Stax SRA-3S Rehab Thread

[ericj]

So as to stop spamming the stax thread, and get everything in one place. 

I recently got a ripping great deal on an SRA-3S in unknown condition. And since all of these are at least 50 years old, there is much to go through. 

There are some existing resources to link, and I will come back and add more as i scrape 'em together: 

 

 

It's a shame there are no component designations on the boards or schematic, so we'll go ahead and muddle through without them. Maybe I'll annotate some images. 

The area in the red box is the phono preamp. It's not much of a preamp. I hear you can pull the board and leave it out. An enterprising nerd could build a better phono preamp to plug into the same slot. 

The modern replacement for all transistors in this box is the KSC1845. These are available from Mouser and other 1st tier vendors so there is no reason to look for them from secondary sources. The pinout is reversed. If one of the four on the driver board is bad you should probably replace all of them. I suppose i should put together a complete BOM at some point. 

You will need to remove the bottom of the case and desolder the RCA connectors on the back in order to angle the main board out to work on it. 

Starting with the power supply in the area in the blue box. The vintage diodes should be replaced with 1n4007, uf4007, or similar. I used BYV95C fast soft recovery rectifiers because i have some. Probably doesn't make a difference what you use here as long as it's a high voltage rectifier. 

My unit actually had 22uf 350v caps in the power supply. I installed 39uf 350v. They are comically small compared to the original caps and it may be wise to anchor them with some hot glue. I used Nichcon UCY2V390MHD, qty2. I am not sure if it is wise or useful to go big here. 

Axial caps are getting harder to find and there are few choices. It might be totally reasonable to get radial caps and attach wire to the positive lead, maybe wrap it in heatshrink or plastic tubing of some kind. 

For the 30uf 150v caps, my unit actually had 22uf. Which i foolishly ordered 22uf replacements for. They were expensive and about the same physical size. I'm thinking about ordering some 33uf long-life radials and adapting them however it will work. 

 

[ericj]

On to the shaded portion, the actual amplifier.

The 0.05uf caps are the large films on the main board. It's likely a good idea to increase these to 0.22uf 630v polypropylene. 

The 0.2uf film caps on the driver board should be upgraded to 0.47uf polypropylene. There are also 0.22uf electrolytics near the center of the board, I replaced those with 0.22uf Wima polypropylene.  I think it's possible that both of these should be upgraded but i am unclear on that, and would welcome input on the matter. One of them is clearly an input coupling cap, the other goes from the base of a transistor to ground. 

The 10pf bypass caps shown on the schematic were 5pf ceramic discs in my unit. Spritzer recommends upgrading to silver mica 10pf which sounds reasonable to me, though there's probably no reason not to use polypropylene. 

The back to back 100uf 6.3v can be replaced with a single 100uf bipolar radial cap. The unused position for each channel should be jumpered. 

The trimpots are probably pretty crusty. Piher "15mm" horizontal-adjustment trimmers fit this position, fwiw. Bourns probably makes a similar trimmer. 

The 33uf 50v 105c axials you can get from Mouser are listed as Vishay, and what you get is a USA-made Sprague cap in clear heat shrink like it was made in the early 80s or something. I'm sure they work but, whaa? I'm not the first person to buy these. I wonder if they are ancient stock or not. It may be worth the hassle of adapting a radial cap here. 

5.1k safety resistors can be added to the outputs per Spritzer's recommendation. $1.70 at Mouser gets you qty10 TE Connectivity RR03J5K1TB - 5.1k 5% 3W 750v rated metal film. I am not yet sure where best to mount these. 

Having read some threads about other stax tube amps, i think it *may be possible to adapt the circuit to accept ECC99 or 6N6P(i), but the heater current on those is somewhat higher - 800mA for the ECC99 and 900mA for the 6N6P vs. 600mA for the 6fq7 / 6cg7 and the transformer might not be up to it. 

Edited February 21 by ericj

[ludoo]

I might be tagging along in your footsteps: I have two of those, one with a busted transformer, and one which was recapped (not by me) but has a hum and random cuts in one channel.

[ericj]

Y'all are making me worry i need to check the transformer windings in mine. I never even plugged it in. I'll check tomorrow. 

