Teisco Checkmate 21 Solid State amp schem + help

[galaxiex]

Update;
Amplifier current draw and voltage.

No reverb module
Idle                      24.3mA   19V
Strumming hard      62mA     19V and down to as low as 15V on peaks

With reverb module
Idle                       191.5mA   10V
Strumming hard       200mA     10V not much change, might vary a few mV, distorted sound. (sounds kinda neat actually, like an old fuzz pedal)

Measurements taken at main filter cap.

So a .5A or better transformer and maybe a 1A or more diode as the existing one's rating is unknown.

Perhaps this one...

http://www.digikey.ca/product-detail/en/241-5-16/595-1046-ND/952916

[galaxiex]

hi galaxiex,
Check the voltage on power pin 7 of belton brick,, is it Positive? 
Phil.

Hi Phil sorry I missed your post earlier.
Thanks, yes +V on the module. As you can see it's getting sorted out. 

[Roly]

The module drags the supply down to 10V or so.
Unplug module... voltage comes back up to 20V.

You will note that I came directly off the main filter cap, not after the 1k decoupling cap for the preamp, but this is not entirely unexpected.

The only way around this I can see is to add more supply, say a 9V or 12V DC plug-pack rated at 100mA or more + 7805, i.e. build it in where space permits.  This is certainly something you could just lash up to confirm that the signal arrangement works as intended.

Or...

But no point yet until I get a transformer with a higher VA rating.

I'd also consider changing the rectifier to a bridge arrangement, half wave is a bit ...erm... "basic".

I have to observe that this module seems to be a bit of a current hog compared to the way most stuff is going these days.

"Every non-trival programme has bugs"
Every non-trivial circuit has errors.

In general you should never check your own work - if you overlooked it the first time you are likely to go on overlooking it.

Surprised no one mentioned the S-51-A Telequipment Serviscope I have.

Oh I noticed all right  , nice score, I'm a CRO collector and buff and if you want to start a specific thread on fixing it up a bit later I'm available to assist.  Some background here;

http://www.ozvalveamps.org/testgear.htm#cros

With the exception of the sweep generator CRO's of this vintage are pretty similar to a stereo valve guitar amp, and the kind of faults that turn up are also pretty similar to what turns up with guitar amps, high resistors leaky caps and duff valves, but dirty switch contacts and pots in particular - they ain't as exotic as they look at first glance.

I have a service manual for it

Which I would love to have a copy of if you could (my e-mail is in my profile).

Presuming you have measured the AC voltage out of the current tranny, that looks like it would do, but with the increased current and a half-wave rectifier I'd also consider bumping up the value of the main filter cap, say 2200uF.

With these inexpensive amps the only way to go is up, and the more they were trimmed down to the last cent the more altitude is available for improvement.

[galaxiex]

Yep I was taking power right off the main filter cap for the v-reg and module hack test.

I've ordered an X-former but for a test I'll rig up a wall-wart and the 5V reg and see what happens.
I'm sure I've got a 12V 500mA wall-wart around here someplace.

Ya I didn't care for the half wave supply either.
I've got some bridge rectifiers and loose diodes so could do either old school 4 diodes on a terminal strip or go "modern" with a bridge chip. I also have some large value caps that need a home.

I emailed you about the 'scope manual but you're probably sleeping right now.

I haven't looked at that link yet but much thanks  for the offer of assistance later in a new thread on the 'scope.
I think I did spray the pots and switches (proper contact cleaner) some time ago, and it helped, but it still needs some attention.

For now gonna focus on getting this amp the way I want it.
Also got 2 more old SS amps in the line up that need attention/mods...


Warning
Random thoughts, background, and other off-topic blather....

Not sure if any other folks are following this thread....

I know I'm polishing a turd here.
Doesn't matter, I want to do it.
Had this amp as a kid bla bla etc...

I got into messing with electronics a long time ago but no formal education in it.
Experimented with circuits I found in books, built all kinds of little battery powered gadgets.

