Peavey repair- change out one of a group of outputs?

[Conrad Hoffman]

Fixing a big old Peavey PV-8.5C amp for a friend. I located the shorted NPN and PNP outputs in one channel. They're 70473180 and 70483180 Peavey house numbers. Reading various posts, it seems a suitable replacement is the MJ-15024 and 25. There are four NPNs and four PNPs wired in parallel groups, with a gain of about 100 (20 mA base gives 2A collector current @ 10V). What are the chances that just replacing the single shorted devices will be OK; how sensitive do you think the groups will be to Hfe will be using 0.33 ohm emitter resistors? Obviously there's a huge cost difference if I have to replace all of 'em.

[Andrew Eckhardt]

" What are the chances that just replacing the single shorted devices will be OK"

Maybe about 20%.

[Zero Cool]

while it WILL run...it just wont last long during real use.

[infinia]

well you can spend the time trying to beta match them to the old ones (unlikely with out a whole groupings of parts), or most would replace a rail at a time from a "lot" with same date codes. the age old adage "time is money" to most. If it's easy to service you could take a chance and check the balance of Vre's at elevated idle temps and decide from that.

[llwhtt]

I used to do warranty work for Peavey and asked that question many times, always told just replace the bad ones. This was on warranty and non-warranty repairs. I don't particularly agree but I never had any returns because of it. To me the outputs are cheap enough to just shotgun them all. Also I used to order all my Motorola/ON semiconductors thru them as they were by far the least expensive but don't think that is the case anymore. You may also consider the PITA factor to replace outputs in the whatever amp you are repairing. If it truly is a PITA replace them all.

Craig

[Conrad Hoffman]

Interesting range of replies! Time isn't a big deal here, so I think I'll order a few and put them on the curve tracer with the originals. If they aren't close, I'll just order more. I wonder if Peavey even bothered to match them in the original production. BTW, I've measured a fair number of power devices and can reliably say that coming from the same batch means nothing. If you truly need matched devices, you have to match them. It can also take a remarkably large number, same batch or not, to come up with say, 8 or 10 that match. Other times you just get lucky.

Being somewhat a-retentive, once I decided to replace all the devices in one channel I'd have a hard time not replacing the other channel, as I don't think the voltage or current margin is all that great on the original parts, nor do I like steel cases from a thermal standpoint.

Thanks!
CH

[infinia]

Originally Posted by Conrad Hoffman

BTW, I've measured a fair number of power devices and can reliably say that coming from the same batch means nothing. If you truly need matched devices, you have to match them. It can also take a remarkably large number, same batch or not, to come up with say, 8 or 10 that match. Other times you just get lucky.

so that blows up some peoples (mine) ideas. seems you should/can get a fairly tight groupings with a mature semi process right off the factory floor, but OTOH I'm also sure a weeks production at ON Semi includes lots of different wafers not to mention how the distrib's hand them out. Be interested in any data you can post, esp currents and instrument used.

[Conrad Hoffman]

No data on recent devices, so maybe things have improved over the years. I have a Tek 575 122C mod curve tracer (OK, boat anchor) that works well and I've always used it to build up power amps. That a-retentive thing makes me test every singe component I use. Back in my Tiger amp days I discovered the THD could be quite dependent on matching of the devices and it's really tough to get the PNP and NPNs close. A friend and I kept buying devices hoping for a decent match. We sorted both flavors by hfe at moderate currents. Most of the tested devices were MJ802, 4502, the similar TI devices and various TIP series for some blameless amps I built later. Quantities were typically 10-20 devices. I had about as much luck with different batches as I did working within one batch.

One thought that occurs to me is that getting the NPN and PNP to match required working near the edges of the distributions. The usual scenario was to blow one or the other, then have zero luck finding a similar one to replace it, so we'd order another batch and hope for the best.

My guess, and it's only a guess, is that devices are not the same across a given wafer, much less wafer to wafer. I also wonder if they cherry pick devices to meet certain companies special order hfe requirements, as I often found devices in amps and equipment that couldn't be replaced even from large batches of supposedly the same jedec number- way off the bell curve. Or, I could just be remarkably attractive to bad luck.

[infinia]

I recently bought 50 ea of MJ15024/25 same date code. but my home made beta tester is buried out there somewhere, mostly broken. so... if I get the gumption I might use a cheap DMM and rig a TO-3 socket. I might trade for some ON drivers and Fairchild TO-216 and small signal stuff.

[Elvee]

Originally Posted by Conrad Hoffman

Fixing a big old Peavey PV-8.5C amp for a friend. I located the shorted NPN and PNP outputs in one channel. They're 70473180 and 70483180 Peavey house numbers. Reading various posts, it seems a suitable replacement is the MJ-15024 and 25. There are four NPNs and four PNPs wired in parallel groups, with a gain of about 100 (20 mA base gives 2A collector current @ 10V). What are the chances that just replacing the single shorted devices will be OK; how sensitive do you think the groups will be to Hfe will be using 0.33 ohm emitter resistors? Obviously there's a huge cost difference if I have to replace all of 'em.

What counts for the sharing is not the Hfe (not directly anyway), but the collector current resulting from a fixed and imposed Vbe.
Not something usually measured.

Obviously, due to to the technology of semiconductors, there will generally be a correlation between this parameter, the Vbe and the Hfe, but correlation is not the same as equivalence.

But with 0.33R emitter resistors, each mV will translate into only 3mA unbalance; not alarming in my opinion.