QSC 1400 Question

[Techknowman]

I looked through the forum to make sure I wasn't replicating an old post and I didn't find any solid answers there.

I have A QSC 1400 that the soldering on the bottom of the board was very thin and many of the output transistors and main caps arched and burned the traces around the cracked out solder. Many of the transistors have failed. anyway that all can be repaired but I'm left with this question.
This unit looks like its straight from the factory, meaning It doesn't look like its been repaired anywhere else before but...
The output transistors are MJ21194 and MJ15025s installed these are not really complementary?
Shouldn't it be MJ21193 and 94 sets or MJ15024 and 25 sets?

The schematic shows 2SB554 and 2SD424s

Thanks in advance

BTW this amp looks almost brand new. very clean internally and externally

[Enzo]

Well, it is not THE schematic, I know of at least three for the 1400. When the 424s became obsolete, they swapped them for the MJ series, no real changes there.

Oh sure, the MJ21 series is more the complement than the MJ15 series, but they are very similar. What you don;t want to do is mix the types on the same row of transistors. You don;t want a row of MJ15024 with an MJ21xxx in it or a 2SD424. But it is less important to have MJ15025 facing the MJ15024 across the polarity divide. You could operate with 2SD424 on one side and MJ15025 facing them. I mean in the same channel. The only concern is when the parts are in parallel, will they share current and track thermally. The ones on the +V side do not share current with the ones on the -V side, they conduct alternately.

[tedmich]

Enzo guided me through a recent QSC 1400 repair. The amp was in production for ~ a decade and looked pretty much like a USA 900 model (except the input PCB) toward the end of its production.

Here is the thread:
http://music-electronics-forum.com/t20867/

FYI for anyone trying to repair older QSC amps, their service manuals are available online for free here:
http://www.cinematech.info/Sound/Amp...als/svcman.htm


they are zipped files and have a password requirement, but the word "power" worked for all the ones I downloaded.

[Techknowman]

Thanks.
This is the third 1400 i have worked on. one looked similar to this one but older and everything was brown inside the other was labeled as a SonitiK it to was all brown inside. This one is sparkling clean.
The service manual and schematic came directly from QSC and the 2sb and 2sd numbers were on them, however the bill of materials listed QSC part numbers for them and I would suspect if ordered from QSC you wouldn't get any 2sb 2sd's.
probably get MJ series. So if its OK i'll use MJ15024 and 25
I'm going to replace all the outputs on this one, good or not because of the damage. its just not worth the risk.
Anyway one channel has both drivers blown as well, so im replacing both channels there.
pulled and checked the bridge diodes, ok
pulled the caps, wasn't hard some just came out due to cracked out solder. 1 cap is open and 1 cap reads low (75%) the rest check OK but I may replace those as well.
I'm going to check all the bias diodes and pcb mount pots from the post you guys show in the previous post.

[tedmich]

as per Enzo's previous post the PCB trimmers often fail and pull the rails high, the dark trimmers are (I believe) the old ones; newer ones are red from QSC and higher values in some cases. A little heatsink grease on the slip on TO220 driver's is good insurance IMO and at ~$1.50 each new drivers are AOK. QSC mentions no problem mixing output transistors in some cases; using same lot, replace them all is often overkill IMHO.

[Techknowman]

Yea I always put the silicon heat sink compound on the drivers. even when I don't replace them. I pull the heat sink and tighten it to fit the to-220 a bit better and then put compound on it and slip it back together.
On the outputs, I'm just worried about the ones that pulled the load of the missing ones till the traces failed. I may re-evaluate that position. The way this unit failed, I don't know. The last two I fixed just lost one output on each rail and blew the fuse.

[Enzo]

All three of the schematics LOOK similar, just little details differ, and often that is what matters so look at the layouts. INparticular the number and exact location of the little trim pots is how I decide if I am looking at the correct drawing.


No, they will not send out 2SD424s, no one makes them anymore. When your schematic has 424s in it, I worry that the other details will not be right. Whatever the thing started life with, it will be perfectly happy with a complete set of MJ24s and 25s. Just as I said before, don;t stick an MJ in the row of 424s.

[Techknowman]

I'm on it!
Thanks!