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Messages - phatt

#31
Amplifier Discussion / Re: Fender 112 Deluxe Plus
May 03, 2022, 07:57:15 AM

Yes it's possible when the power Tr's blew it also effected U1.

U1 is the power stage input, those 4 Pwr Tr's boost the current needed to drive the speaker so both U1 and Pwr tr's could have failed.
The other 2 tr's (Q8 & Q9 I think) are part of the over current protection. 

Now assuming none of the passives or Q8&Q9 are at fault then replacing U1 Should fix the problem.

Hopefully others more qualified will chime in,, if I've missed something.

As you have noted that plugging into the Pwr Amp input still sounds distorted then the Power stage is most likely at fault.

If you want to check if the preamp is working normally then take the Preamp output to another amp (if you have one) if that produces the same Buzz then the problem maybe further up stream in the signal path.
Phil.
#32
Amplifier Discussion / Re: Fender 112 Deluxe Plus
May 03, 2022, 03:10:35 AM
Oh dear, Reset,, that noise is not ground hum/buzz that sounds more like a blown poweramp. :o
I asked if the amp is working and passing signal except for some hum/buzz,, if so you may have a ground issue.

Obviously this is NOT the case.

In your fist post you said you replaced the power transistors but there might still be a problem in the power amp.
My best guess is the opamp at the input of power amp stage might be stuffed.
Check the DC test points on U1. (pin7&4)
Phil.
#33
Good to hear it's worked for you and having the feedback may help others who struggle with these Amps. 8)
A lot of SS Fenders from this era tend to suffer from excess treble response.
Phil.
#34
Amplifier Discussion / Re: Fender 112 Deluxe Plus
April 29, 2022, 02:23:11 AM
Just turn the amp on and with a length of wire with alligator clips on the ends and just probe the Known ground points of the pcb and related ground points back to chassis.
**Just make darn sure you don't short power nodes back to ground.**
Use the schematic to verify test points and ground nodes.

If hum increases then you have created a ground loop.
If it reduces hum then you know there is a ground missing somewhere.

BTW the speaker Neg terminal is NOT Ground. This is a current feedback system and spk NEG is lifted from circuit common via that big 10 Watt resistor, R76.

You can visually work out the ground path but often not obvious.
Hence the wire probe can help define the problem.
Maybe post some circuit pictures and we might be able to help more?
Phil.
#35
Amplifier Discussion / Re: Fender 112 Deluxe Plus
April 24, 2022, 09:39:20 PM
A scope may not help much if it's ground hum.
I'm assuming the amp is working and passing signal except for the hum?

Well as there is no ripple on the supply and voltages are within spec then the most likely issue is one of 2 possibilities.
1/ open ground connection somewhere.
If the star ground node of main supply is not connected back to chassis correctly then it can cause hum issues. (Which is the node between C47 & C48 or Conn CP4)
You can test simply by shorting the main Common back to chassis with a wire. If the hum stops then you found the issue,, if hum increases then you just created a ground loop,, which brings us to,, 2/ Something is grounded that should Not be grounded, which would cause a ground loop hum.
Ground plane issues can be a nightmare to track down.
sometimes in design phase one has to use an alligator clip to find the best ground path before committing to a layout.

Often components like input jacks, FX loops and speaker outputs,  even the PCB need to be *Isolated from chassis*
This will depend on how the circuit common was designed which is hardly ever noted on schematics.

Tiss easy to loose these little isolation washers or forget to reinstall them.

If you acquired this amp with issues then someone may have already worked on it and if small parts have been lost then you have no idea why it hums.

*I would be researching ground path issues long before replacing parts in hope.*

I got caught once, I did not notice an isolation washer dropped off a PCB post and the hum drove me nuts for a few days.
finally found it hiding in the chassis corner. >:(
Phil.
#36
Quote from: SemiConductive on April 19, 2022, 06:20:38 PMThanks, @phatt. I am studying that and will compare to the sound samples I'm getting. Seems to make sense.

Couple more questions: To shift that mid dip... to 400hz as you suggest... is that a one or two component swap, or a rearrangement of the whole tone stack? I've played with stacks a bit in Duncan's Tone Simulator, but this thing doesn't really fit in there.

You can change R15 to 10k which will move the mid dip a bit lower but you will loose a little bass.

Quote from: SemiConductive on April 19, 2022, 06:20:38 PMBack on the capacitor C6, which is effectively filtering out treble: I've installed a switch and a pot for the diodes and played with that in conjunction with the overdrive (IC1-B) switching. It really is cutting a lot of treble. I think they were going for that dark boost tone. Misses the mark.

I know I can change the value of C6 to get more treble. Is it reasonable to just life C6 and see what it's like? Just "eliminate" the bypass? I can also try some lighter values but I'm wondering if it's safe to go to infinity.
Removing C6 will give more treble but it also might oscillate at high gain levels. Maybe try 100pF see what happens?

Phil.
#37
Schematics and Layouts / Re: Samick SM-10 Amp
April 20, 2022, 11:16:12 PM
These are small practice amps and In My Experience a waste of time changing every cap that will likely not make any difference to the sound these cheap amps produce.
Re scratchy pots
I just spray the pot guts with RP7 or WD40 and unless the pots are totally stuffed they usually come back to life.
As example, I scored a home Hifi years ago that had a scratchy volume pot, a squirt of WD40 and still going fine, that was 10 years back.

