Teisco CheckMate 22 schematic

[galaxiex]

Got this off the bay awhile back, just getting to it now.

Seller pics.

The seller said the reverb does not work and I found the tank output coil to be broken.
No surprise, considering the amount of corrosion in there... 

The schematic inside the back panel is "not quite" correct.

I traced the board and drew a correct schematic. (next post) 

 

[galaxiex]

Here's the corrected schematic...

Edit; added main voltage to schematic.

Edit 2; I made a mistake drawing the original schem, so here is the (again  ) corrected schematic.

[galaxiex]

Here's the badly corroded output coil.

I have already managed to get the coil bobbin removed from the core.

I have a pickup winder so I'll make an attempt to rewind the coil.

[galaxiex]

Here's some mods so far....

3 wire power cord and a fuse!!! 
Also got rid of the "death cap", C20 in the original schematic.

C1 and C7 changed to 100nF, altho C7 is not doing much right now as the reverb tank is still out.

Removed the aux speaker jack and put the fuse holder in that hole.

More mods to come....

Input impedance is dismal, so a buffer on the input is in the plan.
Possibly a (slightly) bigger power Tx and full wave bridge rectifier.

Edit; schematic removed, mistakes were made, corrected schematic in post below...

[galaxiex]

BTW, they made these (CM-22) in different versions.

Here's the other type, I call it the "square head".

Not mine, internet pic...

It only has Tremolo speed, no depth control, and has foot switch jacks.

[galaxiex]

Here's a layout for the original board.

[galaxiex]

Here's some modifications to the little CM-22. 

Pics show the added stuff.

The input buffer is just hung on some stiff wires, right on the input jack.

The added C24 and R39 (see schematic) give a large gain boost,
very "crunchy" germanium transistor distortion, almost Fuzz Face territory. 

I tried with just the 100uF cap and it was too much gain, squealing feedback.
The 1K resistor tames it just right.
Thinking to maybe put the C24, R39 "gain boost" on a switch...

Last pic is a size comparison of the power Tx.
The bridge rectifier setup is temporary, I'll make a little board for it.

Can you tell I'm bored with being stuck at home? 

Edit; still haven't fixed the reverb tank, I'll get to that later today... maybe...

Edit 2; schematic removed, mistakes were made, corrected schematic with all modifications in post below.

[galaxiex]

Ok, the reverb coil winding has started.... 

First pic is the bobbin mounted to the winder with the old wire still on it.

Second pic is the new wind started.

Wow this thing is tiny!

Winding going really sloooow cuz I don't want to break the wire and hafta start over...

The new wire is thicker than the old, so I'll over-wind it somewhat.

Old wire measures .015"
New is .025"

I was able to find the end wrap and paid attention to the direction of wind.

New wind is going on the same direction, don't know if it would matter (phase issue, maybe?).

[galaxiex]

Got the coil wound and installed. 

4760 turns and 662 ohms.

It's alive!
We have reverb!
(albeit not very good reverb, but I expected that  )

Now I'll disassemble the tank and clean it up some,
just to get rid of some of the corrosion.

Edit: This is the output coil that I just wound at 662 ohms.
No idea what it was originally, or what it's supposed to be.
Seems to be working fine.

The input coil measures 1K6 ohms DCR and both coils were/are wrapped with blue tape.
I don't know if the tape color indicates coil impedance/DC resistance like the Accutronics tanks.

[galaxiex]

Ok, some more mods. 

Added a buffer at the mix point of the volume and reverb.
The high pass filters at the output of the op-amps (C23, R38 and C26, R43) cut unnecessary low frequencies.
Corner frequency of the filters is approx 88 Hz.

The filters helped a lot to get rid of the amp sounding muddy/mushy/smeared sound.

Now... I think the mix point of the vol and reverb could use some help.

The way it is now the reverb sounds like a completely separate signal sort of "riding on top of" the main signal.
IOW it doesn't sound "mixed".
Hope that makes sense...

