Amp Tech Thread / Ask a tech Q

[Hemisaurus]

mortlock, are you sure it's meant to make a noise without you applying a control voltage to one of the CV inputs? I'd either say read the manual carefully, post the schematic or prepare for a lot of twiddling.

[Hemisaurus]

Here's the schematic:


Here's the input jacks. J1 is on the right.



See how J3 connects to that resistor there, the schematic has it bypassed from J3. But the sound is quite noticeably higher gain in J3 as the schematic would suggest. Looking at this, J2 and j3 are in parallel.

I'm pretty giddy about this things simplicity.

Sorry morty, didn't mean to cockblock.

Well either theres magic going on, the pictures don't tell the whole story, or you are imagining it.

It's a little hard to see, that 33K on the left, is it soldered through the J2 terminal to J3?

What do you read if you clip on end of your ohmmeter on where the two resistors join the cap, and then ohm out from the tip on each J? Maybe there is switching we don't see?

[Pissy]

I read 34k from the J3 to the cap connection as well as continuity to that J2 terminal across the solid wire. J2 across the resistor is the same 34k.

I suppose I should measure to ground to see if there is a difference.   There is noticeably more noise crackling from the J3 terminal. Also, my imagination wasn't influenced by knowing any of this. I played it then opened it up to see this. I had considered the same conclusion of influence.

I had only opened it up to install a 3 prong prep for that pre-amp mod I'm doing. Noticed the noise, then looked at this.

Have to look again when I get home tonight.

[Hemisaurus]

Yeah, it looks like it's only grounded via the chassis, but I'd expect a bad ground would give you less signal not more. Are you measuring from the tip of the socket, or the terminal? I'd try from the tip, or even plug a cable in, and measure from the tip of the cable on each socket, but it does seem kinda weird.

You could always modify J3 to resemble the schematic, for experimental purposes, or just short out that 33K resistor with a clip wire and see how it sounds.

[mortlock]

mortlock, are you sure it's meant to make a noise without you applying a control voltage to one of the CV inputs? I'd either say read the manual carefully, post the schematic or prepare for a lot of twiddling.

i dont have a manual..got it used and i dont know what you mean by control voltage to the cv input..dont forget you are talking to a complete novice when it comes to understanding electronics..

[Lumpy]

There's probably a manual online. What kind of synth is it? Is it the white noise generator that you hear? Also, the Cut-Off Freq (or other filter) may be clamping down on your signal, making it inaudible. It may take some serious tweaking, that's a lot of knobs to figure out with no manual. Has Larry tried it?

CV In can be used for an expression pedal, probably opens up a filter.

[Hemisaurus]

OK analog synths predate MIDI, a lot of the functions are controlled by voltage levels. So your controlling keyboard would output a different voltage for each key pressed.

http://www.analogue.org/network/cvolt.htm

Do you have any info on the type of synth it is, based on a net schemo maybe? Also what type of patch cords is it using? Those look like they ground to the chassis, so I expect its using some kind of mini jack?

[mortlock]

susanoo..or something like that..there was a youtube vid on the site when they were selling it..cant find it now..got it from noise fx..

[Hemisaurus]

Wow there is zero information on it.

http://www.noisefx.com/product_details/susanoosynth_analog_synth_2_fx1078

What are you using for patch cables?

The Susanoosynth Analog Synth 2 is an analog synth with a fat and strong sound. Controls for VCO1:FREQ. (Frequency of saw and pulse), VCO2:PULSE2 VOLUME, SAW VOLUME, PULSE VOLUME, P.ADJ(PULSE ADJUST), VCF:CUT OFF FREQUENCY, FREQUENCY OF THE PULSE2, LFO:LFO ADJ (LFO LEVEL), WAVEFORM, SPEED, and RESONANCE. Has jacks for 2 CV IN (-10V to +10V),PWM IN,SAW OUT and PULSE OUT, 2 CV IN,PULSE OUT, 2 LFO OUT, 2 CV IN(-10V to +10V) and 2 audio OUT (3.5mm and 1/4 inch). There are 4 noise generators including WN (WHITE NOISE), RN (ROAR NOISE), GN (GRAIN NOISE) and DN (DIGITAL NOISE). Each noise has one out and volume. It self-oscillates by tuning CUT OF FREQ. and RESONANCE like VCF of KORG MS-20. It runs on 12-volts. Made in
Japan!

