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Author Topic: Teisco CheckMate25 solid state schematic etc  (Read 581 times)

galaxiex

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Teisco CheckMate25 solid state schematic etc
« on: March 03, 2017, 11:20:32 PM »

TEISCO CHECKMATE 25 SS HEAD
REPORT AND RESTORATION
[Or, how to polish a turd] ;)

Sorry for the long post.  :-[

I found this amp on eBay, cheap.
No matching speaker cab, but there is one of these amps on Reverb(dot)com with the speaker cab, so I grabbed the pictures so you can see what it looks like.

The seller of this one said… “Its old and it works”…
So when it got here I plugged it in and it did “work”.

Very quiet barely even bedroom volume level.

The tremolo works, a bit fast on the speed range though.

The reverb is a very UN-musical clank!

Opened it up to find disgusting mess of dead bugs, cobwebs and general filth.
Cleaned the mess and added a 3 wire power cord and got rid of the death cap.

The Voice channel has the highs rolled off, very dull sounding with guitar.
47nf cap to ground, right on the Voice volume control.
(I assume Voice means Microphone input)

Taking a quick look at the boards I noticed an abundance of 10uf electro caps that appear to be used as audio path coupling caps.
I don’t like electrolytic caps in the signal path so thought I’d replace them with 1uf box film caps.

I’d need a schem to do that….
(Well, I could do it without a schem, but I’m gonna draw one anyways)

However, the input cap C1 right after the jacks was easy to spot without a schematic so I replaced that one first.

Holy Hanna!
After changing just that one cap this thing got very LOUD!
(Relatively speaking)
Driving a Jensen MOD 10-50 speaker.
Changing that one cap convinced me to re-cap the whole amp.

So did that, and replaced the trimmer TR1 100K in the power amp.
The original open frame trimmer was kinda glitchy and rough looking.
Installed a better quality Bourns sealed trimmer.
With a volt meter on the output junction between Q5 and Q6 I adjusted the trimmer for ~ half the supply voltage after warm up.

Amp sounding “not bad” now, but the reverb was still that gawd-awful clank sound.

What to do….

Noticed the tank was installed backwards…
Input was marked on the tank, and input was connected to the recovery side of the circuit.
Switched it around and…. No difference….

Opened the tank and noticed it’s a piezo type. Ah-ha.
Likely makes no difference which way it’s hooked up.

Hmmmm,
The cabinet seemed big enough to install a “normal” full size reverb tank on the inside top.

Have such a tank in my junk and determined that it would fit.

The tank I have has the often used 8 ohm input impedance common in many tube amps.
Did some reading/research and found that this can be difficult to drive.
Connected it to the existing circuit anyway, just to test and… no reverb sounds.

Rattling the springs/tank produced reverb noises, so the recovery side will “probably” work ok.

Well, reverb tanks are relatively cheap so ordered up a 4FB2A1A.

4= Type 4 - 17” long, 4 springs
F= 1475 input impedance
B= 2250 output impedance
2= Medium decay
A= input/output grounded
1= No lock
A= horizontal mount, open side up

So the higher input impedance of the new tank should be good… at least easier to drive…

Connected this new tank directly to the existing circuit, knowing full well that it probably would not work.

Nope, gonna need a drive circuit.
What to do….

I lifted the reverb drive circuit almost verbatim from a Univox U65G schematic that I have. I even have a spare 2SC1317 transistor!

The Univox reverb sounds ok, despite having a smaller (tiny) tank.
I don’t know the AC input impedance of the Univox tank, but the DC resistance is close to what the DC resistance reads on the new tank.

Bread-boarded the circuit and hooked it all up and…
It sounds ok. 
Not a Fender, but sure a darn sight better than the clank noise it had before.

