Author Topic: 1200s Weak Tremolo  (Read 166 times)

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Offline Tdlunsfo

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1200s Weak Tremolo
« on: April 22, 2017, 04:41:16 am »
Ok, need a peer check on a tremolo circuit on my 1200s. I recently recapped this amp and got it running but the tremolo is weak. Suspecting a voltage issue I checked the F supply voltage and zener and it's good at 23v. All tremolo resistors check good. I have good ESRs on all oscillator caps. Concerned with the signal path at this point I begin matching it to the schematic, that's when I realized this is wired more like a 190L with the trem bug moved to second stage plate of the 12AX7. The speed and intensity pots work and I can see it on my scope, but for some reason the bulb isn't translating into large voltage swings in the cds cell. Transistors check good on my dmm. I don't know what else to look at and at this point I think I have a weak trim bug. I do see one for sale on eBay but they are the Sigma datacell -6 version instead of -12. I have 6 volts across the bulb, think I could swing the -6 trem bug if I lower my bulb voltage? Welcome any thoughts or suggestions.

Offline GrannyGremlin

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Re: 1200s Weak Tremolo
« Reply #1 on: April 22, 2017, 05:42:11 am »
The trem is a bit subtle.  And finicky.  But when it works it is pretty nice.

There is also an issue with the ground around it due to the rivetted terminal strip.  The rivet is on the ground terminal (obviously) and does work it's way loose over time causing intermittent conenction.  Drill it out; clean off the area (oxidisation) and replace with a machine screw with toothed lock washer digging into the chassis a bit.

Another issue I have found with mine is that sometimes the trem doesn't work on the signal, but on the noise, and the depth works in reverse (rate is non-linear/jumpy).  I think someone may have crossed some wires at some point in the history of mine.  When this happens sometimes the reverb stops working too (they share a recovery amp).

Offline Tdlunsfo

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Re: 1200s Weak Tremolo
« Reply #2 on: April 22, 2017, 06:35:17 am »
I have similar issue with mine. The rate washes out around 7/8. It's pulsing faster on the scope but you cant hear it through the speaker. Intensity doesnt really show till about 5. It's just not that deep of a tremolo. I'm considering rewiring this thing back to the earlier rev to see if that helps. The cds is pretty simple. The signal comes off the 12ax7 plate to the cds cell, out of the cds cell to the tone stack. Not much to go wrong there. Maybe my light isn't bright enough in the trem bug? I guess I could mess with some of the resistances in driver transistors to see if it makes a change. I'll check my rivets again but already tried this when I was chasing a hum problem earlier. I may just solder all of them just to be thorough.

Offline Tdlunsfo

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Re: 1200s Weak Tremolo
« Reply #3 on: April 27, 2017, 10:47:55 am »
I made some changes to overcome my tremolo issue. The datacell ended up being good as tested from the bench supply. I changed resistor R115 from a 1.5k down to a 360 ohm. This gave me more deflection in the tremolo amplitude and it sounds normal again. I am suspecting weak transistors and the new resistor gives me more current. I have an issue with my scope at the moment and can't tell if I am sending Q103 into saturation. I could had tried changing the transistors but I got tired of fooling with it and I am happy with the work around. Just in case someone has a similar issue..I will report back if this gives me problems down the road.

Offline GrannyGremlin

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Re: 1200s Weak Tremolo
« Reply #4 on: May 01, 2017, 05:22:44 am »
Cool.  Please do let us know how that goes.