Yorkville ap schematic

The Yorkville In-House Method for Repairing AP Series Amps

At Yorkville, we have developed a method of rebuilding AP series
power boards in the shortest possible time. This method is valid for
repairing about 95% of all AP amp failures and is essentially the same
process for each of the different models in the AP line.

This process was not designed to correct problems other than blown
outputs and collateral losses. We still see an occasional failure in other
areas, but about 95% of all amps that you receive in PROTECT mode,
will benefit directly from this method.

This uniformity of approach is possible because all AP series amps
share the same basic circuit topography and protection circuits. The
rail voltages and the number of output devices may change, but overall
they remain more alike than they are different. Only the AP800 &
AP1020 have enough differences to warrant a comment or two.

The entire program rests on the experience we gained while rebuilding
these boards. What became apparent was the fact that we needed to
focus on rebuilding the board and not trying to ANALYZE the
problem. As you can imagine, this practice goes against just about
everything we have practiced as repair technicians. But, by using this
method, we were able to reduce turn-around time and expense by more
than 50%. In fact, with practice, an average technician can do a full
meltdown rebuild in about 90 minutes. The exception to this rule would
be the AP 6040, which runs about 3 hours for an average rebuild.

The root of this system is the practice of NEVER measuring TO-92 style
transistors. They are always replaced because it takes about 5 minutes
to replace them and they cost next to nothing compared to the cost of
analysis and the risk of a failed rebuild. I instituted this policy after
discovering that these little transistors have a really nasty habit of “half­failing”. By this I mean that we were testing these things in the normal
fashion and they were showing “good”, but when powered up to
operating voltage, they would do all kinds of inexplicable things. As
soon as I mandated the practice of always replacing these little
transistors, these strange post-rebuild malfunctions disappeared and
departmental efficiency went way up.

Determining which channel is blown

In order to determine which channel(s) requires repair, it is only
necessary to open the chassis and flip the upper circuit board out of the
chassis without disconnecting any wires ( be sure to insulate the board
from touching the chassis when in this position). Then all you need to
do, is turn on the power and measure the voltage across ZD7 on each
power board. (The location of ZD7 is shown on the M1146 Diodes &
Semi’s PDF) If the voltage here is less than the zener rated voltage of
16volts, the board under test is blown and probably in DC offset.

Locating Defective Parts

Attached to this file are 3 PDF’s that locate and identify all the
components which require measuring. For the purpose of clarity I have
divided the components into 4 groups, each with its own colour. Further
to this, I separated the parts into 2 PDF’s, one for resistors and one for
semiconductors. In order to provide maximum clarity, I have expanded
isolated areas of the circuit board, showing only the area of the board,
which is home to the suspect components. By doing this I have cropped
out half of the output transistors, emitter resistors and tier diodes
(MR854) from view. Please be certain to test these none-visible parts
along with the rest of the output devices , resistors and diodes.

The PDF Colour Coding

Green

Blue
show 4 of the 8 MR854 tier diodes which must all be measured). The
only significant measurement in this group will be a short circuit which
signifies a diode to be replaced.

These are the large transistors and Fet’s.

Simply test these with your diode checker and replace any
shorted units. You will almost never need to replace the tier Fet’s,
they never seem to fail, but we check them anyway. Although it is
not shown on the PDF, you will need to check the output crowbar
TRIAC for shorts, along with everything else.

These are all the diodes to be measured (remember, we only

Red

NEVER MEASURE THESE, Always replace these 8

transistors

Yellow

These are the resistors to be measured. You are only interested
in units, which measure open circuit, all other readings are meaningless
at this time Please note that we are only measuring resistors of 249 ohms
or less, with the exception of the 2 x 3K resistors and the 10K trim pot.

The Process

With the component side up and the heatsinks farthest from you.
We use a continuity checker (you may use a diode checker) and begin
by checking the 8 emitter resistors for open circuit. I immediately cut
out any components that measure defective, We do not unsolder the
lead fragments until I have cut away all the various bad parts.

Next. Measure the MR854 tier diodes and cutout any bad ones.
Now measure all the transistors outlined in GREEN, cut out or remove
any which are shorted. The output devices must obviously be
unsoldered to remove them from the board. Please note that it is not

uncommon to find it necessary to unsolder and isolate all of the output
bases and emitters in order to find all the shorted output devices.

Now measure all diodes in BLUE (except theMR854 that you have
already done) and cut out any that read short. When that is done you
may measure all resistors shown in YELLOW, and cut out any

that

read open circuit. It is also a good idea to check the bias trim pot
(orange colour) just to be safe.

Once you have located and cut out all the short and open
components, it is time to unsolder and remove all the remaining pieces
of the component lead wires from the board. After that, collect all the
parts you need and install them to the board.

For the sake of efficiency, I have small parts bins on the bench
which have quantities of all the values of resistors I need and all the
diodes as well. I only go to the main parts bins to get the big transistors
required for the job. (10 different resistors and 6 different diodes will
cover 90% of the work). I also keep the mandatory TO-92 devices on
my desk because they are always replaced on every job.