Hello Rod,

05 Feb 97 12:40, Rod Gasson wrote to Bob Lawrence:

 RM>>> I've seen amplifiers of all sorts take off, and the problem is
 RM>>> either poor design (read: layout)

 BL>> I see. I've just told you that it is *never* (read: never)
 BL>> layout, based on extensive actual design experience of these
 BL>> things and you choose to contradict me. Okay

 RG> I realise that this is slightly different to the audio amps under
 RG> discussion, and correct me if I'm wrong, but don't many HF circuits
 RG> actually use the PCB tracks themselves as part of a resonant circuit?

HF (3-30 Mhz), none that I can recall (although layout is much
more critical than an audio amp) - but at UHF, yes, common.
The "inductors" at those frequencies in filter and oscillator
circutry are usually PCB tracks... and with power amps working
in class C (dirty), you need a few of them in the output stages.

 RG> Surely layout would then be very important for stability.

 BL>> I supppose you've confirmed this by fixing an oscillating amp
 BL>> or DC-DC converter by changing the layout, and then changed the
 BL>> layout of a good one to make it oscillate? Pig's arse, you have!

 RG> I haven't, but I'm a service tech, not a design engineer. I just fix
 RG> whats there, rather than redesigning things and changing the layout.

"Redesigning" or whatever you want to call it is something I avoid
whenever possible - but occasionally it can't be helped.

 BL>> all you guys have to do is fix something that an engineer
 BL>> designed right,

 RG> That's the theory, but surely you are not implying that everything
 RG> designed by an engineer is flawless or couldn't be improved upon?

It's not usually the engineer's oversight or fault - the real culprits
in "bad design" cases are the accountants and other financial people,
looking to cut the costs.  Save $1 in parts on a radio that sells
a million, and that bumps the company's profit from that radio up
by $1,000,000...

 BL>> and when you find a bad one you use standard cures... slap a 0.01
 BL>> across the collector base (0.01 is bit high, btw).

 RG> Crap. I've repaired zillions of electronic faults, and not once have I
 RG> needed to shonk a repair by redesigning the circuit...

Within one piece of equipment, true - the problems come up when
interconnecting two different things.. in this example, a DC converter
and a two way radio.  If "Brand X" converter consistently gives the
same problem with "Brand Y" radio then it's not a fault (i.e. it
wasn't working for a period of time, and then failed), but rather
a bad design and either a "mod" or replacement of one of the offending
devices are the only cures.

If you've done zillions of repairs and never had to do this, then
the world of VCR's and camcorders is a lot more standardised than
the radio game... haven't you ever struck a problem where both
"brand X" TV and "brand Y" VCR both work fine on their own, (and
fine with "brand A" or brand Z") but not with each other?

 RG> The only service techs that I've known that have
 RG> tried to fix a fault be redesigning the circuit have been failed
 RG> engineers...

OK, what would you do in either of these (real life) senarios?

1) A certain model Arlec power supply, when used to power a Philips
   FM92 local control radio in a base station setup, causes the
   radio to "play up" in some of the digital circuitry, like the
   channel up/down and tone signalling functions.  The Arlec works
   fine with any other radio, and the Philips works fine with any
   other suitiable (12-14V, capable of 10A) supply.   The owner
   dumps 3 Arlecs and 3 FM92's on your counter, and says "make 'em
   work"...

2) Roys case (I'm presuming a bit here, he was a tad light on the
   original details)... a 24-12 converter on a truck, when used
   on "brand X" radio, "takes off" so badly that it wipes out
   reception on a nearby AM radio.  The radio works OK when
   connected to a 12V tap on the battery temporarily, and the
   offending converter works fine with another brand of radio...

 BL>> Unfortunately, design engineers can't work like that. We have
 BL>> all the time in the world to get it right. I offered you a little
 BL>> of that

 RG> Pity that more of them don't actually use this time then. Even a lowly
 RG> service tech like myself could do better than some of the designs I've
 RG> seen  :-(

Very true when it comes to Korean made stuff..

Regards,
Chris.


--- GoldED