One of note was a laser "bug".
Bounce a laser beam off a window and capture the reflected beam with a rifle scope focused on a receiver with a photo diode and small amp to headphones.
I actually got this to work... sort of... picked up a lot of building noises and the occasional garbled conversation.
Was easy to do when I lived in an apartment building with other apartments directly across from my 3rd floor apartment. setup laser and Rx on tripod etc...
Now live in a house... no line of sight windows around... just as well...  :loco

Built test equipment, a Dual + - variable power supply for powering Op-amp circuits . Capable of + - 30V at 1.5A
And a single variable supply 30V @ 3A.
Phase locked Freq generator. 1, 10, 100, 1000 Hz 5V peak square wave.
It phase locks to an FM audio signal.
Built it from plans in Radio Electronics or some such magazine.
I used to subscribe to...
Radio Electronics
Modern Electronics
Popular Electronics and a couple others I don't remember.
Saved all the old issues until a few years ago... long gone now.
Back issues on-line tho... not the same as holding one in your hands.

Went on to fixing friends and others car stereos, car amps, home stereos,and what-not.
Was not afraid to take anything electronic apart, esp if it had a problem.
Diagnosed/troubleshoot by "feel, guess and by gosh", and sticking voltmeter probes everywhere, got lucky most of the time... sometimes not. Smart enough to know where to be cautious about sticking the probes...

"Just enough Knowledge to be Dangerous"

One memorable one was a friends motorbike electronic ignition module. He was a student so no money.
Dealer wanted $350.00 for a new module that had about $5.00 worth of parts in it.
It was also potted and sealed. I cracked it open, (what do we have to lose?) and found a broken trace on the board right at the connector. Poor vibration mount. REAL important on a motorcycle.
Fixed it with some wire and solder. He thought I was a genius. Saved him bucks and got his ride back so he could go to school.

Rarely applied ohm's law and other formula. Copied stuff out of books.

Left electronics for a long time.... 5-6 years or more.
I had/have a whole spare bedroom/hobby room with all this electronics stuff in it collecting dust.
Forgot most of what I had learned.

Then got into radio controlled airplanes. Flew FPV right from the start around 2007.

FPV = First Person View.
Mount tiny security camera and vid Tx in model airplane and have ground station with vid Rx.
Feed vid to VR goggles and bam! you are "in" the plane flying around.
If you care to do a search FPV has really come a long way for the hobbyist.
Early FPV days you had to make stuff work that wasn't intended to work with other stuff.
Now you can buy turn-key systems off the shelf and go fly.
On Screen Displays, Return to home, GPS, Stabilization systems, Head Tracking, etc... it's cool geeky stuff.
Probably you've heard of "Drones" and the "menace" they are reported to be. Blech!
Most are just hobby guys having fun.

Left the airplane hobby and electronics again... still got planes new in boxes stashed away.

It's been the same with playing guitar. Play for awhile... rest... etc...

Right now in a band with a great bunch of guys (and girl singer, man she can belt it out!)
so probably will stick with it for a while.

Then found this amp on eBay. Electronics, Here we go again.

One of my FPV vids....



Cheers!

Edit; looked at your CRO page, Wow! lots of info, cool stuff. Thanks!  8|

[Roly]

you're probably sleeping right now.

These days I seem to spend most of my time sleeping.  As I say to SWMBO, "I'm just off for a nap ... before I have a snooze ... to get ready for a good sleep."   xP

I know I'm polishing a turd here.

You want to do that to make money/a living, you're raving mad.

You want to do it for self education, curiosity, or just the hell of it?  Nuthin' wrong with that.  My place is full of well polished turds.

1256 views in 17 days ain't just us chickens.

[galaxiex]

 :lmao: Thanks for the humor guys!  :lmao:

I needed that just now. 

I had hoped to try the wall-wart power supply last night but... best laid plans and all that.... oh well.

Today the transformer came in (DigiKey ships fast) so tonight I'll get to hooking that up with a bridge rectifier and see how it goes.

Cheers!
Dale

Edit;

Roly, SWMBO?

"You want to do it for self education, curiosity, or just the hell of it?"

Yup, all of that. 

