Troubleshooting a 1392 tremolo circuit that doesn't seem to be working. Just starting but it has a new tube. Am I correct by the circuit that the footswitch closed would turn off the tremolo by shorting the signal to ground?? I will try to trace the signal through circuit tomorrow and have more info.
P.S. How do I resize a pdf to download??
P.S. How do I resize a pdf to download??
From the schematic I found, yes, it is turned off by shorting the output to ground. A lot of times the problem is those .02 caps that are used to make the 6AU6 oscillate. Probably too old. Are they those wax caps?
You should also check the integrity of the connectors. Look for dirty contacts in them.
Is the file too large to upload to a post? You could just put the link in your post and we can download it ourselves. I found a schemo, but it is copyrighted, so Im not sure I can post it.
You should also check the integrity of the connectors. Look for dirty contacts in them.
Is the file too large to upload to a post? You could just put the link in your post and we can download it ourselves. I found a schemo, but it is copyrighted, so Im not sure I can post it.
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Link to it if possible. Is this your circuit?
http://bmamps.com/Schematics/Silvertone/silvertone_1392_1956-58.pdf
Yes, in that, shorting the FS to ground shunts the trem signal, thus turning it off.
The oscillator runs all the time, and its output goes to the pot. if it isn't oscillating, other than a different tube, look for good voltage on the plate and screen. The three 0.02uf feedback caps need to be good, if leaky, that will stop the oscillation. The B+ node C says 227v, I'd take most anything within 50v really, but the screen should have half of whatever C is. Make sure there is continuity through the speed control and its 68k up to the joint between the two 0,02 caps. That is from right at the caps, measure resistance to ground and see if it adjusts from 68k to 1meg plus 68k.
http://bmamps.com/Schematics/Silvertone/silvertone_1392_1956-58.pdf
Yes, in that, shorting the FS to ground shunts the trem signal, thus turning it off.
The oscillator runs all the time, and its output goes to the pot. if it isn't oscillating, other than a different tube, look for good voltage on the plate and screen. The three 0.02uf feedback caps need to be good, if leaky, that will stop the oscillation. The B+ node C says 227v, I'd take most anything within 50v really, but the screen should have half of whatever C is. Make sure there is continuity through the speed control and its 68k up to the joint between the two 0,02 caps. That is from right at the caps, measure resistance to ground and see if it adjusts from 68k to 1meg plus 68k.
Hope this isn't OT. That's the exact one I found. Enzo, see the copyright on that? Is it OK to post it or are we just supposed to give the link?
Yes that is the correct circuit. So the jack is a nonshorted type meaning w/o any FS in circuit the tremolo should be working? It has a new tube but I haven't checked any of the caps yet. Thanks for the help. I'll let people know what I find.
Thanks Enzo. Yes the tremolo will work without the footswitch plugged in. Its only function is to ground out the signal when you want to turn off the tremolo( better explanation is when you don't want to send oscillation signal to the amp). As Enzo said, it's oscillating all the time. It just depends if the signal goes to the amp or not.
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I have checked continuity of the circuit. The pots are all working. They are a little on the high side going to 1.5M instead of 1M. With the strength pot turned up all the way and the speed pot turned up all the way I can see the oscillation on the scope. I do notice large shifts in DC level whenever I turn the pot. The amplitude is much higher on the signal as i first turn the pot and I can hear the tremolo through the speaker. But the amplitude drops and settles to a steady state and at that klevel i can't hear it anymore. I have included scope shots trying to show this. The shots are AC coupled because the dcoffset goes all over the place when I turn the pots. Is all this pointing to leaky caps? They are the old blue-green Sangamos. Thanks for help.
I would change those .02 caps. They sound like they are leaky. How about those voltages Enzo wanted you to take on the 6AU6? Also, make sure that .05 coupling cap isn't leaky.
