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Vintage Tape (Audio), Cassette, Wire and Magnetic Disc Recorders and Players Open-reel tape recorders, cassette recorders, 8-track players etc.

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Old 22nd Feb 2020, 1:47 pm   #1
barrywebb
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Default Akai 4000DS right channel

Hello all i just got hold of a akai 4000DS tape deck that had no right channel i have cleaned and sprayed out all the switches and pots with no joy.
I then lifted out the play/record amp board only to find the old c458 transistors so replaced them with a modern replacement after doing that the right channel came to life but after a few minutes it went back off i have looked at all my joints and all look good .
Would anyone on this forum have any idea what the problem is like i say it was working but only for a short time.
Barry.
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Old 22nd Feb 2020, 2:04 pm   #2
DMcMahon
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Default Re: Akai 4000DS right channel

Hello Barry,

The 2SC458 transistors typically are noise generators (when bad) i.e. do not normally fail completely, so the intermittent right channel output possibly may have existed even before you replaced the transistors.

I assume you are talking about Playback, is the right channel missing on the VU meter, headphones as well as line output to Hi-FI amp?
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Old 22nd Feb 2020, 3:08 pm   #3
barrywebb
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Default Re: Akai 4000DS right channel

Hi yes that is correct I got the deck with no right output and as you say no vu and no headphones and no RCA out put all on the right channel.
Barry
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Old 22nd Feb 2020, 7:07 pm   #4
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Default Re: Akai 4000DS right channel

Is there no output at all or is it just low level/volume ?

Do you get a good record level on the right channel when you set up to record (some of the record amplifier circuitry is shared with the Playback amp) ? If you get a good level and then do a recording, do you then get any right channel output ?

Are you 100% happy that the tape you are using for playback is a good 4 track stereo recording ? i.e. if some of the recordings on the tape happened to be mono for example then you may not get the right channel depending upon the track selector switch setting. If any doubt, then try/prove tape on another recorder (if you have one of course).

Are you happy that the heads are really clean, they may look clean but still may need more cleaning to get rid of insulating films/dirt that may not be visible.

In playback mode try touching with your finger or the end of the small screwdriver the Playback head wire connections at the head, be careful not to short out adjacent pins on the head. To do this just remove the heads cover, no need to go right inside. You should hear quite loud hum/buzz (through headphones/external amp) if PB amplifier circuitry is alive/working and connected.

2 of the 4 pins on the PB head will be live, one for the left channel and one for the right channel.

If tape is good/recordings are good and heads clean/working then unless there is hard failure in the electronic components (unlikely unless a capacitor has failed and dragging down a transistor etc), the most likely suspect area would be the various switches such as track selector, source/tape and pots (which you have cleaned).

If nothing seems untoward then if you have a voltmeter then would be worthwhile measuring Playback amp transistor DC voltages and comparing them to the schematic values.

The voltages shown on the schematic are just ball park typical figures but with a dead transistor, there should be significant voltage differences, you can also compare to the left channel for typical values.

As this involves getting inside and working live, only do this if you are competent and be very aware of the potential danger from the 240 volts mains supply.

David
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Old 22nd Feb 2020, 8:06 pm   #5
barrywebb
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Default Re: Akai 4000DS right channel

Thanks David Heads are spotless .
No recording or playback on the right channel.
Ill try the buzz test tomorrow and report back.
The test recording is a good one.
I have replaced the tape selector switch with a tested good one just to rule it out.
When i power it off both VU meters move.
Then sometimes the channel just comes on but not for long and goes off again.
Barry.
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Old 23rd Feb 2020, 12:48 am   #6
Michael Maurice
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Default Re: Akai 4000DS right channel

These recorders use the infamous and now very scarce LD3141 IC, they could be the cause of your fault.
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Old 23rd Feb 2020, 11:58 am   #7
barrywebb
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Default Re: Akai 4000DS right channel

I have done the test with the wires on the back of the playback head left channel gets a buzz and vu meter goes straight across .
Right channel also get a buzz and vu meter goes straight across.
Would this tell me i have a head gone down?
Barry
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Old 23rd Feb 2020, 11:59 am   #8
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Default Re: Akai 4000DS right channel

Barry, when you say "No recording or playback on the right channel" does this mean "Not able to see/set-up a record signal on the VU meter AND no playback signal heard/seen of that attempted recording" or does it mean "record signal set-up OK BUT no Playback seen/heard of that recording" ?

So sometimes you get the right channel playback for a short period, is this more repeatable if you switch power off and on each time .

Is your 4000DS a Mk. 1 or a Mk. 2 ? the Mk. 2 is easily identifiable as it will say "Akai 4000DS MK. II" on the heads cover next to the track sector switch.

It is only the Mk. 1 that uses the infamous Sanyo LD-3141 Op amp ICs, they are mainly infamous for noise related issues rather than hard failures.

