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Messages - phatt

#1
The Blue and Orange traces are the main audio path from U1 output.
Blue side carries positive wave while Orange is Negative wave.
CR5 to CR8 are the bias diodes for the output Tr's.

C39 & C40 are the Bootstrap setup (Google these things to learn more)

R76 is the ground lift to speaker Neg terminal, Current Feedback is then sent through R77 & C35 and returned to pin 2 of U1

The circuit uses both voltage Fback as well as Current Fback.
R65 sets Voltage FB while the combination of R76, R77 & C35 deliver the Current FB and meet at Pin2 of U1.

Remember these amp designs are Current Feedback via R76 so Speaker neg is ground lifted. *So don't ever ground the speaker neg back to chassis Com*. :-X

I've deleted most of the current Protection parts to help you see the circuit in it's basic form,, *Which is just a current Boosted opamp*
Of course without the over current protection the Output Tr's would instantly blow if the speaker wires were shorted out.
(No need to ask how I know that to be true,,  :'(  )

So if all the parts in my Edited drawing check out ok then I'd suspect the protection setup has malfunctioned when the Pwr tr's blew.
So that would be Q8 & Q9 as well as those associated diodes an Caps etc.

Now I may have missed something so don't take this as completely correct.
hope it helps, Phil.You cannot view this attachment.
#2
Amplifier Discussion / Re: Fender 112 Deluxe Plus
September 28, 2022, 10:08:24 AM
The most likely fail of the power amp was a short circuit or running very low Z speakers at high volume for long periods. Well you could try replacing Q8 & Q9 as they may have blown when the power units failed.
They are just there as over current protection but that setup is not fool proof so now they maybe actually causing the problem. They are not part of the Audio pathway only protection from shorts and over current.
The active audio parts of the power amp are only the opamp front end and the 4 power darlingtons which deliver the grunt to drive the speaker.
Don't hold me to it but I think that the amp might work with Q8&9 removed.
Try at your own risk. :-X  I'm kinda busy at the moment but I'll try to draw up a signal path overlay on the Pwramp section might help you understand the circuit. Phil.
#3
Amplifier Discussion / Re: Fender 112 Deluxe Plus
September 25, 2022, 10:37:51 PM
Then the most likely place is the power amp itself. You said in the first post you replaced the 4 power transistors.
Whatever caused them to fail may well have taken out other parts of the power stage.
We have been over the power amp before but maybe there is a need to look deeper into that. You will need better minds to establish just what has failed.

Phil
#4
An electrical technician repair shop.
Often the music store has connections to repair techs, maybe ask them.

You said you liked the Amp?,, so likely worth fixing as trying to shoehorn another circuit into the chassis might be even more complex. :-\
Phil.
#5
Hi Hombre,
           Yes as already noted  most likely blown the poweramp.

If you don't know which are the Power transistors then my advice is that you are better to send it to a workshop otherwise you may well struggle to fix it, possibly creating even more damage.
If you do wish to fix it yourself then you will need to take many tests and it will take a lot of time.
If you have high DC voltage (i.e. 40 volts) at the speaker terminals then you have blown major parts and at a guess you may have to replace most of the transistors in the power amp section.

The good news is that the preamp sections are likely ok as you only shorted the power amp.

Yes absolutely no mojo gain of any sorts by jumping the FX.
 BTW it's not an FX loop as such, just a passive break loop between Pre out and Power in which can be used in similar fashion as a Dedicated FX loop.
It's a shame the designer placed the speaker outputs right next to the Preamp/Poweramp loop as there is plenty of space to mount the Spk outputs on the other side of that back panel. :-X
Maybe don't try modifying the amp in future, changing chips in hope of improvements is often not as good as claimed by youtube geeks. ;)
Phil.
#6
Amplifier Discussion / Re: Fender 112 Deluxe Plus
September 08, 2022, 08:02:44 PM
Yes The power supply is the most likely suspect but you said it checked ok so we went looking for other issues.
Maybe your meter is not working well,,they do get old and can give suspect readings especially cheaper ones.
PSU is 4 diodes and those 2 main filter caps.
Phil.
#7
Amplifier Discussion / Re: Fender 112 Deluxe Plus
September 05, 2022, 02:47:37 AM
Ok,, Now you had some trouble reading diodes before so maybe go back and recheck the main diodes in the power supply,
that's CR22,23,24,25.

It is the most obvious point of hum and you only need one to fail and induce hum through the whole circuit.
If you are not sure just replace them anyway as they are not expensive parts.
Phil.
#8
Amplifier Discussion / Re: Fender 112 Deluxe Plus
September 01, 2022, 02:49:32 AM
Measuring CP1&2 at this point is not going to tell you where the issue is.
 Divide and Conquer is how you find the problem.
So go lift R82 FIRST
If no change then you move on to the next step.

So lift the top end of R82 as that will isolate the WHOLE switching system.
(top end as drwan in schematic is the AC input from CP5 which powers the switching)

Then power up
If no signal passes through the preamp then try inserting guitar into poweramp as it is separate from the preamp and switching so it should still pass signal.
If you can get a clean signal with no buzz then it's a good bet the switching circuit is bleeding AC back into the circuit somewhere.

