Gates Radio SA-39 limiter restoration and modification

[tubologic]

When I turn on an SA-38/39 with a neon bulb, I have to have the door open and grab the zero adjust so the meter doesn't slam past full scale.  Then I have to continually re-check the zero point for several hours.   With the units I've put a zener in, the meter has exhibited less deviation, and the zero point stabilized much faster.  Meter drift was not a significant factor after warm-up with a neon, but there might be a bit of slow up and down.   The zener's have locked in quicker, and stayed put. 

The neon 'failure' in all my experiences has been entirely based on audible oscillation.  Nothing like an audible tone mixed into the audio.    I've seen frequencies between 14.5K and 26K.   In all cases, 275VDC could still be dialed in, and the voltage divider string was in spec.   

I'll check the part # I used.     Joe-electro may also have something to add. 

Thank you for the info's.  I would be interested to test it with your type of Zener because I bought the last vintage NE-45's available here in Belgium. As far as I can remember I had the impression that the current through a 1.3W Zener I used was too low for it to work properly. The circuit became very temperature sensitive and I checked this by cooling down the zener. Are you using BZ**** or 1N5*** series zeners ? There is still a noticeable warmup meter drift with the neon bulb,but no worse than on other tube compressors of the same era (RCA,COLLINS,G.E...),nothing you can't live with.(no more than two divisions from cold to fully warmed up) I still haven't noticed any instability in the power supply regulator so far. This is a very basic regulator design and it's hard to believe that GATES produced hundreds of such compressors with a critical oscillating prone circuit. Of course a zener can't oscillate but still it would be interesting to understand what has changed in the circuit to make it instable,though my SA-39 exemplars doesn't exhibit this peculiar behaviour.(but I changed ALL the capacitors and ALL out of tolerance resistors before putting the unit into service). 

 

[lewilson]

Speaking of the gates sa39, does anyone have a  nice clean schematic for one of these? 

 

[lassoharp]

Best bet is probably here:

http://louise.hallikainen.org/BroadcastHistory/uploads/GatesSa-39a.pdf

The originals are very big foldout types that aren't practical for scanning

 

[emrr]

probably right.  The 39A and the 39B have a number of differences; both attenuators differ and the 39A lacks the DC filament change. 

 

[emrr]

Is this big enough for you?

SA-39B

 

[echorec]

Thanks Doug and tubologic for your input on NE45 vs zener.

I have an old NE45 in my SA39 and will test both options in the SA38. Is there a SA38 schematic available somewhere? I have the nice foldout version for SA39A but nothing on the 38.

Regards,

Gunnar

 

[joe-electro]

Wow, that's grrrrreeeeeaaaaatttt!!!

Thanks Doug!


Joe

Is this big enough for you?

SA-39B

 

[emrr]

I have used 1N4758A-TP with no problems to date. 

 

[echorec]

Thanks, I will try that one.

Do you (or anyone else) know if the big caps (c10, c11) is supposed to be 20 or 30uF in the SA38?

Gunnar

 

[MatthewleBlom]

I have used 1N4758A-TP with no problems to date.   

I went to the electronics shop and bought a couple of these zeners.
But the man behind the counter asked me about the voltage...
The 1N4758A-TP seems to be 9.1v.
Is that correct?

I'm confused because I know most of the neon bulbs will glow at quite high voltages (+70v or so?).


greetings,


Matthew

 

[tubologic]

I have used 1N4758A-TP with no problems to date.   

I went to the electronics shop and bought a couple of these zeners.
But the man behind the counter asked me about the voltage...
The 1N4758A-TP seems to be 9.1v.
Is that correct?

I'm confused because I know most of the neon bulbs will glow at quite high voltages (+70v or so?).


greetings,


Matthew

Not correct: the 1N4758 is a 56 V / 1W zener and should work in this circuit. The original NE-45 neon bulb has a operating voltage drop of 75VDC, thus a 1N4761 (75V) zener will be closer to the original value. The "A" suffix means a 5% voltage tolerance (instead of 10%) which is not an important factor in this application.

 

[skipwave]

Nice one Doug!

Hahaha..... from the manual: "Limiter Action: Attack Time, essentially instantaneous."

Stock release time from 0.2 to 1.2 seconds, definitely too long for rock n' roll.

What's the reasoning behind not going to a continuously variable release control? Is it finding a pot with suitable resistance/power rating combination?

 

[emrr]

Be an 11 meg linear pot.  Besides; I like to keep as much original as possible, and the steps are more than adequate spacing.   I'm just expanding the existing switch, not even replacing it.  

 

[Gus]

Why not work on the zener part of the circuit to make it more stable?  In an old Motorrola zener book I have they show adding series connected lower voltage zener diode(s) to have the same voltage but lower temperature coefficient

Maybe try three 20VDC zeners in series for about 60VDC total or three 24VDCs for about 72VDC or a combination.  This will also let you use lower wattage zeners because the power will divide between the series connected zeners  The issue is you would need to have the TC graphs of the zener(s) used

 

[emrr]

Gus, I believe thats what Joe did with his.  So far in my experience, I'm not seeing any issues with a single zener.  But mo' bettah'; why not?    55 volts is the target. 

 

[mik]

Hi,I' Have to restor this beast, it is in working condition but it' has HUM at -50db,
I really don't know How to start.

need same Help.

thank you !

 

[emrr]

Did you add the DC filament mod to the 1st stage tubes?  It's on the SA-39B schematic. 

 

[mik]

I didn't do any mod.
What's the mod you'r talking about ?

let me konw.

thank you

M.

 

[emrr]

the sa-38 you have pre-dates the use of DC filaments on the 1st stage.  add it. 

 

[mik]

Are tou mean the 3 resistor of 3 ohm in parallel plus diode and Capacitor ?  ( litics I guest... )

M.