Studer A800 audio noise from PSU

[Pat Maki]

Future Electronics and Rochester Electronics are not "primary" suppliers, but they often have NOS for obsolete parts. I've bought some odds and ends from them over the years and they seemed OK. Can't 100% say they don't sell counterfeits. Rochester seems to specialize in NOS.

https://www.futureelectronics.com/p...s--bipolar-transistors/2n5884g-onsemi-1429786

https://www.rocelec.com/part/01t4w00000NZ6jyAAD-2N5884G

The picture at Rochester is wonky but the specs seem correct. I also don't know what MOQ both companies currently have.

Bri

Thanks Bri, I hadn't found these guys before. Interesting Rochester is a hotbed for electronic components. Looks like they have a $250 min spend. Will give them a call tomorrow to see if there's any flex on this.

 

[Pat Maki]

BTW, Jameco is another source I use for NOS. In the past few years I've bought a "smallish" inventory of 74xx TTL logic chips from them for maintaining some Ampex ATR-100 and MCI JH-110 recorders. Alas, Jameco didn't have the 2N5884.

Bri

Ooooh...those are both beautiful machines! Thanks for checking Jameco, found them when repairing a Lexicon Primetime this year. They seem legit.

 

[Brian Roth]

As I mentioned, I don't know current MOQ for those vendors. I suspect Future may have a lower threshold.

Bri

 

[Brian Roth]

I just checked Onsemi's website and they state the MJ4502G as the replacement.

https://www.onsemi.com/PowerSolutions/pcnPub.do?pn=2N5884G

Click on the link at the left side of the top entry and that new part number pops up. It's actually somewhat beefier if you look up the specs.

Bri

 

[Walrus]

Would a MJ15004G also be a suitable substitute?

 

[zamproject]

Maybe little short ?
The original one is rated for 25A max
Studer designers probably over size it, still it's preferable to not downsize, especially as junction have failed over the time.

Don't know how this power transistor is mounted on this card, I suppose on some radiator, make sure for clean surfaces, eventually with past or mica sheet, and tight mount for max dissipation.

 

[Walrus]

Just checked and farnell UK have 13 2N5884G in stock still, also have the MJ4502G too.
I know we here can buy stuff from Farnell USA stock (at a premium postage charge!), is there a reciprocal system?

 

[Brian Roth]

Mouser shows MJ4592G stock.

https://www.mouser.com/ProductDetail/onsemi/MJ4502G?qs=HVbQlW5zcXXll1Oe0dMDgg==

Bri

 

[Pat Maki]

Mouser shows MJ4592G stock.

https://www.mouser.com/ProductDetail/onsemi/MJ4502G?qs=HVbQlW5zcXXll1Oe0dMDgg==

Bri

Thanks @Brian Roth! I'd done some replacement searching but never trusted those databases on google...that said, never tried searching the actual component vendor's site (big duh on my part!). Got a handful on order. This looks to be the right TO-3 package and all. Not cheap, but worth the investment given the critical role it plays in this machine. Thanks also for sending the future link. A much better price but alas, they have a 200 min requirement. Not sure I could pull together a bulk buy to get that many! (-; Thanks also for your inputs @Walrus and @zamproject

 

[Pat Maki]

Maybe little short ?
The original one is rated for 25A max
Studer designers probably over size it, still it's preferable to not downsize, especially as junction have failed over the time.

Don't know how this power transistor is mounted on this card, I suppose on some radiator, make sure for clean surfaces, eventually with past or mica sheet, and tight mount for max dissipation.

Thanks @zamproject. The two screws mount on directly a heatsink substrate and there's what looks like a gasket spacer between the heatsink and the body of the transistor. The gasket material reminds of an Avon rubber raft material. Seems to be doing a great job as it shows no deterioration after all these years.

 

[kidb]

If not done already, change those tants out for lytics while you’re in there

 

[juanito2008]

If not done already, change those tants out for lytics while you’re in there

Good point. If you don't do it, you will have some smoke quickly.
I advice you to change the cable between the rectifier and the lytics. They get corroded by the heat of the bridge.
To check it, Move the wire on the rectifier side. If the insulation breaks change the wire.
If your power supply breaks quite often, specially when you don't use it for a long time, change the lytics that are behind the audio cards on the mother board.
If you have a back ground noise that appears and disappears when the studer is hot, change the molex from the power supply.
There is quite a lot of things to do on those power supply. That's the weak point of the Studer A800.

 

[zoji12]

Looks like Quest Components is your best bet for onesies. Most places want you to order 100 for $67 or 1 for $67. Quest has various manufacturers for about $6.70 EA

 

[Brian Roth]

Quest is a new one for me which I'll keep in mind. I do see:

1. Minimum order $25.00
2. Minimum line item total of $5.00
3. Flat rate shipping, but least expensive option is $15.00
4. No clear description of order processing time (Mouser ships same day)

Nevertheless, another name to add to where I'll check in the future!

Bri

 

[Cqwet Dbdfte]

That 25A transistor has low gain, and the driver is also at risk if it melts.
One could look for a better alternative in a TOP-3 or TO-247 case.
Glass passivation was not a thing way back.
There is nothing magical about the original transistor.
A likely failure cause could be transistor lost thermal contact with heatsink.
Unless spring loaded, screw contact can loosen up with thermal cycling.
If the equipment is valuable, designing better voltage regulators may be in order.
Maybe SMPS followed by linear post regulators.
There are much better parts available these days.
What is the current load on that 25A transistor?

 

[Walrus]

The +-15V rails are fuse at 6.3AT pre bridge recs.
Never measured the actual current draw.

 

[Cqwet Dbdfte]

Are the rails powering motors?
It is hard to believe low level amps need 150W.

 

[Walrus]

No motors, just cards for repro amps, sync amps, record amps and bias drivers. Each psu supplies 8 channels worth of cards.
I imagine the switch on surge will be quite high, so it has to be fused with that in mind.

 

[Cqwet Dbdfte]

Power draw without motors running can be checked with one of those handy "kill-a-watt" plug in the wall meters.
The storage cap will have some surge current, but 6A ?
The cause for the failure may be thermal. Should be checked out.

 

[Pat Maki]

If not done already, change those tants out for lytics while you’re in there

Thanks @kidb parts arriving today and have some tants here that'll go in!