fender ,93 the twin ,red knob

[sid]

Hi i am new to this forum,i am wondering if there is anyone out there who is familiar with the "fender twin red knob amp " ,I have recently got it to restore ,i am not an electronics expert but will have a go at simple repairs .The main problem on this amp is that the reverb send signal is none existant ,i have swapped valves and also fitted a new driver transformer but still nothing ,I can not make any sense out of the schematic and cannot find a layout diagram for it,could anyone suggest if it maybe a resistor or cap at fault ,any advice would be greatly appreciated, cheers .

[patlaw]

Why did you replace the driver transformer what made you think it was defective it would be best to use an oscilloscope to trace the signal through the circuit to know where the problem is just shotgunning parts is expensive.

[pdf64]

What test equipment have you got?
Are you aware of safe working practice, eg Safety Tips For Working On Tube Amplifiers
Is R56 the V8 reverb driver cathode resistor ok?

[g1]

Also, you mentioned you have no "reverb send signal", but we should begin from scratch.
How did you determine the send signal is non-existant?

[Pedro Vecino]

Is the reverb selector (clean-both-lead) set correctly? Any symptom that can be damaged?

[Mark Black]

For the OP - I just want to mention you must be very careful with the PC boards in these amps - very easy to lift a pad or trace while soldering (much more than most).

[sid]

Hi i am new to this forum,i am wondering if there is anyone out there who is familiar with the "fender twin red knob amp " ,I have recently got it to restore ,i am not an electronics expert but will have a go at simple repairs .The main problem on this amp is that the reverb send signal is none existant ,i have swapped valves and also fitted a new driver transformer but still nothing ,I can not make any sense out of the schematic and cannot find a layout diagram for it,could anyone suggest if it maybe a resistor or cap at fault ,any advice would be greatly appreciated, cheers .

Thanks for your interest and thoughts. As I said I'm not a tech, the reason I changed the driver cus it was recommended on a forum I found it was also easy for me to install, I tested it by wiring a speaker to it as suggested on that forum no sound coming through. I am aware of the dangers of valve amps and after seeing a friend of mine been shot across the room after accidentally touching a cap I am very cautious. I will probably take it to a tech soon just wondered if anything was obvious.

[sid]

red knob

What test equipment have you got?
Are you aware of safe working practice, eg Safety Tips For Working On Tube Amplifiers
Is R56 the V8 reverb driver cathode resistor ok?

Hi i only have a digital multi meter ,as i said i am not a tech im actualy a mechanic ,i got this amp for 150 quid,with two new celestion speakers instaled,it was very beat up but it works except the reverb ,someone has taken the tank out and just instaled in out jack plugs on the back of the cab ,why i dont know ,so i want it put back to original spec .If i touch the return wire tip it makes a noise like an unplugged guitar sounds ,so i presume this is ok ,i read to simply test the feed just hook a speaker up to it and a signal should be heard when a guitar is played ,there is nothing no hiss or anything ,because it was easy to install and not expensive i fitted a new trans driver but no luck.The amp will need servicing anyhow just thourght i would have a go to see if i could fix it .Do you repair amps ?i also live in staffordshire sunny stoke on trent .cheers

[52 Bill]

...someone has taken the tank out and just instaled in out jack plugs on the back of the cab ,why i dont know ,so i want it put back to original spec .

Putting it back to original is a good thing. When the person that removed the reverb modified the amp, he may have removed some parts in the amp that could cause your problem.

If you have a camera, post a photograph of the reverb area of the pc board and perhaps we can see something that has been altered.

[g1]

How about that switch Pedro asked about in post #5?
That is what routes the signal to the driver tube.

[sid]

ok i will get a photo tomorow ,there are two wires that have been cut and cannot see where they went ,one goes from reverb 3 on the pcb (the centre one )the other comes from the mains transformer tag BR its a thin black wire,looking at photos on the web these two wires disapear under the front pcb where the knobs etc are ,but theres no photos of the underside .

[Tom Phillips]

Attached is a gut shot of a stock 1990 "The Twin" which will be useful for you to see how they were originally wired. The resolution is good enough that you can zoom in to trace some of the wiring.


My tip is that you be VERY careful if you ever try to get to the far side of any of the PC boards. This is because the flying lead wires are not properly strain relieved and Fender used brittle 7 strand wire. The strands break easily (Birdcaging) when flexed to flip the board over and it doesn't take long before the wires break completely free.

Very strange that someone would have moved the reverb send & recover connectors to the rear panel of the chassis.

Cheers,
Tom

[sid]

Attached is a gut shot of a stock 1990 "The Twin" which will be useful for you to see how they were originally wired. The resolution is good enough that you can zoom in to trace some of the wiring.
[ATTACH=CONFIG]33184[/ATTACH]

My tip is that you be VERY careful if you ever try to get to the far side of any of the PC boards. This is because the flying lead wires are not properly strain relieved and Fender used brittle 7 strand wire. The strands break easily (Birdcaging) when flexed to flip the board over and it doesn't take long before the wires break completely free.

Very strange that someone would have moved the reverb send & recover connectors to the rear panel of the chassis.

Cheers,
Tom

Hey Tom cheers for the photo ,i know all about those crappy fly leads every time i have the pannel away a lead breaks,would have expected better from fender.any how i now have the pannel away ive noticed that the wire that has been cut must come from channel one on the pcb (reverb control end) as it is marked on the pcb with nothing to it ,it then goes to ch 1 on centre pcb ,another wire comes from the same ch 1centre pcb and goes to ch 1 on the other end of the front pcb ,the white wire goes to channel 2 centre pcb to front pcb.the mystery black wire from the PT marked BR SEEMS TO GO TO AN EARTH ON THE FRONT PCB.Bloody soldering iron has just packed up so ill get a new one tomorow and give it a go ill let you know how it goes cheers

[Tom Phillips]

Fly lead strain relief

Regarding the crappy fly leads in "The Twin." It is interesting to note that the earlier versions of the amp had strain relief holes in both the front panel and the main PCBs which the flying leads could be looped through to provide strain relief. However, in the examples I have seen, Fender did not loop the wire through those holes and the earlier amps had the same birdcaging wire issue. The assembly procedure fix should have been to route the wires through the strain relief holes but Fender revised the PCBs and the holes were eliminated on the new rev.

[olddawg]

I haven't seen one of these evil twins in a while. Do the have an fx loop? If not, it may have been half ass modded for an fx loop. Try taking that send on the back as it exists and running it into another amp. If there is a signal there to amplify, then stick a pedal or rack effect between the existing in and out as is. If it works, you might be better of just putting a cheap stomp box reverb (or multiple effects whatever) there and be done with it.