Probably no easy replacement for the transformer. You could get edcor to make one but it might be more expensive than having the original rewound. Or you could use two transformers, or something. 

[ericj]

In theory, this r-core could work.Might have to reorient the main power filter caps to make space for it. I'm pretty sure if i read correctly it has a pair of 220v 50ma and a pair of 6.3v 0.8a. 

https://www.aliexpress.us/item/2251832814206073.html

Edit: I checked. It'll fit in the chassis in either orientation but it *will block the holes for the original main filter caps. Modern caps are no bigger in diameter than the original axial caps on the board, so you can mount them flat to the board and use that space.

With 2x800mA 6.3v it also has plenty of heater current for ecc99 and probably enough for 6n6p. 

I know Spritzer says that there's no place for hot glue in electronics, but if you use the high-temperature stuff it's Firm Enough to hold stuff even in adverse environments, and with a squirt of isopropyl it will give up and come off so I don't see the downside to using it to secure stuff. 

Edited Sunday at 09:49 PM by ericj

[ludoo]

I'm in Europe, so 220v are easy for me.

[Oneguy]

Just subbed and thanks for creating this @ericj . Will follow your lead when I get the time or when mine starts to show its age. No issue now though. 

[ericj]

I think 5.1k safety resistors would serve no purpose in this amp. There are already a 51k resistor and a capacitor between b+ ("A") and the outputs. 

Mouser has replacement slots. I ordered 1. They have 10 left. No idea if they would re-stock after they are gone - nobody else seems to have them in stock. 

https://mou.sr/3D1kgOo

Good glue and a zip tie seems to be able to repair cracked slots. I repaired both of mine that were cracked at the bottom and i figure the top end will crack some day too. There's very little chance that i will ever have a use for the phono preamp. It's fun to imagine what else you might fit into that spot. Crossfade filter? would need its own preamp. 

Should be possible to rewire the rotary switch if you simply wanted multiple inputs. 

Speaking of, the RCA connectors on mine are in poor shape as I am sure are others. I'm sure there was a time when you could just buy a panel with four RCAs on it from any vendor but not anymore. deoxit and some elbow grease is probably enough but, we can do better. 

Due to the way it is manufactured, most RCA jacks won't mount to the existing panel - holes are too big. 

Maybe these jacks which are secured with an external nut can replace carefully removed 50+ year old tin jacks: 

https://www.aliexpress.us/item/3256805498973752.html

I ordered a pair. I also have basic CAD skills and a laser that will cut plywood, so it would be no big deal to manufacture replacement panels to accept contemporary jacks. 

[spritzer]

The 5k1 output resistors are very much necessary, you clearly have no idea why they are there. 

[ericj]

5 hours ago, spritzer said:

The 5k1 output resistors are very much necessary, you clearly have no idea why they are there. 

OK. Did i claim to? 

I can see where a mosfet shorting out in a solid state amp would be very bad. That's not a typical failure mode for a tube. 

Stax used one 5k1 output resistor per channel in the SRD-5 but not in the 4, 6, or 7. I've seen threads where you said you wouldn't put them in a transformer energizer. 

So, enlighten us. 

[spritzer]

You said they would serve no purpose, that is clearly wrong.  Stax never put any protection resistors in any SRD unit (or anything else for that matter) as they weren't introduced until 1994 with the T2. 

I don't need to enlighten you about shit but I will call out stupidity when I see it. 

[ericj]

4 minutes ago, spritzer said:

You said they would serve no purpose, that is clearly wrong.  Stax never put any protection resistors in any SRD unit (or anything else for that matter) as they weren't introduced until 1994 with the T2. 

I don't need to enlighten you about shit but I will call out stupidity when I see it. 

The SRD-5 does have them. Here's the schematic. Do you want to see pictures of the board in my SRD-5? 

[ericj]

Tell me what green brown red gold means to you and how it differs from output safety resistors on later products. 

We don't deserve to know why but these matter. splice them into the output wires and seal with heatshrink - the traces on the sra-3s mainboard are far too fragile to support extra components. 

[spritzer]

You think those are output protection resistors... on half of the signal?  Yup, makes sense.    Notice the cap there, with the resistor?  Might that be a fucking RC network?