Boston Acoustics CR400 Plate Amp Troubles

ouimetnick

Hafler Fan!
So I have this Boston Acoustics plate amplifier from a CR400/Micro90pv subwoofer. It WAS working (though seemed weaker than I remember it being) so I recapped it since several caps tested out of spec. After recapping it, I fired it up on my DBT with a 40w bulb. The bulb was somewhat bright, but not fully lit. Smoke shortly started rising from the amp board.


Resistor R121 was the one that smoked the most, and in 2nd place is R122. 1 leg of R122 and 1 leg of R122 are tied together and connect to a capacitor (C115) and a resistor R119.

The other end of R121 connects to the emitter on Q102 (D1563A), and the other end of R122 connects to the emitter on Q103 (B1086A)

The fact that these resistors burned up I’m guessing means those transistors are likely shorted (wonder how I did that) Where would I find replacements/drop in substitutes for these transistors (2SD1563A and 2SB1086A) and how do I figure out what actually killed them. I think they are driver transistors (right before the output transistors) but how do I go about this? Do I start pulling transistors and testing them for shorts/open circuits? I’m trying to avoid soldering as much as possible since the board traces are really fragile (cheap board too) Do I start by testing the outputs and work my way down? If the outputs need replacing, where can one obtain a replacement or substitute for 2SB688 and 2SD718. Boston doesn’t share schematics with customers and the service centers don’t have access to old schematics (this CR400 had a 1998 build date)

I'd like to actually learn and fix the problem (not throwing parts at this) so I need help and advice on how to troubleshoot. It's not the end of the world since it's just a plate amp and replacements are cheap but I'd like to learn. The photos show the resistors, etc. I have 2 of these amp boards (tried fixing a broken 1 I found on ebay a few years back with no luck. Whoever BA contracted production and design of this amp to seems like they used the same board in other designs since a lot of the board is not populated. I did try to remove the conductive/corrosive glue and replace some corroded components. The 4558 op amp probably should be replaced judging by the corrosion on the pins.
 

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You might opt out and use the box and speaker and find a new amp. board. and bench the old one for experiments to learn on.

with no schema you're stuck in test modes.. check soldering. Note pic.. looks like it's bridged with solder...
also see something on cap c10x? 4700 25v? what is that at the base?
m6xR55N.jpg
 
Replaced the transistors, but after rereading EchoWars Testing a transistor, the outputs seem fine, to I reinstalled them. One of the driver transistors tested open. Replaced both drivers, and all of the other semiconductors on the board. The resistor STILL smokes. With the preamp board connected and the OFF-Auto-ON switch in off, nothing happens. The DBT's 40W light glows brightly for a second, then dims down (very dim), and then when I turn the switch on, the bulb glows slightly brighter (still quite dim), the relay clicks, and R121 begins to smoke. :( No schematics for this old unit through BA. Anyone else have advice?
 
I have one of these in pieces up on my shelf. I always tell myself in the back of my head that I will fix it one day.
Did you ever get anywhere with yours?


By the way, I would like to find the inventor of that black stuff they decided to contract out onto their boards in a dark alley.
 
I have one of these in pieces up on my shelf. I always tell myself in the back of my head that I will fix it one day.
Did you ever get anywhere with yours?


By the way, I would like to find the inventor of that black stuff they decided to contract out onto their boards in a dark alley.

I have one on my bench now. It had blown fuse on arrival. I found one of the FETs shorted so I replaced them both. But it still won't produce sound. It acts like it wants to. Auto-on works with a signal and I can get some hissing sounds while fussing with the volume. If I jam the volume the signal will pop in and out very intermittently. Anybody have a schemo? I don't want to shot-gun parts without a schematic.
 
I have one on my bench now. It had blown fuse on arrival. I found one of the FETs shorted so I replaced them both. But it still won't produce sound. It acts like it wants to. Auto-on works with a signal and I can get some hissing sounds while fussing with the volume. If I jam the volume the signal will pop in and out very intermittently. Anybody have a schemo? I don't want to shot-gun parts without a schematic.
From everything I have gathered about these (and the majority of plate amps) is almost none of them are still functional and that no one can seem to give a reason on this model. Most plate amp issues are obvious but these seem to baffle most people.
 
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