Sony LED level display - UK Vintage Radio Repair and Restoration Discussion Forum

Hampus · 9th Oct 2015, 11:52 pm

Hi all,

I just obtained cheaply a sony cassette deck, model TC-U30. It both records and playback fine but the right channel lever meter is stuck at full scale. All LEDs are illuminated even when I don't record or playing. I have found a circuit but this is over my head.

Any suggestions where to start?

Thanks.

julie_m · 10th Oct 2015, 1:25 am

The metering circuit is a bit more sophisticated than some (there are cassette decks that drive the multiplexing from the power transformer secondary) and employs a digital section with a memory to indicate peaks. This means the problem could equally plausibly be the analogue amplifier bursting into oscillation, or the digital system misbehaving.

I think you'll need to 'scope pins 11 and 10 of IC401 (assuming by the vertical lines up the middle that the RH channel is a duplicate of the LH channel, just blanked out in your diagram). That should give you a clue as to whether the fault lies in the analogue or digital domains.

Radio Wrangler · 10th Oct 2015, 7:03 am

The "Meter drive board does both the Left and Right channels.

IC401 is a sort of DVM chip with a built-in multiplexer. It has two inputs, pins 10 and 11, and it runs analogue-to-digital conversions on them alternately. It stashes the results in two small memories.

Not only is the conversion multiplexed, the display is too. The LEDs are split into L and R meters, and each of those is split into the top 8 LEDs and bottom 8 LEDs.

The anodes of each of the four groups of LEDs are powered in turn via the four transistors Q405 etc. So the LEDs flash, strobed as a quarter of the meter group at a time. For each group, the ADC converter data is fetched from the little memory of the appropriate L or R channel, along with the current conversion result which is converted into 'Thermometer code'. the upper or lower 8 diode's worth, as needed, is fed to the LED cathodes. Only those LEDs which get both anode drive and cathode drive light up. The purpose of the memory is to leave one LED behind to indicate recent peaks. This is looked after by the ethnically stereotyped "Compalator circuit" - such circuit names are of course completely unacceptable nowadays.

This is great for you, because all the fun bits are time-shared between L and R. You only say R is faulty, so I assume that means L is working OK. This means the fault must be before the meter chip (unless the very input of the chip has been damaged, or you happen to have 16 dead LEDs all on the one channel, or both of two transistors up the spout)

So, look at the voltages on pins 10 and 11. It looks like pin 10 is the R channel and I bet it's stuck high... which points at the "meter log amp" circuit for the R channel, which isn't shown.

David

Hampus · 26th Oct 2015, 8:47 pm

Hi again,

Today I took a look at this fault again, thanks for your explanations. I have also found the service manual. I have been probing around with my oscilloscope and the signal into IC401 seems to be OK from both channels. I switched the input so left signal=right meter and vice versa. The right meter is still stuck at full output. The left works well as before but now driven from right channel.

Hope you understand how I mean

This indicates that something on the "meter drive board" is at fault.

The "timing control circuits" output all have nice square waves.

10th Oct 2015, 1:25 am	#2
julie_m Dekatron Join Date: May 2008 Location: Derby, UK. Posts: 7,735	Re: Sony LED level display The metering circuit is a bit more sophisticated than some (there are cassette decks that drive the multiplexing from the power transformer secondary) and employs a digital section with a memory to indicate peaks. This means the problem could equally plausibly be the analogue amplifier bursting into oscillation, or the digital system misbehaving. I think you'll need to 'scope pins 11 and 10 of IC401 (assuming by the vertical lines up the middle that the RH channel is a duplicate of the LH channel, just blanked out in your diagram). That should give you a clue as to whether the fault lies in the analogue or digital domains. __________________ If I have seen further than others, it is because I was standing on a pile of failed experiments.

10th Oct 2015, 7:03 am	#3
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,903	Re: Sony LED level display The "Meter drive board does both the Left and Right channels. IC401 is a sort of DVM chip with a built-in multiplexer. It has two inputs, pins 10 and 11, and it runs analogue-to-digital conversions on them alternately. It stashes the results in two small memories. Not only is the conversion multiplexed, the display is too. The LEDs are split into L and R meters, and each of those is split into the top 8 LEDs and bottom 8 LEDs. The anodes of each of the four groups of LEDs are powered in turn via the four transistors Q405 etc. So the LEDs flash, strobed as a quarter of the meter group at a time. For each group, the ADC converter data is fetched from the little memory of the appropriate L or R channel, along with the current conversion result which is converted into 'Thermometer code'. the upper or lower 8 diode's worth, as needed, is fed to the LED cathodes. Only those LEDs which get both anode drive and cathode drive light up. The purpose of the memory is to leave one LED behind to indicate recent peaks. This is looked after by the ethnically stereotyped "Compalator circuit" - such circuit names are of course completely unacceptable nowadays. This is great for you, because all the fun bits are time-shared between L and R. You only say R is faulty, so I assume that means L is working OK. This means the fault must be before the meter chip (unless the very input of the chip has been damaged, or you happen to have 16 dead LEDs all on the one channel, or both of two transistors up the spout) So, look at the voltages on pins 10 and 11. It looks like pin 10 is the R channel and I bet it's stuck high... which points at the "meter log amp" circuit for the R channel, which isn't shown. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done