Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: donahue on November 26, 2011, 08:43:22 am
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Hi, I've got a Fender Delux Reverb Reissue with a tremolo that sometimes just comes on for no reason and other times doesn't work when you want it to...so checking it out I took some readings. V3 has 424vDC on pins 1&6; V5 has 419vDC on pin 1 and 370vDC on pin 6. These look really high to me. Does anyone know if this is normal??? The schematic show a lot of the other voltages and everything jives with my readings but the schematic is showing VAC at V3 (which I'm wondering why?) and 300v pK-pK at V5 (which means nothing at my level).
I have done some reading on the trem problem and came across some info that a dirty PCB board can cause leakage the across traces. Clean the board no difference. Been searching for my voltages questions but haven't found any thing from a source I trust. I searched here but didn't come up with an answer to my questions. Any input would be appreciated! Thanks!
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The VAC readings are the results of a 1KHz test signal plugged into the amp for signal tracing purposes. See note 8, last sentence.
Those DC voltage readings are normal. They are supplied by point 'Z' which is TP1 which is 396vdc according to the schematic. V5 is the only tube that will affect the tremolo. If the pin 1 and 6 voltages are steady, then the trem oscillator is cut off. Whenever the oscillator is working those voltages will fluctuate wildly on a DVM. This is because of the 300v pp trem signal that is also on pin 1. You would need an oscilloscope to see this signal. A very common reason for no trem on an AB763 amp that's on the bench would be the footswitch is not plugged in, or the footswitch plug/jack are not making good contact, or the VIB switch is open (off position). This trem circuit requires a ground from the footswitch to have trem.
Intermittant tremolo problems are very often caused by a poor connection somewhere between the footswitch jack to footswitch. See the white wire on the FS jack? Use a jumper clip lead to connect that wire to chassis and I bet the trem will work correctly.
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Thank you very much sluckey! I must have glazed over when reading note 8. Here's the thing that's stumping me...the trem occasionally comes on by itself with NO foot switch plugged in. I was concerned about the voltages because they are higher than the spec'ed max on the data sheets I have. You have set my mind at ease...I thought perhaps I had a larger problem than just the trem!
As far as the 300v pK-pK...I don't own a oscillascope let alone know the proper use of one.....
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the trem occasionally comes on by itself with NO foot switch plugged in
Now that's something you can troubleshoot. There are two prime suspects that would cause this. (There are other suspects too, but rule out these first.) Unplug the footswitch while making these checks...
1. Loss of negative bias thru R38. It's not likely that you're actually losing the bias voltage because you would probably notice the output tubes melting down. However, R38 could be intermittant or may have a poor solder connection, cracked pcb trace, etc. Monitor the voltage at the junction of R37, R38, R39, and R40. This voltage should be approx -6vdc when trem is off, or approx 0vdc when trem is on. Now poke around the R38 area and see if you can make the trem come on. Don't be afraid to put enough pressure on the pcb to cause it to flex slightly.
2. An intermittant ground at the junction would also cause the trem to come on. Inspect the FS jack carefully to assure there are no stray wires touching the chassis or sleeve connection on the jack. C18 would be another likely source for an intermittant short. Tap on it a bunch.
Intermittant problems often require patience. Good luck.
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I believe that got! Had 7.2vdc at the junction so that looked good. Re-flowed the solder joints at the jack and the components...playing through it, it's been working for the last hour or so. Thanks again. Still I wonder why Fender runs that reverb driver at so high a voltage...probably going to go through driver tubes often?
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Still I wonder why Fender runs that reverb driver at so high a voltage
So we could have that neat surf music! :icon_biggrin:
Hope you got the problem fixed.
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its been working all day long! Much appreciated sluckey!