Hitachi VT-6500A Audio Playback

[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.

[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.

[JohnCT]

Originally Posted by ThePlague

Is it possible that without the counter belt attached, the system is not producing audio?

I never say never as I've seen stuff in my 50 years in this business that would appear to make no sense.

Most of the time, a bad counter belt would cause (other than no counter movement) a shutdown in a few seconds as most of the counters had a hall effect sensor to inform the micro of tape movement, but of course, not all did.

Now, that machine looks suspiciously like the RCA convertible VFP series. We did have an audio problem with the RCA convertibles, and there are two tiny relays that can cause either noisy audio or no audio. I think they were identified as R400 or R1400 or so - maybe R1400 and R1401. See if there's any on the board marked something like that.

If that's your problem, there's a *small* chance I may have a couple in an RCA box.


John

[ThePlague]

Originally Posted by Electronic M

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.

No large multi-pole switches on this unfortunately. All soft touch logic. As much as I enjoy this portable systems, the weird design choices made to build to a size always incur the strangest servicing quirks. I will go ahead and try and see if there is an audio signal coming off of the audio playback head to verify the audio board is actually getting a signal.

Originally Posted by JohnCT

I never say never as I've seen stuff in my 50 years in this business that would appear to make no sense.

Most of the time, a bad counter belt would cause (other than no counter movement) a shutdown in a few seconds as most of the counters had a hall effect sensor to inform the micro of tape movement, but of course, not all did.

Now, that machine looks suspiciously like the RCA convertible VFP series. We did have an audio problem with the RCA convertibles, and there are two tiny relays that can cause either noisy audio or no audio. I think they were identified as R400 or R1400 or so - maybe R1400 and R1401. See if there's any on the board marked something like that.

If that's your problem, there's a *small* chance I may have a couple in an RCA box.


John

This unit seems to have the hall effect sensor on the pulley attached to the take up reel instead of on the counter side pulley itself. Lets the unit play without having a working counter I suppose.

Looking at the audio board in the unit and the schematic, it would seem that you are correct. There are two small relays labeled RL402 and RL401. I am not sure if these would produce an audible click, but I do not here any actuation noise. When I get the chance I will try and test these out.

If you do happen to have a couple, I would be more than happy to pay you for them. Even if it isn't the issue, it would be nice to have a spare set.

[Ed in Tx]

Originally Posted by ThePlague

Looking at the audio board in the unit and the schematic, it would seem that you are correct. There are two small relays labeled RL402 and RL401....

While playing back a tape try tapping on the relays with a toothbrush handle or small screwdriver handle. You may hear a change or even proper audio.

I've cleaned and made relays work at least temporarily with a piece of thick paper wet with Deoxit spray back and forth a few times between the contacts.

[ThePlague]

Originally Posted by Ed in Tx

While playing back a tape try tapping on the relays with a toothbrush handle or small screwdriver handle. You may hear a change or even proper audio.

I've cleaned and made relays work at least temporarily with a piece of thick paper wet with Deoxit spray back and forth a few times between the contacts.

Sorry for the delay in my response. Finally have some hours to put into to troubleshooting this deck. I was able to check the relays, and they are clicking away. My next idea is check the output of the audio head and if that is good, try and trace out the playback audio path from the audio circuit diagram. I will report back.

[Ed in Tx]

Just because a relay "clicks" doesn't mean the contacts are making good electrical contact.

[ThePlague]

That is a fair point. It looks like in the meantime the FW/REW Idler rubber has given up the ghost. I managed to get a hold of a replacement assembly. Looks like it requires dropping the reel motor so that may take a bit. I'll take a closer look at the relays after getting it mechanically running.

[ThePlague]

Update - I unsoldered the RL401 and RL402 relays. I used some DeOxit D5 and a piece of paper to go back and forth on the top and bottom contacts of the relay. There was this bodge wire installed on the back of the board. It looks factory as no traces lead out from the lower left pin on the relay. Upon re installation, there is still no audio coming from the player. However, I can hear more of machine's noises if you will on the audio output so I believe this may have been part of the problem.

Is it possible that the audio/control head is out of alignment? I don't have a scope to check for any waveform on the head output.

I will try to interject an audio signal in place the output of the audio head and see if there is some circuitry that is stuck in a mute mode possibly.

I have a feeling it is a simple fix hidden somewhere.

[ThePlague]

Upon further inspection of the characteristics of the RL401 and RL402 relays, here are my findings.

If I am reading these diagrams correctly, the switch shown in the relay should be normal state (or resting). When operating the unit, I observe the following behavior:

1. Power On
RL402 is activated.
RL401 is not activated.

2. Press Play
RL402 is active.
RL401 is activated.

By looking at the schematic, RL402 is circled in yellow. Being activated places it in the (PLAY) position. Which should be the correct behavior.
RL401 is circled in green. When this relay is activated, it is in the (REC)ing position. My understanding of the circuit is that RL401 should remain unactivated when I press play to remain in the (PLAY) position. When RL401 is in the (REC)ing mode, following the circuit shows that it is grounding the playback audio coming out of the audio head.

Somehow, somewhere, the audio portion of the recording circuitry is getting activated. Does my diagnosis seem logical?