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Pet Peeve Peavey (GPS 3500)
Pet Peeve Peavey (GPS 3500)
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Old 8th March 2017, 05:33 AM   #21
PRR is offline PRR  United States
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> why does there seem to be a ground symbol on what I would think is the speaker output?

We normally run power output stages as emitter-followers, which is really Grounded Collector, ground at the battery CT. This is unity gain so it needs a hundred volts of drive signal.

We can also run Common Emitter. Ground at the emitters. The collectors fling the battery around and the battery CT is the "speaker hot". This only needs a few volts drive.

The difference is that in CE mode the driver is a 19 cent chip instead of some messy discrete collection.

The drawback is you can not run two channels from one battery.

Because this is wall-power, the conceptual "battery" is a PT winding, FWB, 2 Caps. All this must be duplicated for *every* channel.

At this insane power level, they would probably need many-multiple rectifiers and caps anyway, so the alternate topology does not add big-stuff cost and does reduce cost in the small stuff.
____________

I used to fix blown-up power amps for salary. _I_ would not begin to attempt this brute, especially with previous repair failures. Too much brain-work and smoked parts to suss-out what is *really* wrong. 3,500 Watt amps with warranty are not "that" expensive today (he says with a shudder).
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Old 9th March 2017, 03:22 AM   #22
wg_ski is offline wg_ski  United States
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Yet still another flying rail amplifier. In this case a big one. These amps weren't around very long - perhaps there is a reason. From what I've seen PV's flying rail amps go off like a brick of Black Cats when something goes wrong, many times taking out the whole thing. "Empty cans and charred remains" like Indianajo's PV 1.3k. You're lucky if just a couple of power transistors went out. The QSC (and clones) implementation is much simpler and less subject to catastrophic cascade fails.
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Old 9th March 2017, 04:19 AM   #23
Enzo is offline Enzo  United States
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QSC simpler? having been authorized service centyer for both brands, I feel exactly opposite. QSC circuits with commutating rails controlled by little surface mount housekeeping boards.

My experience is that when an output Xstr shorts C-E, one part blows, and maybe one on the other polarity. Might take out a driver. WHen one shorts B-C, then it takes out the entire row.
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Old 9th March 2017, 05:08 AM   #24
wg_ski is offline wg_ski  United States
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The circuit in question here is also commutating rail. Added onto a topology that some would question the wisdom of. IMO, there are only two reasons to do a flying rail - a) you really like lateral mosfets but you don't want to pay a voltage swing penalty, or b) you use CFP that you can directly ground the collectors of. This is neither, and only makes things more difficult than they need to be. Two power supplies, more crosstalk, and you don't even get to bolt the trannies to the heat sink without an insulator. And it's track record isn't as favorable as a standard Lin. Or the QSC flying rail.

The rail switches that I've seen the least amount of trouble with with is the 311 comparator on a bootstrap. When things do go wrong, there just seems to be less collateral damage. I've put those on 'standard ' amplifiers, and are almost fool proof. I've seen one and only one case where they just wouldn't work (and to this day never could figure out why not).
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Old 9th March 2017, 12:17 PM   #25
Peter S is offline Peter S  Canada
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Thanks All, for all the info!
I have only just grasped the Class H concept...and now 'Flying Rails'! I hate to reveal my ignorance (further) but I was actually thinking
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Old 9th March 2017, 01:59 PM   #26
Peter S is offline Peter S  Canada
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Thanks All, for all the info!
I have only just grasped the Class H concept...and now 'Flying Rails'! I hate to reveal my ignorance (further) but I was actually thinking the ground symbol on the speaker output was just a link to some other connection point (or even a typo!).
I have a Crest CA-18 which has been tamed for home use (quieter fans installed and the noisy torroid dipped 20 times in electrical varnish)--I compared schematics and see that the Crest shares a common power supply for the low rails of channel A and B.
Indianajo;
Thanks for your info re 'flying rails'. It will take a few more coffees to absorb this, but at the very least, you have saved me tremendous confusion, right from the beginning. The next time I'm looking for victim, I will search for the schematic, not just the specs. If the primary advantage of this configuration is bridging, then I'm going to avoid it like the plague, if for no other reason than reduced damping factor.

