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Author Topic: Sears/Silvertone 40XL solid state amp schematic  (Read 6843 times)

galaxiex

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Sears/Silvertone 40XL solid state amp schematic
« on: October 10, 2016, 12:37:47 AM »
Finally got this one done.

EDIT; removed schematic from this and other posts, see later post #19 for correct schematic. Thanks!

The schematic for the solid state version of this amp is unobtainium.

This amp may be better known as the 1422 and started life in the late 60's as a tube amp.
By the early 70's it became Solid State and has a model # 14222.

Schematic for the tube version is easy to find.

I've seen a few folks asking on other forums for the SS version schematic, so, here it is!  :)

Found on ebay with a seller made cab and a non original vintage speaker.

Tremolo and reverb were not working. Fixed now. Bad reverb tank and a broken trace for the trem.

Note the weird tremolo circuit, Q8 collector goes to ground.

Cheers!

« Last Edit: October 14, 2016, 08:30:07 PM by galaxiex »
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galaxiex

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Re: Sears 40XL solid state amp schematic
« Reply #1 on: October 10, 2016, 12:43:20 PM »
Here is the layout I created by scanning the board trace side and using Paint.net for the art work.

Seems like a convoluted layout and it was confusing to create the schematic from this.

EDIT; removed layout, see later post #19. Thanks!
« Last Edit: October 11, 2016, 06:28:34 PM by galaxiex »
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galaxiex

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Re: Sears 40XL solid state amp schematic
« Reply #2 on: October 10, 2016, 03:04:47 PM »
Oooops  :-[

Do I have Q7 collector and emitter backwards?  ::)
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galaxiex

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Re: Sears 40XL solid state amp schematic
« Reply #3 on: October 10, 2016, 06:14:42 PM »
Corrected Q7 and slight clean up of the schematic.

Q5 remains a mystery. It has markings;

5981
274
6912

I think the 6912 is the date code. 12th week of 1969.

Unsure of the rest.
Its a TO-5 metal can package, board mounted with a fair heat sink.
It gets quite warm. Hot even. Uncomfortable to hold your finger on it hot.

Q6 and Q7 barely get warm.

Edit; removed schematic, see later post #19. Thanks!
« Last Edit: October 11, 2016, 06:28:52 PM by galaxiex »
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Enzo

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Re: Sears 40XL solid state amp schematic
« Reply #4 on: October 10, 2016, 08:16:25 PM »
I don't know what it is, but it is just a transistor.  It needs to handle the entire power supply voltage, which in this case is only 26v.  So a TIP31 or TIP41 shold work fine.  Those are TO220, but you could use them anyway and put TO220 heatsinks on them.  You call it TO5, I call it TO39, I'd wager.  Same size as the center part of a 40409?

In a TO39, a 2N5320 or a 2N2102 should work, and fit your heat sink.  I have those types in stock, hence their choice.

It gets hot because too much current flows through it.  So how hard is it turned on?  Did you test it?  R17 is 68 ohms, does it measure OK?  And how much voltage is dropped across it, ie what voltage is on its base.

The fact it has a heat sink implies it gets warm.

galaxiex

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Re: Sears 40XL solid state amp schematic
« Reply #5 on: October 10, 2016, 08:42:27 PM »
Thanks Enzo!

I called it a TO-5 as I am not overly familiar with all the different transistor packages.
I have seen a TO-5, so that was the closest thing to what I know of, and it "kinda" looks like that.  ::)

A quick search for a 40409 got me this image.

This is *exactly* what it looks like, heat sink and all.
Of course the numbers are different.  ;)

I suspect the numbers I am seeing on Q 5,6,7 are "house" numbers since searching those yields few results.

I have not yet checked any of the parameters you mentioned.

I will do so and report.

Thanks again!  :)
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J M Fahey

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Re: Sears 40XL solid state amp schematic
« Reply #6 on: October 10, 2016, 08:49:22 PM »
Q5 is dissipating 3W continuously which is *a lot*  for a small size transistor.


 Dear galaxiex: congratulations on your efforts on saving these early SS amps, some day they will be justly recognized and you´ll become a folk hero.
No kidding,In truly believe that.

That said, Q8 is still upside down ... unless designer did that on purpose to make it distort a little less.
An upside down transistor is still a transistor and does everything, with 2 not so small problems:
1) current gain drops catastrophically to about 5 or even less (test it yourself by testing upside down on multimeter Hfe test socket)
2) Vce also drops catastrophically to about 5V (which is reverse Vbe "zener"  votage).

On compensation, I vaguely remember it was more symmetrical when using the transistor as a variable audio resistor or attenuator, but wouldn´t swear on that.
Please recheck datasheet pinout vs. actual position on PCB.
In any case, if it fails you can use any small signal Si NPN transistor there.

As of Q5, a heatsinked TIP something is fine , but given that you scrounge all old SS amps available, if I were you I´d make a list of old obsolete transistors , chat a little with Enzo to see what he still has availble and buy a small backup stock of old hard to find parts, just in case.

