Tube Amps / Music Electronics
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|5/29/1999 7:30 PM|
||BFSR Vibrato Intensity "lag"|
A friend brought his SR over today because the vibrato had quit functioning. After installing a new tube, it worked great with one exception: the vibrato doesn't really get to a noticeable level until the intensity control is up to almost five. Under that setting, it's not apparent it's on. I checked everything out (caps, resistors, pot) and it all seems copacetic and within tolerance. The only thing I found out of spec was the voltage on pin 6 (P2) of the vibrato tube. With the vibrato switch off , the voltage (according to the schematic) should be 280--this one measures 425. Could this be the problem? If so, why is this voltage so high?
Thanks in advance.
|5/31/1999 7:03 AM|
Looks like 380 volts on pin 6 to me, although the schematic is a bit fuzzy. What's the voltage on the other side of the 10 M resistor? If there's no voltage drop across the resistor, then the tube's turned off. Could be a leaky cap in the previous stage, dirty tube socket, poor ground connection at the 56K cathode resistor, bad tube.....what's the voltage at the cathode?
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|5/31/1999 12:23 PM|
>Looks like 380 volts on pin 6 to me, although the schematic is a bit fuzzy.
I meant pin 1. Sorry for the typo.
>What's the voltage on the other side of the 10 M resistor? If there's no voltage
>drop across the resistor, then the tube's turned off.
I haven't checked it, but I will this afternoon. The vibrato does work--the bulb flashes and once I turn up to about "5" on the intensity control it comes through loud and clear.
>Could be a leaky cap in the previous stage,
I replaced them with new just to test it out and it didn't help.
>dirty tube socket,
Used some Caig DeOxit on it, though that doesn't necessarily guarantee a fix. I'll give it a closer look.
>poor ground connection at the 56K cathode
I've checked all the grounds and they seem intact with no excessive resistance.
Replaced with new (a few times).
>what's the voltage at the cathode?
K2 -- 0-2 when vibrato's on; 0 when off
K1 -- 20 (on); 0 (off)
BTW, when my friend first bought this amp a few months ago, the vibrato didn't work at all. I traced the problem to a bad roach (broken lead on the LDR) and replaced it with one from Hoffman. The vibrato worked great after that until now.
How bright should the neon flash be? It's the normal orange color right now.
Thanks for your reply and any further assistance.
|5/31/1999 1:30 PM|
Most of these types of trem are not as good sounding, or work as good as others. There was a problem that was in the Fender amp manuel about some models of these getting out, and needed changed.I fooled around with a bandmaster, and found that some of the other Fender schematics have different value resistors in parts of the circut. There is on some models a 100k with a bypass cap in this circut that goes to ground. If you have the 100k in yours, try a 56k or 47k. It will inhance the trem.You can play around here to find what works best. Some amps sound ok.. and some are weak.Also have a good 25uf bypass cap.Hope this helps some.And most people run the intesity full up on the fenders any way, you won't hear much trem effect untill its turned up. Changing the value of caps in the circut.. .01 to .02uf will slow the speed down too. [Richie]
|6/1/1999 7:29 AM|
Pin 1, 280 volts is normal. K2 should have 5.2 v according to the schematic. I believe this is the oscillator "off" state . Seems to me that the lamp flashes white, but I'm relying on memory here. The lamp shouldn't light in the off state. I agree with Richie about the intensity pot usually having to be turned up quite high to get a useable effect. When I see a problem of low intensity vibrato with this circuit, it's usually the LDR cell that needs replacing. Also, if light is reaching the LDR, it will limit the effect, so make sure it's sealed up light tight. Hope some of that's useful...Jerry
|6/1/1999 10:37 AM|
Thanks, Jerry. It's working OK once I turn the intensity up, and that seems to please my buddy who owns the amp. It sure doesn't come on as fast as my DR does, that's for sure.
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