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.

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

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.

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.

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.

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.

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.

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.

That sounds reasonable, maybe in the diy subforum?

I would just leave the ties in place. There's no downside. The only reason I'm not tying the other end is because that might just mean it breaks somewhere harder to fix.

Ah, i used ~22awg solid copper and twisted the ends until it closed the sides with the superglue. I guess the drawer full of wire was closer than the container of zip ties. I kinda feel like cyanoacrylates aren't what they used to be -- "bonds instantly" used to mean i didn't have to clamp it for half an hour. sigh. I don't see evidence that there was foam holding them in at any point but i have some dense foam i will install. All of the resistors in my unit look fine to me. Nothing ran hot enough to show. I suppose i should lift a leg of the one big carbon comp resistor to see if it is still in spec. Can anyone fill me in on the adjustment procedure for the trimpots?

When i pulled the main board out of my sra-3s further so i could work on it more comfortably i finally noticed that the reason the cards fell out in shipping was probably the fact that both slots were cracked at the bottom end. I've fixed them with superglue and wire ties, but it looks like you can still get a direct replacement: https://mou.sr/411lhxZ

So while I'm here, i have the ksc1845 transistors that are the recommend current production replacement for all of the transistors in the sra-3s. Should i just replace the driver transistors on principle because they are likely lower-noise? There is very little risk that i will ever use the phono preamp.

I'm updating the SRA-3S i bought recently. It was pretty cheap ($65) in as-is condition untested, so they said. I've never powered it up. I read through the thread in the diy subforum and saw Spritzer's recommendations. Looking at my driver board, I think I don't have enough capacitors? There are four small ceramics that just have a "5" on them which i presume to be 5pf, and appear to be in the positions in the schematic that specify 10pf. I'm getting some mica caps to replace those. But where are the four 0.05uf caps that he recommends to upgrade to 0.22uf? There are two ceramic blobs on the back marked "5-D" that i don't really understand, unless there were supposed to be two more of them? I suppose i should desolder one and measure it. The trimpots were removed for replacement. EDIT: Nevermind, I'm a dumbass, they're on the main board.