SZ13C zener replacement for 1964 JMI Vox T60?

Hi all,

New member here, first post!

Recently acquired a 1964 Vox T60 chassis that I feared was headed for the scrap bin. Get the feeling the chassis was unloved for the last 30+ years as it sat on a shelf and was very grimey and dirty.

I get it; not a very reliable amp and thus probably not very desirable. Also not very powerful. It had a lot going against it... But if I can get this repaired, would probably make a cool little recording amp for bass.

Anyway, the original Arrow power switch plastic bat handle was broken and so was the voltage selector. The front panel was bashed in which may have been the reason for the aforementioned.

But the goal is to save this from the trash.

Did not know if the amp was funcitoning upon arrival as various components in the output section (including the output transistors) were replaced rather amateurishly.

The driver transformer was measured having DC resistances of 32 ohms on the primary with 4.5 ohms on each of the secondaries so I'm presuming that is fine. Power transformer confirmed at 55 VAC on the secondary (27.5VAC CT).

Disassembled down to the chassis. Worked the front panel dent back out (carefully; aluminum chassis!). Straightened the faceplate out, hardwired the PT to mains voltage, new power switch. Replaced all the eletrolytics and a few of the resistors in the output (R23, R27, R30, R31). Will be using silicon PNP MJ15025G for the outputs (of course need to rebias).

I attempted to reuse the original G.E.C. SZ13C zener (stud mount) but the cathode lead broke off inside the body...

I tried to find a datasheet for it, no such luck!

Is it safe to assume that this is essentially a basic 13V 1W zener? If so, would there be a problem using something like a BZX85C13 or 1N4743A?

Some pics before and during the refurb:













And the schematic if anyone's curious:

The only reference I have seen for that Zener is that it is a medium power zener diode, whatever that means.
http://www.wylie.org.uk/technology/semics/GEC/GEC.htm

If you have a 1 watt 13V zener then try it, check how hot it gets.
If you have to buy one then why not get a 5 watt 13v zener diode, it will look chunkier and that it belongs in the amp.
You can get 5 watt 13 v zeners for under $1 if you shop around so that is what I would do.(Mouser)

Thanks Tessieviking for this!

FWIW/FYR: here's the broken one that came out of the amp:



How's this through-hole axial look as a replacement?

Would like a stud mount like the original but me thinks these went the way of the dinosaurs... 

Quote from: fazeka on March 19, 2024, 05:11:11 PMThanks Tessieviking for this!

FWIW/FYR: here's the broken one that came out of the amp:



How's this through-hole axial look as a replacement?

Would like a stud mount like the original but me thinks these went the way of the dinosaurs... 

Looks good to me!  I think you're right about stud mount, I don't think I've ever seen those for sale in the 20+ years I've been doing this.

I take it back, found one at DigiKey if you're willing to pay $7.50 for it: https://www.digikey.com/en/products/detail/solid-state-inc/BZY93C13/14547303?_gl=1*lg2n2t*_up*MQ..&gclid=CjwKCAjwkuqvBhAQEiwA65XxQAnaCSY-UfLQ0asVQqmoPnxE8L2QX5VvT-PRfi2zSy823wjqdrrGshoCOBAQAvD_BwE

That one looks good to use, If you look on Ebay or similar you will find many stud mounted diodes you can use, just make sure you get one with the correct orientation.
The stud mounted diodes are still used but mainly in industrial situations where higher power is involved.
Old car alternators used to have stud mounted diodes in them as well.
I have replaced some that had a 16mm thread on them when I worked as a maintenance electrician for a large manufacturing company.

Update: just finished the refurb of the T60.

Appears to be an October or November '64 as it has the link added (late Sept. '64) but still has R14 = 330k (changed to 82k in mid Dec. '64).

Good amount of work put in and it's not the most desirable of JMIs but if it means keeping it out of the dumpster then it's worth it.

Was in a rather sad state upon arrival but gave it some TLC and it turned out pretty good, best it's probably been in decades I reckon.

Everything disassembled and cleaned. Front of the chassis was bashed in but was carefully "body worked" nearly straight. Also took care of the various front panel dents.

Replaced all the electrolytics, a few of the input resistors, rectifiers, the zener and the output transistors. Used MJ15025s and rebiased according to R.G. here.

Plastic power switch bat was broken off so replaced that with a Carling. Also, the voltage selector was cracked and non-functional so it's hardwired to 115 VAC (US).

Still needs a few minor things to finish 'er up (hardwire the speaker cable and mains cable, etc.). Sounds pretty good, actually!

Output before clip is 33W @ 9.5 ohms (that's what the big wirewound resistor I have measures at), 26W @ 16 ohms. "T60" (as in 60 watts), huh? Hah!  

Anyone got a spare empty T60 head cab lying about? 

That is some very nice work congrats 

Beautiful amp thanks for the pics