Read that last line. I'd try hooking one of the noise generators to the output.

[Lumpy]

Make sure you're using a 12 volt adapter, too.

[Hemisaurus]

After looking at that vid. Try hooking one of the noise generators RN GN WN DN to one of the jacks marked INPUT play with the RESONANCE and FILTER knobs and go from there.

[mortlock]

ha!! i hope you guys have cracked the code..

[mortlock]

got it to work..i had to pause the vid and hook everything up the same way..i feel like im reverse engineering an alien piece of equipment..its cool. thanks for the help as usual. 

[Pissy]

Put the mod in.

My soldering skills are rusty, but they do shine.





Added the pot (50k) and a 270 ohm straight to the Spkr jack. The 8 ohm brick is paralleled to gnd with the pot leg.

I still got some testing to do cuz, while it sounds good, I'm still not confident I'm bleeding enough off. It's pretty hot into an amp.

I plan on putting an ammeter on the signal to see, but how much is too much here?

I plugged it into a cabinet and can here it, which makes me think that's too much.

Just got the Stewart in today.  $275 shipped.




Edit:  replaced that 270 with a 150k, but I'm thinking I need to make the brick a 6 ohm to really get the current going that way.

[Hemisaurus]

Yeah, I think you misread that schematic a little, the 270 ohm is meant to be between the pot wiper and the pseudo ground, the 150K resistor should be in series with the pot between the pot and the output jack, most of the voltage will be dropped across the 150K.

You worry about current don't, using a 270 ohm as the series resistor was your mistake, you had the speaker output across a 270 ohm resistor in series with your 10K amp input, which would mean that about 2.5% of the signal voltage was dropped across that resistor and the other 97.5% was at the amp input.

Compare that to a 150K, where 94% of the signal voltage is across the resistor.

The big power resistor of 8 ohms is just to sink the current to prevent the output transformer melting, it's not dropping the signal voltage for you, and changing it's value won't have any real effect on the output signal.

[Pissy]

True about the schematic. I actually didn't look at it when I was installing. DOH!


So I should probably play with the varible resistance leg to get the signal where I want it, rather than messing with the static load leg?

I'm having a blast doing this btw. After I get this signal thing squared away, i'ma put a switch in it to bypass all this new stuff then put it back in the cabinet so I can still use it as an amp, be it through the stock 6" or otherwise.

[Hemisaurus]

So I should probably play with the varible resistance leg to get the signal where I want it, rather than messing with the static load leg?

If by variable resistance leg, you mean the one that connects to the output jack then yes. There should be a large resistance between the pot and the output jack. The schematic you pasted on the last page says 150K, with a .005 brite cap across it on a switch, you can leave off the brite cap, but you need a large resistor in there. Otherwise with the pot up full, the line output jack is effectively the same as the speaker out.

The big-ass power resistor is just a dummy for the speaker, it doesn't really affect the line output circuit.

[ryansummit]

sorry bout that
thanks for the info

[Hemisaurus]

Not a tech question, looks like a Peavey 212, ask in a buyer/seller thread?

[moose23]

Think I may have bought a broken Sound City L120 earlier. Will no doubt need help from you guys to get it sorted. Guess it's time I learnt how to fix amps as well as the pedals and guitars.

Guess I start with Oli's trick of stripping and cleaning everything and taking it from there.

[VOLVO)))]

Rebuild thread plz

[dunwichamps]

i went overzealous in my strip down of the SC120R, im doing a full rebuild into a Hiwatt/Matamp GT120 mashup with El84 driven verb

[moose23]

I won't be overzealous I don't think. Looking forward to seeing what you do though Nick.

[dunwichamps]

heres the stripped down chassis



The preamp is a mix of the a GT120 but phase inverter and driver are right out of a DR103 with the exception that the PI is a cathodyne which is DC biased using the Hiwatt scheme. Then I will put in a parallel verb path, which is post EQ using an EL84 to drive it much like a 50s standalone fender reverb or the Or/Mat verbs. non master volume beast.

[VOLVO)))]

That isn't a SC anymore...