If that kills the hum/buzz then you at least know the switching circuit is likely at fault.  I'm assuming it could be any place but seems obvious that Q3,4,5 or 6 are possible suspect points for AC to bleed through.
If they are working then maybe C44, C45 or C46 might be bad.
As I said better minds may know more.
Phil.
#9
Amplifier Discussion / Re: Fender 112 Deluxe Plus
August 25, 2022, 09:07:36 AM
Yep diodes look fine,, moving on,,,,
You mentioned in the first post that you replaced u4 which fixed the channel switching.

well the switching circuit power comes from the main AC at CP5.  As u4 was dead there maybe other damaged parts causing ac to bleed through to the audio path.

Switching circuits always confuse me and Others here will likely know more but if it was me I would lift the top end of R82 which would remove power to the whole switching circuit and see if the buzz goes away.

If it does kill the buzz then I'd be checking for dead parts around U4. maybe one of those diodes are fried.
your amp won't work per normal but it may help to isolate where the buzz is actually coming from.
maybe check C44 ,C51,C52
Phil.
#10
Yes TassieVikingman,, fp is short for Flame Proof. ;)

@ Scooby,
Those resistors often run warm but as they drop a fair amount of voltage they cop heat, if the 130-Ohm resistor has burnt then the resistance may have dropped and then  C285 has likely been running over the voltage rating and died, as you have found. I'd replace C286 while you are there. Even better replace with 35volt caps.

If those 2 resistors are running very hot that can shorten there life.
If excessively hot in normal use you can use 2 watt resistors which will run cooler and hence last longer.
Phil.
#11
Amplifier Discussion / Re: Fender 112 Deluxe Plus
July 24, 2022, 07:59:04 AM
I get the impression you are not clear on diode testing, If so then
You check diodes with power OFF and lift one end of the diode.
Meter set to diode test.

If you get a reading both ways it's dud.
To save writing it all,, go here;

https://www.fluke.com/en-us/learn/blog/digital-multimeters/how-to-test-diodes
HTH,Phil.
#12
Quote from: Loudthud on July 18, 2022, 04:04:16 AMThis effect is not what I would call "Diode hash". It's not any fault with the diodes. The circuit is doing exactly what the math says it will do. To get rid of the "1+" effect, use the inverting form of feedback. See attached. The one disadvantage is the low input impedance, so you really need an input buffer like the TS pedal.
You cannot view this attachment.
Hi , thanks for that, yes I worked out years back that using inverting circuits tends to produce a better result, but was never quite sure why, so ta that makes sense.
I think I got slewed with the term crossover and Expansion at zero crossing. I'm Still not totally clear on that point and also, if I remove the diodes in the sim then the jagged edge on peaks is gone. Of course it just square waves but the flat top peaks show no hash. A bit hard to compare because it hits the rails so early.
It's certainly and interesting subject.
#13
Hey it's not too bad but likely to struggle if you want distortion.

Hey you have a BBoard,,Maybe test some of the Marshall circuits that *TassieViking* mentioned on first page?
FWIW,
     In My 30 plus years of building guitar circuits I can say that if you just want to strum a few clean chords then bandwidth and tone shaping will not be much of an issue but if you want OD/Distortion, crunch, big fat sweet singing leed guitar tones then you will spend years building land fill if you don't research the bandwidth issues of Distortion,, an Why do I know this,,Well I have draws full of land fill circuits that went nowhere. In all those years I've only had a few success circuits that actually worked well. It's a long journey but I'm glad I persevered in spite of all my failures as those fails taught me stuff that I may never have known otherwise.
Phil.
#14
Amplifier Discussion / Re: Fender 112 Deluxe Plus
July 22, 2022, 07:37:31 AM
Yes, maybe check them.
Phil
#15
Yes I have often wished I had more bench gear but at older age and
eyes failing It becomes harder to work on fine detail.
I'm happy to have been able learn enough to build a whole rig that I'm
very happy with and I'd rather sing and play music nowadays than to try
and reinvent the fuzz box. 8)

FWIW,
My understanding about this quirk is, it's just diode hash (often mentioned in pedal forums) as Grit,Fizz,Hash and like terms.
I think you will find that what you are hearing is diode switching.
Using this circuit as example (See Screen shot) you will find at levels below ~200mV you get a smooth flattening of the signal But once you increase the input signal those diodes obviously struggle. Hence the peaks display a jagged edge.
This is high freq hash from the diodes switching state.
And Yes as you have noticed when you increase the input it sounds like Gravel.
the bigger the input the worse it gets.
If the bandwidth is not limited at higher frequencies it can sound gawd damned awful, no longer musical. :-X

I get around this by using 3 OD pedals but Never turn the gain/dist knob too high on any of them.
This issue is not limited to TS circuits it seems to happen to most  Dirt circuits that use diodes in this manner.
Phil. You cannot view this attachment.