Fender "The Twin" Switching Channel Not Working ?

[gtrplayr1976]

Hey folks ,
Got this Twin back in with another issue. Doesn't seem to be completely switching to the "Dirty" channel. Originally I had recapped , and replaced the input jacks on channel one ( Switching Channel). Channel one works when plugged in for clean ,and channel two works as designed with both channels mixed. Just when you are plugged into channel one ,and try to switch to channel two it doesn't completely switch?
Owner says he can sometimes jiggle the jack ,and it will kick in ? I have it apart ,and checked solder joints ,and such. I was thinking this could be an Optocoupler issue ? I removed LDR1 ,and 2 ,and tested them.
I connected a 9v battery - side to the - ,and used the + side of the battery along with the 1k resistor to turn it on /off. I tested with the - lead of the DVM on one end ,and the + on the other leaving the center open. On one of the Optos(LDR2) was reading 8.8k ,and the other (LDR1)around 5.6k. Tested again with the meter set a little lower. With the + lead of the DVM connected to the center (common?) I am getting 4.8K ohms on LDR2 opto ,and LDR1 is getting 2.4K. I think according to what I'm seeing on another site at 10mA current when it's on I should get 5k ,so could be the one is bad at 2.4k ?
I may swap positions on these to see if the problem moves.
Any other ideas appreciated.

[Tom Phillips]

...I am getting 4.8K ohms on LDR2 opto ,and LDR1 is getting 2.4K. I think according to what I'm seeing on another site at 10mA current when it's on I should get 5k ,so could be the one is bad at 2.4k ?...

I say that VTL5C3 Vactrol is good. 5kΩ is the typical expected resistance in the on state per the original EG&G spec sheet. Yours, being 2.4kΩ, is better so it is good. The LDR is not a precision device so all the specs are a fairly wide range. In addition, the ON state resistance depends upon the part's light adapt history during the previous 24 hour period.

The customer said that wiggling the input jack made a difference so I'd look there. Solder cracks, dirty internal switch contacts in the jack, bad jack etc.

[gtrplayr1976]

I say that the VACTROL is good. 5k would be either the typical or the worst expected resistance in the on state. Yours, being 2.4k, is better so it is good.

The customer said that wiggling the input jack made a difference so I'd look there. Solder cracks, dirty internal switch contacts in the jack, bad jack etc.

Jacks in that channel are new ,and I checked the joints again. I was thinking that wiggling that jack could affect about anything in that area since that whole front control panel is a circuit board.

[Tom Phillips]

Agreed. Could be anything in that area. The troubleshooting clue points to that area.
Also...always remember that "new" isn't a 100% guarantee of "good."

[g1]

The replacement jacks were the exact same type?
New parts can still be faulty. Verify by testing the same way you would if they were old.
Otherwise get out the chopstick and start poking around that board.

[gtrplayr1976]

Agreed. Could be anything in that area. The troubleshooting clue points to that area.
Also...always remember that "new" isn't a 100% guarantee of "good."

Yes sir I agree new doesn't mean it's good. Also found a couple of suspect wires from the preamp tubes in that area. They seem to break pretty easy. Had that issue the first time with this amp.

[gtrplayr1976]

The replacement jacks were the exact same type?
New parts can still be faulty. Verify by testing the same way you would if they were old.
Otherwise get out the chopstick and start poking around that board.

Yes they are the same Fender jacks. Testing these type jacks have always confused me ,but I need to figure out how. It might be chopstick time

[Tom Phillips]

...Testing these type jacks have always confused me ,but I need to figure out how. It might be chopstick time

You could monitor a point down line in the switching path and look for the proper effect or variance in the parameter as you plug into the jack and wiggle the contact. Then work your way back towards the jack until you isolate the section that is malfunctioning.

What is the build year was the twin you are working on? I'm wondering if it is one without the strain relief for the PCB connecting wires.

