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Fender 112 Deluxe Plus

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

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Quote from: FenderDeluxe112Plus on May 30, 2022, 07:31:21 AMWith a jumper to ground at cr3 anode the buzz is there but quieter.
Better if we can put some numbers on it.  Measure AC volts across the speaker terminals with and without the jumper.


1.48vac with jumper to ground.
3.90vac no jumper

These are the readings that it settled at. Both readings started higher then dropped.


Initial ac voltages
1.72vac with jumper
4.5vac no jumper


That is a substantial difference.
I searched for "100Hz squarewave" on youtube and it sounds pretty much the same as your soundfile in post #41.
Are you on 50Hz 240V power?

Can you run the 'preamp out' to another amp and see if it has the same sound?


Yes, am on 50hz 230v mains in the U.K.
I looked up 100hz tones on youtube and thought it sounded close.

I don't have another amp here to test it at the moment. I might be able to dig out a sampler to test it though.


If you have a stereo receiver, you could even run the preamp out of the amp to an AUX input on th receiver for a listen.


Have got the 100hz tone coming from the preamp out.


So there is excess 100Hz hum in both the preamp and the power amp.  That pretty much narrows it down to the power supply.
Recheck again the DC and AC volts on the power supply rails.


Tp1 30.31 vac 
Tp2 30.31 vac

Tp3 30.12vdc    00.04vac
Tp4 - 30.20vdc  02.45 vac

Tp5 16.40 vdc    00.02vac
Tp6 -16.42 vdc  00.01vac


Just double checked ac on collectors of both tip147s
Q11 and Q2
2.34vac 2.35vac respectively.


So it seems the problem is on the -48V rail.  I'm not sure why it didn't show up when you measured the AC volts there before.

Check that the solder is good and there are no broken pads at C48.  Sometimes the pads break near the cap and you can't see it because the cap is glued down.  Measure resistance from the (-)lead of the cap to Q2 collector just to be sure.
If no solder or pad/trace issues, replace C48.


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Thanks g1,

I guess either I misread or recorded the ac voltages incorrectly, or something changed after I replaced parts.

I reflowed the solder on the main filter caps and checked resistance to all connections when I originally pulled the board so will assume c48 is bad. Will order new filter caps!

Thanks again


Hi all,

I finally got hold of the new filter caps and had time to install them. I also replaced q9 while the board was out.

Long story short - I still have the same or similar buzz happening.

After cap replacement I checked for dc on speaker 0.023vdc

Tp1 30.05vac
Tp2 30.00vac
Tp3 39.02vdc
Tp4 -39.03vdc

Ac voltages

Tp3 0.066vac
Tp4 0.209vac


I also checked for broken pads/cracked solder and all checked out ok.



Checked the ac on speaker 3.59vac
It drops to 1.32 wiith a jumper from cr3 anode to ground.