Nakamichi CR2 Cassette Deck

 Nakamichi CR2 Cassette Deck

 

Saw this going on ebay relatively cheaply, but not without concerns or probable faults.

 

 

The usual scratches were noted, but my main concern was the 'dead' left channel; so what was happening?

Before the 'dead' channel was investigated properly, I needed to clarify if the faulty channel was in the audio chain or just part of the LED meter circuit? After putting in a test cassette, audio was confirmed in both channels, leaving the LED meter circuit to be looked at later.

Transport

The transport appears to be near identical to the the BX-150E, although at this moment I cannot confirm 100%, possibly more on this later.

{This section is to be written up later}

Dead LED Meter in Left Channel

Initially I suspected the meter IC TA7612AP, or one of the two 1N4148 signal diodes that facilitate signalling to the TA7612AP.

By default the meter displays the -30dB segment as 'ON', but this was only active in the right channel. To check that the above IC wasn't failing I swapped the two signalling diodes and found that the left channel was okay, this time the right channel was inactive. So the problem was not the TA7612AP LED driver.

Studying the 'Meter Amp' was a little confusing at first as it appeared that both left and right channels (post meter processing) shared just one line to the LED meter system. At which point I realised that this was a rather unique way of managing the metering system. 

Nakamichi had designed a simple Time Division Multiplexing 'TDM' system to manage audio level monitoring.

The principle is to allocate each channel a segment of time to a single transmission line which is syncronized at both ends (from meter amp, to LED meter circuit) by a 50Hz/60Hz 'clock'. The source of that clock is of course - a secondary tapping off the transformer. 

It was then I realised that it may be possible that one of the two full wave rectifier circuits may have been interferring with effective synchronisation? It just might be possible that one of the internal diodes to the rectifier bridge circuit was an open, or a short circuit?

This time I didn't bother analysing the possibilities, but went straight to the circuit board and replaced both bridge rectifiers. The result? - a success!, both channels metering as they should.

 

 
Connection CN-13 supplies the TDM 'clock' signal sourced from one of these full wave bridge rectifier circuits.

Meter display now working. 

After studying the schematic for the Nakamichi BX-150E, I noted too that the same metering technique is employed on that deck. And perhaps all the BX, and possibly the CR models too?

06/09/2026.