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.
After changing just that one cap this thing got very LOUD!
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.
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!
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)
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.