Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Apexelectric on February 15, 2019, 01:14:43 pm

Title: Fender Princeton Reverb II with weak lead output
Post by: Apexelectric on February 15, 2019, 01:14:43 pm

Having an issue with the lead channel on a Princeton Reverb II. The normal channel seems fine and all of the functions on the amp do too including the reverb with exception of the lead channel which drops considerable volume when engaged. I suspect one of the two Vactrol Optocouplers is the issue but am not sure how to go about eliminating them as a suspect. They both seem to change resistance value when the amp is turned on and off as well as when the lead channel is engaged and disengaged.

I’ve gone through and checked tubes, pots, caps, resistors etc and there are some slightly out of spec components that I will eventually address but nothing that raised a red flag. The big issue is the lead channel so I would like to resolve that first.

Doug has the schematic in the database which is listed as “Princeton Reverb II boost”

Any info on testing the optocoupler and resolving this issue is appreciated.

Thanks!

https://el34world.com/charts/Schematics/files/Fender/Fender_princeton_rev_ii_boost.pdf (https://el34world.com/charts/Schematics/files/Fender/Fender_princeton_rev_ii_boost.pdf)

Edit: Added schematic link. Willabe

Title: Re: Fender Princeton Reverb II with weak lead output
Post by: Slimtim on February 15, 2019, 06:49:55 pm

check the led side like any other diode or led.meter on diode test, leads one way then reverse them.if you get a reading no matter polarity the internal led is bad.the other side resistance varies if the led is working properly.led off resistance is high i think then it goes low when led is lit.so you either need 2 meters or build a small circuit with the voltage for your optos model.is yours a 4 lead or like a 8 pin socket.mine is 4 pin and i used this method and my led was shot.

Title: Re: Fender Princeton Reverb II with weak lead output
Post by: Apexelectric on February 15, 2019, 07:04:25 pm

Pretty sure it’s not the led since there is a change in resistance when the lead channel is switched on and there is about a 1V voltage drop across the led. 6V is present at the led terminals as well. If it is the opto, I think it would have to do with the LDR side and not the LED side. I just don’t know how I can determine what the right resistance range would be and if there is an easy way to bypass the LDR to take it out of the equation.

Title: Re: Fender Princeton Reverb II with weak lead output
Post by: Slimtim on February 15, 2019, 08:58:46 pm

there's a thread in here somewhere that sluckey went into pretty good detail on optos.

Title: Re: Fender Princeton Reverb II with weak lead output
Post by: Apexelectric on February 15, 2019, 10:44:19 pm

I tried a 100K resistor across the LDR leads on the optos and got more gain out of the lead channel. Looks like the opto’s LDR has drifted out of range.