Note; some tecks will frown on using RP7 or WD40 but Cricky it's a cheap guitar amp.

If you want the best pots then track down the conductive plastic units as they last much much longer.
Phil.
#38
yep that's sounds close to how the maths work. you are obviously already better than me at the maths,  :-[
Just remember that this only tells you what the rolloff point is at that point in the circuit.
By the time it gets to the speaker it will have altered, often by a large amount.
Much like the Duncan tone stack app as a good example, Yes a big help but only tells one part of the whole system tone shape.
Phil.
#39

From my experience a lot of Hi-Freq hash comes After the pre stages. I've found a lot of hi freq crap can be removed before power stage which in most SS amps runs clean.

I would try adding a higher value cap at C26 at U5B (Preamp output). It's 22pF on the Deluxe 112 Plus Schematic I'm working from which will likely be similar as most of this era SS fenders use very similar circuitry so just find the Preamp output section. 
You can just leave the 22pf in place and bridge with higher values until you find a sweet point. You may even need 1nF.

The input section on most of this era of SS fenders are close to identical, it's just a hi pass filter which is helpful but it and the rest of the circuit does not have enough suppression of the high frequency,, so the result is a very harsh treble so when in distortion modes they sound crappy and harsh.
I resolved this crap hifreq issue in a Fender Performer 1000 by doing the above and swapping to a darker speaker.
I doubt these amps will ever win awards but you can improve them somewhat.
Phil.


#40
Amplifier Discussion / Re: Fender 112 Deluxe Plus
April 19, 2022, 07:06:11 AM
It's a long shot but,,, maybe check you have continuity from Mains plug earth pin back to chassis. For that matter try another wall outlet they can indeed fail on rare occasions? (if earth pin in an outlet fails equipment still works,, just no earth :o )
Phil.
#41
Here is my edit of Epi-Regent schematic, there may well be things I've missed.
Adding a screen shot of the tone curves.
Note; looks like C11 is 220nF which seems to work in the Simulation.
Screen shows 3 plots;
Yellow trace; Treb 10, mid 1,  Bass 10
Green trace;  Treb 1,  mid 10, Bass 1
Blue trace;   Treb 5,  mid 5,  Bass 5

Could be improved if Mid dip was shifted closer to 400hZ
Phil.
#42
Amplifier Discussion / Re: Fender 112 Deluxe Plus
April 17, 2022, 06:52:24 PM
When you reinstalled the PCB you may have inadvertently disconnected the Chassis Com from circuit com.
If the amp is otherwise working I would be checking that you have Circuit common in contact with Chassis.
If that connection is open it will cause hum.
Phil.
#43
I said,
Re R5,R6 and C6;
These are not in the signal path, R sets the gain of the stage and C sets the hiFreq roll off.

*This is the FEEDBACK path Not Signal path.*

You said:
" In this case it looks like it's bleeding it around IC1-B (?). "
YES Correct ;)

C6 basically dead shorts hi freq (set by the value of Cap)
So the stage does not amplify above a set freq.
Higher values will wipe off at a lower freq.

Valve Amps used to have a Presence control built into the power amp stage.
Simply by adjusting a pot which progressively added or subtracted a Cap in the FEEDBACK path, More cap less treble.
Nothing magic Just another place to alter tone.
------------------
If the tone knobs can't give you good results then it's a matter of tweaking what I call *Whole System Tone*
The feedback path is often used to do this.

Side note;
 I did try to simulate your tone control setup but it does not seem to work well.
I can't read some of the values so I guessing.
If you can give me the right values I maybe able to work it out. 
--------------
Something that is often missed ,,well at least I did when learning how this stuff works, LOL;
When you look at a schematic you are actually looking at 2 circuits intertwined on one drawing.

There is the AC path, (signal you hear) Which floats on the DC potential which comes from your DC supply. You have to set the DC points so as to pass the best signal. 

As I mentioned before just Google stuff.
Also down load a sim program and just fiddle with circuits relating to your Q's.
Often sims come with a library of circuits to help you get started.
Meantime trawl through old book stores you may find some electronics books.

I believe you can download Art of Electronics in PDF.
I have that book and it is not too hard to grasp.
How fast you learn depends a lot on what you already know.

For me it was a lot of reading, and Bread board testing.
But once I used sims my understand went ahead very fast. 8)
When I build/design a circuit I simulate it while also breadboarding it. The advantage is you not only see a plot on the screen but then you can hear it live.
Sims are not perfect but seeing and hearing at the same time speeds up the build process big time.

with BBoard and Sims you are able to tweak stuff to taste as well as find all the problems before you commit to a PCB. 
Phil.
#44
Amplifier Discussion / Re: Fender 112 Deluxe Plus
April 14, 2022, 08:00:15 PM
@ FenderD112Plu, Regards Test point 7 & 8
Those AC test points need to be taken with a signal at input.
Read Note 5 bottom left corner for clues.
Phil.
#45
Welcome Marcello, :)
My guess try this;
Remove C21 (lift one end to test)
That will remove some hi Freq

Also you can raise the value of C25 (Add 5 or 10uF there)
That Should (hopefully) add a lot more bass.
those 2 mods will help balance the system tone shape.
Phil.