Technics RS-B48R cassette deck- lights on, nobody home - UK Vintage Radio Repair and Restoration Discussion Forum

AlanC · 21st Sep 2021, 8:07 pm

Hi all,

This unit has not been powered for a very long time and was stored in the owners attic. It was apparently working when stored away. In view of the long time without power I did basic safety checks on it before bringing it up on a dim bulb tester.

The fault is that when powered on, the VFD level meter lights up, as does the forward transport direction indicator (it is an auto reverse machine) but it is completely unresponsive to any of the function keys. When you insert a cassette the capstan motor runs, this is controlled directly by a simple switch in the transport. I have the service manual for it.

The power rails are all up and very close to the values shown on the diagram. The microcontroller chip signals are shown in the manual for each pin; it has 5v power on pin 39 as it should, the clock on pin 40 is very close to the waveform shown in the manual. Since the clock is set by an external RC network, the oscillator must be in the chip and is clearly working. The reset is clean and rises to the full 5 volts.

However the keyboard scan outputs on pins 31 to 35, which should go high in turn with a 2mS pulse every 11mS, are all stuck high at 5V. Disconnecting the keyboard connector does not change this.

It looks to me like the chip is faulty, but is there anything else I should check before I condemn the machine? - I assume the processor would be unobtainable by now. I can post a screenshot of the relevant parts of the circuit if it helps.

Thanks, Alan.

Nickthedentist · 21st Sep 2021, 9:07 pm

Presume the timer switch is in its central position?

AlanC · 21st Sep 2021, 9:35 pm

Quote:

Presume the timer switch is in its central position?

Thanks, it is indeed.

ben · 21st Sep 2021, 11:17 pm

Dirty cass-in leaf switch. Had this before. Try bridging the contacts and see if the transport works. There may be more than one switch to 'fool'.

Nickthedentist · 22nd Sep 2021, 6:33 am

Good point. There will be several leaf switches like this. Clean their contacts with thin card moistened with Servisol, and remove the PCB mounted ones and clean their legs and the pads on which they sit with a cotton bud.

AlanC · 22nd Sep 2021, 7:44 am

Thanks Ben and Nick will do.

21st Sep 2021, 8:07 pm	#1
AlanC Hexode Join Date: Jul 2006 Location: Sheffield, South Yorkshire, UK. Posts: 312	Technics RS-B48R cassette deck- lights on, nobody home Hi all, This unit has not been powered for a very long time and was stored in the owners attic. It was apparently working when stored away. In view of the long time without power I did basic safety checks on it before bringing it up on a dim bulb tester. The fault is that when powered on, the VFD level meter lights up, as does the forward transport direction indicator (it is an auto reverse machine) but it is completely unresponsive to any of the function keys. When you insert a cassette the capstan motor runs, this is controlled directly by a simple switch in the transport. I have the service manual for it. The power rails are all up and very close to the values shown on the diagram. The microcontroller chip signals are shown in the manual for each pin; it has 5v power on pin 39 as it should, the clock on pin 40 is very close to the waveform shown in the manual. Since the clock is set by an external RC network, the oscillator must be in the chip and is clearly working. The reset is clean and rises to the full 5 volts. However the keyboard scan outputs on pins 31 to 35, which should go high in turn with a 2mS pulse every 11mS, are all stuck high at 5V. Disconnecting the keyboard connector does not change this. It looks to me like the chip is faulty, but is there anything else I should check before I condemn the machine? - I assume the processor would be unobtainable by now. I can post a screenshot of the relevant parts of the circuit if it helps. Thanks, Alan.

21st Sep 2021, 9:07 pm	#2
Nickthedentist Dekatron Join Date: Jan 2005 Location: Oxford, UK. Posts: 17,843	Re: Technics RS-B48R cassette deck- lights on, nobody home Presume the timer switch is in its central position?

21st Sep 2021, 11:17 pm	#4
ben Dekatron Join Date: Aug 2006 Location: Madrid, Spain / Wirral, UK Posts: 7,498	Re: Technics RS-B48R cassette deck- lights on, nobody home Dirty cass-in leaf switch. Had this before. Try bridging the contacts and see if the transport works. There may be more than one switch to 'fool'. __________________ Regards, Ben.

22nd Sep 2021, 6:33 am	#5
Nickthedentist Dekatron Join Date: Jan 2005 Location: Oxford, UK. Posts: 17,843	Re: Technics RS-B48R cassette deck- lights on, nobody home Good point. There will be several leaf switches like this. Clean their contacts with thin card moistened with Servisol, and remove the PCB mounted ones and clean their legs and the pads on which they sit with a cotton bud.

22nd Sep 2021, 7:44 am	#6
AlanC Hexode Join Date: Jul 2006 Location: Sheffield, South Yorkshire, UK. Posts: 312	Re: Technics RS-B48R cassette deck- lights on, nobody home Thanks Ben and Nick will do. __________________ Always expect the unexpected...