High current TO220 diodes

[ceharden]

I'm looking for an alternative to the MR2402 diode for use in repairing a Crest CA12. It's a 200V, 24A device in a TO220 package.

The replacements from Crest are seem rather over-priced and the diode doesn't look anything special from the specs, just a general purpose rectifier diode. It's used for commutating between the rails (class H amplifier).

I was wondering whether I could use something like the IXYS DSEP29-03A. Datasheet here: http://www.farnell.com/datasheets/51865.pdf

30A, 300V, fast recovery rectifier. Anybody see a problem with that choice? About £1.50 each compared with £8 for the old MR2402 (which I can't find for sale other than from Crest themselves).

[EchoWars]

Looks like a fine replacement. Why are you assuming there might be problems?

[ceharden]

I always have in the back of my mind that there might be some reason why the manufacturer chose that particular component. In reality, cost is probably that reason in a lot of cases!

I'm not that knowledgeable about diode technology and what types are suitable for what jobs. Just have to trust the datasheets I guess.

Especially as this is not my own amp, if it were my own, I wouldn't mind so much using what I think is the closest alternative.

I'll give the IXYS one a go, certainly looks the business.

[wg_ski]

The MR2402 is rather dated. And so are the hexfets they use. The CA amps are an older design that's been around a while and original parts are going to be overpriced. I've used IXYS, Fairchild, and IR/Vishay ultrafast types in my class H prototypes and they've all worked fine. Chances are anything that fits and has sufficient ratings will do.

[megajocke]

Those look like good replacements as long as they fit physically.

Does anyone know a good replacement for the NEC 2SC2336 and 2SA1006 pair used in these as VAS and predrivers? I have thought about the Toshiba 2SC4793 pair but they are only 230V parts but I need 250V as the rails are +-120V in the CA9 I'm repairing right now. I'm going to try replacing the predrivers with MJE1503x parts, but those are a bit slow and have a little low gain for VAS duty if those were to need replacement.

ceharden, which preamp board does your amp have? If I understand it correctly the CA12 has the same preamp board as CA9. In the CA9 I'm repairing there is a bug on the preamp board which causes the clip limiter and current limiter to stop working. They fixed it in a later revision of the preamp board by adding a darlington transistor. It seems current pulses from charging a capacitor in the clip limiter destroys the internal transistor of the OTA chip.

[wg_ski]

MJE15034/15035 seem to work just fine for VAS duty. They have higher gain than MJE340/350. For predrivers, the 230V parts will work in a class H stage, because the needed VCEO is high rail plus low rail, not 2X high rail. But I'd just use the 15034/035 ayway.

I popped the hood on one of my new/used CA18's. The predrivers (and VAS) are MJE13007 and MJE5851! Those are like TIP series parts in terms of gain, linearity and SOA. But they work. With a triple darlington and a buffered VAS, you can get away with a lot. They just didn't have 15034/035's or anything like them when those amps were designed so they used what they had.

[megajocke]

These run the predrivers from the high rail all the time so high voltage is needed there. Drivers and outputs run from the cascoded rail though. I had forgot that the bigger CA:s used those parts you mention The smaller ones will probably be okay with MJE1503x parts then...

[ceharden]

I was going to ask about the driver transistors actually. I've been buying the actual replacements at great cost so it's good to know I can replace them with the modern equivalents (what I use in my own designs) without any issues.

[wg_ski]

Originally posted by ceharden
I was going to ask about the driver transistors actually.

Yeah, just you try finding a 2SC4029 (that ain't FAKE) for a CA18 You pretty much have to use MJL21194's.

[ceharden]

ceharden, which preamp board does your amp have? If I understand it correctly the CA12 has the same preamp board as CA9. In the CA9 I'm repairing there is a bug on the preamp board which causes the clip limiter and current limiter to stop working. They fixed it in a later revision of the preamp board by adding a darlington transistor. It seems current pulses from charging a capacitor in the clip limiter destroys the internal transistor of the OTA chip.

Sorry, I missed this the first time round.

This example seems to be a bit of a Franken-amp. It's an older chassis but has been retro-fitted with the newer surface mount PCB preamp board. I think it's been badly repaired before so it's going to take some time to return it to full working order.

I've got a CA9 under the bench to repair one day, it's taken out two sets of replacement output transistors so far Admitedly, one set was an error/accident on my part.

What is most annoying about working on the CA series is there have been so many revisions that the part numbers on the schematics available from Crest just don't match up.