% of the time.  Most modulations systems provide 100% sine variation over the entire LFO cycle, so this will sound different.
If you say theory and practice don't agree you haven't applied enough theory.

galaxiex

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Re: Teisco Checkmate 21 Solid State amp schem + help
« Reply #19 on: July 31, 2014, 05:31:48 PM »
My pleasure.   :)

.
A 10k (or so) goes between the external input socket and the op-amp input pin.  It and the cap are in series so it doesn't matter which is first.

When you have diode clamps you also have to have some series resistance or the diodes could be overwhelmed by what could possibly come up the front end (e.g. accidentally hooking the output of another amp to the input of this one, has been known to happen in the rush of setting up; walk across a carpet and pick up a few kV of static charge, then touch the end of your guitar lead before you plug it in).  The resistor limits the current flow, and it's the current that does the damage.

Since it's a -ve supply rail these clamp diodes go across the input with their cathodes pointing towards the +ve/ground end so they are normally off.  If the input voltage tries to go outside the supply bounds these diode clamp it.

.
Now looking at your op-amp-killed-the-radio-star problem.

Because the input bias resistors on the op-amp bias the input to about -10V, and because it is a 100% feedback voltage-follower, the op-amp output will also be at -10V.

When it is first turned on the output cap C2 10uF will be discharged, so when the op-amp output drives it to -10V the other side, the input/trem/diode side, will also go to -10V and present that to the amp input.  This totally bogs up the critical biasing of the trem diode and kills its action by reverse biasing it by -10V.  Signals go past just fine, they just don't get modulated by the diode which is cut off hard.

Eventually the cap will charge up via the diode leakage (cos there is no other path), the input voltage will fall to something reasonable, maybe minutes to millivolts, and the trem should start working again.  But 10uF charging at microamps will take forever (and maybe never if the cap leakage discharging the cap is larger than the diode leakage charging it.  It may end up floating about at a few volts, depending on the temperature and general mood, and never fully charge back to zero volts).

The simple answer is to provide a (better) charging path.  The impedance at this point is already obnoxious, and the buffer makes it unimportant anyway, so we can't do a lot to offend it.

If you add a ~100k resistor across the (old) input, that is from the end of C2 not connected to the op-amp output, to ground, it should charge C2 and settle to around zero in about 2-3 seconds after switch on.  This cap should have its -ve end to the op-amp output and +ve end towards the trem diode.

.
I would expect it to be slightly louder and a bit more lively with the guitar isolated from the previous loading effect of the low-Z input.

.
This trem BTW only modulates downwards on peaks about 20-30% of the time leaving the signal full and unmodulated for 80-70% of the time.  Most modulations systems provide 100% sine variation over the entire LFO cycle, so this will sound different.

Wow, I even understood all that too.  ;) I can't wait to get home from work to try it!  :) :) :)
Fear leads to Anger, Anger leads to Hate, Hate leads to Suffering.

galaxiex

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Re: Teisco Checkmate 21 Solid State amp schem + help
« Reply #20 on: August 01, 2014, 12:06:14 PM »
Sorry for the delayed reply...

The trem works!  :) With the buffer!  :) The 100K did the trick.  :)

More later... I'm at work right now...
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Roly

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Re: Teisco Checkmate 21 Solid State amp schem + help
« Reply #21 on: August 01, 2014, 04:17:07 PM »
Cool.  That's what we like to hear.   :dbtu:
If you say theory and practice don't agree you haven't applied enough theory.

galaxiex

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Re: Teisco Checkmate 21 Solid State amp schem + help
« Reply #22 on: August 03, 2014, 02:34:22 AM »
This amp now sounds fantastic!?!?!  :loco

Well.. not really... but it sounds waaay better than when I first plugged it in.  :)

Thanks again Roly for all your help. I really appreciate it. I hope I will be able to give back to this forum someday.
Thanks to all others that contributed as well.

Here's some pics of the buffer and installation.

Yes, I know, I still haven't added a 3 wire grounded power cord yet. Gotta do that next.

