Stepper Re: [sdiy] And another amp question

Batz Goodfortune batzman at all-electric.com
Sat Jun 15 18:02:42 CEST 2002


Y-ellow Cynthia.
         Good call. Well your second guess anyway. I'm pretty sure it's a 
servo. It might even be some cheap-assed toy motor. It has but 2 pins and 
at this stage I can't tell where it's being driven from on the board.

Looking in the other direction the remote socket is a TRS type mini jack. 
Tip and ring of which seem to pull down on a transistor each. The 
transistor then in turn pulls down. One of the transistors is connected to 
the serial in(s) of the shift register. The other on the clock. The clock 
pin is also indirectly connected to another transistor and network of bits 
closer to the shift register. Though because of the cheap-assed design of 
this thing I'd have to reef the board out of the chassis to trace it any 
further. And also because I have no idea what these transistors are, I 
can't even be entirely sure we're talking about signals coming or going. 
Story of my life really.

There is another one of those typically Japanese single in-line packages as 
well. I have no idea what this is. It could be a resistor network for all I 
know but it does have a BA part number. Unfortunately, 2 of the numbers 
seem to be unreadable so that's not a great deal of help.

The design of the amp is kinda cool in a cheap-assed king of way. In that 
instead of employing 6 VCAs or anything, they've used this motorized pot. 
Which was probably significantly cheaper to implement than servicing 6 VCAs.

It's got 2 LED bargraphs on the front panel board and I can only assume 
that they sum the 6 channels together to make it light up. But it looks 
like, whatever the remote was supposed to do, it's basically up or down. 
Two indicators either side of the knob seem to suggest thing but then why 
is there a whacking great shift register in there?

Another thought had crossed my mind also in the interim. That the remote 
socket isn't really for some corded remote control gizmo as such, but 
rather a control feed from another unit. A DVD or preamp or something. This 
would make sense in that there is no power pin to the remote, only ground, 
so any buttons on the remote would simply pull down and that's it. But if 
the remote signal came from another box, it could easily supply the 
positive going pulses. Or not even that when I come to think of it. They'd 
really only need to pull to ground in the required sequence.

Which begs the next question. Why would you want to? It's an 8 bit shift 
register. So far I can only understand a need for 2. That leaves 6 bits. I 
wonder what they do?

There are 5 TDA7298 power amp chips in the thing. They're supposed to be 28 
watts each but I'd expect some serious sag given the size of the power 
transformer. This is rat-shack style engineering at it's best. Do all the 
tests on a lab supply but ship it with a transformer half the size it 
actually needs. But these amps do have a mute pin. 6 mutes plus up and down?

I bought the amp because it was described as a Prologic amp. Well prologic 
is prologic but the guy who sold it to me didn't know that. I didn't really 
care too much about the amps I was just interested in the decoder. It's not 
like I couldn't muster better amps if I need to. But it's impractical to 
try and roll your own prologic decoder. But to cut a long story short, it 
obviously wasn't anything like a prologic amp and I was pretty pissed off.

After a careful week's worth of diplomacy, the guy offered reparations in 
the form of a yamaha amp with a REAL prologic decoder and even 3 half 
decent power amps. (2 rear + centre but no sub) But he didn't want this 
thing back and even though I got the Yamaha amp significantly below market 
value, I haven't exactly come out on top. So I'm trying to work out what to 
do with this piece of crap. And I'm not going to go out and buy a boat just 
so I can use it as an anchor. Besides I'd still have to modify it with a 
couple of house bricks before it was heavy enough. Doh!

Thanks Cynthia 'n' all.

Be absolutely Icebox.

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