Voltages look good at 230 and 120 vdc. I plan on replacing with .022uF. That shouldn't be a problem should it? Is there any problem with the pots being a little on the high side? What amplitude should the oscillator run at (about) when turned up all the way. BTW these measurements were taken right at the 390K ohm splitter at the output tubes
The .022s will be fine.
I might think about replacing the pots that are 1.5Meg. Even at 20% spec variance, it still is only 1.2Meg max. If you adjust it for around 500k, does it still quit oscillating? If so, then this isn't the problem. This would simulate a half way rotation on the pot.
Try taking a reading right at pin 5 of the 6AU6. If it still quits oscillating, then the .02 caps are bad. Most guys replace all of them as long as you're at it. Pretty sure it's the .02 caps though. It starts to oscillate then quits because they leak or are out of spec.
I don't know for sure what amplitude it should be at full up. Probably about the max it is at the beginning of your scope readings before it starts to fall off. At that point, do you hear the oscillation through the speaker? If so, then this amplitude should be close.
I've never come across this type of tremolo before. Nat Daniel had some different ideas. I'm pretty sure this is a Danelectro made amp. Looks like this adds and subtracts to the signal to the power tubes from the PI.
I might think about replacing the pots that are 1.5Meg. Even at 20% spec variance, it still is only 1.2Meg max. If you adjust it for around 500k, does it still quit oscillating? If so, then this isn't the problem. This would simulate a half way rotation on the pot.
Try taking a reading right at pin 5 of the 6AU6. If it still quits oscillating, then the .02 caps are bad. Most guys replace all of them as long as you're at it. Pretty sure it's the .02 caps though. It starts to oscillate then quits because they leak or are out of spec.
I don't know for sure what amplitude it should be at full up. Probably about the max it is at the beginning of your scope readings before it starts to fall off. At that point, do you hear the oscillation through the speaker? If so, then this amplitude should be close.
I've never come across this type of tremolo before. Nat Daniel had some different ideas. I'm pretty sure this is a Danelectro made amp. Looks like this adds and subtracts to the signal to the power tubes from the PI.
This isn't rocket science, 0.02 or 0.022 is a trivial difference. That is the difference between 33 pounds of air pressure in your tires or 33.3 pounds.
The 0.02 caps and the speed pot and resistors just determine the speed. if the pot is a little high, then the trem will be a little slower that otherwise. If those three caps are a teeny bit larger, the trem will be a teeny bit slower.
The oscillator runs full out all the time, the intensity control has nothing to do with the oscillator. All the intensity control does is act like a volume control for the trem signal leaving the oscillator. Turn it all the way down, and the oscillator still would be cranking for all it was worth. I have no idea what level the signal will be there, it will be a large signal. It will be oscillating at a reasonable speed or it will not. Just get it going, worry about how large it is later.
For the price of three 0.02uf caps, or 0.022uf if you prefer, just do it.
The 0.02 caps and the speed pot and resistors just determine the speed. if the pot is a little high, then the trem will be a little slower that otherwise. If those three caps are a teeny bit larger, the trem will be a teeny bit slower.
The oscillator runs full out all the time, the intensity control has nothing to do with the oscillator. All the intensity control does is act like a volume control for the trem signal leaving the oscillator. Turn it all the way down, and the oscillator still would be cranking for all it was worth. I have no idea what level the signal will be there, it will be a large signal. It will be oscillating at a reasonable speed or it will not. Just get it going, worry about how large it is later.
For the price of three 0.02uf caps, or 0.022uf if you prefer, just do it.
I had those as couplers in my 1959 Silvertone 1451. I read most are leaky and they were.
Thanks for all the help on this guys. Replaced all the caps last night and things are working great now. Does anybody know what type of capacitor those Sangamo's are? Since they look like a sealed type I figured they would still be good.
They are the old paper type with plastic coating. Since they are charging and discharging all the time even with the amp not using the tremolo, I think it's pretty good that they last this long. They were probably made in the mid to late '50s. You might also check the coupling caps for leakage or being way out of spec.http://www.silvertoneworld.net/amplifiers/1392/1392.html
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