Etched up a circuit board for the reverb drive and got it all installed.
Left the disconnected piezo tank in place for posterity. ;)

In hindsight I should have used a shorter 9 inch tank and mounted it away from the power supply side.
The long tank has the tank output close to the power transformer.
(Mounted as far away as possible)
There is a slight hum that increases when turning up the reverb control.
Moving the tank away from the chassis the hum goes away, so it is picking up from the power transformer.
The hum is not noticeable when playing, but I may change the tank out later for the short one.
Or maybe rig a shield between tank and transformer.

On to the tremolo circuit… I want to slow it down a bit…

Replaced C19,20,21 470nf caps with 1uf caps and the oscillator stopped working at slow speeds.
High speed still works and slowed down to a useable rate.

Replaced Q6 with a higher gain MPSA-18 and the slow speed came back.
It’s slow to start at initial turn on, but works reliably once it starts.

Speed range is now more to my liking. Works great!

Side note;
Have a look at Q7, Q8 and the circuitry around them.
Seems like a lot of rigmarole just to drive a speed indicator lamp!
Node “C” from the P/S is solely for this function!

I took power from this node for the reverb drive circuit because when I tried taking power from any of the other supply nodes; it would load down those nodes too much. (Several volts)

Node “C” only dropped from ~ 14V to 12V with the reverb drive circuit added.
The power at that node does pulse a bit, but does not seem to affect the reverb. Maybe it even adds some ambiance. ;)
Side effect; the speed indicator lamp is a bit dimmer than before.
Who watches that lamp while playing?


Input impedance looks dismal.

Rewired the #1 and #2 input jacks to Fender style with a 1 Meg and 68K resistors. (Shown on the schematic as J1a, J2a etc)
Much better.
Guitar noticeably brightened up on the thin strings.
Was gonna put a buffer on J1 and 2 but the Fender style input works pretty good.

Not sure yet what to do with the Voice channel.
Maybe an input buffer, or make an adjustable gain input with clipping since the control is already there.



« Last Edit: March 12, 2017, 01:59:57 AM by galaxiex »
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J M Fahey

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Re: Teisco CheckMate25 solid state schematic etc
« Reply #1 on: March 04, 2017, 03:04:20 AM »

WOW!!!!!Got tired just reading half of what you did, must have taken a LONG time  :o

Congratulations on your patience and dedication  :dbtu:
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galaxiex

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Re: Teisco CheckMate25 solid state schematic etc
« Reply #2 on: March 04, 2017, 12:50:05 PM »

Thank JM!  :)

Sorry again about the long post.

I got this amp in November and worked on it, off and on, since then.
I kept track of what I did and I thought to put it all in one post.
That way everything is in one place and easy to find, not spread all through a long thread.

Here's another internal pic with the original reverb tank in place (not hooked up),
and the wiring neatened up with cable clamps and zip ties.

I still have not decided what to do with the Voice channel.
A Boost or Drive channel with adjustable gain still appeals to me, just not sure yet how to do that.

I've learned a lot working on these cheap Japanese and other eBay amps.
As a hobby it is fun for me and a sorta/kinda therapy as well.
Keeps the mind engaged and interested.  :)

Cheers!
Dale
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galaxiex

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Re: Teisco CheckMate25 solid state schematic etc
« Reply #3 on: April 05, 2017, 11:31:00 PM »

So I put this amp aside for a couple of weeks while doing other things....

Plugged it in today and something is not right...

Very "fizzy" sounding distortion follows all notes but more noticeable on Bass notes,
especially noticeable if you just pick a single note and let it decay.
As the note decays the "Fizz" becomes very obvious and also seems to modulate, or pulse.

For a quick check I put a scope on the output and got a "kinda sorta" normal waveform,
so I scoped the DC power supply.

The audio is modulating the DC power supply.

Where there should be a nice flat line of DC, the audio shows up on the power supply as a very distorted waveform.
Also, when not playing any notes, the DC shows a saw-tooth waveform.

All voltages look normal when checked with a DVM.

