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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 23rd Jul 2020, 6:21 pm   #1
Larry4911
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Default Akai 4000DS, oscillator problem, any help?

I've gone through my admittedly shallow pool of electronics knowledge and am still having problems with my Akai 4000DS.
When first acquired, I made 4 small, 1"x1", PCB's for replacing the amp modules that were playing up. For 20+ years it has recorded and played nicely at least 1hr once a week. Suddenly, it will not record on one channel. Having scoped the head block I've noticed a discrepancy in the oscillator to the head block on one channel. I have lots of signal on one channel (measured from the erase head inductors), so much so my scope cannot show all the waveform, and only 10v p2p on the other. I have no idea what this means or what the erase head signal should ideally be. Does anyone know what is normal for the erase head voltage?
I've cleaned all the switches, sw9 and track selector also, to no net gain. I've spotted that one of the "to pre amp " lines from the oscillator board is 0.5v but the other is 1v p2p. The transistors in the oscillator are both fine. Any ideas what could be dragging it down and could this be the cause of the issue?
Thanks
Larry
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Old 23rd Jul 2020, 10:07 pm   #2
DMcMahon
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Default Re: Akai 4000DS, oscillator problem, any help?

Hello Larry,

The Service manual states Erase voltage to be around 52 Volts measured on a V.T.V.M. so would be an r.m.s. measurement, only 1 erase voltage connection to both erase heads.

For the record issue it would be the Bias voltage that is important, no voltage spec for that in the manual apart from frequency of 105kHz +/- 5% (by C8 value change).

There is a frequency response check where recordings are done using a signal generator input and on tape playback the bias trimmer C6 is adjusted for correct line output signal (and C7 for the other channel but it is not mentioned) and it says that the bias voltage will be around 11 volts (V.T.V.M) at this time.

David

Last edited by DMcMahon; 23rd Jul 2020 at 10:17 pm. Reason: Update
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Old 24th Jul 2020, 2:25 pm   #3
Larry4911
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Default Re: Akai 4000DS, oscillator problem, any help?

Thanks for the advice. I didn't see that. I'll bear that in mind.

Well, I ruled out all the oscillator / power board components, minus one cap with a mildly over spec esr. I guess it must be something in the audio board bringing it down and messing up my voltages.

Can I disconnect the "to pre amp" lines and fire up the unit to check the outputs without the output dragging it down? That would be ideal to test the oscillator board.

I have to get the scope on it again I guess, when my physical condition allows.

Thanks again.
Larry

Last edited by Larry4911; 24th Jul 2020 at 2:30 pm. Reason: Spelling correction.
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Old 24th Jul 2020, 4:56 pm   #4
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Default Re: Akai 4000DS, oscillator problem, any help?

Difficult to know for sure but I would not think that the pre-amp outputs are likely to be loading down the oscillator signals. Yes disconnect and see what happens.

David
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Old 24th Jul 2020, 7:32 pm   #5
Larry4911
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Default Re: Akai 4000DS, oscillator problem, any help?

Sorry, I meant the pre amp outputs "from" the oscillator board "to " the pre amp board, not the outputs of the pre amp board. If there is a fault in the pre amp board it could drag the output of the oscillator board down, yes?
Larry
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Old 24th Jul 2020, 11:47 pm   #6
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Default Re: Akai 4000DS, oscillator problem, any help?

Sorry Larry am getting a little confused with what you mean by " pre amp outputs "from" the oscillator board "to " the pre amp board.

The pre-amp output signals from the Record amps go to the Oscillator where they are added to (modulate) the record bias signals, the combined signals are then passed through to the record heads.

David
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Old 25th Jul 2020, 3:40 am   #7
Larry4911
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Default Re: Akai 4000DS, oscillator problem, any help?

If you look at the oscillator / power board photo, not schematic, there are 2 oscillator signals passing to the pre amp. White/black and grey/black. They are labeled "to pre amp".
Larry
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Old 25th Jul 2020, 10:26 am   #8
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Default Re: Akai 4000DS, oscillator problem, any help?

I would say the most likely candidates are :
1. SW 9 contact not making on record
2. P11/J11 contacts

less likely

3. C6 or C7 (according to which channel has stopped)

John
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Old 25th Jul 2020, 10:36 am   #9
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Default Re: Akai 4000DS, oscillator problem, any help?

Thank you for the clarification Larry, I can see this board layout, from the wire colours quoted you must have the Mk. 1 4000DS.

Just because it says "to pre-amp" does not necessarily mean they are outputs and they are definitely not outputs "from the pre-amplifier to the pre-amplifier".

I would call them connections to the pre-amplifier board and they carry the pre-amp (record amp) output signals to the oscillator board where after passing through C4/L1 & C5/L2 the combined modulated audio record signals/record bias signals connect out to the record heads by the J11 4 pin connector.

Later today I will try to get one of my 4000DS machines out and get some real/actual erase and record bias voltage levels.

