Gates SA-134/MO-3638 Issues

[andYz00m]

Hey there! Looking for some troubleshooting help on an old Gates remote amp mic pre.

Currently, the issue is that it passes a VERY weak and distorted signal when at max gain.

This thing was in pretty bad shape when I got it, and had a diode bridge rectifier soldered on to the back on the 6X5 rectifier tube socket. The power transformer is not stock but is a proper Triad replacement.

What I have done so far:

• Removed diode rectifier and restored original rectifier circuit.
• New fuse and grounded power cable.
• Replaced all caps. New filter cans and film caps, coupling, etc. (Copied Doug's part choices on a build he posted)
• Replaced all tubes. New tested working units from Bowie.
• Cleaned and reflowed most of the old and dirty looking solder joints. Removed old solder and soldered on some corroded connections.
• Deoxit on many of the socket connections. The rectifier tube wasnt getting heater power and turned out to be old socket pins oxidized.
• Tested power supply and verified correct-ish heater (2.7/3.8 on either side) and high voltage (a bit high at 263VAC). 6X5 rectifier tube is working properly ~300VDC
• Rewired grid input for grid cap on V1.
• Tested nearly the entire signal path for continuity where possible just to make sure it wasnt a cold joint or some other corrosion issue on the jumper plugs.

I have NOT replaced the input and output transformer. Im not sure how to test if these are still functional.

I don't really understand this 3 tube input circuit minus what I have read from Doug's posts on his own repairs and restorations.

Any ideas of where/what to test next? Summoning the might emrr, please help me DougywanKenobi.

Schematic here for reference.

 

[emrr]

Measure the resistance of the transformer windings. First thing one should do on any old restoration like this....unless it's some of that magnetizable Euro junk : ) , then I don't know what to recommend. : ) Had to add that part since someone will undoubtably have a fit over the idea of measuring a transformer with a multimeter.......

 

[beatnik]

You mention the problem is at maximum gain - is it working well at lower gain ? If this is the case it might be the potentiometer

 

[andYz00m]

Measure the resistance of the transformer windings. First thing one should do on any old restoration like this....unless it's some of that magnetizable Euro junk : ) , then I don't know what to recommend. : ) Had to add that part since someone will undoubtably have a fit over the idea of measuring a transformer with a multimeter.......

Yeah I need to do this. I haven't even measured a transformer (I'm still learning the basics!) but this is a good time to learn. Will report back.

You mention the problem is at maximum gain - is it working well at lower gain ? If this is the case it might be the potentiometer

I have to increase the gain pot to max to be able to hear the signal at all, and it is quite distorted.

 

[emrr]

My assumption was correct, only way you hear anything is wide open, but to test the pot, leave it at that position and jumper the sweeper to the high side and see if anything changes. Probably not, but if it does, the pot's possibly bad.

Harder than resistive measurements, you can also just jumper audio throught the transformers with the unit turned off.


Make sure the input XLR is to modern standard for pin out. They aren't as delivered.

 

[andYz00m]

Thanks for those points, fellas.

Make sure the input XLR is to modern standard for pin out. They aren't as delivered.

The schematic shows pin 2 going to the brown lead and pin 1 going to black lead of the input transformer. Should this be opposite? Kinda hard to determine the colors of the tranformer leads on this thing... It was in an old FM radio station in its former life and I imagine its been SMOKED IN.

I think its the output transformer. Im measuring the resistance between the lead on each side, which I believe is the correct way to test??

Input transformer is 29R (input side) - 3.4K (output side)
Output transformer is 34R (output side) - 0 M (no signal on input side)

I believe this means the output transformer is bad, but in my mind that wouldn't allow any signal to get through, right? Am I testing correctly?

I'll also try bypassing the pot.

 

[emrr]

Ugh that does look like a bad output transformer, open primary. Should be about 850Ω DCR. You would still hear leakage. Is there plate voltage at the 6SN7? No easy replacement options out there for that one. 12K:600 with 25mA DC in the primary. Too high a B+ from the SS rectifier is one possible culprit. The other version of these is a 7K7:500 transformer.

Pin 1 is ground in the modern world, so that's wrong. Fix that first and see.

