Sunn Setura solid state trem module?

[stevesamps]

Hi I recently aquired this Sunn Sentura II seems the trem is not working
another tech had done a lot of work and said the module was bad
anyone have any experience with these circuits? Thanks

I was wondering if a standard fender type roach can be used..?
or if anyone has re-manufactured this thing or found something that works
I am not positive the module is bad I have to get my scope on it to see
if the oscillator is working first but just checking for parts and ideas

[Bishop]

Bump! I have a Sentura II, (who knows maybe it's the same amp) that I suspect may have a bad sm-1 tremolo unit. All voltages seem normal, the signal is not effected, there's just no trem at all. I read that if the light goes out you can lose signal, and therefore if you ground the base of the 2n2012 transistor you should notice signal loss as well. I did this and there was no effect on the signal. (I guess that is supposed to turn the light off and tell you if your photo-resistor is working or not?) Does this seem like a valid test? Are fender style tremolo roach units usable here?

[Chuck H]

I'm sorry the OP didn't get a response back in 2006!?! But since the prescribed test didn't prove a fault in the photo resistor I think it's best to review the oscillator circuit as well. I couldn't find an image to download, but here's a link to a PDF.

Prowess Amplifiers - Sunn - Schematics - Sentura 2

There aren't many voltages offered. None for the oscillator other than the source voltage. There is a voltage given for the roach though.

What about the reverb? Is it working?

Check voltages for the oscillator circuit and see that there is actually voltage on the roach circuit.

[loudthud]

The Fender roach won't work because it uses a neon lamp, the Sunn module uses a low voltage incandescent bulb. A simple resistance check would verify if the lamp has burned out.

[Bishop]

The Fender roach won't work because it uses a neon lamp, the Sunn module uses a low voltage incandescent bulb. A simple resistance check would verify if the lamp has burned out.

Thanks for the responses guys. No go on the fender roach. Do the photo-resistors typically fail shorted? That seems to be where I'm leaning right now based on the fact that I'm losing no signal.

[Enzo]

I would think that a rare failure mode. There is no DC on it, you can meausre resistance across the cell while it runs. But why not explore the drive side?

It runs on the 16v supply. Is ther 16v more or less on the bulb leads? Find that 1.5k resistor. Do you have about 4.5v across it? Grounding the base of the 2102 should turn it off, which should make the whole bulb circuit zip up to 16v. SO monitor the collector of the 2102. Does it vary from 16v down to a very low voltage and back when you ground the base or not? Have the trem OFF for this.

Turn the trem on, is the oscillator working? Is there some sort of AC signal at the base of the 2102? At the collector? If there is no oscillation going on, ther will be no trem action.


The key is to find the problem then fix it, NOT just start replacing parts hoping to hit a bad one in the process.

[Bishop]

I would think that a rare failure mode. There is no DC on it, you can meausre resistance across the cell while it runs. But why not explore the drive side?

It runs on the 16v supply. Is ther 16v more or less on the bulb leads? Find that 1.5k resistor. Do you have about 4.5v across it? Grounding the base of the 2102 should turn it off, which should make the whole bulb circuit zip up to 16v. SO monitor the collector of the 2102. Does it vary from 16v down to a very low voltage and back when you ground the base or not? Have the trem OFF for this.

Turn the trem on, is the oscillator working? Is there some sort of AC signal at the base of the 2102? At the collector? If there is no oscillation going on, ther will be no trem action.


The key is to find the problem then fix it, NOT just start replacing parts hoping to hit a bad one in the process.

Thanks Enzo. The 16v supply is there. The bulb and 1.5k resistor both have 4.5v across them. So the bulb has 16v on one side and 11.5 v on the other give or take. Now, at the collector I have basically nothing, under 1v. The 1k resistor there is dropping all of it. If I ground the base (from the depth pot wiper), the collector shoots up to around 30v.

I also measured the resistance of the cell and it's about 8k when the amp is on, but trem is off, and 14k when the amp is off. I can get it to start varying from about 8k to 14k if I mess with the trem knobs, but it doesn't seem to do anything to the sound that I can tell.

[52 Bill]

I also measured the resistance of the cell and it's about 8k when the amp is on, but trem is off, and 14k when the amp is off. I can get it to start varying from about 8k to 14k if I mess with the trem knobs, but it doesn't seem to do anything to the sound that I can tell.

I'd suggest that you retest the photocell to see if it ever goes high in resistance. Turn off the amp and be sure that the power supply filters have discharged enough to turn off the lamp.

This design, puts the resistance in series with the signal path, so the default off state needs the lamp to be on and the cell resistance to be low. So either the lamp never goes dark or the cell is not responding anymore. If you test the cell with the lamp definitely turned off you will know what's going on, and what to look for.

[Bishop]

I'd suggest that you retest the photocell to see if it ever goes high in resistance. Turn off the amp and be sure that the power supply filters have discharged enough to turn off the lamp.

This design, puts the resistance in series with the signal path, so the default off state needs the lamp to be on and the cell resistance to be low. So either the lamp never goes dark or the cell is not responding anymore. If you test the cell with the lamp definitely turned off you will know what's going on, and what to look for.

Looks like it is the photocell, I pulled the unit and it measures the same. The light works though, so I will look to install a new cell in the original unit. Thanks a lot for everyone's help.

[Enzo]

So much for my theory then. Good work.