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Hi there,
I 'fixed' a Fender Twin RedKnob recently, some on the forum may remember, where a solder braid had been used to band together 3 power resistors which had been replaced in place of one. The solder braid had slipped off and conducted where it really shouldn't. I fixed that and replaced a few bad components. Now it works fine on the Low setting, but cuts out at low volume on the high setting.
On measuring the voltages, especially the different voltages for Hi and low power operation the switch no longer adjusts them to their specified voltage.
I'll sum up the original fault quickly; The erroneous bridge was between C111 positive and R132, right side of it on the schematic. This essentially put B+ on the lights and switches circuit, the bottom of the power transformer secondary on the schematic. D108 zener, over on the left blew and created a path to ground. I replaced that, replaced r130 that was open, and R306 that was dying.
After that the amp went from silent to sounding 'almost great' on the low power setting, the high power setting is silent at low volume, can be kicked by increasing volume or gain, but still is only about the same volume as the low power setting.
On the attached photo of my annotated schematic I've labelled the 3 power lines away from the power circuit X Y and Z (yup I scrubbed out calling them A, B, C as the schematic uses these labels for other voltages later!)
X is fine
Y is +236v ish (correct for low power, high power SHOULD be +469v)
Z is +473v ish (correct for high power, low power should be +236v)
So something in the switching circuit has gone and not applying the correct effect when the switch is in one or the other position.
R304 and R305 showed an odd reading in circuit, but pulled they are correct at 220K. C301 and C302 when pulled charge and discharge on a meter's voltage fine.
I can't see enough what this circuit does to be able to make proper sense of this. So my question is, do I suspect the big caps? As they are crucial for bridging things in the High and Low power circuits operation? It looks to me like they are the ONLY components that can cause such a big voltage shift in the power-switch's operation, as the inductor and stuff on the right (C306, C307 etc) would impact the circuit evenly, on both high and low power if failing. The big caps C301 C302 would have been abused by the shorted B+ in the fault, I think
Or maybe it's something further up in the (screen? voltages) that those power rails (X, Y, Z) provide...
I can't see R128, which would be next in line after the failed R130, but figured as it works nicely on low power that is PROBABLY fine. I get correct voltages at A, B and C, on the bottom right of the schema now anyway.
SO, my focus is basically on, something I missed? Am I looking at the big caps (C301, C302). I wanted to get a little input before I bought those big ole ones as, FWIW, they test OK out of circuit. I know that doesn't say much about their operation at 400v! But really, they are the only thing that I can see, in my fairly basic 'ohms law and not much else' approach that could cause the voltage rails Y and Z to be unaffected by the HIGH/LOW power switch!
Fender Red Knob TheTwin Annotated (dragged).pdf
PS - and I realise it may be a big PS... The PCB traces on the power board are all wrinkled up, maybe that just happens, but I'm wondering if it was from heat of the short circuit, and I might be looking at a mechanical fault. It LOOKS like everything is ok, it hard to look at the bottom of the pcb easily without desoldering all of the transformer pins, but I guess a split somewhere may cause the high/low switch to be ineffective, though I'd need to get my thinking cap on to work out where...
I 'fixed' a Fender Twin RedKnob recently, some on the forum may remember, where a solder braid had been used to band together 3 power resistors which had been replaced in place of one. The solder braid had slipped off and conducted where it really shouldn't. I fixed that and replaced a few bad components. Now it works fine on the Low setting, but cuts out at low volume on the high setting.
On measuring the voltages, especially the different voltages for Hi and low power operation the switch no longer adjusts them to their specified voltage.
I'll sum up the original fault quickly; The erroneous bridge was between C111 positive and R132, right side of it on the schematic. This essentially put B+ on the lights and switches circuit, the bottom of the power transformer secondary on the schematic. D108 zener, over on the left blew and created a path to ground. I replaced that, replaced r130 that was open, and R306 that was dying.
After that the amp went from silent to sounding 'almost great' on the low power setting, the high power setting is silent at low volume, can be kicked by increasing volume or gain, but still is only about the same volume as the low power setting.
On the attached photo of my annotated schematic I've labelled the 3 power lines away from the power circuit X Y and Z (yup I scrubbed out calling them A, B, C as the schematic uses these labels for other voltages later!)
X is fine
Y is +236v ish (correct for low power, high power SHOULD be +469v)
Z is +473v ish (correct for high power, low power should be +236v)
So something in the switching circuit has gone and not applying the correct effect when the switch is in one or the other position.
R304 and R305 showed an odd reading in circuit, but pulled they are correct at 220K. C301 and C302 when pulled charge and discharge on a meter's voltage fine.
I can't see enough what this circuit does to be able to make proper sense of this. So my question is, do I suspect the big caps? As they are crucial for bridging things in the High and Low power circuits operation? It looks to me like they are the ONLY components that can cause such a big voltage shift in the power-switch's operation, as the inductor and stuff on the right (C306, C307 etc) would impact the circuit evenly, on both high and low power if failing. The big caps C301 C302 would have been abused by the shorted B+ in the fault, I think
Or maybe it's something further up in the (screen? voltages) that those power rails (X, Y, Z) provide...
I can't see R128, which would be next in line after the failed R130, but figured as it works nicely on low power that is PROBABLY fine. I get correct voltages at A, B and C, on the bottom right of the schema now anyway.
SO, my focus is basically on, something I missed? Am I looking at the big caps (C301, C302). I wanted to get a little input before I bought those big ole ones as, FWIW, they test OK out of circuit. I know that doesn't say much about their operation at 400v! But really, they are the only thing that I can see, in my fairly basic 'ohms law and not much else' approach that could cause the voltage rails Y and Z to be unaffected by the HIGH/LOW power switch!
Fender Red Knob TheTwin Annotated (dragged).pdf
PS - and I realise it may be a big PS... The PCB traces on the power board are all wrinkled up, maybe that just happens, but I'm wondering if it was from heat of the short circuit, and I might be looking at a mechanical fault. It LOOKS like everything is ok, it hard to look at the bottom of the pcb easily without desoldering all of the transformer pins, but I guess a split somewhere may cause the high/low switch to be ineffective, though I'd need to get my thinking cap on to work out where...
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