Teisco Checkmate 21 Solid State amp schem + help

[galaxiex]

Hello all, newb here... go easy on me  Here is a schematic I have generated after extensive searches failed to turn up anything usable.

I tediously/laboriously followed the board layout and made a drawing of all component placement and traces.
With that in hand I sat down and created the schematic in ExpressSCH. Took a couple tries but I think its good now.

Hoping someone can help with the crappy reverb sound I'm getting from this amp. Yeah, I know, it's a crappy little amp but I'd like to make it sound a bit better that it does right now. (Sentimental Value ya know?)

This amp actually has a pretty good deep sounding Tremolo tho the speed range is a bit narrow.

As you can see from the notes on the schem I have already made some changes as the amp was very quiet when I first got it.
Putting a jumper in place of R16 made the biggest difference in volume, but the already bad, tinny, feedback prone reverb got even worse.
I also replaced D2 with a new germanium diode that feeds the trem circuit and got a tiny increase in volume from that change as well.
I suspect the original diode was leaky.

I'm wondering if it's possible to replace the stock single spring piezo reverb tank with maybe the Belton Digi-Log module.

http://www.accutronicsreverb.com/

Might need to add a small driver/recovery board. I'm uncertain about what circuit would be needed to interface with the amp and where to tie in.
If you have looked at the schem you no doubt noticed the odd (at least to me) use of PNP transistors and the funky positive voltage on the chassis. Kinda weird compared to more modern designs.

I don't expect miracles but a bit more volume would be nice too.

Thanks for any and all reply's, help, advice, suggestions, ideas etc 

Edit: Oh ya, check the funky transformer phase splitter (at least I "think" that's what it is) to the "power amp" section.

[DrGonz78]

Not sure this will help that much but I figured it would be interesting to post checkmate 66 schematic. Sorry the quality of the file is not perfect by any means but it shows an example of a solid state amp, hopefully similar to your amp.

Edit: Actually looking closer this looks to be very close if not exactly like your output. Examine how they have the 2.2, 1.8k and 33 ohm resistors in place in this schematic. Try to do the same in your layout and I think you find that this is what you have there in your checkmate 21.

[Roly]

Welcome galaxiex.


The application of the tremolo directly to the input via D2 is ... um  ... shall we say "unique".

Which draws attention to the amp's input impedance, or lack of it.  I make it about 40k ohms which must be the lowest I've ever seen, and would tend to make the guitar sound dull and lifeless.  Add a buffer, or always use a pedal (without true bypass).

Tremolo tho the speed range is a bit narrow.

This is the classic problem with all Phase Shift Oscillators that only vary one of the tuning components - lack of range.

I suspect the original diode was leaky.

All germanium semiconductors are born like that.

If you have looked at the schem you no doubt noticed the odd (at least to me) use of PNP transistors and the funky positive voltage on the chassis. Kinda weird compared to more modern designs.

Yes, but there was a time when this was very common with Asian builds, late 60's/early 70's era.

The single driver transformer arrangement is also very in keeping with the thinking of that time - DrGonz's circuit is a classic of the type.

I'm wondering if it's possible to replace the stock single spring piezo reverb tank with maybe the Belton Digi-Log module.

It's possible, but note that it requires a supply of 5V @ 100mA, so you will require a regulator or some other power supply for it.

a bit more volume would be nice

About the only thing you can do is to fit a more efficient (dB/W) speaker.


But I have to agree with JM.  This amp isn't just old, it was a dated design when it was built, and to be brutally honest it would have been built down to the very last cent.  Teisco's corner of the market was cheapest of the cheap.  Not badly built for what they are, it's just that they aren't much.  It may be a classic now but it was only a throw-away item back then, not much better than a toy.

Even if you kiss this frog, it's still going to be a frog I'm afraid.


If you're really that keen to hot-rod a small amp, then build one of the many chip-amp based designs, and go from there; better prospects I think.

[galaxiex]

Much Thanks to everyone for your replies.

I will try to address them in order.

JM Fahey;

“1) the driver transformer is improperly drawn, it needs a primary winding and 2 independent secondary ones.”

Yes, I see now my error in drawing it like I did.
My not knowing how the six pins on the transformer are wired internally, I simply assumed each side of 3 pins were simply a coil with center tap.
I understand now how that is wrong, and I will redraw the schematic to correct this.

“2) the supply shows a 1K resistor in series with *everything*”

Yes… that IS how it’s wired.
I thought it very odd so I shorted the 1K and the amp did not work properly. I don’t now recall exactly what happened, but I think the Trem quit working when that 1K was shorted, so I put it back to original.

