GATES SA-38 Restore project

[VoxRick]

Greetings from Memphis! First time to post here.

I am wondering if anyone might know where I can find a schematic for GATES SA-38 #Mo-2996A.
Thanks to info in this group I have found the manual and drawings for the 39A and B.

This past summer I acquired this limiter from the first radio station i worked at in high school. I'll post before and after paint job pix.
I have replaced the caps and tubes and I am waiting on some new resistors that had fallen out of spec. (The Post Office is not my friend right now)
I can get audio in and out but once I get past a point it begins to distort instead of limiting. I am hoping that the new resistors will help.
I also need to track down why the front Meter slams into the red when I put the VU in Limiter mode. It did not do this before my re-cap attempt.

I would love to get her up and running on my mic chain.

It's good to find you all. I look forward to learning more about how to get this beauty back to it's former glory.

Rick

 

[VoxRick]

This is what she looked like when I found her in the transmitter room, next to a broken window where birds had access.

 

[VoxRick]

She's pretty clean on the inside

 

[VoxRick]

rear view

 

[VoxRick]

When I first open one of these units... a cock roach ran out!
ICK

 

[emrr]

Did you get the SA-40 too, or just the PSU? I see there's also a monitor amp. 

That's a replacement power transformer, pretty common as stock is under spec. 


The SA-38 is in one of the threads around here, yours will be just like the SA-39 schematic, only the earliest is significantly different.  The rest is the additional metering option wiring.  The manual (online) has alignment info.  You found the meter zero pot inside?  The resistor across the meter itself is selected for meter adjustment, after all else is done.  If the neon bulb is bad the voltage adjust won't work.  Sometimes the current set resistor parallel to the meter in the first stage path has to be adjusted, there were 2 different factory values and a meter scale change, and I've seen alot of different actual values in that position. 

 

[VoxRick]

Thanks Doug, for the  txfmr info

I do have the SA-40! It is the actual board that I learned how to do radio on as a teenager. My hope is to get it up and running and run my mics through the mic pres. I have also wondered (since it has Program and  Audition sides) if I could configure it so that I could run the L/R output of my DAW through it.

I did have one bit of drama with the PSU for the Sa-40. I got it cleaned up and powered up in my studio. I left the room for a few minutes and returned to find that one of the caps had exploded and sprayed black gunk all over one wall. Thank goodness it was not facing my mics and monitors. That's when I learned why re-cap is job #1. I left the remaining cans in place for appearance, with new caps inside. 

The amp you saw is the SA-22. It was our monitor for cueing up records.

But first things first. The SA-38 is my learning project. If I can get it going then I will be more confident tackling the mix desk.
I did find the R-3 knob. Before I recapped, this knob did adjust the meter. Now the meter pegs and the R-3 has no effect. I will double check my soldering and make sure I have the right caps in place.

The neon bulb is glowing, so I assume it is working as it should.
I don't see a VU resistor, unless its the one on the VU-switch between LMT and Output. (pic 399 ohm)

There are a couple other resistors that I don't see on the 39 schematic. One is on V-8 pins 3-6 (5k) and one on V-5 pins 4-6 (12.99k)
Do you know what these values should be?

rt

 

[Rob Flinn]

In my experience you need quite well matched valves to avoid distortion in this box.

 

[emrr]

https://groupdiy.com/index.php?topic=44868.msg944161#msg944161
https://groupdiy.com/index.php?topic=71715.0
if you haven’t seen those. 

I do have the SA-40! It is the actual board that I learned how to do radio on as a teenager. My hope is to get it up and running and run my mics through the mic pres. I have also wondered (since it has Program and  Audition sides) if I could configure it so that I could run the L/R output of my DAW through it.

Sure, Audition feeds the monitor amp which has a 500Ω output, Program feeds the Program amp. 

is it the single chassis or modular version?  Single chassis doesn’t have an interstage volume control in the preamp, modular version does.    If it’s the smaller natural aluminum transformers you DEFINITELY want the output caps replaced before powering, DC leakage can kill those output transformers. 

I did have one bit of drama with the PSU for the Sa-40. I got it cleaned up and powered up in my studio. I left the room for a few minutes and returned to find that one of the caps had exploded and sprayed black gunk all over one wall. Thank goodness it was not facing my mics and monitors. That's when I learned why re-cap is job #1. I left the remaining cans in place for appearance, with new caps inside. 

Was it connected to the console?  If not, voltage rises far beyond the cap rating with no load. 

The amp you saw is the SA-22. It was our monitor for cueing up records.

Yep, I have a pair.  Basically a Fender Champ in a rackmount!

But first things first. The SA-38 is my learning project. If I can get it going then I will be more confident tackling the mix desk.
I did find the R-3 knob. Before I recapped, this knob did adjust the meter. Now the meter pegs and the R-3 has no effect. I will double check my soldering and make sure I have the right caps in place.

The neon bulb is glowing, so I assume it is working as it should.
I don't see a VU resistor, unless its the one on the VU-switch between LMT and Output. (pic 399 ohm)

There are a couple other resistors that I don't see on the 39 schematic. One is on V-8 pins 3-6 (5k) and one on V-5 pins 4-6 (12.99k)
Do you know what these values should be?