 

[beatnik]

I have just measured one unit I have here and the output transformer primary reads around 780ohm.
bear in mind I have no tubes and no filter capacitors installed as they all need replacing.

make sure there is good contact with the probe tips as the old soldering joints often have oxidation

in all honesty the soldering to the ground bar doesn't look too good, you might have some dry joints there. you should try a more powerful soldering iron. and scraping some of the oxide also helps making a more solid joint

have you checked the tube sockets ? these often have loose contacts which need re tensioning with a pair of fine nose pliers

 

[beatnik]

possible replacement opt ? (hope that's not your case)

https://www.sowter.co.uk/specs/1160.html

 

[emrr]

B+ on the 6SN7 is the next biggest giveaway now that you have an open reading.

 

[andYz00m]

in all honesty the soldering to the ground bar doesn't look too good

You're right. I went back resoldered them. Was mostly impatient and not wanting to burn out the caps as it takes a while to heat up that bar. Not sure I like the bar star design. I found my cap heat sink thingy and resoldered them. Some of them were from the previous "tech" as well.

I see what you mean about the XLR pinout, it is currently correct for modern standards (Pin 1 ground) as opposed to the schematic which is pin 3 ground.

make sure there is good contact with the probe tips as the old soldering joints often have oxidation

I desoldered and re stripped the primary leads for the output tranny just to make sure I had a propper connection, no dice.

Bummer about the transformer as I believe that is one of the major mojo makers of these things.

Checking B+ on V4, plate voltage (pin2/5) reads as -1.1VDC and .005 VAC

 

[emrr]

One could also choose to replace the output on one of these with an indirect feed combo of choke load, cap, and Edcor 10K or 15K:600, if you can figure out how to mount all that.

 

[andYz00m]

Too high a B+ from the SS rectifier is one possible culprit.

B+ from rectifier tube is 305VDC before filtering. (i replaced the SS recti and brought it back to stock with the 6X5)

One could also choose to replace the output on one of these with an indirect feed combo of choke load, cap, and Edcor 10K or 15K:600

I dont know how to do that, but willing to learn. Wonder how (if at all) that would affect the tone.

I think at this point, it sounds like I bought a bunk unit.

 

[andYz00m]

No dice on the jumping of the pot either (which makes sense given to dead opt)

 

[emrr]

No sense doing anything else here until the output problem is solved.

Bummer about the transformer as I believe that is one of the major mojo makers of these things.

Checking B+ on V4, plate voltage (pin2/5) reads as -1.1VDC and .005 VAC

Yep that is definitively open, from 2 different types of measurement.

The input trans defines the sound the most. Gates used 3 different input transformer types and 3 different outputs with this basic circuit.

I dont know how to do that, but willing to learn. Wonder how (if at all) that would affect the tone.

Look at RCA OP-6 output for the concept. You would need a choke that can take more than 30mA, I've measured 20H with an LCR meter, there are some Hammonds that are 20-40H I think. More the better on L. The cap value tunes the low frequency shape. Plenty of posts about this around here.

The Sowter or similar may be the easier path, I don't know the cost, but have heard their delivery time was around 6 months recently.

You could beat on some of the vintage dealers and maybe come up with an original at not too insane a price, though the wiring is frequently dicey from age.

 

[andYz00m]

I’m talking with the guy who I got this unit from. Do you know the model of this opt? They might have one I can replace this with.

 

[emrr]

Should be a number on it. or in internet pics. AO-something

 

[andYz00m]

AO-3039

Can’t find any on poopbay/fartverb, maybe he will come up with one.

Thanks for your help Doug!

 

[lassoharp]

It's been a while but I'm pretty sure Sowter had 2 single ended types that were rated for that range of unbalanced DC in the primary. The one beatnik posted is likely one of them and there was another that was rated for 40 ma. Never tried them but they're worth checking into. Sorry to hear of the casualty.

Maybe donate the dead OT to CJ for a teardown?

Or better yet, see if you can get that shop in Nashville to sell it for you on consignment. They'll probably ask a couple grand for it and since they like to sell things "Untested As is" there's no need to worry ourselves over such details - just make you some money man . . . .

and yes, I'm being sardonic on that last suggestion, for the most part . . .

 

[beatnik]

Maybe donate the dead OT to CJ for a teardown?

+1

If 4:1 ratio is suitable there are a couple more candidates

https://www.lundahltransformers.com/wp-content/uploads/datasheets/1671.pdf
https://www.sowter.co.uk/specs/9705.htm