“3) the tremolo diode can't be wired that way”

Respectfully, it is indeed wired that way. I checked the traces on the board and the components connected to them at least a dozen times or more. Probably more…..
It really is wired like that. I can’t explain it. The Trem does work.
Perhaps later I will upload a video demo so you can hear how amazingly effective the Trem actually is. Aside from the narrow speed range it works great. Very deep throb. The only other Trem issue is the Intensity control needs some “expansion” of the range. It goes from full OFF to almost Maximum Intensity with about a quarter turn of the knob.

Pics will be posted of the “piezo” reverb tank.

As you can see, this is a Head type amp. I didn’t get a speaker cab with it, but the original was likely a 2X8 to make this a “Mini Stack”

For now I am using a Vintage @ 1974 Traynor GuitarMate 12” speaker cab. I unplugged the GuitarMate amp and made a cable to use just the speaker.

I will explain here that my interest in this amp is because I had the Sears Silvertone version when I was about 13 years old. Mine was exactly the same as this Teisco but for a silver front panel and the Sears name on it. I had the 2X8 speaker cab too. The Head and cab have these little suitcase type latches to clip the head to the top of the cab. I looked hard to find the Silvertone version with no luck, and when this Teisco came up on eBay I grabbed it. 

I’ll see if I can find some pics. There are other versions of this amp out there.

I plan to build a 2X8 speaker cab for it as similar to the original as possible based on pics I have found of similar amps. I even ordered some of those suitcase latches to clip the head on top.  I’ve built speaker cabs before so that won’t really be a problem.


DrGonz 78

Thank you for the schematic.  I will use it to redraw the output stage. I’m sure that is how it should be.

Roly

Yep, the Trem thru D2 is "unique" 

Input impedance;
I measured 18.75K directly on the first guitar input jack using my Fluke meter.
All other amps I have checked like this measure 1meg or more, so ya, it’s gotta be loading down the guitar. Perhaps a small buffer stage could be placed on the input… or… just always use a pedal… hmmm….

Trem lack of range… I’ll live with it, as I said above it works ok to suit me.

D2 diode leaky;
The original diode forward voltage drop is 2.43 and the replacement measures 3.70 with my meter on diode check. Don't know if the low V drop is an indication of "leaky" but the higher drop diode made the amp louder. I tried a silicon diode but then the Trem quit working.

Reverb;
I can cobble up a 5V reg to power the reverb brick, but I’m uncertain where and how to interface with the amp. Reverb drive and recovery amps needed? I don’t know… Will do more research into this (and get a brick ordered so I have something to play with). Any help along these lines will be much appreciated. 

I’ve been researching efficient 8” speakers and the Celestion 8-15 looks ok at 95dB but if you have any suggestions I’m open.

Yep, it’s a dated design and not really much better than a toy, but I like it, Frog warts and all. I certainly won’t gig with it but might use it as a “novelty” sound for our jam/practice place. 

Thank again for all your input, it is much appreciated. 

Hmmm having trouble getting the pics to post... bear with me...

Ahh... ok... got it.

[galaxiex]

Way cool factor amp 

If at all possible, post a couple close ups of the reverb transducers, from different angles.
Can't even imagine how it works, but never saw anything like that.
Do they have any magnets inside?
Thanks.

Thanks!  I do think its kinda cool myself.

Sure I'll get more pics of the tank later. Magnets? I'm not sure...
I "think" only piezo crystals... I'll test by placing a tiny metal bit next to each transducer.

Right now i am in the process of re-drawing the schem.
WOW I found more mistakes than I first thought...   
I musta been real tired and more than a little cross-eyed from squinting at tiny traces and parts.

[Roly]

Something like a TL071 voltage-follower right at the input J1 would give you as much input impedance and you need.  Me, I'd kludge it up on a small bit of stripboard and mount it on stiff wires directly on the socket, bring power down to it.


The D2 results are odd.   You should get around "100"mV for a germanium forward on a DMM diode test, and the PIV should be at least 20V and more like 50V, so I donunnerstan.  Could be leaky or normal, hard to say with silicon-oriented instruments.


Main thing I'd do with the reverb module is put a 5V zener clamp across the in and output (or similar) to make sure it is not subjected to voltage outside its pay grade.

I suspect that with an active module like this you will be looking for attenuation rather than gain once you try it, but even needing a transistor or op-amp is no big deal.

Interestingly that springline drive looks like it is side-to-side rather than the twist that Belton use.


95dB ain't so hot.  For a small amp like this I'd be shopping around 100dB/W, or better if you can find it - every dB really counts when you're short of watts - and maybe a bigger diameter if it presents.


R9 and R10 are already attempts to fix the abrupt Depth control action.

I certainly won’t gig with it

Might be time for OneBaldBob's story about using a micro amp into a PA mike, hidden behind a cold Marshall stack at a big concert.   