If nothing else obviously wrong and transistor voltage checks indicate nothing untoward then it gets more complex, you either then have to consider injecting a test signal in to the PB amplifier circuit using a signal generator and/or use an oscilloscope to check the signals.

This is not easy if you are not not used to doing it/do not have the relevant equipment. I personally would use a scope and use your line input music as the test signals.
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Old 23rd Feb 2020, 12:19 pm   #9
barrywebb
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Default Re: Akai 4000DS right channel

Hi no play back or record on the right channel.
But when i power down the machine both VU meters jump.
Did the head test give you any ideas both left and right channel get a buzz and the VU meters go straght over.
Barry
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Old 23rd Feb 2020, 12:19 pm   #10
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Default Re: Akai 4000DS right channel

Quote:
Originally Posted by barrywebb View Post
I have done the test with the wires on the back of the playback head left channel gets a buzz and vu meter goes straight across .
Right channel also get a buzz and vu meter goes straight across.
Would this tell me i have a head gone down?
Barry
It is possible but generally I would say unlikely. It would have to be open circuit (or seriously extremely worn out) for it not to work at all, the fact that sometimes you get a short PB operation indicates the head should basically be OK.

You can measure its DC resistance, the Service manual says 95 ohms, both PB heads should be very similar value. If you do this, ideally use a digital meter rather than an analogue meter.

There is some parallel circuitry in the PB amplifier but this should not affect the PB head resistance reading as there are capacitors in between.

Talking about capacitors would also be worthwhile doing a visual check of the PB amp electrolytic capacitors making sure none of them are showing any signs of leaking/splitting etc.

David
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Old 23rd Feb 2020, 12:20 pm   #11
barrywebb
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Default Re: Akai 4000DS right channel

Sorry its a MK 1
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Old 23rd Feb 2020, 12:29 pm   #12
barrywebb
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Default Re: Akai 4000DS right channel

I have done a visual test of all the caps and they all look to be fine i must say that someone has been in there before as it looks like they may of replaced the push pull switch assy and there are soldering signs on the right record amp but nothing on the play back only were i replaced the 458 transistors might just give up on it and get on with some other project.
I started to work on a Revox a77 2 track the high speed version but stopped as i was waiting for a few parts to arrive whats your thoughts on the Revox?
Barry
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Old 23rd Feb 2020, 12:29 pm   #13
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Default Re: Akai 4000DS right channel

I have a Revox A77 Mk. 3 standard speed, I really like it.

Unless you have more than 1 fault then the fact that the record signal does not set up as well as playback not working would suggest a common cause which would most likely would be the PB amplifier, namely TR3b, TR4b & IC1b circuitry for the Mk. 1 and TR5b, TR6b, TR7b & TR8b circuitry for the Mk. 2
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Old 23rd Feb 2020, 12:35 pm   #14
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Default Re: Akai 4000DS right channel

Feed a suitable signal into the line input, set to record and follow the signal through until it disappears.

Lawrence.
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Old 23rd Feb 2020, 12:37 pm   #15
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Default Re: Akai 4000DS right channel

This all sounds like an exact re-run of David’s problems in the https://www.vintage-radio.net/forum/...d.php?t=162904 thread.

Barry, have you read that? The same tests are relevant to your problem. You seem to have established that the PB amps are working - at least well enough to give you an output of some sort if the heads are working.

But, have you tried playing a previously recorded tape that you know is OK? If that works, then you can discount any fault in the playback process, and be confident that the problem lies in the recording process. Also, if it’s a two-head machine. Then you will know that the record/playback head is OK, and the problem must lie in the record amp itself, in the switching, or possibly the HF erase/bias oscillator.

So do the simple, non-invasive operational checks before you start worrying about individual components and circuitry.

Mike

Last edited by Boulevardier; 23rd Feb 2020 at 1:00 pm.
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Old 23rd Feb 2020, 3:20 pm   #16
barrywebb
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Default Re: Akai 4000DS right channel

My problem is odd just tested it again and right channel is back but only for about 30 seconds then off again I then rewind the tape and try again but on right again.
Seems if I leave it for an hour or sow the right pops back but only for a few seconds.
Barry
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Old 24th Feb 2020, 10:18 pm   #17
DMcMahon
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Default Re: Akai 4000DS right channel

Maybe temperature/charging related ?
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Old 11th Mar 2020, 3:57 pm   #18
barrywebb
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Default Re: Akai 4000DS right channel

Hi not sure what you mean by that sorry
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Old 11th Mar 2020, 4:36 pm   #19
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Default Re: Akai 4000DS right channel

Maybe temperature related, i,e. something warming/heating up then going bad and recovering after a shut down period.

Charging I meant possibly a capacitor problem related.
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