mikeeboy 7/12/2018 3:03 AM
Fender Stage 112 SE Distorted Sound output
Greetings of Peace everyone great forum!

acquired an old Fender Stage 112 SE Amp, all stock no repair inside and out.

i have isolated the problem to be on the Power Amp section tru the ff.

1.Plugged in the input1 then input2 the guitar sound output was distorted.
2.tried plugging into the Power amp In still distorted.
3.while plugged into input1 of the connected the preamp out to a Peavey Bandit power amp in and sound was ok, pristine and clean.
4. tested the stock Fender Stage112 Speaker with another amp and sound was Clean and OK.


I have the ff as suggested by Enzo in a separate thread

"Q1, Q2 are limiters. They work through CR17, CR28. Unsolder one end of those two diodes and lift them from the board. Now Q1, Q2 are disabled and out of the circuit." - the sound improved a little bit but still distorted. so i soldered it back to original

The voltages on the PA transistors are: B=+/-1.14 VDC C=+/-44.5 VDC and the E's=+/-4.8mV.

checked the voltages on Bias Diodes BYV26D all are 0.56 to 0.57

i have checked Vce ( Collector to Emitter Voltage)
Q1= 1.13V
Q2= 0.88V

i have also taken voltage readings across on almost all diodes a
Vcr17 = 10mV
Vcr28 = 103mV
Vcr22 = 140mV
Vcr23 = 36mV

Zeners
Vcr21 = 158mV
Vcr25 = 34mV
Vcr19 = 267mV
Vcr27 = 243mV


Any input of knowledge you could give would be greatly appreciated.
 
mikeeboy 7/12/2018 9:08 AM
Searched on other thread and saw others with Vre = 8mV - 9mV
Im thinking of increasing the bias voltage by inserting 22ohm resistor between the Bias Diodes.
 
J M Fahey 7/12/2018 9:37 AM
1) welcome to the Forum

2) congratulations, you have already done the initial troubleshooting and taken important measurements

Through morning coffee now, later will search for schematic and check whether those voltages look normal or weird.
 
Dave H 7/12/2018 11:37 AM
[ATTACH]49687[/ATTACH]
 
The Dude 7/12/2018 4:01 PM
It's a good idea to post a schematic or schematic link when you start a new thread so we know what you're talking about.

http://www.electronicstudio.net/sche...tage_112se.pdf
 
mikeeboy 7/13/2018 7:17 AM
Hi guys sorry for not posting the schematic. i think i have found the solution. Distortion was caused buy drifting bias due to components aging, inserted 17ohm resistor between bias diodes and distortion is gone. Voltage on 0.47ohm resistor is now 5.2mVsound is clean.