3-core mains lead, earth very secure to the chassis, out with that "death cap"    on the mains infeed (which I note you have already disconnected  ).

[galaxiex]

Thanks much for your help Roly. 

Your suggestion for the buffer is close to what I was thinking,
cept I was more on the lines of possibly a super simple single fet and a couple R's and cap on a small perf board.
I'll look at the TL071 too.

I should mention here that I have NO formal education in electronics, only what I have gleaned from messing around and reading Forrest Mims books, Data sheets, and app notes... and not that I always understood what I was reading.... 

Ya, I don't get the D2 results but it seems to work so I'll let it be. 

For the reverb I was pondering a zener or even a little 3 leg regulator but the zener sounds easier.
I ordered the Reverb Brick so while waiting for that I'll mess with the input buffer.

Here's more pics of the piezo tank.

For the speaker I'm having trouble finding an 8" with 100dB or better... I think I could go to a pair of 10's in the cab I plan to build.
Looks like that Sorrento has 10's in it so they will probably fit.
Considering the output stage, any suggestions on ohms and series or parallel?
I don't know what the original speakers were.. 4 ohm, 8 ohm or what...

I suspected R9 and R10 were attempts to fix that, as they are mounted on the pot. It's ok, just need to set the depth "carefully".   

Heh! Micro amp to a PA, I shudda thought of that! 

Death cap unhook was first thing I did so I did learn sumpthin from reading after all. 

3 wire cord is in the plans but I was unsure how that works with the Pos voltage on the chassis?

Here's some more pics of the reverb tank for J M Fahey , sorry, that's as close as my camera will go, and a schem I copied of a Checkmate 10.
Looks like the same output stage as the CM-21, still working on putting that in my CM-21 schem.
Note the 10V and 13V annotations on the CM-10 schem, they were there already, I just copied it like I saw it.
Odd they didn't put a minus sign on them.

[galaxiex]

Ok, here is the redrawn schematic for the Teisco 21.

Hopefully no errors this time.

I did a complete new layout rather than try to fix the first one.

I hope this is easier to read. 

Arrrgh! I missed a couple of values and part #'s I must be getting tired...

Ok, here it is again... last one for tonight.

[Roly]

Much easier to follow.

Earth missing on Intensity pot.   {doncha just hate that?}


"All non-trival programmes have bugs."  Programmers truism.

"All non-trivial circuits have mistakes."  Tech's truism.

Once upon a time in Ye Olde Days of Yore I was building industrial safety systems.  One such (thankfully not of my design) went through prototype testing, company validation, then validation by a government department, and into production before the massive and potentially tragic blooper was spotted.

If there is one thing that a tech must always be, it's suspicious; is my meter fooling me, is that a circuit drafting error, is that solder joint really as good as it looks at a distance?  {is the client telling me the truth about how the amp blew up?}

"Just concentrate on the facts Watson."   


Circuit Drafting Muse

After you have looked at a lot of guitar amp circuits there emerges a bit of a very loose convention about how the circuit is laid out.

Input starts top left and runs across to the output stage top right.  The power supply is bottom right just above the title block, and the tremolo and reverb are placed bottom left.  Very generally.

I note that you have already done some of this re-laying out in your rev1 (and thanks for the rev no., I use the date in reverse as a meaningful date/serial yymmdd  140730 appended to drawings, file names &c.  You leave a developmental history trail that can be reviewed if an errors has crept in, and you always know which is the latest.)  I like the ground at the bottom, even if the supply rail happens to be negative in this case (but this should be clearly marked because it's not what most techs would assume these days).

Naturally you have to be driven by the task at hand, so if it has a particularly complicated preamp you might set the first part of the preamp across the top of the sheet, the rest of the preamp and the main amp across the middle, so the signal path is "backwards-S" shaped.

Similarly there are sub-conventions for drawing different stages/functions so that they are instantly recognisable.  The Phase Shift Oscillator for example is typically drawn;



This highlights the Phase Shift Network and instantly identifies it as an oscillator stage, and of what type.

It is quite possible to draft a circuit where it is very hard to work out the various signal and power flows, what each stage does, and how it does it (just as spin doctors can use real words to obscure what they are actually saying, "the aircraft suffered a systems malfunction" i.e. an engine dropped off.)

The primary object of a circuit, or any technical drawing really, is to communicate to other techs down the track, and to do so quickly and effectively, and that must always remain your number one goal.  There is no law against drafting an awful circuit, you will just be cursed to the seventh son of the seventh son by every tech who sees it.  So every circuit is a bit of an artwork, and some elegance is required in its functionality.

[g1]

3 wire cord is in the plans but I was unsure how that works with the Pos voltage on the chassis?

