Symetrix 501 noisy

[phantombox]

Hello guys.
Someone gave me a Symetrix 501 to see if I could fix it. It started to get noisy...
My guess is that after 30 years, a recap job would be in order. I did some research on the internet and could find some things but nothing specific to my problem.
I could not find the right schematic for this compressor, all there is on the internet is a schematic of the latest version that has an output transformer.
Mine has no output tranny. The one I have is grey-faced, has push buttons, a blue Valley People ECC101 VCA, another 16 pin IC IRC-8703 (no idea what this is, I couldn't find anything about it), and 7 pcs 4560 op-amps.
I already "cranked up" the PSU (diodes over the volt regulators, 2200uF caps instead of 470uF before the regulators and 220uF instead of 4.7uF after the regulators) and shorted out the 2 4.7uF 63V NP input caps which apparently aren't really needed. There are 5 more 4.7uF 63V NP caps in the circuit, and I'm having trouble finding them for a reasonable price. The guys here want 2+ bucks for each cap, and they are not even in the area that I live. Seems that the damn caps are quite unique. Do they have to be this value or can I use larger caps (like 10uF even 47uF)?
I also tried with other op-amps like the TL072 or TL082, even NE5532, same noise pattern. I cannot find any OPA1642s, as someone said these would lower the noise floor a few dB.
Any suggestion is welcome.
Thanks in advance.

 

[pvision]

The only 501 schematic I have is for the later version with the dbx 2150A VCA but would think the Valley people version would be very similar. The main difference between the two is the CV response - IIR one is logarithmic so there would need to be some changes in the sidechain

The audio path is as simple as they come. I'd assume the NP caps are in the audio path. You can go for bigger values with those if you want to or have problems finding exact replacements. One is in the (unbalanced) insert return so I'd assume the other four are input & output (balanced)

I'd just recap everything with the same values, but bump up the psu caps as tou have, and keep the NP caps in the audio path

The cleverness on these is in the sidechain. They sound very similar whether VP or dbx versions

 

[JohnRoberts]

Did it just start to get noisy, or was it always noisy and just noticed?

It is the nature of compression to raise the noise floor so check that it isn't just compressing.

I recall Paul Buffs TA-101 VCA and used it in one Loft Compressor back in the 80s. If you have a faulty TA-101 good luck.

Capacitors are unlikely to cause noise in a compressor unless one screws with the side chain pushing up the VCA gain.

JR

 

[Script]

+1 on swapping out all elcos.

NP's for 2+ bucks is nuts.

Pretty harsh verdict on that comp (later version though) by this guy, but he claims 20dB of noise improvement. He also swapped the regulators.

https://dbelousov.com/blog/symetrix-501-mods/
(TA-101 has been discussed somewhere around here.)

 

[phantombox]

Where exactly on this particular compressor do you adjust the VCA gain? Is it a trimpot or are these fixed resistors?

 

[Script]

https://groupdiy.com/attachments/501_sch-pdf.20371/
If this one, where it says Output Gain R11 50KB into Ec2 (=Makeup). Control signal goes into Ec1.

R5 and R20 define idle gain thu the VCA. Here they chose 13K.

Or you have another version?

 

[analogguru]

....
Pretty harsh verdict on that comp (later version though) by this guy, but he claims 20dB of noise improvement. He also swapped the regulators.

https://dbelousov.com/blog/symetrix-501-mods/

After reading all of that nonsense (in the link's article), I got the impression that this guy doesn't have a clue about (audio) electronics.

"Recapping everything" blindly is only a desperate attempt of those who are unable to find the real reason of the fault. "Recapping everything" has never cured a noise problem (BTW, hum is not noise and there doesn't exist a "static noise").

Old electrolytics (1976) can even be better than their new replacements (2019) - here is the proof:

 

[scott2000]

After reading all of that nonsense

Maybe a plumber?

 

[phantombox]

My 501 is indeed a different version. There is no output trafo. Here are some pics, I apologize for the crappy quality.

 

[phantombox]

There is no ID printed on the PCB, nor is there any ID for the components, either. There are 4 resistors that measure around 13K, close to the in- and output connectors.

 

[phantombox]

Hi guys, I'm back on this project.
Today this guy gave me another Symetrix 501 he had as a pair to the first one. In theory they should sound the same with equal parameter setting, but reality has different idea...
The one I just received makes almost no noise but also has less output and compresses ok, sounds a little thin thou. The one that I mentioned in this discussion (the one I had before) obviously is louder and noisier and also compresses ok, but the peak LED turns on with gain at center position or 12 o'clock and output gain put shuts off at about 3 o'clock. I tried interchanging the 3046 IC (visible in the last pic) but nothing changed. All other ICs in the 2nd 501 are soldered directly to the PCB but the first one has nice sockets installed instead, so I switched each 4560 op amp with a TL072, one by one. Also nothing.
Now, I'm not sure if the second Symetrix 501 is supposed to sound this way or not, the first one definitely has something wrong the peak LED and the output gain circuit. I took the 50K gain pot out of the PCB and it checked out OK.
Any help, again, please...

 

[JohnRoberts]

If you have two identical units you can try some experiments. Maybe shorting the two side chains together so the VCA get the same feed could confirm the VCAs are the same or not...

compare and contrast...

JR

 

[phantombox]

Do you mean linking the 2 units together via the side chain connectors or the stereo interconnect?
There is still the issue with the output gain control that shuts off the first unit at 3 o'clock position and the clip LED that turns on at 12 o'clock.
And now that I'm thinking a little more, the 2nd one sounds thin or without body, just as if the polarity is inverted, even thought that the XLR connectors are original and properly soldered to the PCB.

 

[JohnRoberts]

if you have a scope you can feed identical inputs to both units and see where it gets squirrely...

JR

 

[phantombox]

Yeah, well, I don't have a scope...

 

[JohnRoberts]

I've pretty much had a scope forever, but there are other troubleshooting techniques. One I used way back when, was a crude audio "sniffer". I fashion an audio probe using a small film cap (say 0.1uF) and a couple k resistor (say 2k). I'd feed this probe audio into a low power headphone amp... With two identical units you probe around and listen to the audio starting at the input until it gets nasty or different.

JR

 

[phantombox]

I had another VCA from a Harrison MR3 console and installed it into the 1st 501 and now it works all right. The original (fried) VCA was made by Valley People and the newer one was an Allison Research (really old VCAs...) The clipping LED also turned off and I can crank the output gain all the way up. Nevertheless, I can't get the clipping LED to turn on, even if I feed the input signal from the headphone output of an ordinary radio and turn the volume all the way up...
Both 501s sound very similar now, the first one is a little bit louder at the same parameter settings. I'm not sure that this might be due to the recap job on the first one or the fact that the VCA is slightly "younger" than the original one. Or if it's just that both 501s have gotten into the years.

 

[Script]

Could feed steady 1kHz into unit and measure AC at different points of audio path in order to compare and see where the drop occurs.

Do units have the trim for adjusting offset at output of VCA buffer ?

 

[phantombox]

I think they do, yes.

 

[abbey road d enfer]

I can't get the clipping LED to turn on, even if I feed the input signal from the headphone output of an ordinary radio and turn the volume all the way up...

The "headphone output of an ordinary radio" is not necessarily very high. Most headphones require minimal voltage to deliver loud SPL, because they are low-impedance.
You need to get a minimum of test equipment. That could be a computer soundcard, used in conjunction with an adequate software, such as Audacity.