Servicing a SONY DAT machine (Pictorial)

[hdrobien]

DTC-750: I can't seem to get the left part of the signal as you describe (if I am not mistaken). I did readjust the guides and it seems to play fine but will need to listen properly again to make sure. I have set the scope to read 20mv per division and the Time/Div at 1 ms. I shifted the image to be able to get a good image. I did unscrew the left guide and then adjust the right one to have a uniform equal square image. Is it missing something? Thanks for all your help today

Here is what you should have on the scope: rectangular, symetrical shape with burts at each end and smaller rectangles outside.

If the left side part is missing it is because the tape is too low on the drum so need to raise the guides (unscrew).

 

[jascha]

Here is what you should have on the scope: rectangular, symetrical shape with burts at each end and smaller rectangles outside.

If the left side part is missing it is because the tape is too low on the drum so need to raise the guides (unscrew).

Hi, thanks for your response. Right, I had another go and ended up having to dismantle the guides as they were so stiff to turn to start with. Had to lubricate the posts that drive into the bottom base on which the 2 guides sit on (S1,T1). I have moved the posts up and down till I got this picture now. It is sounding good, no drop outs as such so far. Did this improve you think? I can't seem to get the very thin edges on both sides of the scope pattern, unless I am doing something wrong still. Thanks again

 

[hdrobien]

It could be good but there is maybe a lot of interference.

Did you ground the probes ?

Could you raise the timebase on the scope to see what it looks like between 2 frames ?

 

[jascha]

I have taken some more shots again for you. The gnd connection in the dat is connected to the scope gnd post. Then I have one jumper less connected between swp and the trigger circuitry and another between pbrf and ch1 of the scope. I have changed the timebaee setting and also the volts/div to vary the reading on the screen to be able to get a full shot. Hope this helps.
Thanks again hdrobien

 

[hdrobien]

My concerns when you raise the time base:
- there should be no signal (just a ligne) between 2 frames. Look at my picture in post 482. Your shots show a lot of noise.
- all frames should look identical. Yours are not (IMG 5355).

Could you raise again the time base to show 4 frames ? With the SWP signal ?

In fact the SWP signal swaps the amp to each head alternatively. So one frame out of 2 is from head A, the other from head B. Your shot 5355 shows a frame way stronger than the other. Could be a dirty head or a worn/damaged head.

If you switch the slope trigger from + to - you should switch from head A to head B as a single frame.

 

[jascha]

My concerns when you raise the time base:
- there should be no signal (just a ligne) between 2 frames. Look at my picture in post 482. Your shots show a lot of noise.
- all frames should look identical. Yours are not (IMG 5355).

Could you raise again the time base to show 4 frames ? With the SWP signal ?

In fact the SWP signal swaps the amp to each head alternatively. So one frame out of 2 is from head A, the other from head B. Your shot 5355 shows a frame way stronger than the other. Could be a dirty head or a worn/damaged head.

If you switch the slope trigger from + to - you should switch from head A to head B as a single frame.

Hi, I tried to take 2 shots as you suggested, changing the timebase and inverting the slope. Not easy with the rolling shutter but hope you can see enough of what is happening.
I understand what you are referring to as interference compared to your images and can assure you that it have cleaned the heads many times recently.
I did recap the rf amp as I had 4 out of 6 caps with high esr. Could it be that it's these caps again that I used? I used NOS but that measured well within the esr range.
If I had to change the head is it an easy case of swapping a head or does it require a different alignment? I have a spare head that is in a pcm-2300 which I use for spares but not sure it's a good head on it.
Cheers

 

[hdrobien]

Not easy to see clearly what is going on on the shots. Either there is noise due to the probes or there are parasites in the signal from the rf amp.

Can you use visualize SWP (used as trigger) ? It is a square signal. Each change should start a frame, alternatively head A and heab B. All frames should look identical as the one I uploaded. If one frame out of two is different there is a problem on the head.

You can set the timebase to visualize only one frame and compare head A and B just by inverting the trigger slope.

And the ultimate question: do you still have parasite in the music especially at high level ?

Keep in mind that a good test for tracking adjustment is to listen to music in accelerated speed (push >> during play). The sound should be chopped but without any digital/parasite noise. If possible do this test in test mode with ATF disabled.

 

[jascha]

Happy New Year everyone

Thanks hdrobien for your help yesterday. I shall give the 750 a go when I have some time to do some servicing again, being New year and all that at the moment however i have discovered something very strange today with the PCM-2700

I was recording a live concert from Vienna on the 2700 and realised that something was not quite right when I was comparing the Input signal to what was recorded on tape. The playback during recording (I presume the signal from the playback head?) sounded degraded. I had to stop the recording due to this fault, only to realise that actually the playback of the recording came out very well. Only DRURING recording and monitoring off the heads just does not sound right, but playing back the tape (not during recording) sounds fine.

Any chance of some help on this one please and what could this be? I presume that the same heads are being used for monitoring during recording as the ones during playback? Particularly the high frequencies were affected as if the machine had dirty heads or in analogue terms as if the bias was set wrong.

