1968 Fender Silverface Bandmaster (Fender_bandmaster_aa763_layout.pdf)

[Markbou]

Hello~

I apologize, but my original post was not meant to be posted in Schematics, please delete.

I’m in a situation and was hoping to get some guidance with.  I did a lot of looking around prior to posting, but I was stumped.

I have my uncle’s ’68 Silverface Bandmaster (Fender_bandmaster_aa763_layout.pdf, https://el34world.com/charts/Schematics/files/Fender/Fender_bandmaster_aa763_layout.pdf).  I went through it replacing most of the electrolytic caps and some resistors.  When I finished, the amp sounded great.  I took it back to him and mentioned that the tremolo channel wasn’t working and asked if he wanted me to replace the optocoupler.  He said yes, even though he never uses that channel, so I ordered a replacement roach and installed it.  Didn’t work.  I reported my results to him and he said no problem, so I left it as is and gave it back.

A week later, he told me the sound was distorted and the amp sounded terrible.

I took it back and the mistake I made was to cut the entire optocoupler out.  Now the amp plays nice and clean now, but the volume is very, very low.  I suspect I took one or more of the gain stages out of the signal loop.

What I was hoping to find out was if I should have only cut one of the leads to the neon bulb to keep the signal flowing into the remaining gain stages. Or possibly, if someone knows another way to install another component or two, or manipulate the roach in some other way that would enable the overall circuit to run properly even though the tremolo channel won’t be used.

Thanks in advance!

[tubeswell]

I … cut the entire optocoupler out.  Now the amp plays nice and clean now, but the volume is very, very low.  I suspect I took one or more of the gain stages out of the signal loop.

Your description of what you did isn’t clear Please provide a schematic of your mods.

[Markbou]

I took a snapshot of the schem and put red X's where I cut out the part.

[Markbou]

I'm guessing I should have just cut the leg going to the 10M resistor.  Am I correct?

[Markbou]

My apologies.  Here is the relevant portion of the circuit.



I realize this is a can of worms and I don't want to draw this out too much, so if the answer is leave the roach in there and troubleshoot elsewhere that's what I will do.  However, I very much appreciate you responding to this thread and thank you.

[mresistor]

You have posted a Bandmaster LAYOUT and should post a schematic and both would be cool.
https://el34world.com/charts/Schematics/files/Fender/Fender_bandmaster_ab763_schematic.pdf  the tremolo snippet is from the schematic tough.


What do you mean you "cut out"  the tremolo roach?  YOu  removed it  or you clipped the wires and removed it would be better language to use   

Is the roach still not present? Did you replace it?  If not replace it. 


Is the normal channel distorted now?


 

[stratomaster]

Removing that component will do nothing to the underlying tone.  When the lamp is off the LDR goes high, so it's as if it weren't there at all.  Unless you installed it backwards, in which case you basically shorted signal to ground through a light bulb filament.  This would only affect the vibrato channel and would have gone away once you removed that part.

Your problem is elsewhere in the circuit.  We'll need a good voltage survey and then we'll be about to get you sorted out.  Until then, do not throw any more parts at this.

[Markbou]

Understood.  I will get to this tomorrow and provide as much info as I can.  Thank you.

[Markbou]

Happy to report after soldering the roach back in, the amp played as expected; I reminded again of how good they sound with a fender guitar. 

I had to have put the optocoupler in wrong the first time is all I can think of.

All is well now, so thank you so much for helping out.  My uncle is going to be thrilled, he's 78 and loves his amp.