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June 05, 2023, 01:57:43 AM

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Fender 112 Deluxe Plus

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

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Just ran the preamp out into another amp. The buzz is there but it is quieter now and does not sound like the previous 100hz hum.
With the main filter caps replaced, could the next suspects be diodes breaking down under load in the power supply?


Yes, maybe check them.


I am reading 30.00vac at r82. Does this   suggest the rectifiers are faulty? You cannot view this attachment.



Look at your schematic.  R82 connects directly to CP5, which the schematic says has 31.9va on it.  Close enough.


Thanks Enzo,

Where would be the next logical points to test?


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;



Thanks Phil,

This is the method I have been using so far, I thought there might be other ways to test in circuit.


Cr22,24 and 25 all have forward voltage of 0.5v or slightly above and read infinitely with leads reversed.
Cr23 slightly lower hovering around 0.47v and reads infinitely with leads reversed.


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


Diodes around u4 18-21 tested good.
C44 51 and 52 tested good.
I haven't yet lifted r82 but will do so.


With regard to ac in signal path - should I expect to read similar vac measurements between cp1 and ground and cp2 and ground when nothing is plugged into the headphone jack?


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.


The diodes may be producing some RF induced hum. Try connecting 0.01 uF capacitors across each of the diodes to see if it reduces the hum.


Buzz still present with r82 lifted.


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.