Hi all! I got this 50w Fender Hotrod Deville on the bench and I am losing my mind!
Came in for a blown up output section, output transformer was blown, screen resistor absolutely self destructed and it was loaded with crappy IC filter caps.
I replaced the output tubes, output transformer, screen resistor, grid resistor, and flyback diode on one of the output tubes (R61 went nuclear, R64, and CR4) as well as doing a full recap I finally managed to get the amp to kind of function. After many hours I finally figured out that having the speaker jack not connected to the chassis the amp outputs almost perfectly except for a nasty hum. With the speaker out jack connected to the chassis the output is extremely low and sounds like the output is dead shorted. to simplify the issue I have connected only the 8ohm tap, feedback line, and speaker common tap to a single output jack.
What would cause the amp to not function when the common tap on OT is connected to ground (as per schematic)?
I have tested and verified the following:
-All supply voltages on the tubes, ICs, output transformer, and heaters are correct with no AC ripple in the supply
-Bias supply is rock solid with no ripple and the output tubes are biased properly
-All diodes test good
-All ground connections are solid
-When the "shorted" issue is present signal is going all the way to the grid of output tubes as it should
-The new output transformer is a direct replacement for the old one and is connected exactly the same (except for how I simplified the output jack, wich did not affect the issue at all)
I will attach the schematic for this amp. Any help is GREATLY appreciated!
Came in for a blown up output section, output transformer was blown, screen resistor absolutely self destructed and it was loaded with crappy IC filter caps.
I replaced the output tubes, output transformer, screen resistor, grid resistor, and flyback diode on one of the output tubes (R61 went nuclear, R64, and CR4) as well as doing a full recap I finally managed to get the amp to kind of function. After many hours I finally figured out that having the speaker jack not connected to the chassis the amp outputs almost perfectly except for a nasty hum. With the speaker out jack connected to the chassis the output is extremely low and sounds like the output is dead shorted. to simplify the issue I have connected only the 8ohm tap, feedback line, and speaker common tap to a single output jack.
What would cause the amp to not function when the common tap on OT is connected to ground (as per schematic)?
I have tested and verified the following:
-All supply voltages on the tubes, ICs, output transformer, and heaters are correct with no AC ripple in the supply
-Bias supply is rock solid with no ripple and the output tubes are biased properly
-All diodes test good
-All ground connections are solid
-When the "shorted" issue is present signal is going all the way to the grid of output tubes as it should
-The new output transformer is a direct replacement for the old one and is connected exactly the same (except for how I simplified the output jack, wich did not affect the issue at all)
I will attach the schematic for this amp. Any help is GREATLY appreciated!
The two 1/4" female output jacks have internal contacts. Make sure those contacts are working properly when a 1/4" male plug is inserted. If that checks out OK, but the problem persists, you could have a defective resistor somewhere in the output section.
I replaced the 2 speaker out jacks with just 1 switchcraft (not enclosed) 1/4" jacks with the 8ohm tap and common tap wired directly to it. The jack is working fine because the amp outputs normally just not when the jack is screwed to the chassis. Just tested all resistors in output section, they all test good.
If it works except when the jack is mounted to the metal chassis, you must be getting a ground loop through the chassis.
So, I am assuming you wired the speaker hot wire and the speaker ground wire properly, where the ground is also the female jack's sleeve.
Use mica or other insulators on the female jack to isolate its sleave from the chassis, then go from there.
I am also assuming you left the feedback loop connected to the OT secondary's 4 ohm tap.
But the "nasty hum" you mentioned is yet another question.
So, I am assuming you wired the speaker hot wire and the speaker ground wire properly, where the ground is also the female jack's sleeve.
Use mica or other insulators on the female jack to isolate its sleave from the chassis, then go from there.
I am also assuming you left the feedback loop connected to the OT secondary's 4 ohm tap.
But the "nasty hum" you mentioned is yet another question.
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Sorry I should have been more clear. The amp originally had these switchcraft jacks mounted directly to the chassis, I just removed one and got rid of the 4ohm line to simplify until I figure out the issue. The amp SHOULD be fully functional with this jack connected to the chassis.
I moved the feedback line to the 8ohm tap with the speaker out jack, and the common tap is connected to speaker out ground like it was originally.
How would I test for a ground loop in this scenario? Would a ground loop even cause a "dead short" symptom like this?
I moved the feedback line to the 8ohm tap with the speaker out jack, and the common tap is connected to speaker out ground like it was originally.
How would I test for a ground loop in this scenario? Would a ground loop even cause a "dead short" symptom like this?
I'm now stumped on the dead short too. Without being there to see for myself, I can only guess. You will probably figure it out if you keep going.
However, you should have kept the feedback connection on the 4-ohm tap, as shown in the schematic, since the feedback circuit (which includes the presence control) was designed for the signal from that particular tap, not for the signal from the 8-ohm tap. There's a pretty big difference.
Anyway, good luck. Hopefully, someone more experienced than me, and who has seen this problem before, can help you here.
However, you should have kept the feedback connection on the 4-ohm tap, as shown in the schematic, since the feedback circuit (which includes the presence control) was designed for the signal from that particular tap, not for the signal from the 8-ohm tap. There's a pretty big difference.
Anyway, good luck. Hopefully, someone more experienced than me, and who has seen this problem before, can help you here.
Connect the choke directly to the 8 ohm speaker. Leave the feedback connection. Disconnect capacitor 24 as shown..... and try to connect a suitable audio signal to capacitor 24 and ground. See if the amplifier output is OK and normal. Is there any noise?
should first check the audio power section. with a 1K hz signal and8 ohm load and a scope, or by playing music through a c24 capacitor and ground, and the speaker. If you don't do this, repair will be very difficult. Did you do it?