Cheers

 

[hdrobien]

On Sony 4 heads DAT one pair is used for recording and normal playback, the other for playback during recording (monitoring).

So you can be fooled by a bad monitoring during a good recording.

 

[jascha]

Oh Gosh! So is it a case of the pair of heads being faulty or something to do with electronics?

 

[hdrobien]

Could be anything.

 

[jascha]

Have you had experience with a similar fault? Shall try to clean the heads again but it sounds like either the heads or an eq problem. Where would one look you reckon? Worth trying to trace the fault if possible of course

 

[jascha]

Not easy to see clearly what is going on on the shots. Either there is noise due to the probes or there are parasites in the signal from the rf amp.

Can you use visualize SWP (used as trigger) ? It is a square signal. Each change should start a frame, alternatively head A and heab B. All frames should look identical as the one I uploaded. If one frame out of two is different there is a problem on the head.

You can set the timebase to visualize only one frame and compare head A and B just by inverting the trigger slope.

And the ultimate question: do you still have parasite in the music especially at high level ?

Keep in mind that a good test for tracking adjustment is to listen to music in accelerated speed (push >> during play). The sound should be chopped but without any digital/parasite noise. If possible do this test in test mode with ATF disabled.

I have gone back to working on the 750 and have realised that because I was not using a proper probe (but just a multimeter tester cable with crocodile clips instead), I was getting different measurements. Indeed it seems that the cable I used to read the pbrf signal had interference due to the cable. What do you make of the new measurements now? They seem closer to what I'm trying to achieve I guess. Also the A/B reading seem close to being equal.
One question please: how can you tell what's a perfect setting while adjusting the guides regarding the edges of the image displayed on the scope. I mean once you get image to be rectangular and showing a pulse on each edge of the rectangle, how far more due you turn the guide? Should there be more pulses showing or just one? Difficult to explain in words as I don't know the technical terms yet.
Cheers

 

[hdrobien]

Well done for the probe. It is way better.

I believe the left (supply) is ok but not the right one (take up) as the frame is not rectangular shaped. Did you do this in test mode, accelerated speed, ATF off ?

 

[jascha]

Thanks . I did put it in test mode yes. Did that by shorting XTESTMAIN to GND and pressing the AMS >>> key. The display showed DPG75 and went into play mode but meters showed no audio signal. This is how I did my test. I set the REC mode to Standard and the Timer to PLAY. I am not sure if I understood that I have to put it in accelerated mode in test mode (by pressing I guess the fwd button?). When I did that the display DPG75 altered it's value and did not know what that refers to but i set it back as it was. Do i try accelerated play not in test mode?

 

[hdrobien]

Do not hit the fwd key (>>) but the ams key (>>I). If you hit the fwd key you will modify the DPG value (but you can go back with rewind key <<).

In test mode the ams key will put the deck in accelerated speed (x1.5).

The "ultimate" test (ie listen in fast play and hear just chopped music without any parasite) is to be performed not in test mode. My mistake, it can not be done in test mode.

 

[jascha]

Do not hit the fwd key (>>) but the ams key (>>I). If you hit the fwd key you will modify the DPG value (but you can go back with rewind key <<).

In test mode the ams key will put the deck in accelerated speed (x1.5).

The "ultimate" test (ie listen in fast play and hear just chopped music without any parasite) is to be performed not in test mode. My mistake, it can not be done in test mode.

This is the best shot I could take. I have tried fwd during playback and if I pick up a parasite here or there during ffwd it's probably normal no? It seems like it's tracking well during normal playback.
Indeed not for the faint hearted this stuff!!

 

[hdrobien]

Not bad but if the tracking is good there should no parasite in sound (only after 3-4 seconds when the speed gradually raise).

Try also to record a tape and to play it on your other deck.

 

[jascha]

Not bad but if the tracking is good there should no parasite in sound (only after 3-4 seconds when the speed gradually raise).

Try also to record a tape and to play it on your other deck.

I have done the alignment again as I had to dismantle the transport again as one of the guides was not sitting right on its white plastic tracks. The tiny white plastic bases seem to crack quite easily when looked under a magnifying glass! I have a pcm 2300 for spares so I swapped one. I am listening to a recording made on the 2700 and playing back on the 750. It seems all good and although the scope image flickers during alignment I carefully positioned the guides until I got a uniform image.
Hope this is an improvement. Just did the ffwd test and I don't pick up any parasites, as far as I can tell.
I will do a recording on the 750 and playback on the 2700 and see what happens.

What I wish to sort out is the playback monitoring during recording on the 2700, as it does not sound as sparkly as the input source. Dirty heads? I did clean them some time ago but maybe worth looking into the cleaning again?

Cheers

 

[hdrobien]

Good job, the shot looks pretty good.

If the ffwd test is positive and the tapes play on other decks we can conclude that the alignment is conclusive.

Concerning the monitoring on the 2700 I don't have any idea.