At least you don´t look like you are going to stop collecting any time soon ;)

Loudthud

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Re: Sears 40XL solid state amp schematic
« Reply #7 on: October 10, 2016, 09:02:11 PM »
Are R19 and R20 really 30 Ohms? Seems kinda low.

galaxiex

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Re: Sears 40XL solid state amp schematic
« Reply #8 on: October 10, 2016, 09:11:18 PM »
Thanks very much J M!  :)

Just a quick note here as I gotta go get dinner.  ;)

The amp works fine, I just noticed/commented on Q5 getting hot. Maybe it will fail? Last indefinitely?

re: Q8
I carefully checked pinout and orientation on the board and got confused because it was/is indeed installed like that.
Checked it *many* times re: my confusion at seeing the collector to ground.

As its function is to wobble the signal for tremolo it seems to work being installed upside down.
I suspect a JFET would work as well?

The 2n5133 transistors all test with very low hfe in the area of 63 or so.
At least the ones I checked so-far. Q8,9,10.

Thanks! Gotta go eat!  :)
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galaxiex

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Re: Sears 40XL solid state amp schematic
« Reply #9 on: October 10, 2016, 09:12:34 PM »
Are R19 and R20 really 30 Ohms? Seems kinda low.

Yep, didn't measure them, but they are cement block 5 watters and the value clearly printed on the side.
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J M Fahey

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Re: Sears 40XL solid state amp schematic
« Reply #10 on: October 11, 2016, 06:45:21 AM »
Are R19 and R20 really 30 Ohms? Seems kinda low.
It´s the value actually needed there.  :o

Way back then transistors were expensive, so saving a couple was worth it.

In this case notice the class A driver Q5 is directly driving the power transistors, NO drivers involved and they are not Darlingtons either so Q5 must pass a high idle current to be able to drive outputs (class A, remember) .

I remember many old Hi Fi home use amps designed and built that way.

In fact a friend of mine worked at a Hi Fi amp factory where they initially made stereo 10+10W or 15+15W amps, basically a couple TIP 31/32 driven by a Tip29 with, say, 60+60 ohm resistors.

Then they upgraded to 25/30W using TIP41/42 and 30+30 ohm resistors, basically what you see here and the limit for this cheesy design.

I once visited him at work, resident Engineers were mumbling about making a premium 60+60W version and I jokingly asked them: "what will you use now? a couple TIP35/36 driven by a heatsinked TIP3055 and a couple 10 ohm 10W resistors?"
They looked at me glassy eyed and asked "is there any other way?"  :loco

Q5 idle current must be the peak current needed to fully saturate power transistors, and that´s a relatively high level, that´s why it dissipates 3W at idle and works very hot.

In any case, these designs didn´t look so outrageous to early designers, they were supplanting transformer driven designs, where you also have a high dissipation class A driver, so complementary transistors allowed them to avoid the expensive and bulky driver transformer .

Loudthud

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Re: Sears 40XL solid state amp schematic
« Reply #11 on: October 11, 2016, 01:19:40 PM »
I hadn't looked at the second schematic where it says the rail Voltage is only 26V. Using that, R20 dissipates about 7W when the amp is making a square wave. That's not unreasonable for a 5W wire wound resistor. In a HiFi amp it's no problem.

Is that a 4 Ohm speaker?
« Last Edit: October 11, 2016, 01:21:30 PM by Loudthud »

galaxiex

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Re: Sears 40XL solid state amp schematic
« Reply #12 on: October 11, 2016, 03:14:33 PM »
I hadn't looked at the second schematic where it says the rail Voltage is only 26V. Using that, R20 dissipates about 7W when the amp is making a square wave. That's not unreasonable for a 5W wire wound resistor. In a HiFi amp it's no problem.

Is that a 4 Ohm speaker?

The non original speaker that came with the amp measures 7.8 ohms. It's marked 8 ohm.
I have no idea what the original speaker was.

I suspect it may have been 4 ohms cuz into 8 ohms this amp is *not* loud.  :o

I will hook it to a 4 ohm cab and see if it gets any louder.
'course I'll monitor for excessive heating.
« Last Edit: October 11, 2016, 03:16:08 PM by galaxiex »
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galaxiex

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Re: Sears 40XL solid state amp schematic
« Reply #13 on: October 11, 2016, 05:03:42 PM »
Ok, hopefully for the last time,  ::)  here are the revised layout and schematic.

I included a couple of voltages and descriptions.

Edit; BTW Q7 is *very* leaky.

I swapped in an ST 2N3792 TO-3 that I happened to have.
The amp got a tiny bit quieter and cleaner sounding.
I know the 2N3792 is Si but nothing seems to overheat or other perceptible ill effects.

Edit; removed schematic and layout, see later post #19, Thanks!
« Last Edit: October 11, 2016, 06:29:14 PM by galaxiex »
Fear leads to Anger, Anger leads to Hate, Hate leads to Suffering.

Loudthud

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Re: Sears 40XL solid state amp schematic
« Reply #14 on: October 11, 2016, 05:30:12 PM »
Are you sure about Q3? It would make a lot more sense if it was a PNP.

 

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