  There is no voltage on the chassis.  It is positive ground.  The supply is negative instead of positive.  Many old cars used to be positive ground.
  Voltage and ground can be tough to wrap your head around, all voltage is relative to something, we usually use ground as that reference, but it is not necessarily so.
  In this case, everything is in reference to the positive, so we call that ground, and you will be connecting that point to the chassis.

[galaxiex]

Much easier to follow.

Earth missing on Intensity pot.   {doncha just hate that?}


"All non-trival programmes have bugs."  Programmers truism.

"All non-trivial circuits have mistakes."  Tech's truism.

Once upon a time in Ye Olde Days of Yore I was building industrial safety systems.  One such (thankfully not of my design) went through prototype testing, company validation, then validation by a government department, and into production before the massive and potentially tragic blooper was spotted.

If there is one thing that a tech must always be, it's suspicious; is my meter fooling me, is that a circuit drafting error, is that solder joint really as good as it looks at a distance?  {is the client telling me the truth about how the amp blew up?}

"Just concentrate on the facts Watson."   

Earth missing....       ahhh knew it was too good to be true.. 

I work as a automatic transmission re-builder and the hardest thing to do is trouble shoot something that someone else rebuilt/messed up.

"Objects are not always as they appear"

Circuit Drafting Muse

After you have looked at a lot of guitar amp circuits there emerges a bit of a very loose convention about how the circuit is laid out.

Input starts top left and runs across to the output stage top right.  The power supply is bottom right just above the title block, and the tremolo and reverb are placed bottom left.  Very generally.

I note that you have already done some of this re-laying out in your rev1 (and thanks for the rev no., I use the date in reverse as a meaningful date/serial yymmdd  140730 appended to drawings, file names &c.  You leave a developmental history trail that can be reviewed if an errors has crept in, and you always know which is the latest.)  I like the ground at the bottom, even if the supply rail happens to be negative in this case (but this should be clearly marked because it's not what most techs would assume these days).

Naturally you have to be driven by the task at hand, so if it has a particularly complicated preamp you might set the first part of the preamp across the top of the sheet, the rest of the preamp and the main amp across the middle, so the signal path is "backwards-S" shaped.

Similarly there are sub-conventions for drawing different stages/functions so that they are instantly recognisable.  The Phase Shift Oscillator for example is typically drawn;



This highlights the Phase Shift Network and instantly identifies it as an oscillator stage, and of what type.

It is quite possible to draft a circuit where it is very hard to work out the various signal and power flows, what each stage does, and how it does it (just as spin doctors can use real words to obscure what they are actually saying, "the aircraft suffered a systems malfunction" i.e. an engine dropped off.)

The primary object of a circuit, or any technical drawing really, is to communicate to other techs down the track, and to do so quickly and effectively, and that must always remain your number one goal.  There is no law against drafting an awful circuit, you will just be cursed to the seventh son of the seventh son by every tech who sees it.  So every circuit is a bit of an artwork, and some elegance is required in its functionality.

Thanks for your insights and explanation of this. 

I may at some point redraw (ver2) the schem along those lines. I like the yymmdd. Very clever. I may borrow that for future versions.

btw, that first schem, is my first one! I have never drawn a schem before that... although I've looked at lots of them.

[galaxiex]

3 wire cord is in the plans but I was unsure how that works with the Pos voltage on the chassis?

  There is no voltage on the chassis.  It is positive ground.  The supply is negative instead of positive.  Many old cars used to be positive ground.
  Voltage and ground can be tough to wrap your head around, all voltage is relative to something, we usually use ground as that reference, but it is not necessarily so.
  In this case, everything is in reference to the positive, so we call that ground, and you will be connecting that point to the chassis.

Thank you!

I thought that was so, and of course thinking about it now it only makes sense. Being in the automotive repair business, I of course, know about "positive earth" cars but have never worked on or dealt with them.

Earth is relative....   

[galaxiex]

This might seem stupid... the "positive earth" thing has me stumped again.

I'm trying to make a input buffer using a TL071.
I get that the chassis is at positive potential and I hook that to pin 7 of the TL071.
Pin 4 goes to "negative" on the board somewhere, probably after the 1K resistor R37.
Pin 6 to 10uF cap to guitar input on the amp.
Guitar in to .1 cap to pin 3.

So where do my "signal grounds" go?

Here is the circuit I am trying to use.
This can't be that hard but I can't wrap my head around it.

[g1]

  The point marked +9 goes to ground.  The points marked with ground symbol go to your negative supply voltage.  The TL071 should be able to handle up to 36V difference between pins 4 and 7 so you should be fine.
As you see, pin7 will still end up being "positive" in relation to pin 4.
Pin3 still ends up being at half the supply voltage (due to R1/R2 voltage divider).

[galaxiex]

Thanks! I'll try it. If I blow something up, well... it's a 70 cent IC. I bought a bag of 10.