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Plessey CT-80 Cart machine issue

Hi there, wondering if there's anyone one there who may have experience with these old machines. I've got a Plessey CT-80 mono machine which works as it should except that when recording and replaying, the cue tone is audible, i.e. coming out through the output, which it shouldn't! Have done eveything I can with my limited electronics knowledge and I'm stumped. Perhaps somebody might know someone UK based who has the expertise to sort it out. It's such a niche machine I don't think the average vintage hi-fi technician would know where to start. Thanks in advance!

Comments

  • edited July 2

    Are you playing mono or stereo carts ? You say that this machine records too and is mono - i.e two track pickups on the playback head ? One for content and one for cue tones ? If you're playing a more contemporary stereo cart with three tracks (left, right and cues) then depending on the head configuaration you could be playing back part of the cue tone track.

  • There should be no electrical connection between the audio circuitry and the cue circuitry so it's probably the replay head azimuth out of alignment (i.e. the audio head is picking up the cue tone from the tape). You'd need a mono line-up cartridge (which might be difficult to find these days) and an audio frequency meter or oscilloscope to re-align it.

  • Graham - wouldn't the opposite be true? Playing a mono cart on a stereo machine would be more likely to cause bleed from the cue track...? (I'm racking my brains trying to visualise the head layouts.)

  • Yes it's a two track mono, I'm trying to record and play back, and can hear the cue tone through the output whilst doing both. You could be right about the head alignment, is it wise to tinker with such a thing without an oscilliscope or test tape? I'd imagine no is the answer to that!

  • GuyGuy
    edited July 2

    I'm intrigued that you can hear the cue tone while recording: that suggests a different fault, unless the monitor output still comes off the playback head. I'll see if I can find my CT-80 manual to see how the output is derived. Probably not wise to align the head without a test tape although you might be lucky and cure it, there's no guarantee you won't make things worse. If you have a bit of clear leader tape that you can load into a cart, you might be able to do a rough visual alignment check of the pickups on the playback head. Whereabouts are you geographically?

    Edit: is the cue tone continuous or just a short burst? If continuous, it may be some other fault: the manual suggests that the replay amplifier may go into oscillation; the cause is no earth connection between deck and chassis and the remedy is "tightedn deck screws". You might want to try that! The stop cue should be a 1kHz burst (just slightly higher pitch than a Greenwich Time Sig pip). If you are hearing any other tone it suggests a different fault.

  • I don't think azimuth has anything to do with it - this is the angle of the head to the tape travel direction and with mono equipment is largely irrelevant anyway. Vertical head positioning relative to the tape might be an issue but I agree with Guy, if you can hear 1kHz tone as you record then you have circuitry or audio chain issue. GMS pips are at 1kHz so a cue tone should sound familiar, but more like the sixth, longer pip.

  • @CharlieSalvidge said:
    Yes it's a two track mono, I'm trying to record and play back, and can hear the cue tone through the output whilst doing both. You could be right about the head alignment, is it wise to tinker with such a thing without an oscilliscope or test tape? I'd imagine no is the answer to that!

    As someone once said to me--"I didn't get to be this old by sticking screwdrivers into things I know nothing about"...:) By the way I sent you a PM..

  • A friend of mine has suggested that athough the machine will record, it does not erase. Are you using wiped carts to experiment with ? Also the connections to the heads use mini-jacks that are simply plugged in, so you may simply have them in the wrong sockets. If you are hearing just tone then this might be the issue. If it's tone plus audio, and everything else is correct, then it's most likely a short somewhere on the record/repro board.

  • i have no idea how plessey set this thing up. my only knowledge of plessey was when they managed to ruin the garrard line....with that said, based on u.s. brands, some spaced the heads in a way so that 2 were used. 1 for record and the next for play. with little effort you could get an echo effect doing head pingpong. the audio for program was on 1 (the top) track. cue lived at the bottom of the tape. most heads were on rcs plugs. look on the board or back of the head to be sure the right line is going to the right point. a simple problem to track down but a pain in the butt to track....all else seems to work right?

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