David
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Old 25th Jul 2020, 9:51 pm   #10
Larry4911
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Default Re: Akai 4000DS, oscillator problem, any help?

That would be great, tbanks.
I think I have to study the circuit a lot more. I broke off a couple of recording head wires today when cleaning the head deck and may have mixed them up while re-attaching them to the switch.

I hate it when something like this happens, it throws your whole day off.

Thanks for the advice.

Larry
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Old 26th Jul 2020, 10:09 am   #11
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Default Re: Akai 4000DS, oscillator problem, any help?

Ok, not sure what happened but the oscillator board started smoking when I tried testing it without the pre amp connections. I guess it drew too much current through the 18 ohm shunt.

So, I decided to strip the track selector switch to try and determine what, if anything, I had messed up. I had indeed connected the record head wrong. But not so wrong as to cause the above smoke.

While smoking however, my levels on j11 were as they should be, level! So, that would indicate I'm looking at the wrong board as I was beginning to suspect.

While trying to rebuild, the now correctly wired, switch I realised I had no idea which holes the spring contacts came out of! I know, I should have taken more photos. So, now I'm stuck. Why did they put 6 holes in the wafer if they only used 3 of them? Anyone know which holes they go in?

Thanks for all your advice.

Larry
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Old 26th Jul 2020, 10:38 am   #12
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Default Re: Akai 4000DS, oscillator problem, any help?

Oh dear Larry, that sounds like a bit of a disaster

I am not entirely sure what the 6 holes you refer to are, but I will take some photos.

David
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Old 26th Jul 2020, 11:12 am   #13
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Default Re: Akai 4000DS, oscillator problem, any help?

Struggling to see 6 holes/spring contacts ? I assumed you meant at the Track selector switch, can you please clarify further/send photo.

David
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Old 26th Jul 2020, 11:15 am   #14
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Default Re: Akai 4000DS, oscillator problem, any help?

Unless another fault was created when disconnecting the outputs from the pre-amp board to the oscillator board the 25 volt rail shouldn't be affected.

Lawrence.
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Old 26th Jul 2020, 12:09 pm   #15
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Default Re: Akai 4000DS, oscillator problem, any help?

Yes I am of the same opinion, nothing obvious that should cause excess loading of the 25 volt supply (assume it was 18 ohm R3 that was smoking) by just disconnecting the pre-amp connections.

David
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Old 26th Jul 2020, 12:24 pm   #16
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Default Re: Akai 4000DS, oscillator problem, any help?

Not easy to get much detail but 4 views of the Track Selector switch.
Attached Thumbnails
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ID:	211859  
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Old 26th Jul 2020, 6:55 pm   #17
Larry4911
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Default Re: Akai 4000DS, oscillator problem, any help?

Thanks for all your help guys.
Yes, the 18 ohm wire wound resistor was very hot.
Inside that switch are 4 contact plates. One pair each for record and play heads. An upper plate, with 3 large contacts and a lower plate with 9 small (single position I'm assuming) contacts comprises the main part of each switch. Each pair contains, in between each contact plate, a wafer attached to the switch spindle. This wafer carriers 3 tiny Z shaped spring contact wipers. These are housed in 3 holes on each wafer. Because there are actually 6 holes in this middle wafer it was not immediately obvious which ones the springs came out of. I hope I've managed to work it out properly. Only 3 holes are in constant contact with the contacts of the plate with 3 large contacts so logically those must be where the spring contacts came from, yes?

Those photos are going to be a big help, thanks again.

Larry
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Old 26th Jul 2020, 9:02 pm   #18
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Default Re: Akai 4000DS, oscillator problem, any help?

Hi Larry,

Do not fully follow your description because I have never had to dismantle the Track Selection switch, so very hard to advise about correct spring contacts positions ?

David
Attached Files
File Type: docx Track Selector switch.docx (172.3 KB, 68 views)

Last edited by DMcMahon; 26th Jul 2020 at 9:23 pm.
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Old 26th Jul 2020, 9:22 pm   #19
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Default Re: Akai 4000DS, oscillator problem, any help?

Does this drawing section from the SM help in any way ?

Update - Somehow the attachment got added to previous Post ?

David
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Old 27th Jul 2020, 3:00 pm   #20
Larry4911
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Default Re: Akai 4000DS, oscillator problem, any help?

Cannot open it on my phone. But if you mean the section of the schematic with the selector switch on it, yes, I've seen it. However, the PDF version of SM I have is very unclear and I cannot tell the difference between a cross over and a connection. If someone has a real, printed version, a photo or high res scan from that would be very useful.

I realised what I did wrong, re: smoking, I didn't see an insulator on the transistor when I removed the heatsink. So, stupidly didn't insulate the screw when I put it back together. I just hope I didn't blow something, but it all seems fine with my multimeter. And I checked the transistors with a component tester and they are fine.

I'll try to get back to it today.

Thanks.

Larry
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