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June 05, 2023, 12:18:58 PM

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Fender 112 Deluxe Plus

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

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Tp16 (U1 pin 7) and Tp17 (U1 pin 4) give the expected voltages of +25.7 vdc and -25.7 vdc.
Could the U1 still be causing problems?


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.


As real MC1436 are hard to find, and I think the circuit is picky about the exact part, post your DC voltages for U1, pins 2,3, and 6.


Thanks, will do. I do have an Mc1436 in my parts box from a supplier that I have bought good components from in the past. Can I test it in any way to check if it would be suitable?


Quote from: FenderDeluxe112Plus on May 03, 2022, 03:57:37 PMThanks, will do. I do have an Mc1436 in my parts box from a supplier that I have bought good components from in the past. Can I test it in any way to check if it would be suitable?

Throw in a socket on the board and then try it.  If it works, you can leave it in the socket, if not, take it out :-)
Life is what you make it.
Still rockin' the Dean Markley K-20X


Hi all,
I have finally got the amp out to work on again.I have installed an ic socket as suggested by Joe. Unfortunately the solder pad for pin 8 has lifted. Am I right in thinking this is not a problem as it is not on diagram?

You cannot view this attachment.You cannot view this attachment. 


For the MC1436, pin 8 is a no-connect (N.C.) so you can ignore a connection there.

However, be careful. Often, the supply pins are omitted from the main schematic page(s) because layout techs usually put all the op-amp power supply connections and all of their bypass caps in one area, usually separated from the other pages. This makes the schematic cleaner and easier to read, but can be easily mis-interpreted.

- Will


With the new op amp in U1  the amp seems to be drawing more current through the 100w bulb. Not a huge amount, but the bulb is glowing slightly more. This is with the speaker still disconnected. What is a sensible way to proceed next? Does this suggest power amp diodes or q7 and 8 breaking down under load?
Should I connect the speaker at this stage? I would prefer not to blow the entire power amp again :)You cannot view this attachment.


You cannot view this attachment.
This is the bulb previously.
I did check all of the new solder joints with a mm for continuity to other components.


Check for DC on the speaker output and DC Voltages around the preamp. Something must have changed.


As mentioned yes check for DC on output but tiss a fair bet that now that the circuit is correct it maybe well be just drawing more idle power.
I would suggest, try a speaker  (with bulb still connected)
If the bulb glows very bright at idle (no signal passing) then there is still an issue.


Thank you both, will check for dc later.
Was hoping it might be the case that amp is now running correctly and just drawing more idle current!


Some very slight fluctuations. And has increased since opamp change.You cannot view this attachment.

Out of interest what would be considered acceptable dc?


Less than a hundred millivolts DC at output is fine.  You have about 50mV so no problem.
On the bulb limiter with power on, connect the speaker.
Some amps won't start up right with load connected when on the bulb, so that's why I said connect speaker while power on.


Just fired the amp up- still making the same noise.   :(
Bulb did not brighten any further with speaker connected.