As you can see I took your suggestion to use a small strip board and mount it on stiff wires.

I also added connectors to make board removal easier. (I have no doubt I will be working on it more)
The amp is amazingly quiet even with the rats nest of wires and general lack of shielding.

It still lacks volume but a more efficient speaker will help.
However... the most efficient speaker I could find is a 10 inch Eminence Red Fang at 102.1 dB but it's approx $200.00 Canadian!!!!
And I need 2 of them if I build a "replica" speaker cab.

If anyone has any suggestions for a reasonably priced 10 inch guitar speaker of 100 dB efficiency or better please let me know.

Cheers for now!
« Last Edit: August 03, 2014, 02:41:28 AM by galaxiex »
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Roly

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Re: Teisco Checkmate 21 Solid State amp schem + help
« Reply #23 on: August 03, 2014, 08:13:30 AM »
All very neat.

Quote from: galaxiex
I still haven't added a 3 wire grounded power cord yet.

You do that FIRST goddamit!   :trouble



Just leaving aside your worthless hide for a moment, tender little IC's like TL071's don't like floating about at roughly half your mains voltage.  Even if it is current limited enough so you don't get electroluxed, the op-amp input's won't take too kindly to such excessive voltage.

"Excessive voltage?  I don't feel no excessive voltage"

Well no, you may not, or it may just seem that the chassis is vibrating at mains frequency when you touch it lightly.  Well it isn't vibrating, you are, due to leakage current.

A mains transformer is very well insulated against resistive leakage between windings, but having some capacitive coupling, and therefore some leakage current, is unavoidable.

One side of the mains is connected to actual ground, the neutral, while the other side is the active.  Now since one side is grounded this is an unbalanced signal to ground and the effect of the stray capacitive coupling between the windings is to create a capacitive divider between active and neutral/ground, meaning that without grounding everything connected to the secondary winding will float at about half mains voltage.  If you touch your neon screwdriver to the chassis it should light up dimly (and these are a literal life-saver, a must have, and a must use).

The capacitance may be quite small, so the amount of current that can flow at mains frequencies will also be very small, but without a ground there is nothing to limit the voltage, to harmlessly sink this leakage current.

Now you are standing on ground and you touch the input to the op-amp, but the whole of the amp circuit is going up and down half mains voltage, so from the point of view of the op-amp FET input you have just applied half mains voltage to it, and it won't like that at all.

That little green wire is your last line of defence against your IC's (or indeed yourself) being unexpectedly blasted into the hereafter, so take it very seriously indeed.  If you are going to check and correct the mains wiring at any time, always do it first.   :dbtu:
If you say theory and practice don't agree you haven't applied enough theory.

galaxiex

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Re: Teisco Checkmate 21 Solid State amp schem + help
« Reply #24 on: August 03, 2014, 09:57:28 AM »
SIR! YES SIR!!!!

Seriously, I had no idea... which I realize shows my lack of knowledge/understanding in this area.

I mean, I know grounds are important, but once again, your clear simple language puts a new perspective/understanding on the subject.

I guess we (the uninformed/unaware) become complacent since we see so many 2 wire appliances that apparently "don't need a ground".
Then too there are the 2 wire power tools (Drills especially) that have the label "Double insulated" ...
...and maybe we think it has something to do with the cold....  :lmao:

Ok, not really a laughing matter since you could be killed,
but I am old enough to have lived in an age where almost everything only had a non-polarized 2 wire plug.
I'm NOT old enough to have seen houses that were wired with 2 BARE wires and NO ground, running thru the walls and terminated at glass insulators.
But I've obviously heard of them...

Anyway, point taken. Understanding received. The amp will not be plugged in again until said 3 wire is installed.

Once again, Thank you!  :)
« Last Edit: August 04, 2014, 10:04:33 AM by galaxiex »
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Bakeacake08

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Re: Teisco Checkmate 21 Solid State amp schem + help
« Reply #25 on: August 03, 2014, 12:32:56 PM »
Wikipedia has a concise explanation on grounding classes that made a lot of sense to me when I was learning about grounding techniques.