PRR;
Thanks for your in-depth explanations also. I'm a little discouraged by your final remarks, but I bought this amp as a learning experience (and to be honest, I was hoping the repair might be something simple!) I've already learned a lot, and I'm not going to give up until it's an unidentifiable smoldering mass on the bench!
Enzo;
Thanks for your concise guidance on this project even before I have done the very basic 'legwork'. As soon as the boss's (wife's) head is turned, I will compare all devices, in circuit, comparing chan A to the functional chan B.
wg_ski;
Glad to have your input on this project, and thanks for your advice on my other projects. This amp has not appeared to have failed in a catastrophic manner---but I'm not done yet! BTW, with Class H amplifiers, is it not necessary to have devices insulated from the heat sinks anyway?, because of the multi-tiered rails, or am I 'out to lunch'.
BFN, Peter, in Hillsdale ON, Canada
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Old 9th March 2017, 11:10 PM   #27
Enzo is offline Enzo  United States
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Like any circuit, the insulation matters if the circuit demands it. QSC amps had their transistors "inside out", meaning the collectors were at the speaker common and emitters towards the powr rails. So with the "output" grounded for flying rails, we could ground the collectors, which is the part on the heat sink. But if the emitters are the common element, then the collectors still must be insulated.
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Old 9th March 2017, 11:25 PM   #28
AudioFreak88 is offline AudioFreak88  United States
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You Blew apart one of those Thermistor's? Lol
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Old 13th March 2017, 01:44 PM   #29
Peter S is offline Peter S  Canada
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Latest Update:
Tested in circuit it seems that Q 113, 115, 117, 119 and 121 are all dead shorted, Base to Emitter. This is the entire low rail PNP bank. Q 117 also appears to be shorted across all 3 pins. These five outputs where previously replaced.
I have read about "counterfeit" 2SA1302 transistors, but the bad outputs look identical, in every way to the remaining original transistors, except there is small arc bent into the leads, (I presume to allow for thermal expansion, avoiding stress on the PCB?).
OK: On removing 3 of the outputs (the other 2 to be removed anytime now) It seems that 2 of transistors are still good....I feel foolish for not realiziing that if one device shorts, they all appear shorted! (at least, Base to Emitter, I think).
I was about to smash open one of the defective trannies, and thought to do a quick, 'out of circuit test. It was still good! I will attempt to open the shorted device to compare it to an on-line video showing the so-called 'counterfeit' device's smaller chip size.
If the replacement devices where authentic Toshiba parts, then I am banking heavily on the problem being non-matching (un-balanced current sharing) in the second set failing.
I will draw indianjo's test rig, for selecting matched transistors and attach, asap....There is also some question as to the best substitute for these units, I read that they have been discontinued. Any input on the 2SA1302 debacle would be greatly appreciated!
Thanks again, Peter
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Old 13th March 2017, 07:07 PM   #30
indianajo is offline indianajo  United States
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Lacking input from Turk182 Enzo or other amp repair pros, I'd say attack the obsolete part problem by buying from Peavey parts dept. If they will sell you a few, then they think their matching program is good enough. If they insist on the whole set of channel outputs being replaced, then that proves they didn't solve the obsolescence problem either.
IMHO requires a phone call, I don't think they respond to internet orders.
I replaced all 20 outputs on my PV-1.3k with something cheaper than MJ15024/25, realizing that the bias spreader resistor value suggested by peavey might be wrong now. I used MJ21193/4 which is about $1 each cheaper with better SOA.
I have to say if you didn't do an Iceo test on each output to check if the junction is stessed, you don't know how many bad ones you really have. Just because they pass a double diode DVM test doesn't mean much. My stressed ones leaked too much current even at 12 v Vce.
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Dynakit ST70, ST120, PAS2,Hammond H182(2 ea),H112,A100,10-82TC,Peavey CS800S,1.3K, SP2-XT's, T-300 HF Proj's, Steinway console, Herald RA88a mixer, Wurlitzer 4500, 4300

Last edited by indianajo; 13th March 2017 at 07:13 PM.
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