Welcome to Solid State Guitar Amp Forum | DIY Guitar Amplifiers. Please login or sign up.

November 26, 2022, 10:51:23 AM

Login with username, password and session length

Recent Posts

 

Sears/Silvertone 40XL solid state amp schematic

Started by galaxiex, October 10, 2016, 12:37:47 AM

Previous topic - Next topic

BBailey

Got it. Thanks. I have been restoring this 40XL that was totally inop when I started. Now, everything works except the tremolo, which will work for a minute or so and then quit. It's proving to be an interesting and elusive problem. I also hope to improve the reverb as the original tank sounds terrible.

phatt

If the reverb is weak then I doubt the reverb tank is at fault. The way the reverb Driver is setup it would be a very weak signal that drives the reverb tank.
As with a lot of reverb circuits (even some used now) the Drive is Critical to performance.
the schematic shows the reverb Drive is taken straight from the passive input, only relying on 2 transistors,
So it will be weak.
Adding even more problems is the return is also passive.

If it was me I'd rebuild the reverb section or just use a pedal
Phil.

BBailey

I have a second, fully-functional 40XL that I use as a comparative tool. The two amps are identical, with the same PCB revision (134-5 rev C).  The reverb actually sounds fairly good on the working amp.  I have swapped the PCBs as a test of the chassis and reverb on the "inop" amp, and with the working PCB installed, the reverb on the inop amp works but sounds brassy and a bit distorted - very ugly - but not necessarily weak.  That makes me think the tank itself has an issue.


phatt

As I'm not sure exactly what you meant  :-\
So does the Bad Rev Tank work when run by the good PCB?
Is that what you mean?
Phil.

BBailey

Yes, that is correct. It's just that the reverb sounds brassy and shrill rather than a good reverb.  When I put the repaired PCB from the "inop" amp into the working 40XL chassis, the reverb in that amp works fine, so I am convinced the reverb drivers on both PCBs are working well.  The only difference is in the quality of sound coming from the two reverb tanks.

galaxiex

Quote from: BBailey on February 12, 2022, 10:17:11 AM
Yes, that is correct. It's just that the reverb sounds brassy and shrill rather than a good reverb.  When I put the repaired PCB from the "inop" amp into the working 40XL chassis, the reverb in that amp works fine, so I am convinced the reverb drivers on both PCBs are working well.  The only difference is in the quality of sound coming from the two reverb tanks.

See post #26.  ;)
If it ain't broke I'll fix it until it is.

BBailey

Now for the tremolo problem: the trem comes on strong for a minute or so, and then fades out to nothing.  The audio is not effected, just the tremolo effect.  Using the schematics found in these posts, I have replaced each component in the trem circuit, one at a time, but the problem still exists.  I have also examined the board closely for bad foil traces and solder joints but everything looks good.  When the trem fades, it seems to speed up first just before fading out, which leads me to believe a failing capacitor, but there are only four cap's in the trem circuit and I have swapped each one with no change.  I also noticed that if I shut the amp off and leave it off for a while before powering back up, the tremolo lasts longer before fading, whereas if I shut it off and right back on it cuts out quicker.  The longer I leave it turned off, the longer the tremolo lasts before fading.  Again, this seems to point to cap discharging.  Has anyone experienced this issue with the 40XL tremolo?


Jazz P Bass

"Using the schematics found in these posts"
Can you post the schematic that you are working with?
You may have a resistor problem if you replaced all of the timing capacitors.

BBailey

I have attached the schematic I am using - I downloaded it from this string of posts.
All the resistors are new, but there could be one that is failing under load.
There are some differences in this schematic and my two 40XL amps, but none that I see within the tremolo circuit.

galaxiex

#39
Quote from: BBailey on February 13, 2022, 04:21:44 PM
I have attached the schematic I am using - I downloaded it from this string of posts.
All the resistors are new, but there could be one that is failing under load.
There are some differences in this schematic and my two 40XL amps, but none that I see within the tremolo circuit.

Replace cap C12.
My schematic from post #19
If it ain't broke I'll fix it until it is.

Jazz P Bass

Nice job on that schematic, Galaxiex.
Serious work went into that.
Thanks for all of that!!

BBailey

Okay - I did replace C12 once already.  I'll try another.

BBailey

I should add that these schematics are very helpful! Thank you for creating them.

galaxiex

Quote from: BBailey on February 13, 2022, 04:43:40 PM
Okay - I did replace C12 once already.  I'll try another.

If that doesn't help, monitor the voltages at "A" and V+ when the problem occurs.
May give a clue as to what's happening.

Did you replace any of the power supply decoupling caps?
Specifically C10 and C20.
If it ain't broke I'll fix it until it is.

galaxiex

Quote from: Jazz P Bass on February 13, 2022, 04:36:14 PM
Nice job on that schematic, Galaxiex.
Serious work went into that.
Thanks for all of that!!

Quote from: BBailey on February 13, 2022, 04:48:31 PM
I should add that these schematics are very helpful! Thank you for creating them.

You are welcome guys.  :)

Kind of a labor of love to go to the trouble, but I like doing it.
If it helps someone out, so much the better.
If it ain't broke I'll fix it until it is.