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My SA - 7100 Drops in and out of Protection

New Components Have arrived !

Now I have all the new components I need to do this, I have some questions before I begin. I want to get prepared properly before I tackle it and hopefully keep it mistake free. All advice is appreciated.

Qu.1. What wattage soldering iron is required for this delicate situation ?

Qu.2. Do we trim the legs from the new capacitors, etc before or after soldering?

Qu.3. Does the heat generated from the soldering process degrade the components, or do we need to shield them somehow ?(I have read that alligator clips placed on the underside work well as a heat sink. Is this practicle with these components given the clearance to board ?

Qu. 4. I am looking to buy a DC power supply. What spec should I keep in mind when looking?. I was thinking either a 30v 5a, or 30v 10a. Why would you buy either? Which would suffice for this kind of work?

More questions to follow. Thanks.
 

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1. 30w but temperature controlled, it's a few bucks more, but you won't be fighting the iron or burning pads loose (unless you really try) and it's a long term investment, ge6t one with replaceable tips (but you'd REALLY have to try hard to find one that doesn't have replacable tips).

2. trim after soldering usually, It's always easier to remove wire than to ADD wire!!!

3. components are generally specified for how long they are expected to resist soldering heat, most are for several seconds, so don't hang around on a joint. There are exceptions, like the polyester RIAA caps in (usually) the phono amp, they are silver bodied, and have clear ends, THOSE you DO have to clip a heat sink onto the leads first. Also thermal fuses. a small locking pliers is dandy, or if you want to splurge, get a set or two of surgical hemostats, they lock nicely.
The heat sink HAS to be between the heat source and the component, so my default it HAS to go on the component side of the board, and you need to LEAVE the clearance for it, that's why the hemostats are nice, long thin jaws.

4. you are not really at the level where power supplies are that useful, and 30v hasn't been enough for me when I wanted to use them. I have a 30v 3a el cheapo, and a 24v 12a H.P. , and I almost never had need of that amount of current.
What I did lately is modify a SX-1250 regulator card to a fixed current limit, which gives me 65v, and isn't as easy to blow out as the unmodified card.
given my druthers, I'd get an adjustable 90v 500mA supply or two, with adjustable current limits, but good luck on finding them.I'll probably end up building them, from scratch (when I find (haha)the time)...
 
Thanks again Mark for your valuable input.
The reason for the qu on the DC supply was for when we need to adjust bias current. Correct me if i'm wrong, but I recall reading somewhere that a DC voltage supply was required to help effect this. I know I'm jumping too far ahead of myself. I think I'll come back to you when I get the boards sorted first.
Cheers
 
Awesomeaudio said:
Thanks again Mark for your valuable input.
The reason for the qu on the DC supply was for when we need to adjust bias current. Correct me if i'm wrong, but I recall reading somewhere that a DC voltage supply was required to help effect this. I know I'm jumping too far ahead of myself. I think I'll come back to you when I get the boards sorted first.
Cheers
Click to expand...

You only need a DVM for that.
 
no external power supply needed to set dc offset, OR idle current. or anything else I can think of... in the silver faced pioneers....
 
Ok, Roger that.

Regarding the soldering iron, I have the standard plug and use type, but I will purchase a temp controlled unit. I've seen these on Ebay. I might look elsewhere too.
I gather this is to regulate heat as you go smaller in components??
 
I don't mean to butt in, but one of your photos shows four MJ21194G transistors; IIRC the shopping list called for two each of MJ21194G and MJ21193G. I hope there wasn't a mistake in your order?
 
ezhik said:
I don't mean to butt in, but one of your photos shows four MJ21194G transistors; IIRC the shopping list called for two each of MJ21194G and MJ21193G. I hope there wasn't a mistake in your order?
Click to expand...


GOOD eyes!! the one board has both channels on it, not two separate boards.
 
ezhik said:
I don't mean to butt in, but one of your photos shows four MJ21194G transistors; IIRC the shopping list called for two each of MJ21194G and MJ21193G. I hope there wasn't a mistake in your order?
Click to expand...

Wow ! Great spotting. Thanks for pointing this out. I've just checked my order and I don't see the (MJ21193G) anywhere. I did miss these. Don't know what happened.
Dumb question, Can I install all 4 of the MJ21194G ?? or I will have to get the other 2.

I actually have two items from my order on back order, I can still contact them and maybe get these tacked on.
Thanks for your interest.
 
they are opposite polarity, they will NOT work in the other two positions.

Yes, you need to get them.
 
Pin connection Question

I'm just about ready for surgery. Just making some final preparation before I commit to this. I have a question relating to those connecting wires/pins on the boards. Some boards I can avoid undoing these but others will require removal. What is the best way to remove the wires without making a meal of it ? How are others approaching this ?
Thanks
 

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Undo all the wire ties (bundle ties) and make a lot of slack in these wires. You may get enough so that you can rotate the board enough to desolder, clean and solder.

LAST RESORT is removing the wire from the pins. It takes a special tool and a LOT OF PRACTICE to do it right. 1 step up is desoldering the pin from the board and removing the pin (with the wire attached noting the hole # on a piece of tape attached to the wire) . Then clean up the remaining solder from the board pad and the pin.

If at all possible, leave the wires intact. Any time you cut them, it shortens them and some are tight enough now that you don't have much if any slack.

Larry
 
just an FYI, when I did mine, all boards can be flipped and worked on. I only unwrapped to do some testing.
 
I have managed to never need to unwrap, I DESOLDER the pin from the board, they push through nicely when the solder (slobber?) is melted. Might have to reheat to insert because of hardened flux altering the hole dimensions..

Careful, the wires are preconditioned to BREAK, (ALWAYS short too!!) by the mechanics of the wire wrap - the square corners end up with TONS of force nicking the SOLID wire with gastight cuts. just a little flexing and there she goes!! A good wrap should have a stress relief un-nicked turn between the nicks and the wire going elsewhere.
 
Thanks for the info guys. I'm going to start on the control board first and work my way around. I'll report back soon.
Cheers
 
Phono board QU.

Just remembered something.

On the Phono board, I noticed on the schematic that C3 & C4, C13 & C14 are not of the same value to those "actually" on my board.
I did notice this a while back but forgot about this when I ordered the bits, and as a result I have the wrong caps for these, well maybe at least two. I have ordered the ones as per the schematic.
I need to order the caps as per my board.
The schematic shows C3, C4 as 100/35. C13, C14 as 220/16.
On my board they appear as C3, C4 110/10. C13, C14 330/16.
I guess I can use the 100/35 in place of the 100/10, but definitely NOT the 220/16 over the 330/16.
Question is, would someone kindly provide the Mouser part number for these caps please. I'm not sure on the finer detail and hence am relying on the learned people for this.
Thanks
 

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