You should have voltage adjusted for 275V at the output of 6X5 as shown on schematic. 

V-8 pins 3-6 is R50 4K7

V-5 pins 4-6 is R21 10K

R6 47Ω is in parallel with the meter.  Earlier versions it’s 220Ω.  That current flow is what the meter senses.  R47 is directly across the meter on a 39, it may be on the switch on a 38….see what it’s connected directly to.  R47 is a secondary parallel adjustment, no value quoted. 

 

[VoxRick]

No, the board was not connected - good for the board - not so good for the PSU!

I would guess that the SA-40 is modular but I'll post a pic for your assessment.

Good to know about the amp. I sold my VOX amp last Christmas. I've been playing acoustic all 2020.

I couldn't find the 38 schematic here but I did locate the manual at this link

https://funkwerkes.com/web/wp-content/techdocs/MixedProAudio/Gates-SA-38A-Limiter-Amplifier-Manual.pdf

I am a newb when it comes to schematics, so I may be mis-reading. In my unit there is a capacitor between V10 and V9 that I do not see on the drawing. There is a C9 between V9 and V8 on the drawing but not in the unit.

R46 (390 ohm) is on the meter switch

Still trying to suss out the pegging meter thing. I saw in a post where you said  lowering the value of R2 would bring the meter up t zero. Would the converse be true? Raise the value to lower the meter?

Thanks

 

[emrr]

Modular, and very clean!

I probably did say that about R2 at one point, ignore that and look at what I said here again.  Modify R6 to a lower value with R3 adjusted to center.  R47 (SA-39) is an R6 modifier, you don't have to actually change R6 in your case.  Use someting like a 1K pot as a rheostat across the meter terminals, then measure the working value

That's probably C9 you're seeing. 

 

[VoxRick]

Well, it wasn't very clean when I rescued it. It had been out of service for a few decades and it was covered in nastiness. (pic)

I will put the 1k pot on the meter. Thanks for that.

Rob mentioned matching the valves. My multimeter shows the voltage but the manual lists the current in amperage MA

After being on for a couple of hours here are the current readings
V1 & 2 : 5.65
V3 - 6.60
V4 - 5.70
V5 - 27.60
V6 - 24.24

>>>You should have voltage adjusted for 275V at the output of 6X5 as shown on schematic. <<<

Do I measure at Pin 5 and 1?

rt

 

[emrr]

Well, it wasn't very clean when I rescued it. It had been out of service for a few decades and it was covered in nastiness. (pic)

Rob mentioned matching the valves. My multimeter shows the voltage but the manual lists the current in amperage MA

After being on for a couple of hours here are the current readings
V1 & 2 : 5.65
V3 - 6.60
V4 - 5.70
V5 - 27.60
V6 - 24.24

>>>You should have voltage adjusted for 275V at the output of 6X5 as shown on schematic. <<<

Do I measure at Pin 5 and 1?

rt

pin 5 sounds right, whichever shows the 275V, and the ground buss bar. 

V1-2 is where balance is most important.  I look at zero compression, and also 5dB compression with a tone. 
The 6V6's need to be reasonably matched. When ordering new ones, I specify matched and I request a lower current number, as SA-39's run at the limits of the original power transformer current capability, that's why, like this one, they burn out.  There are many out there with R-14 replacements that do 120mA rather than the R-11 at 90mA. 

By clean I also mean the paint, there's no rust showing through the transformers, most have some, look at mine again.  The dirt we can clean off doesn't even factor into my meaning of 'clean', I mean 'withstood the test of time'. 

 

[VoxRick]

Yes, there doesn't appear to be rust or leakage. Although that could change once powered up I suppose.

After recapping the SA-22 I powered her up (on the back porch this time) and the xfmr got hot as a $2 pistol. It leaked and didn't smell too good either. Hopefully I won't run into that in the mix desk. It's a shame the new xfmrs don't come in cans like the UTC did.

Still waiting on parts I ordered 3 weeks ago via mail. I re-ordered and used UPS this time.

I appreciate your guidance. My education in these matters came on an "as needed" basis in my radio production/VO biz. I tried to enroll at Elkins Institute to get a FCC 1st class phone in the 70's but they were going out of business. Then my career took a production path. Until recently, I have only had solid state gear in the studio. 


Merry New Year!

rt

 

[VoxRick]

Question: When you say put the "1k pot on the meter terminals" - There are 6 terminals on my VU meter. Which one do I connect to the pot?

 

[emrr]

2 are unused
2 are the light bulbs

 

[rackmonkey]

Quite the score, and with the UTC versions of the transformers throughout (or nearly so).

 

[emrr]

On the console, I would advise acquiring an adjustable bench supply and restoring/testing each module on it’s own before attempting the whole thing under power.  It’s the only way to be safe and eliminate variables. 

Sounds like the 22 PT may be shorted. Try it again without any tubes if you haven’t.    It can be replaced, and you can even de-core and restuff the original can if you want to go that far. 

After recapping the SA-22 I powered her up (on the back porch this time) and the xfmr got hot as a $2 pistol. It leaked and didn't smell too good either. Hopefully I won't run into that in the mix desk. It's a shame the new xfmrs don't come in cans like the UTC did.