Technics PR305 Blank Display

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Hi all :wave:

I have a Technics PR305 which has had a fading display for a while, and now it no longer displays anything – just the backlight.  Everything else seems to work…

It’s a long shot, but I’m hoping someone out there has some experience with repairing these things.  I am pretty handy with electronics/hardware/soldering etc. but before I break out the multimeter I thought I would seek advice from the Internets.

Does anyone know if there is a service manual or circuit diagrams available for these?

As a first port-of-call I have cleaned the LCD zebra strips with IPA, but no joy there.  Now I’m wondering power supply?  Any resources or ideas?

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Hi

Having recently got my Technics PR804 startup problem fixed, I can recommend Mike Barden. Although based in the UK, he has fixed many keyboards / pianos located around the world using the mailing / courier service to ship the failing part.

https://www.mikebarden.com/technics-electronic-digital-kn-pr-px-piano-keyboard-ensemble-repairs/

 

 

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jinajon

Thanks yatawai – I was hoping to fix it myself, but if I run out of ideas Mike Barden looks like a good option.

It seems I can’t insert images to this reply for some reason, so I’ll attach instead.

Here is my PR305 main board, QJBG2173:

Corrosion doesn’t look to be too rife.  I have +15.2V, -15.2V, +5.0V, and +3.3V, so all the power rails I can find look to be good.

The LCD connector is on the far left.  I have done some preliminary fault-finding, and it seems that the LCD is not getting sent any data or commands.

The SED1330FBA is a CMOS low-power dot matrix LCD controller. Its CS (Chip Select) line is supposed to be active low, but is held high.  Meaning that the LCD driver is effectively turned off, and the LCD is not being told to display anything.

I’m flying blind here, but educated guessing is better than giving up.

Anyone with any pointers feel free to chip in …  In the meantime I’ll keep posting …

My next move is to find out WHY the CS is held high.

 

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I managed to track down the Service Manual for the PR307 which is very similar to the PR305.  I’ll upload it here soon.

In the Service Manual there are some instructions for running some basic PBITs which is pretty cool.  Most of them involve turning on the keyboard while holding down two particular keys, but for some of the tests you need to make a “Checking Device”. The manual describes it, but either the manual is wrong, or the PR307 is backwards to the PR305.

I grabbed a low-power LED and a 1kΩ resistor to make an indicator, and some component lead off cuts to make a ‘switch’.

Pretty rough and ready, but worked a treat.  NB the manual has the pins backwards to the PR305 main board.

PR305, Main PCB QJBG2173, Connector CN3 pinout for ‘Checking Device’:

1. = <span style=”text-decoration: underline;”>GND</span> (Switch)
2. = <span style=”text-decoration: underline;”>DSR</span> (Switch)
3. = DTR (1kΩ resistor)
4. = +5V (LED Anode

Here’s what you should see:

RAM & ROM test:

CPU test:

PR305-CPU-Test_480p

You should see +5V ‘data’ on all four of the test points.  Not too specific, but I’m assuming that if there was a fault you wouldn’t see anything, or you’d see a mess …

My unit passed all the other tests except the LCD test.

 

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