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Fender 112 Deluxe Plus

Started by FenderDeluxe112Plus, April 12, 2022, 04:39:01 PM

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Thanks Phil,
I will take another look at cr22-25.
Do the readings for those diodes in post #112 look off?

 With the buzz still present with r82 lifted does it narrow the fault down to the power supply?


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.


New main filter caps and psu diodes
Cr22- 25 replaced.
Buzz still there. 


Tps 1-6 check out as per schematic

But excess ac still present on tp4
Ac on tp3 0.068
Ac on tp4 0.235


Ac across speaker leads 4.5vac


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.



Thanks for your help Phil-  I will have another look at the power amp- will pull the heatsink diodes cr6-8, plus cr10/12 and check the electrolytic caps.
If anyone has any ideas about how to track this fault down, I would be glad to hear them. Thanks, Andy


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.


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.


Thanks a lot for this - will start working through this over the next few weeks.


Great explanation Phil, helped me understand it better.
There are no stupid questions.
There are only stupid mistakes.


Glad it helps,, I'm not qualified enough to understand some of the more complex power amp designs but yes once you remove the current protection it makes more sense.
I've designed a few plain and simple power stages and had great success just by installing a PTC on the output. In this case one PTC would replace 18 extra parts and as likely in this case the Protect circuit may well be adding to the problem.
If a poly switch fails the amp stops working and you only need to replace one part.
PTC is a Poly Thermal Capacitor, Often called a poly switch because they switch to high resistance when they reach the set threshold. They do this switch faster than a power transistor can fail, effectively doing the same job as the rather complex current limit in the power amp.
BTW Tassie I buy them at Jaycar if you want to source some. ;)   
Link,,,  https://www.jaycar.com.au/rxe185-ptc-fuses-speaker-protection/p/RN3468?pos=2&queryId=4192c822761e620ef506c2f53a24ce71 

Re my drawing;
I'm still miffed about R69 & R66. ???
I think R66 is added Fback from output but R69 is really weird as they connect to
C37 & C38 which are filter caps for the opamp supply which I would assume would normally go to ground but they are summed at R65 which goes back to main output.
While R66 takes it back to the junction of R62 & C35.
Maybe better minds might like to comment on that subject.


I am looking at replacing the power transistor bias diodes cr5-8.

Can anyone advise me whether I can substitute the Byv26d diodes for Byv26e in this application?

The Byv2de is easier for me to get hold of and a lot cheaper.
Datasheet attachedYou cannot view this attachment.


The only difference between the diodes is the breakdown Voltage. In this amp, the diodes are not being operated anywhere close to the maximum rating so substitution is OK.