Hitachi VT-6500A Audio Playback

[Electronic M]

Originally Posted by ThePlague

Hello all,

I am currently working on a Hitachi VT-6500A that had all but the loading belts turn to goop. I have replaced the capstan belt, take up reel belt, and pinch roller. I didn't change out the idler wheel as it feels fine and looks to be a pain to replace, though I did manage to find a NOS assembly.

The only belt I wasn't aware of was the counter pulley belt which of course also turned to goop. While I am waiting on a replacement belt, I am trying to determine the source of my issue.

The issue: Video plays back perfectly with all the new rubber, but there is no audio out of the line out, earphone, or RF circuits. I can hear amplifier noise so I don't think that is the issue.
I have been digging through the circuit diagrams and traced the counter/memory switch to the flip-flop circuit that is handling the memory control. One of the outputs of this circuit is OSC STOP. Looking in the service manual, that signal is described as "Stops bias oscillation of audio circuit in modes other than REC, Audio dubbing modes" (when HIGH). This sounds to me that if this signal is high, there will be no playback audio out.

Is it possible that without the counter belt attached, the system is not producing audio? It seems like an odd connection, but it is all I can think of right now.

I don't think that's your problem.

The audio recording bias Oscillator is only supposed to run while recording. That osc runs at above audible frequencies to facilitate correctly magnetizing the tape during recording...If you run the record osc during playback it may damage the existing recording.

If you can identify the audio playback head (its often clustered with the tracking control track head) you could try injecting low-level audio there and see if it makes it through.
Some decks especially mid-80's and older had long multi-pole switches on the PCBs that look like those found in 70's and 80's cassette decks. Those PCB switches have to connect into the mechanism correctly to play properly....Case in point I worked on a sony SL-5800 Beta deck where the bottom PCB had such switches and if the bottom PBC was hinged down into service mode (which I needed to do) the switch on the PCB that switched audio from tuner to tape would not be engaged by the mech and stay in tuner mode (I could manually toggle it).

Not familiar with your model so I can't offer much more insight.