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 yesterday at 02:47 AM by ericj