I have a Fender Blues Deville in my shop that is giving me fits. I normally take care of retubing and basic maintenance at my shop but am trying to learn more so I took this on. The problem is very low and distorted output. It never gets above what it normally would on around 2. It has all new tubes, all of voltages are within proper range and when I hook it up to the scope I am getting a nice waveform going in to pin 5. I have changed out the coupling capacitors from the phase inverter and double checked the bias. I checked if it was the output transformer by swapping in a known good one and changed the 8 pin tube sockets. I am running out of options. I have checked the resistors around output tubes and they read fine. Any help would be greatly appreciated.
Follow the schematic. Insert a 1kHZ sine wave at 4mV on input 1. Follow the voltage listings and it will be obvious where the fault lies.I have been repairing Fender since 1959 and only once had to replace a valve base due to arcing.
Don't jump in, work slowly, with lots of coffee and breaks, from one end to the other. It's only an audio flow chart!
Don't jump in, work slowly, with lots of coffee and breaks, from one end to the other. It's only an audio flow chart!
I agree with basreflex. Make sure the speaker is plugged into the correct jack. There are two speaker jacks, the main one and the extension one. the main one has a shorting contact. If you plug the speaker into the extension jack by mistake, then the main jack remains shorted across the output. That results in a low output level with distortion.
There is a label on the inside cabinet wall that identifies which jack is which. I THINK the main jack is the one nearest the wall.
Other things we always check on this series of amps: look CLOSELY at the solder on the power tube sockets. I usually have to resolder at least some of them. Also, plate resistors like to open on the phase inverter tube, so check to make sure B+ is present on both pins 1 and 6 on V3, the 12AX7 next to the power tubes.
There is a label on the inside cabinet wall that identifies which jack is which. I THINK the main jack is the one nearest the wall.
Other things we always check on this series of amps: look CLOSELY at the solder on the power tube sockets. I usually have to resolder at least some of them. Also, plate resistors like to open on the phase inverter tube, so check to make sure B+ is present on both pins 1 and 6 on V3, the 12AX7 next to the power tubes.
Your'e right Enzo.
I once received a party of those amps - brand new for repair.
I think it were 10 or 12 pieces.
They all had the described problem.
After some time spent I found out that the wiring of the jacks was the way round.
The main and the extention jack wiring had been reversed.
The shop who bought them in an auction at a redicolous low price was very happy to hear of the sollution and the price I charged.
So, OP, check where the speaker plug is inserted.
If it solves your problem we are deligthed to here it.
Cheers
I once received a party of those amps - brand new for repair.
I think it were 10 or 12 pieces.
They all had the described problem.
After some time spent I found out that the wiring of the jacks was the way round.
The main and the extention jack wiring had been reversed.
The shop who bought them in an auction at a redicolous low price was very happy to hear of the sollution and the price I charged.
So, OP, check where the speaker plug is inserted.
If it solves your problem we are deligthed to here it.
Cheers
Thank you for all of the replies. I double checked that I was in the correct output jack. I am getting good B+ voltage on all of the tubes and when I input a sine wave and trace it with my scope it looks perfect all the way going in to pin 5 of the output tubes, but is tiny and garbled coming out of pin 3. I have replaced both of the output tube sockets because they had a little bit of scorching around pin 6 on both of them so all of the solder joints are new.
pls try with 2 loads, each connected to one output jack . looks like a short in the tranny or output circuit. or try without a load by plugging in 2 open jacks, and run a "chase the signal" test with very low level signal, observe anode voltage with a scope first ..
another remote possibility option is that the overvoltage protection diodes D4 D5 are shorting , but I expected that more if they were connected to bplus.
another remote possibility option is that the overvoltage protection diodes D4 D5 are shorting , but I expected that more if they were connected to bplus.
I will try using a load in each jack. When you say it looks like a short in the tranny are you talking about the output or power transformer? I swapped out the output transformer for a good one to see if that what the issue with no result. I was at the point where I was looking at those diodes, but they test good in circuit and I didn't have any spares to throw in there, so I thought I would ask the experts before ordering some in case there was another place I should be looking. They do have 488V on them.
The diodes from the output tube plates to ground are called flyback diodes by many of us. They are a safety device. You can unsolder one end of each and lift it out of the circuit to see if sound is restored. It is like a car seat belt. You really ought to use them, but the car will drive fine for testing without them. Same with these diodes, the amp will function without them for testing, you just no longer have protection from open loads.
It is possible one of the output jacks is faulty, even though you are in the correct one. it may be shorting across the output. A test for that is to remove the OT wires from the jacks, and just use clip wires from the bare wires to a speaker. For this test, don't worry about the negative feedback wire.
It is possible one of the output jacks is faulty, even though you are in the correct one. it may be shorting across the output. A test for that is to remove the OT wires from the jacks, and just use clip wires from the bare wires to a speaker. For this test, don't worry about the negative feedback wire.
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