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Old 24th October 2009, 03:42 AM   #1
slor is offline slor  United States
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Default Sunn Sceptre: Weak Vibrato Cause?

Hi there,

I have a 1970 or so Sunn Sceptre. The vibrato was essentially non-existent when I bought it; someone had replaced two of the electrolytics in the vibrato section with the wrong values, so I went in and replaced all the electrolytics for the heck of it (several looked a bit iffy).

Now, I can hear just a "shadow" of the vibrato superimposed over the signal, which is an improvement I suppose but not much of one. I was going to investigate the transistors in the circuit, thinking maybe the 2N2102 isn't putting out sufficient voltage to fire the LDR completely? Just shooting in the dark, really. I would try a different photocell but it's an obscure model. Any tips at all would be appreciated!

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Old 24th October 2009, 11:36 AM   #2
m6tt is offline m6tt  United States
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Coupla ideas...

Do you need the footswitch to activate? (ala Deluxe Reverb)
Is the bulb good?
Can you replace all of the transistors involved?
Do all the resistors measure spec?
Take the LDR out and measure light and dark resistance?
How does everything measure after (and including) the 1n2070?

Just some guesses. Interesting design, good use of SS when they use it.
The 6AN8 driver circuit is fairly unique!
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Old 24th October 2009, 12:50 PM   #3
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I fixed a Gibson vibrato with this kind of circuit.

Quickest way to a fix was:

unwrap the LDR/lamp module so you can see the bulb. power up and set vibrato to SLOW - see if the lamp lights. It only has to be gently lit, with pulsed brightness at the vibrato frequency.

If that's OK, suspect the CDS cell (the receiver section of the LDR). My Gibson went open circuit. I pulled a CDS cell out of one of those joke gnomes that fart when you cast a shadow on it - resistance 1M (dark) & about 40K in some grey English daylight. The farting gnome's CDS worked perfectly, with no adjustment needed.
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Old 25th October 2009, 09:45 PM   #4
slor is offline slor  United States
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Thanks Gents,

Yes, the footswitch is operative, it turns off and on the "shadow" vibrato. I'm gonna see if I can puzzle out the transistors and make sure they're delivering what they should;
I'm not sure if I can disassemble the LDR / lamp module, it's pretty burly compared to the plastic-wrapped Fender version. With any luck, I won't have to resort to farting gnome technology!
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