[Jukebox-list] Rowe R84 computer troubleshooting

[Ray Finch]

Hi Jeremy,

I'm not sure what processor is used in the R84, but I can give you a few 
tips for general processor based troubleshooting.

If the processor or other chips are socketed, (one at a time and with the 
power off, of course) carefully pry the each chip out of the socket with a 
small flat blade screw driver and then carefully re-socket the chip back 
in place.  Be very careful not to bend any pins.  This should be done one 
part at a time, as it is easy to mix up chips if they have the same number 
of pins.  If you have a digital camera, take picture of the board first so 
there is no chance of mixing up chips. Often "reseating" chips can fix 
many problems because the reseating rubs away tarnish that has collected 
over the years on the pins of the chips and the sockets.  Sometime it may 
take twice or more of reseating the chips to get things to work.  For a 
device that is 10 or 20 years old, reseating the chips will fix most 
issues about 75% of the time.

Get on-line and Google the processor part number and "PDF" and you should 
be able to find a datasheet for the part.

If there is no activity with the processor check the following in order:

Check the power supply connections for 5V.  Some processors have more than 
on connection for 5V so check all of the power supply pins.  Some 
processors (restively few) have other voltages that need to be present as 
well.  Also check for continuity with system ground and the ground pin(s) 
on the processor.

Check the reset circuit:  Start from the reset pin on the processor (Look 
at the data sheet you downloaded from Google) and work backward. Most 
processors are active low on the reset (meaning that the reset pin is 
normally high (5V) but will go low (ground) briefly on power up.  Very few 
processors are active high on the reset pin.  When powering up the unit, 
you should see the reset pin pulse.  Depending on the processor and the 
design of the unit the reset pulse may be very short (milliseconds long) 
or fairly long (like 1/2 second) but usually it will happen 1/2 second or 
longer after power up to give the power supplies a chance to settle.  If 
the reset pin is stuck in its active state (stuck low for active low or 
stuck high for active high) then the processor will do absolutely until 
the reset pin is "released".

Check the local oscillator:  All processors have one or two clock (or CLK) 
pins. Some may have more than two but this is uncommon.  This pin is the 
"heartbeat" or pulse that "clocks" the operation of the processor.  Each 
pulse causes the processor to execute one operation.  The clock pin on 
most processors operates is in the Megahertz range.  Checking the clock 
can be tricky.  If the clock is a quarts crystal (2 pin small metal can 
close to the processor), because of the extra capacitance that will be 
added to the circuit, testing either pin with a logic probe or 
oscilloscope can stop the oscillation.  This can lead you to think that 
there is a problem with the quartz crystal when in fact that is not the 
case.  Sometimes you will have a separate oscillator circuit that simply 
feeds a square wave into the clock input of the processor.  This is 
simpler to test as the logic probe or oscilloscope won't stop the 
oscillation at the processor pin.  Just like the reset pin, start at the 
CLK pin and work backwards.

Check every pin on every chip for either 5V or ground or an oscillation 
between 5V and ground:  You will need an oscilloscope to do this.  Every 
pin should have 5V or ground, or an oscillation between 5V and ground. 
Most processors will use what is called "tri-state" on some outputs 
meaning that at some points in normal operation the outputs of the 
processor (or other chips) are turned off.  This can give readings that 
are not 5V or ground but something in between, often a gradual rise or 
fall or "squiggle" on an oscilloscope.  What you want to look for here is 
a stair step signal.  If you find a stair step, this usually means that 
the input of a chip has issues and is "loading" the output of another 
chip.  If this is the case then the chip with the stair step signal at the 
input needs to be replaced.  Refer to the repair manual or google for the 
suspect part to find which pins are inputs.

Note that often logic levels will not always be right at 5V or ground 
level.  Often a high level signal (5V) will be something between 3.5 to 
5.5V and a low level signal will be between 0V and about 1 volt. 
Anything below about 1 volt for a low or above about 3.5 volts for a high 
is acceptable.  Also (looking at the data sheet or the schematic in the 
repair manual), some pins may be not used and marked N/A.  Obviously these 
pins may give weird readings.  If a pin is marked as N/A or not used, 
don't bother looking at it.

Note that these tests can only be done with an oscilloscope.  Sometimes a 
logic probe will see a stair step signal and mistakenly call it a normal 
oscillation.  With an oscilloscope you can be certain of what kind of 
signal you have.  You can often find a very basic oscilloscope on ebay for 
$50.00 or less.

These are just some basic things to check for on any processor based 
device.  If these things check out you are welcome to e-mail me about 
specific issues you may bump into - I'll be happy to help.

Ray Finch



On Thu, 30 Aug 
2007, Jeremy Agema wrote:

> I am working on a Rowe R-84 computer. I understand it would be quicker and
> easier just to send in the computer for repair but I like a challenge. I do
> have the schematic for the computer.
>
> I have replaced all the small signal transistors, removed the battery, and
> just for fun replaced all the small signal diodes, and I have re-soldered
> all the joints. I also cleaned the service switch.
>
> The processor seems to be locked up (no data going in or out). When the
> service switch is on or in the service position the record basket just keeps
> going around picking up random records and replacing them. But the processor
> is still locked (verified with a logic probe).
>
> Does anyone know what inputs I need to give it so it will at least go into
> audit mode on the bench? (the juke is 30 miles away) I have a 8 volt power
> supply. I think I have it figured out but I wanted another opinion.
>
> I think I just need to connect the 8 volts and ground to get it into audit
> mode. The 8 volts "on" signal pin should get it into play mode.
>
> Thanks
> Jeremy Agema
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