Rocktron Velocity Issues

[gtr-fxr]

next question what can i use to replace U1, U2 it's a ssm2131

[J M Fahey]

Are you sure it's an SSM2131?
Datasheet says it's a 20 pin package quad analog switch.
Schematic says it's a 741 pinout Op Amp.
The OPA2131 is a dual Op Amp, but with typical dual Op Amp pinout, it does not match.
The OPA141 does match pinout, but not schematic label.
Unfortunately the schematic was cut in an awkward way.
Still an unsolved mystery.

[gtr-fxr]

that's what it says on the ic unless I'm reading it wrong

PMI SSM
2131 P
S9310

[olddawg]

You really should be using a variac with an amp meter rather than a light bulb limited for a big SS amp.

[gtr-fxr]

i would if i had one

[Chuck H]

OK... Check it out... Every time you fire it up something new overheats or fails. Did you check the conditions for C8??? What caused it to fail??? Is there an overvoltage at the +50V point??? Did you check all the power supply components as prescribed??? Check the rectifiers too. Is there any AC present on the rail where it shouldn't be???

I think if you study all the working conditions in the power supplies you'll find a problem. And until you fix the power supply your just going to keep burning up parts.

JM2C

[J M Fahey]

Itīs confusing.
Yesterday I googled SSM2131 and the first result was:

SSM2131 ssm2404p FEATURES "CIickless" Bilterl Audio Switching Four SPST Switches 20-Pin Pckge Ultrlow THD+N: 0.0008% rms, Chrge Injection: High Isoltion: -100 kHz) Crosstlk: kHz) Resistnce: Supply Current: Single Supply Opertion: Gurnteed Brek-Before-Mke CMOS Comptible Logic Inputs Cost-Pe

which is the 20 pin 4 CMOS switch IC I mentioned .
And searching for "2131 Operational Amplifier" mentioned OPA2131 ... which is a dual Op Amp.

Today sarching for SSM2131P yielded the correct Op Amp, unfortunately obsolete.
Oh well.
Datasheet:
http://www.analog.com/static/importe...616SSM2131.pdf

*If* itīs bad, (hope not), you *should* be able to replace it with another high slew rate Op Amp of similar characteristics, this one has 40V/uS and 10MHz unity gain bandwidth.

Anyway, as suggested above, you still seem to have supply problems.
Thereīs also *a lot* of transistors to check, not only the Outputs and Drivers.
Oh well 2.

[Steve Conner]

Oh no, not the SSM2131!

It is a weird op-amp, famously used as the front end of the Alexander current-feedback power amp, with NFB taken to its output pin, and its power pins acting as outputs.
http://www.analog.com/static/importe...17334AN211.pdf

From the schematic I see Rocktron have used this same topology, so chances of finding a modern replacement that works are slim. I'd try a TL071 and keep a fire extinguisher handy!

Likewise to check the op-amps, you can stick them in some other piece of equipment like a guitar pedal, in place of a TL071.

I have a DIY Alexander amp here, built to the application note, with the original SSM2131s in it, and I could test some candidate replacements.

[gtr-fxr]

thanks guys. so far Ive been replacing transistors as i go along. the ones that test bad anyway. i'm pretty sure the supply's are ok i did find i had a bad diode D5 and a bad trans Q13 . im thinking that could explain some of the voltage problems i was having before. so far everything else i tested looks good.+/- 50v,18v all where they should be as far as i can tell. im just waiting on a replacement 2SC3281 so i can fire this thing up again . hopefully the 2131's aren't bad. but well see. Ill let ya know how it goes

[Steve Conner]

Well be careful and keep the light bulb limiter ready!

If the SSM2131 is bad it'll probably try to take out everything else with it. Its supply current sets the bias currents for the rest of the circuit.

Likewise, if the transistors feeding its power pins were blown, it's probably toast too, and maybe anything else connected to the 18V rail, if 50V got fed back onto it.

These trannies were a weak link in the original design. When the amp is driven into clipping they overheat. Rocktron don't seem to have done anything to address that in their version.

[Chuck H]

i'm pretty sure the supply's are ok i did find i had a bad diode D5 and a bad trans Q13

So what caused the failure of that diode??? And what caused C8 to fail??? It's just a decoupling capacitor.

I don't know what the installation of an incorrect part for U1 or U2 would cause in this circumstance but it seems to me with the failure of D5 and C8 that there are problems with the power supply.

[gtr-fxr]

as far as what caused them to fail...i dont know that diode could of been bad when i got it . this thing was in pretty bad shape when i bought it.

[gtr-fxr]

so far i'm thinking they are bad. no signal past pin 3 of U1,U2

[J M Fahey]

Ugh.
It means they donīt amplify signal (AC).
Before getting into that, how do they behave, DC wise?
Meaning:
* do they receive +/- 15V on pins 4 and 7?
* what DC voltage do you measure on pin 6?
* for that matter, what DC voltage do you measure on pins 2 and 3?

In my book, Op Amps are incredible DC amplifiers, which "just happen" to work on Audio, and in a more limited way.

So checking DC performance will tell us something about "rapidly varying DC", a.k.a AC/audio.

[Jazz P Bass]

And you really should not look at the input pin (AC signal wise)
Look at the output pin.