Arfa Daily wrote:

In my workshop, I have four four-drawer filing cabinets absolutely
jam-packed with service manuals that I have collected over more than 40
years in the business.

** Wow, I have hardly any after 40 years in the same business.

Here in Australia, service manuals for most pro audio products have long been hard to get and/or cost real money. The info (other than a schematic) provided in the vast majority is so poor as to be worthless. I am thinking of manuals from Crown, Yamaha, QSC, Phase Linear and others.

One problem is that the folk who write these manuals are never service techs. Often the product designer is given the job or some one associated with the design team who picked the short straw. If you have never made your living doing bench servicing, how in hell can you expect to advise those that do ?

Another problem is that at the time of writing, no actual products have been sold so none of the product's design faults or reliability issues have come to light. Most manufacturers are unwilling to put anything potentially damaging to their reputations into print in any case - lest a customer might see it!

I needed a schem for the EV 2.0kW power amp a few year back and was offered a full manual for $20 from the local EV agents. When it arrived it was a joke. All the schematics were drawn without dots - you simply could not tell if any two lines that crossed at right angles were joined at that point or not !!

I rang to complain and was immediately told they are all like that - sorry.

The manual for the Crown PSA-2 amplifier has procedures for testing the amp after repairs have been done. The final test is for the VI limiter and involves driving each channel at 20V rms into a 1 ohm load. What the manual writer did not take into account is that if the VI limiter circuit *fails* to operate correctly, that channel will instantly blow up! Hell of a test - eh.

Manuals for many QSC amps ( USA series etc) give details on connecting an output transformer to convert the amps for 70V line operation. QSC even sell a 300W transformer for the job. But don't you dare do it !

The current limiting scheme QSC use is a real oddball and works only with resistive loads. Soon as a low or subsonic frequency comes along, the iron core in the output tranny saturates and instantly blows up the amp. One of my customers found out the hard way after and simultaneously destroying four brand new USA900s.

Over the years, I have had to evolve my own methods of safely bench testing amplifiers, before and after repairs, including non invasive setting of bias levels and checking VI limiters for correct operation into reactive dummy loads.

Far as I am concerned, Service Manuals are mainly for light entertainment reading.


..... Phil