Fender The Twin Reverb Problem

[plexi50]

I turned on the amp 2 days ago and the reverb began to crackle and then fade away. I can barely hear any spring action moving the springs. I have replaced the reverb tranny. The reverb send and return tubes showing a good bias. The resistors as well in both circuits read good on there values. Reverb tank is good and shows the right impedance. I tried other reverb tanks anyway

Im now looking at LDR 101. Im starting to hate this amp

[phsyconoodler]

Quote:"The reverb send and return tubes showing a good bias."

  i hope you didn't rely on voltages and changed the tubes anyway? I've had to change a reverb transformer once and only after checking everything else,so likely yours was good.

[DummyLoad]

i hate those amps. worst. ever. the only amp iv'e given back to a customer not repaired. the one i had would cut out, couldn't find the culprit - reverb was also feedback prone at any volume.

in a nutshell, those amps are junk. sorry, had to rant. good luck and i do hope that you get it sorted.

--DL

[plexi50]

V8 has 375+ on it as on the schematic. V9B plate shows 232VDC on the schematic.  I am only getting a 113VDC plate.

[phsyconoodler]

It must be 'Red Knob Twin' week here at Hoffman's. I have one on the bench right now too.The bias supply has been extensively modded by a real hack.It's a mess beyond words.
  Count yourself lucky it's only the reverb gone funky on yours.This banana head has added a whole network of resistors,caps and even a little transistor added onto the input PC board.It's a rats nest in the extreme added to the worst amp Fender ever built next to the 'Super 60' and 75.
 

[DummyLoad]

V9B plate shows 232VDC on the schematic.  I am only getting a 113VDC plate.

*something* is loading that stage... check the bypass cap (C105) and plate load resistor (R114), if not, check LDR. lift the 100K coming off the reverb level pot. see if that get the B+ up. if it does, suspect LDR101.

btw, on another forum, can't remember which one, the owner gutted everything but the PS PCB and built an AB763 turret board.

--DL

[plexi50]

V9B plate shows 232VDC on the schematic.  I am only getting a 113VDC plate.

*something* is loading that stage... check the bypass cap (C105) and plate load resistor (R114), if not, check LDR. lift the 100K coming off the reverb level pot. see if that get the B+ up. if it does, suspect LDR101.

btw, on another forum, can't remember which one, the owner gutted everything but the PS PCB and built an AB763 turret board.

--DL

Checked R114,C105,and lifted the 100K off the reverb level. No change. I have 1.6VDC going to LDR 101. How do i check the LDR on the opposite side of the + lead?

[DummyLoad]

V9B plate shows 232VDC on the schematic.  I am only getting a 113VDC plate.

*something* is loading that stage... check the bypass cap (C105) and plate load resistor (R114), if not, check LDR. lift the 100K coming off the reverb level pot. see if that get the B+ up. if it does, suspect LDR101.

btw, on another forum, can't remember which one, the owner gutted everything but the PS PCB and built an AB763 turret board.

--DL

Checked R114,C105,and lifted the 100K off the reverb level. No change. I have 1.6VDC going to LDR 101. How do i check the LDR on the opposite side of the + lead?

if you lifted the 100K from the wiper of the pot the LDR is out of stages circuit. the only thing loading the stage then is the reverb pot - is the coupling cap leaking? is the pot in spec? trace short on the pot? you do have ~370V feeding R114 right? 

a test is to lift the coupling cap C103. if the plate voltage doesn't go to >230V after that - there's no load on V9B with the coupling cap lifted.  tube or R114 is bad. while you're in there, check C104 (.0022uF hanging off pin 7) if it's shorted that could be your culprit.

--DL

[plexi50]

Thanks DL. Yes i have 370+ feeding R114. I was just proding around C105 & C104. Will lift and check them out later today.

[plexi50]

I lifted C105 and i noticed that R114 is a 220K. Do i have the right schematic here? I think i do. This is not a red knob amp.
R114 & R106 both look like they have been changed to 1 watt 220K's.  Another mystery.  

I just lifted the board from the chassis and yes those 220k's have been manually soldered in place after it left the factory

Should i go ahead and replace them with 100k values?

I replaced them with 100K values and now have the 230+VDC on the plate of V9A & V9B

Problem still persists. I have send working but no recovery

[DummyLoad]

Bypass the LDR. Lift the LDR out and tack a jumper in just to test.

[plexi50]

Bypass the LDR. Lift the LDR out and tack a jumper in just to test.

DL it works. I cut the three legs on PITA LDR and the reverb sounds great. The 3 position reverb switch works as well. I dont notice any tone loss other than me going deaf. My ears are still ringing. I hate that.

R111 & R112 are joined together going to the level pot so i just cut the three LDR legs and did not have to jump anything

Should i call it finished? Tell me i dont need that LDR? 

[DummyLoad]

cool... but now you have reverb on all the time. i see that now, had to look at schema again. and i'm on the road. it's a shorting LDR. is it the switching logic, or do you even care at this point?

--DL

[plexi50]

Humm! Well the pot cuts the level on and off. I have one of those LDR's here some where. I ordered some and used them in a Super 60 a few months back. Let me see if i can even find it. Good thought DL. Im just glad to hear it working again at this point