[Jukebox-list] Rowe R84 computer troubleshooting

Fri Aug 31 04:59:59 PDT 2007

Wow, thanks Ray for that great troubleshooting information. 

The chip used on this board is a Mostek MK3870 (same chip as a Seeburg SMC)

After much playing around the chip is still locked. The thing that clinches 
it for me is when I force a reset the only pin to change it's state is the 
reset pin! No other pin or output on the board changes at all.

Just for fun I put in a Seeburg programmed 3870 processor (I know - 
different operating instructions). But I was able to get the Seeburg chip to 
reset and it changed it's state when I forced a reset, and I saw clock 
signals and data going into and out of the chip.

I may play with it a bit more but I am going to give the owner the option of 
sending the board in or waiting for something to come up on ebay.

I welcome any more ideas or suggestions.

Thanks
Jeremy Agema

On Fri, 31 Aug 2007 00:14:32 -0600 (MDT), Ray Finch wrote
> 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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