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

galaxiex

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Re: Teisco Checkmate 21 Solid State amp schem + help
« Reply #26 on: August 04, 2014, 12:26:58 AM »
Wikipedia has a concise explanation on grounding classes that made a lot of sense to me when I was learning about grounding techniques.

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

Bakeacake08 thanks for that link. Very concise and makes sense.
Thanks for contributing to my understanding of this.  :)
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Roly

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Re: Teisco Checkmate 21 Solid State amp schem + help
« Reply #27 on: August 04, 2014, 11:08:25 AM »
Quote from: galaxiex
Seriously, I had no idea...

And that is what we are here for. 

"Double insulated" instead of a safety earth seems to have worked out in practice but I'm not a great fan because I've been inside many items where a manufacturer's idea of "insulated" doesn't really coincide with mine.

Let me give you another sobering thought.  The strings on your guitar are deliberately connected back to the ground/screen side of the guitar electronics.  Via the guitar lead this is connected back to the amplifier chassis.  So if it goes live, so do you!

More common and annoying is when you are singing as well as playing, and while floating leakage may only be a very small and harmless current it can bite your wet lips if you happen to touch the mike on a properly grounded PA system.  Not dangerous, but definitely off-putting.  Nibble and tingles are warning signs that something isn't right and needs speedy attention.
If you say theory and practice don't agree you haven't applied enough theory.

galaxiex

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Re: Teisco Checkmate 21 Solid State amp schem + help
« Reply #28 on: August 04, 2014, 11:50:24 AM »
Yep, I have been reading the safety pages on your website Oz Valve Amps and saw that bit about the guitar strings.
Sobering indeed. And ya, I've had that microphone "tingle" before and knew it was a ground issue.
We used to reverse one of the 2 wire cords in the outlet (usually the guitar amp) to make it stop.
Now of course most modern stuff is 3 wire and the tingle should not happen but of course there can be mis-wired mains and other faults that still have this come up from time to time.

Ok, so back to this amp...

I want to have all 3 inputs functional.

Can I use the 1 buffer for all 3 or is there a need to make 2 more buffer boards?

As an option I'm supposing I could make one of the buffer boards have a little gain, so 1 input could be "High Gain" ???

Thoughts?

Edit; Further thought perhaps 1 high gain input, 1 "Normal" buffered input and 1 stock original low Z input.

I would number them L > R

1. Normal
2. High
3. Low

The Low-Z input would be pretty useless for the most part... but allows a quick easy way to compare from original to the others.
« Last Edit: August 04, 2014, 12:07:21 PM by galaxiex »
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Roly

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Re: Teisco Checkmate 21 Solid State amp schem + help
« Reply #29 on: August 05, 2014, 12:22:26 PM »
In Australia, which has always had three-wire "Live-Earth-Neutral" practice, mike "tings" should never happen at all, but particularly at venues it is not uncommon to encounter amateur wired extension leads and power points with Active-Neutral transposed, or no earth at all.

First time the venue operator gets told, but a couple of times after a second gig the supply authority got told (these days it would be WorkSafe).  Most venue operators are responsible about providing a safe workplace for musicians and will make it good, but the few who aren't deserve an official ferret up their trouser leg.  {I spent years in industrial safety gear so I'm rather hard-nosed about irresponsible management.}


How about;

In1 -> gain op-amp -> In2 (switch socket) -> buffer -> In3 (switch socket) -> old input.

With nothing in In3 it connects the old input to the buffer output.
With nothing in In2 it connects the buffer input to the gain op-amp output.
{you don't need a drawing do you?}

Actually a lot of amps have a Low-Z input available on one of the sockets, generally the insensitive one, which is effectively a Line level input for playback input from tape, CD, Pod, laptop, whatever.  This can be handy at "weddings parties anything" where the band is the whole show and has to also provide playback music between sets (and PA for drunken speeches etc  ::) ).
If you say theory and practice don't agree you haven't applied enough theory.

 

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