A77 EOT problems - UK Vintage Radio Repair and Restoration Discussion Forum

Linnovice · 5th Aug 2017, 7:20 pm

Hi all, I'm having having a problem with my A77 end of tape sensor. It's not working and I'm confused as to why. I've checked the voltages on the sensor (28mV uncovered, 45mV covered). I've cleaned the relays on the board. One of the Rifa caps had blown and taken out a resistor with it. I've now changed all the Rifa caps and the blown resistor. I've checked the transistors and the diodes and all seems ok there. So, what else should I be looking for. The machine runs, records and plays perfectly apart from that.
Any pointers and advice would be most welcome.

monaro0162 · 5th Aug 2017, 11:20 pm

Is the dummy plug in?
Is the bulb lit?
I had to diagnose a similar fault recently, according to my notes I had much higher voltages of 27v across LDR R155 with the sensor covered, and 10v with it uncovered.
+27v rail present? Can be checked via pin 13 of relay A and B, or pin 14 relay C.

Humber888 · 6th Aug 2017, 9:50 am

If all the above are OK, it is worth checking the resistance of the LDR both in light and darkness. I can't remember the figures offhand, but I seem to remember that you should be able to get the resistance down to a couple of kOhms when well lit. The LDR leads are push-fit to the PCB, so can easily be disconnected and a multimeter used to check the resistance swing. These LDRs can degrade over time and not provide a low enough 'lit' resistance to trigger the stop circuit. It is possible to tweak the other fixed resistors in that part of the circuit if the LDR is marginal.

5th Aug 2017, 7:20 pm	#1
Linnovice Hexode Join Date: Jun 2014 Location: Chelmsford, Essex, UK Posts: 386	A77 EOT problems Hi all, I'm having having a problem with my A77 end of tape sensor. It's not working and I'm confused as to why. I've checked the voltages on the sensor (28mV uncovered, 45mV covered). I've cleaned the relays on the board. One of the Rifa caps had blown and taken out a resistor with it. I've now changed all the Rifa caps and the blown resistor. I've checked the transistors and the diodes and all seems ok there. So, what else should I be looking for. The machine runs, records and plays perfectly apart from that. Any pointers and advice would be most welcome. __________________ Small Scotch, New Aids and Happy Ears

5th Aug 2017, 11:20 pm	#2
monaro0162 Pentode Join Date: Sep 2016 Location: Stansted, Essex, UK. Posts: 185	Re: A77 EOT problems Is the dummy plug in? Is the bulb lit? I had to diagnose a similar fault recently, according to my notes I had much higher voltages of 27v across LDR R155 with the sensor covered, and 10v with it uncovered. +27v rail present? Can be checked via pin 13 of relay A and B, or pin 14 relay C.

6th Aug 2017, 9:50 am	#3
Humber888 Heptode Join Date: Jul 2012 Location: Bromley, London, UK. Posts: 702	Re: A77 EOT problems If all the above are OK, it is worth checking the resistance of the LDR both in light and darkness. I can't remember the figures offhand, but I seem to remember that you should be able to get the resistance down to a couple of kOhms when well lit. The LDR leads are push-fit to the PCB, so can easily be disconnected and a multimeter used to check the resistance swing. These LDRs can degrade over time and not provide a low enough 'lit' resistance to trigger the stop circuit. It is possible to tweak the other fixed resistors in that part of the circuit if the LDR is marginal.