Where should I start to look?
All caps are new and this amp was working fine a couple weeks ago.

Bad bridge diodes? Something else?

Thanks for any help.  :)
« Last Edit: April 05, 2017, 11:32:21 PM by galaxiex »
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nomeX

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Re: Teisco CheckMate25 solid state schematic etc
« Reply #4 on: April 07, 2017, 05:59:24 PM »

I have the Kay Version (K-849) of the Checkmate 20 on it's way to me.  I was all excited to get another tube amp when I saw that I was looking up the wrong Checkmate 20.  we'll see how it is when it gets here.  I've been getting a lot of low watt tube amps of late, but my only bass rig is a GK M115 that I love to put my guitars through as well.

Seems like no one is willing to work on Solid State around here (Houston, TX)
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J M Fahey

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Re: Teisco CheckMate25 solid state schematic etc
« Reply #5 on: April 10, 2017, 07:29:28 PM »

Plugged it in today and something is not right...

Very "fizzy" sounding distortion follows all notes but more noticeable on Bass notes,
especially noticeable if you just pick a single note and let it decay.
As the note decays the "Fizz" becomes very obvious and also seems to modulate, or pulse.
Crossover distortion.
That said, bias is not adjustable and even if it were, you might risk thermal escalation and eventual transistor burnout, doubly so because those are Germaniums.
Small consolation: amp probably works the same as when it left Factory in the 60´s  :o

You *might* add a 100 ohm trimmer in series with 200 ohm bias resistor , put out low power 1 or 2 V RMS into a load, scope it, and increase bias slightly until crossover notch/kink disappears but tou´d better epoxy thermistor to one heatsink so it tracks temperature better.

**Personally** I´d replace power amp with a single supply LM3886 ; sound will not change at all, butvreliability will improve 1000% .
Same supply, in/out capacitors and gain of course.

OR mod that amp to a Silicon version using same PCB and 90% of current components and add the proper biasing network.

Again this would not change sound, SS power amps in general are just "large OpAmps"  and by definition "all sound the same".

Quote
For a quick check I put a scope on the output and got a "kinda sorta" normal waveform,
so I scoped the DC power supply.

The audio is modulating the DC power supply.

Where there should be a nice flat line of DC, the audio shows up on the power supply as a very distorted waveform.
Also, when not playing any notes, the DC shows a saw-tooth waveform.
Perfectly normal.
I bet you increased Scope gain a lot just to see that, we are tlking  few hundred mV at most (on the powe rail), a couple V ripple when full blast.
Quote
All voltages look normal when checked with a DVM.
o no problem there.

Quote
Where should I start to look?
All caps are new and this amp was working fine a couple weeks ago.

Bad bridge diodes? Something else?
You probably only *noticed* it now ... unless a speaker is scratching which is something else.
Try it with another known good speaker.

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galaxiex

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Re: Teisco CheckMate25 solid state schematic etc
« Reply #6 on: April 10, 2017, 08:20:36 PM »


Crossover distortion.
That said, bias is not adjustable and even if it were, you might risk thermal escalation and eventual transistor burnout, doubly so because those are Germaniums.
Small consolation: amp probably works the same as when it left Factory in the 60´s  :o

You *might* add a 100 ohm trimmer in series with 200 ohm bias resistor , put out low power 1 or 2 V RMS into a load, scope it, and increase bias slightly until crossover notch/kink disappears but tou´d better epoxy thermistor to one heatsink so it tracks temperature better.

**Personally** I´d replace power amp with a single supply LM3886 ; sound will not change at all, butvreliability will improve 1000% .
Same supply, in/out capacitors and gain of course.

OR mod that amp to a Silicon version using same PCB and 90% of current components and add the proper biasing network.

Again this would not change sound, SS power amps in general are just "large OpAmps"  and by definition "all sound the same".

JM, Thanks for the detailed answer!  <3)

Yes, I think you are correct that the amp is working the same as when it left the factory.