Nice to know I'm not the only one that polishes turds...  :lmao:

[galaxiex]

Well I got the schematic done with a few additional minor changes.
Cleaned up the trem circuit a bit and added the bridge and reg's for the reverb.
Found and fixed a mistake that crept in... R38 on the trem pot somehow got changed to 47K, should be 4.7K

Any other mistakes I missed? 

Perhaps add a couple of small bypass caps on the reg outputs?

Have not got the actual changes/mods done to the amp.
Got the transformer mounted and now puzzling out where to put everything else.

Circuit/component changes/repairs are relatively easy compared to (some) hardware changes.
It's a challenge figuring out where/how to mount all this stuff... and have it look neat.
Not much room in this chassis....

I'll post pics when I get it all installed. 

[Roly]

SWMBO - "She Who Must Be Obeyed"

It derives from "Rumpole of the Bailey" by the British writer and barrister John Mortimer, and starring the late Leo McKern in the TV series.

Rumpole's wife Hilda, the daughter of a judge, was a dragon who was permanently disappointed that Rumpole was content to be a rather shabby barrister in criminal practice with no ambition to "take silk" and become a QC, and thence be elevated to the bench.  Despite his "common" tastes in food and more than a little wine, Rumpole was a man of letters given to literary allusions and quoting the classics, and his tacit references to Hilda as "She Who Must Be Obeyed" is a reference to the fearsome queen in the adventure novel She by H. Rider Haggard.

{My step-father similarly would refer to my mother, an avid gardener, as "The Grounds Committee" because we were often roped in for the heavy lifting on her projects.}

Perhaps add a couple of small bypass caps on the reg outputs?

Yes.  I habitually put a small (0.01-0.1uF) cap on the input and output to ground right at the regulator ever since I had one take off at 50-odd MHz and cause no end of hair tearing in a logic system build.  (both regs in your case)


2v2 erratum:  module pin 5 "ground" should go to the "ground" connection on the 5V regulator, i.e. -ve supply, not amplifier ground.  {I'll be glad when you finish this project and I can stop standing on my head to look at the circuit.  }

[galaxiex]

Ahhh, SWMBO... 

I found some clips from the show on YouTube, not watched enough to form an opinion, but it looks like something I would like. 

{I'll be glad when you finish this project and I can stop standing on my head to look at the circuit.  }

:lmao: :lmao: :lmao: :lmao: :lmao:

I've not laughed so hard in a long time.

THAT was FUNNY!!!  :lmao: :lmao: :lmao: :lmao: :lmao:

It'll be done soon... I hope... or I could take a break from this and get to building the speaker cab. 

I fixed the schematic above.

Ground on the reverb module
Added caps to the reg's
Changed the V+ and V- pins on U1 input buffer. Good catch from J M Fahey. Thanks! 

Edit;
The original transformer output measured 16.5 VAC with no load, nothing connected to it.

The new transformer output is rated as 16 VAC at full load, (800mA).
With no load connected it measures 20VAC.
This will certainly give me some PS headroom, but I'm wondering if it might not be too much?
Once I get the bridge rectifier and filter cap hooked up I'll check the DC output with no load.
I know the unloaded DC output will be more, and I'm sure there is a formula... 

Forgot to mention, the original PS diode had markings on it indicating a 1A rating.
Still gonna use the bridge instead.
Also will look at all the cap voltage ratings in case the new PS goes over any of them.

I can, of course, always obtain yet another transformer with a lower voltage secondary.

Edit 2;
The new supply has 26.5V DC unloaded.
With only the reverb module (and the 2 regulators) plugged in it drops to 22.6V DC
so I think we are good to go! 

Next; plug in the amp board and see if we let the smoke out of anything. 

[galaxiex]

Success! ... sort of...

Reverb and amp all hooked up. Idle 22V
Hard strumming at full volume and it never drops below 21V.
The amp is still "not loud" at full volume, but it's again much better than before.
The increased PS stability seems to help.
With some decent efficient speakers it should be very good.

The reverb "sorta" works...
It hums very loudly and cannot turn it up too much. The hum gets very loud past 3.
There is a reverb effect in behind the hum.
It's more of a distinct "echo" or like a double note.
Almost like a digital delay with a medium/long single slap-back echo.
But the echo is at least the same volume if not louder than the first picked note.
Not pleasant at all...

