Gates Radio SA-39 limiter restoration and modification

[emrr]

yes.  Make sure the diode can handle the constant current load, and I would go for a slightly larger voltage cap than specified. 

 

[mik]

..I see, Any advice for a particular diode ? and.. the capacitor schould'nt be polarized I guest.

M.

 

[mik]

ok, with the DC mod. Hum, is 20db lower, But It still have gross distortion in low band..

maby the tube are too old ?

M.

 

[emrr]

did you measure the current balance of the tubes for various conditions?  Will point out most compression related distortion immediately.  I usually go through about a half dozen input tubes when getting one of these set up, and I don't even listen until the current meters are matching reasonably well. 

 

[mik]

No I didn't, but I may use my Hickok tube tester to see if they are still matche or not.
I see also the band isn't so flat, not as drammatic, but still not proper flat as it schould be. I think this beast has got to be totaly rebuild, at leat recapped, they mesure ok, but I see some sine wave jump on scope passing through different condition during diagnostic test.

thank you emrr.

M.

 

[emrr]

current test via the current jacks on the back is the only sure way.  A tube tester gets you nowhere here. 

When checking response, be sure to check with attenuators at different settings.  I see radical changes in response based on attenuator leakage sometimes. 

It looks like you have a replacement choke or filament transformer on the inside.  The power trans already blew up once, and was replaced. 

Clean the tube sockets too; exercise the tubes in and out at least. 

Pull the 6H6 out and see how it sounds as a straight amp. 

 

[mik]

OK, I will do, and let you know.

M.

 

[emrr]

Here's another fresh out of rebuild and mod surgery.

 

[Mike Cleaver]

Again, great looking work and I'm glad to see another tech who dates the parts he replaces.
Makes it so much easier for the next guy who has to open up the chassis when there are problems.

 

[plumsolly]

Hey Doug - What did you use for the 'attack' and 'release' labels?

-Ben

 

[emrr]

Printed transparency and poly-crylic clear coating. 

 

[Marcocet]

It looks beautiful Doug

 

[lassoharp]

I dig the 3 dimensional effect on the TC labels.

 

[johnhancock]

Makes me want to get my SA 38 fixed finally.....
Anyone in Brooklyn up for the task?

 

[emrr]

Funny, that one I just showed moved to Brooklyn, and I have several people in Brooklyn sending tube comps down here for service.    Round trip shipping of an SA-38/39 from Brooklyn to here and back runs around $130 lately.

 

[johnhancock]

Sounds tempting, wish I knew how much the total repair would run. It's hard to tell with this baby what's wrong with it, needs new caps for sure.

 

[emrr]

Like you said, depends on what it needs and what extras you want. 

 

[emrr]

I have an SA-38 on my bench that is giving me some issues that I hope you could help with.
Voltages at pin 3+4 of the 6V6's on the output are sitting at 440V. and pins 3+4 of the 6L7's are about half the voltage of my sa-39.
Could this be related to the NE-45 bulb, or do you suspect something other?

6V6's not drawing current, so full B+ with no load present:
1) bad 6V6's
2) cathode R open
3) current test jack not closing, resulting in open condition. 

6L7's are part of the regulated power supply feed:
1) regulated PSU not correctly adjusted or broken
2) plate or cathode R off of value
3) meter zero incorrectly set, along with one or both of the first two problems

 

[dustbro]

Thanks Doug!
do you have a schematic for the SA-38? A few things are different from the 39 schematic I have.
A bunch of wires have the insulation falling right off them... I guess I'll have to replace a few things while I'm in there.
It looks as if someone has poked around in there a bit before it arrived. I wonder if some of the upgrades were improperly done.

 

[emrr]

I do, but they are way down in the scan pile here.  I am seriously behind on scans I owe different people.  I would swear there's an SA-38A pdf floating around somewhere online.  Probably by the MI-#.  Anyway, not a lot different from any SA-39 variation, just not like the SA-39B in exact details.  The extras you see are almost entirely related to the different metering.  You can get by very well with the SA-39B schematic.