Teisco Checkmate 20 repair/Questions

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Teisco Checkmate 20 repair/Questions

Postby the19thbear » Tue Oct 20, 2015 6:08 am

Hi. On a flea market I found an old tube guitar amp. Thanks to the good people at this forum i found out it was a Teisco checkmate 20.
Did any of you ever own one? Can it distort on its own? I have seen several youtube videos, but i isnt really clear if they have a pedal before or not.
Old thread: http://www.diytube.com/phpBB2/viewtopic.php?f=8&t=6713

On the front it has this:
3 inputs
Treble pot
Bass pot
Volume pot
Reverb pot
Tremolo depth
Tremolo speed
A 3 way switch with an lightbulb on each side. One green one red. None of them work.
In the middle position i a really low sound, with lots of distortion on it. Which maybe could sound way cool, but its just way low.
In any other position (the two extreme ones) i get sound.
Im thinking this is an Off/on/standby switch, but it just seems odd to have "off" in the middle... meh.

I swapped the 2 12ax7s around, but no improvement. I even did the same with the 12au7 and swapped that around, but no improvement.
Maybe the tubes arent even in the right places. Who knows.

This is the amp part only. I dont have a speaker, but plugged in a set of crappy headphones to the speaker output jack and kept the volume WAY low.

First things first. I can follow a schematic, and solder, and have done some analog synth diy, but never messed with tubes before.
So i need to discharge what ever needs to be discharged. How do i go about that?

First off i'll start by replacing the caps. Should i replace all or are there some types i dont have to change? Some that dont deteriorate with age. What about sound? Are there any types i should avoid? Im located in Denmark (the little blob of land above Germany, so there might be some stuff that i cant get here...who knows).
Right off the bat, i should change the two grey ones that are right next to the transformer in the first picture, and the 3 grey ones in the lowest picture, right?
Thanks a lot:)
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Re: Teisco Checkmate 20 repair/Questions

Postby Gingertube » Mon Dec 21, 2015 10:21 pm

Since no-one had replied I thought I'd put a couple of thoughts down for you. Was'nt sure of you level of prior knowledge so tried to keep it simple.

The 3 way power switch looks like it is a centre OFF with either side being ON. The difference is ONLY is the mains noise suppression cap (0.05uF) being switched from active to ground or neutral to ground. This is the cap which is often called the "Death Cap" on amp forums although it is normally on it own 2 way switch. If that has to be replaced you need what is called a Class Y safety rated cap to replace it. It is most likely OK though.
If the lamps do not work then I would suspect simply blown lamps.

The 2 main power supply caps shown on the schematic as 100uF/250V in series after the rectifier diodes are the main power suppy caps for the voltage doubler power supply. They are the cans which poke thru' the chassis and can be seen on Pic #1.

Check the power supply diodes are OK and then replace these 2 caps first. Make sure you get +ve and -ve lead around the same way else boom. Electrolytic caps blow up if you reverse connect them. This can be spectacular, as in loud bang and spreading its guts everywhere, but often is just a pop with a venting of fumes and a nasty smell. Safety glasses are a good idea for a 1st power up after they have been replaced.

Next run along the power supply line in the schematic and you will find a series of caps separated by decoupling resistors, 20uF/450V, a 10uF/450V and then a 2nd 20uF/450V. These are probably the 3 caps next to the power transformer in the bottom picture, replace those too making sure you have the polarity around the right way (all + to the rail and - to ground).

Hint: modern electrolytic caps are MUCH smaller than those on the amp. To keep the looks the same it may be possible to leave the can caps ( the 2 x 100uF/250V) in place, fit a couple of tag strips to their securing bolts underneath and simply move the wires from the can caps to those tag strips and fit the replacement caps to the tag strips - make sure you get + and - the right way round. That is, leave the cap cans in place but remove the wiring from them.

There are 2 more electrolytic caps which should be replaced too. In the pictures these are the blue'ish looking things.
One of them is the output tubes cathode bypass cap which is 100uF/15Volt. 15V is right on the limit, replace with at least a 25V rated cap. You may find a 50V or 63V easier to find. They will be fine.

The other is the cathode bypass cap for the reverb driver and is 30uF/15V on the schematic. Replace that too. In this case 15V is plenty.

Electrolytic caps of that vintage were typically +/-50% or even +/-80% tolerance at best so don't obsess about fitting 22uF in place of a 20uF or even a 22uF in place of that 30uF etc.

Replace the power on globes.

At that point you should be ready to fire it up. Watch the 7189 output tubes carefully and if the structure inside starts to glow orange switch it OFF immediately. Its called red plateing even though its normally orange. This can happen if those 0.005uF coupling caps to the output tubes have gone leaky but I would suggest that they should be OK.

I had a careful look at the pictures to see if there were any caps labelled "Toshiba Oil". Many Japanese tube amps of that vintage, particularly HiFi Amps, used these. They were great when new but don't last and if you ever see any in Vintage Japanese Amps just rip them out and replace them straight away, don't bother to test them coz 90% will be no good.

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Re: Teisco Checkmate 20 repair/Questions

Postby the19thbear » Tue Dec 22, 2015 12:18 am

You are the man! Thanks:)
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Re: Teisco Checkmate 20 repair/Questions

Postby Geek » Tue Dec 22, 2015 10:51 am

I missed this one (???)

Those seem to be flooding the used market, as I've had a few through my shop, all modded. Seems losing the 7189 for a pair of 6V6 is the most common mod, as is putting in an Accutronics tank instead of the little Japanese reverbs of the day.

As well as replacing the caps, replace the diodes as well. The ones that came through my shop all had shorted bias diodes, making the tubes redplate =:o

-= Gregg =-
Fine wine comes in glass bottles, not plastic sacks. Therefore the finer electrons are also found in glass bottles.
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