Any suggestions on how to "more properly" mix the 2 signals?

Thanks for any help!   

Edit; connecting the reverb output to the top of the volume pot seems to work better.

Here's the corrected schematic...

[galaxiex]

Ok, more polishing the turd.... 

I just can't leave well enough alone.   

I wasn't happy with the reverb sound of this amp.
It's mostly due to the limitations of the single spring reverb tank.
At almost any reverb setting it sounded more like a metallic "clank" than reverb.

So I thought the way to get a more musical/realistic reverb sound,
was to have some control over the frequency's being sent to the tank.

I first tried the reverb tone network you see in the schematic,
without the trim pot and no buffer.
2 X 51K resistors in place of the trim pot and output taken from the junction of them.

This worked somewhat,
but the tone network loaded down the circuit too much, reverb too quiet.

So I took the volume buffer (from previous schematic) and swapped it to the tone network.
Then adding the trim pot as a reverb tone control.

This works great!

The tone network adds a mid notch and the trim pot lets you roll off the highs.

The boards are supported on solid core stiff wires.

[joecool85]

Ok, more polishing the turd.... 

I just can't leave well enough alone.   

I wasn't happy with the reverb sound of this amp.
It's mostly due to the limitations of the single spring reverb tank.
At almost any reverb setting it sounded more like a metallic "clank" than reverb.

So I thought the way to get a more musical/realistic reverb sound,
was to have some control over the frequency's being sent to the tank.

I first tried the reverb tone network you see in the schematic,
without the trim pot and no buffer.
2 X 51K resistors in place of the trim pot and output taken from the junction of them.

This worked somewhat,
but the tone network loaded down the circuit too much, reverb too quiet.

So I took the volume buffer (from previous schematic) and swapped it to the tone network.
Then adding the trim pot as a reverb tone control.

This works great!

The tone network adds a mid notch and the trim pot lets you roll off the highs.

The boards are supported on solid core stiff wires.

Great job!  I always love seeing people make old gear work / work better. 

[galaxiex]

Great job!  I always love seeing people make old gear work / work better. 

Thanks Joe! 

This one has been fun for me.
I don't even mind that it's barely 2 watts (if even that) with those tiny germanium output transistors.
I've learned a lot after tinkering with all those other amps that are similar to this one. (still learning! )

I've been running this thru a 12" sealed cab loaded with a 8 ohm Celestion G12H.
Great sounds at bedroom levels with the vol on max.

I did find out it doesn't like 4 ohm loads.
The output transistors go into thermal overload after about 20 seconds. 

Cheers!

[galaxiex]

Bumping this cuz we seem to have reached the limitation of the tiny output transistors. 

The new 18VAC transformer and resulting 25VDC rail is just a bit too much for them. 
Playing the amp for more than 30 minutes at full volume causes the output transistors to go into thermal overload.

At less than full volume I have not reached the limit...
but this thing sounds so darned good at full. 

So options are...

1. Better heatsinking on the output transistors
2. Better (higher rated) transistors and better heat sinking.

I'm leaning toward option 2 but might try cobbling up a better heat sink for the originals.

I do have a couple of NOS T0-3 PNP Germanium transistors looking for a home... 

[joecool85]

Bumping this cuz we seem to have reached the limitation of the tiny output transistors. 

The new 18VAC transformer and resulting 25VDC rail is just a bit too much for them. 
Playing the amp for more than 30 minutes at full volume causes the output transistors to go into thermal overload.

At less than full volume I have not reached the limit...
but this thing sounds so darned good at full. 

So options are...

1. Better heatsinking on the output transistors
2. Better (higher rated) transistors and better heat sinking.

I'm leaning toward option 2 but might try cobbling up a better heat sink for the originals.

I do have a couple of NOS T0-3 PNP Germanium transistors looking for a home... 

I'm always a fan of giant heat sinks: https://www.ssguitar.com/index.php?topic=175.msg293#msg293