I like the idea of making the bias adjustable, if only to further my own learning about these things.
So did you mean to put a 100 ohm trimmer in series with R10 the 220 ohm that is in parallel with the therm1?
Incidentally that therm1 is nowhere near the heat sink as it is mounted on the circuit board.

Quote
For a quick check I put a scope on the output and got a "kinda sorta" normal waveform,
so I scoped the DC power supply.

The audio is modulating the DC power supply.

Where there should be a nice flat line of DC, the audio shows up on the power supply as a very distorted waveform.
Also, when not playing any notes, the DC shows a saw-tooth waveform.

Quote
Perfectly normal.
I bet you increased Scope gain a lot just to see that, we are tlking  few hundred mV at most (on the powe rail), a couple V ripple when full blast.

Yes, scope gain quite high,
I did not know that was normal as I don't think I have ever scoped a DC supply while playing any amp.
Another lesson learned!  :)

Quote
All voltages look normal when checked with a DVM.
o no problem there.

Quote
Where should I start to look?
All caps are new and this amp was working fine a couple weeks ago.

Bad bridge diodes? Something else?

Quote
You probably only *noticed* it now ... unless a speaker is scratching which is something else.
Try it with another known good speaker.

Yes, again you are correct.  :) I only just noticed it now.... the "fizz" was probably there before but I didn't notice it then...

Only now after not playing the amp for a couple weeks, it's like you notice something that was there all along...

Speakers ok, I did try a couple different speakers, but at loud volumes things rattle and buzz in the small room I work/test in.
Very annoying as several times I have thought something was wrong with some amp or speaker when it turned out to be stuff in the room buzzing/rattling.

I'm gonna have to start taking these things outside to test them!  ;)  :lmao:
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J M Fahey

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Re: Teisco CheckMate25 solid state schematic etc
« Reply #7 on: April 11, 2017, 12:38:49 PM »

In the old days they often underbiased everything so thermistors or diodes only had to track ambient temperature, now once you set the proper current the transistors start dissipating even if you do not drive them, so things become critical.
Yes, glue thermistor to one heatsink (or clamp it with a small strip of aluminum or even a piece of thick copper wire) so it *really*  tracks it , add a 100 ohm trimmer and start raising bias until crossover notch (which is visible at *low* power) just disappears.
Doublecheck by measuring current, you may measure voltage drop across emitter resistors, once you get them passing, say, 10mA they are beyond the crossover area.
Play at good volume for a few minutes, so heatsinks get warm to the touch, and measure idle current again.
If same or lower, you are fine; if higher the system is not stable and yu should bias it colder.

FWIW most old transistor amps, specially those transformer driven such as VOX or Standel *did* have crossover distortion ... or else.

Part of the early rejection of transistor amps, at least this one problem was real.
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galaxiex

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Re: Teisco CheckMate25 solid state schematic etc
« Reply #8 on: April 11, 2017, 07:35:45 PM »

Thanks again JM, for the awesome help!

The way the amp is right now,
the output transistors are barely warm even after playing at full volume for some time.

I will add the mods and do as you suggest, monitor the V drop on the emitter R's and scope for X over notch.

I'll report back, but may take some time.

Right now I have my 1978 Silverface Deluxe Reverb apart on my bench for some clean up and maintenance.
It's getting a electro cap job and some new metal film resistors in a few key places.
Already got the MOD 12-50 speaker installed.

Cheers!
Dale
« Last Edit: April 12, 2017, 08:49:02 AM by galaxiex »
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nomeX

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Re: Teisco CheckMate25 solid state schematic etc
« Reply #9 on: April 13, 2017, 08:27:06 PM »

I just got mine in and played for 20 minutes-  I notice the distortion galaxiex has mentioned.   The amp does break up a bit, which I enjoy.  The horn is definitively piercing.   This is a unique amp which definitely has a place with my unique low watt tube combos.
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