I'm not sure if the problem is on the drive side, recovery side, or both.
Or maybe I damaged the module with the first test when the voltage dropped?
I didn't get the trimmer installed on R16 yet. Not much room on the board there.
It will take some fiddling to get that done. I'll try that next.

BTW, the heat sinks on the regulators for the reverb get pretty hot.
Cannot hold your finger on them for long, couple of seconds and it's very uncomfortable.

[Roly]

Excessive hum?  Hummmm.  I go back to my earlier suggestion of a DC plugpack or similar (module ground to amp ground).

[galaxiex]

Dear J M, this is the current schematic and how everything is hooked up.

The new transformer has a center tap so, yes, that's a great idea to use that for a low Voltage supply. 

Roly, I didn't post this last night... it was late and I was tired,
but I tried a large value resistor is series between pin 1 of the reverb tank and the reverb pot.

The reverb and hum both got better so I kept increasing the R value until I got up to a 5.1M resistor. Yes 5.1M.
Hum was almost gone and the reverb sounded "more ok". No more double note and more echo-y like a reverb should.
I have higher value R's I think... (at work right now) but I stopped there as I said, it was late and me tired... 

... but another issue came up.
It was sounding good so I kept playing... and after 5-10 minutes the voltage dropped and the amp sound cut out completely.

I think the regulators overheated. I quickly switched off, waited and switched back on. worked for 20 sec and cut out again.
That's when I called it a night.

[galaxiex]

Thanks J M, 

I made those cap changes and some other slight re-draw of the schematic.

Of note, I added +Ve symbols to help keep in mind the "positive ground chassis".

I will not re-draw the reverb PS, until I have a working setup on the bench. (polish, polish, polish....) 

I just happen to have a small, 300mA 12V transformer in my stock of parts.

I'm thinking to put back the original transformer to power only the amp board (with bridge rectifier and 3300uF cap) and adding the 12V transformer, bridge, 5V regulator, and filter cap for just the reverb module.

[Roly]

Some mods work out, others don't.  When things start getting complicated it's time to back off and try a different tack.

Attached is a sketch of what I think will work.

The reverb module supply is independent of the main amp supply, only sharing the amp ground.

I've added an emitter follower to provide a lower impedance drive to the 10k module input impedance, and a cap on the output (coz I'm a little dubious about the module internal caps)  A resistor to ground might be required at the module input and output, say 47k each.

[galaxiex]

Roly,
Thanks very much for that, and I certainly will try that. But..... new problem...

I think there is a basic problem with the amp itself.

I decided to try powering the amp only (no reverb module) from the new supply.
Monitored voltage right at the supply connection to the amp board. 25.XX volts powering just the amp.
All good so-far.
(I found the data sheets for the 2SB175's and the 2SB324 output transistors.
They are in Japanese but fortunately the numbers are not.  )
2SB175 max -Vcbo 30V
2SB324 max -Vcbo 32V

A bit close to the 25V of the supply but within the rating? I think?

So, amp plugged in playing guitar, sounds good,
Louder that ever, slight pleasant distortion, voltage at PS output 24-25V ok.
30 seconds in the voltage drops to close to 0 and the amp (of course) goes quiet.
No nasty noises, it just fades to nothing.

Please correct me if I'm wrong, but I think one (or more) of the transistors is breaking down and "shorting".
I "think" this amp should be able to handle the 25V supply.
IF that is so, I need to find the fault causing it to cut out.
(a chance to practice my diagnostic skills) 

Or...

Do you think I could possibly replace the transistors with silicon? Possibly 2N3906's?
Or would we have to extensively re-bias everything?
At this point I don't care about the work involved.
I've spent so much time on it already I may as well go whole hog.
Why not 

Your thoughts please. 

Edit;
I'm gonna go put the ammeter in series with the supply (and monitor voltage)
and see what the current is doing at fade out.
I'm gonna guess current goes up.... lots.... and maybe I'll let the smoke out of something....

Will report back...