Symetrix 501 No Bottom

I have an old 501 compressor with the Valley People VCA, and this thing has no bottom (or very little). I recapped it, changed op amps, changed just about everything...still no bottom. I compared it to a 501 with a DBX 2150, and the difference was huge; the one with the DBX VCU has bottom for days.

What am I missing here? Even in bypass, the bottom is gone. And this thing is supposed to be good on bass (go figure). Could it be the VCA? I suspect not, but one never knows. It compresses and limits fine, just no bottom. Cap values? I didn't change them, and I can't imagine these things were sold like this.

Any help would be appreciated.

Steve

What is feeding the signal into input?

check the 100uF caps C13/C14 in case it is not the transformer version.
This unit has no hardwire bypass and the VCA is always in circuit.
To check if the VCA is faulty, pull the chip out and link pin-1 to pin-8.
Have a look at http://support.symetrixaudio.com/cgi-bin/symetrix.cfg/php/enduser/std_alp.php, search for 'schematic'

You definitely have a problem with it. I have one of these & it is quite good used on bass guitar.

Thanks guys for the reply.

Craig: I'm playing a Jazz bass into the hi-z of a Chameleon Lab "neve-ish" pre, into the compressor. I a/b'd the signal into two 501's, and there is no comparrison. One clearly sounds good, the other doesn't.

Harpo: Symetrix has the schematic for the "push-button" 501 (2150 vca), but not the "toggle-switch" 501 (valley people vca). There are some significant differences between the two (completely different vca's, bi-polar caps in the push button one, fewer transistors, some different op amps, etc). Nothing I can't figure out, though. What really sucks is that the PCB is not silkscreened, so tracing is a real drag. Interestingly, the orientation of C13/14 were reversed compared to the schematic. I did change the polarity (with new caps) and it didn't make any difference. With regard to the VCA; it's very different from the in-line DBX vca's. It's square, with a bunch of pins. And, to the best of my knowledge, there are no replacements available. The compressor does compress and limit, but, again, the bottom is very weak. It's like the bass is rolled off from about 200 cycles below. It really changes the tone, and not in a good way.

Rob: I hear ya'. The comparrison between the two 501 is not subtle. One sounds really good and heavy; one sounds really thin.

I'm curious about whether it's the vca, and what it will take to either (a) find a valley people replacement or (b) modify the circuit for a 2180.

Thanks again for the replies. Any other info would be helpful.

Regards,

Steve

[quote author="lookn4tone"]

Harpo: Symetrix has the schematic for the "push-button" 501 (2150 vca), but not the "toggle-switch" 501 (valley people vca).
[/quote]

Just to throw a spanner in the works my 501 is uses push buttons but has the valley VCA.