[gtrplayr1976]

You could monitor a point down line in the switching path and look for the proper effect or variance in the parameter as you plug into the jack and wiggle the contact. Then work your way back towards the jack until you isolate the section that is malfunctioning.

What is the build year was the twin you are working on? I'm wondering if it is one without the strain relief for the PCB connecting wires.

Not sure of the year ,but it does have the holes in the board to loop the wires through if that's what you mean by strain relief. Whoever worked on it before didn't use them. I now have the wires soldered directly to the back of the board. I didn't loop them to the underside. They are brittle. I've had to strip back a few of them. Once I get this back together I will try what you suggest.

[Tom Phillips]

Not sure of the year ,but it does have the holes in the board to loop the wires through if that's what you mean by strain relief. Whoever worked on it before didn't use them...

Yes. That's what I meant. I have only worked on a handful of "The Twins" and none of them had the wires routed through the strain relief holes. I think it was a Fender factory assemble screw up. The PCB designer included the holes but the assembly floor didn't include the proper instructions (or QC). The problem is made worse because they used 7 strand hookup wire vice 19 strand and that 7 strand wire is very brittle. I have replaced the wires for two customers that really liked the amp and planned to keep it long term. It was a lot of extra work but has paid off in reliability and easier follow up service. It's interesting that Fender eliminated the strain relief holes completely when they up reved the PCB. Go figure.

[gtrplayr1976]

Yes. That's what I meant. I have only worked on a handful of "The Twins" and none of them had the wires routed through the strain relief holes. I think it was a Fender factory assemble screw up. The PCB designer included the holes but the assembly floor didn't include the proper instructions (or QC). The problem is made worse because they used 7 strand hookup wire vice 19 strand and that 7 strand wire is very brittle. I have replaced the wires for two customers that really liked the amp and planned to keep it long term. It was a lot of extra work but has paid off in reliability and easier follow up service. It's interesting that Fender eliminated the strain relief holes completely when they up reved the PCB. Go figure.

That sounds about right. I may still replace that wire. I want to fire it up ,and see if I can find the switching issue first.

[gtrplayr1976]

So I got it all back together. Tested voltages. All preamp tubes are within limits. On the LDR1 negative side there should be 2.8Vdc with channel 1 selected. This reading was correct. When I switch to channel 2 that should change to .59Vdc. I am getting .005 ,but when I plugged in a guitar it seems like it's switching ?

[Tom Phillips]

...On the LDR1 negative side there should be 2.8Vdc with channel 1 selected. This reading was correct. When I switch to channel 2 that should change to .59Vdc. I am getting .005...

I see the 0.59Vdc value written on the schematic but I don't understand how that could be because the schematic also shows that S7A/B switches that point to ground. Therefore, the reading should ideally be 0Vdc which is much closer to the reading you got. Schematic mistake?

...but when I plugged in a guitar it seems like it's switching ?

Since it appears that the switching is working I wouldn't worry about those voltage readings since they are going in the correct direction. Hopefully, you don't have an intermittent problem.

[gtrplayr1976]

I see the 0.59Vdc value written on the schematic but I don't understand how that could be because the schematic also shows that S7A/B switches that point to ground. Therefore, the reading should ideally be 0Vdc which is much closer to the reading you got. Schematic mistake?

Since it appears that the switching is working I wouldn't worry about those voltage readings since they are going in the correct direction. Hopefully, you don't have an intermittent problem.

They have been known to make mistakes This thing was on for a good while ,and I switched back ,and forth many times. Worked the same every time , so I'm hoping it's working now.

[Tom Phillips]

They have been known to make mistakes ...

That wouldn't be the first mistake I noticed on the published "The Twin" schematic. Take a look at the attached which shows a direct connection path from the plate of V7B to the top of the treble pot in the tone stack. Much of that 220Vdc would make its way to the grid of the next triode if the connection was really in the amp. Fortunately, the connection only exists in the schematic and it has survived through multiple revisions of the schematic.