Tascam MS-16 , selectively passing audio

[man-bot]

I have a Tascam MS-16 in reasonably good shape I have aquired however I am having an issue with 4 of the rec/play cards.

On these 4 cards they will record 10khz just fine, however as the frequency is moved lower the level goes down as well.  At 2khz there is no signal registering. 

The problem is definitely with the audio cards as it follows them to know good channels, and vice versa - good cards work fine in the channels where the faulty cards typically reside.

This is where I have traced the problem to:

I have swapped C22, C24 and R49 as well as the bias trap with no luck in fixing this and am feeling a bit stuck.  Everything seems fine up to where C22 goes to C24 and R49 - this is where I see a voltage drop as the frequency is swept lower.

I am using the oscillator on my mixing desk to provide signal and tracing with a fluke scopemeter.

I you have any thoughts of what to try I'm open!

You can grab the full schematic if requried at home.cogeco.ca/~michael.lett/

Mike

 

[radardoug]

At the point you are looking at there is H.F. boost provided anyway.  look at U3 pin1. The signal should be flat there 20 to 20K.
Most likely loss of l.f. is dry caps.
Do you know that your repro side is flat? have you run a repro test tape on the machine?

 

[moamps]

Hi,
did you check the HF bias generator (U7)?
Regards,
Milan

 

[man-bot]

Heyo,

Radardoug - at U3 pin 1 all is good throughout the sweep.  In repro mode (be it playback from a calibration tape or recording at certain frequencies) only 10K to 2k shows level... nothing a 1K and below.

Moamps - I haven't checked U7.  I'll probe voltage on a good card and check against the problematic ones.  But if I still don't have 1K and below in playback would bias be the culprit?  I understand in record mode that could be an issue but if playback is the same would it still point to that?

Thanks for the help so far.

 

[Brian Roth]

Heyo,

Radardoug - at U3 pin 1 all is good throughout the sweep.  In repro mode (be it playback from a calibration tape or recording at certain frequencies) only 10K to 2k shows level... nothing a 1K and below.

Just trying to 100% understand here.....A known good calibration tape (MRL or whatever) shows the low frequency loss on playback?  If so, then the problem is in the playback path, unless I'm totally confused.  Do you see the same problem with Sync playback?

Bri

 

[man-bot]

Hi Brian,

OK let me clarify as I just retested.

No playback from calibration tape at all in SYNC.  Only playback from calibration tape in REPRO at 10 khz and above - nothing below.

When I tested it before - recording in REPRO - a +4 signal at 10kHZ reads 0db on the meter then will gradually go down in level as the oscillator is swept down until at about 2 khz there is no level on the meter (nor sound at the monitors).

Hope that makes it clearer - I know it is a bit muddy my description.

 

[Brian Roth]

OK, if the "known good" test tape does NOT play back properly at low frequencies on the bad channels, then you need to chase THAT problem in the repro circuits before worrying about the record circuits.

Bri

 

[moamps]

..... But if I still don't have 1K and below in playback would bias be the culprit? ......

No, of course. You should first confirm flat frequency response in the playback mode on the test point TP1.
If there is a LF drop, the problem might be due the worn, dirty relais contacts. I had once lot of issues with the relais in the Tascam 85-16.
Also check out DC offset on TP1.
Regards,
Milan

 

[man-bot]

Okay bit more info...

Brian - to be honest I'm struggling with where I need to be looking!  Clearly the section I posted before was in the recording portion of the circuit.  Below is another image of what I have found (in what I believe) is the playback portion.

X means no 1k present whe probed, 0 means 1k is there - this is playback in repro from my MRL

Does this make any more sense?

Moamps - at TP1 there is no 1k on playback in repro from the MRL.  I have changed the relays already.  I'm not sure how to check DC offset on this machine.

Mike

 

[Brian Roth]

I don't have the service info on that machine, so no schematic of the repro amp for me to examine.  Is the full schemo available somewhere?

Bri

 

[man-bot]

Hi Brian,

You can grab the full rec/play card schematic off my site home.cogeco.ca/~michael.lett/.  The file is MS-16 audio schematic.

Thanks!

Mike

 

[Brian Roth]

Hmmmm...a few odd things in that schemo...oh well.  Are you using an o'scope for your pobing/testing?

On a working channel, look at pin 3 of U1 while running the test tape.  The response will NOT be flat, since U1 and U2 do the EQ correction.  But, you will have a "baseline" for comparison with a funky channel.  See if the funky channel follows approximately the same response curve.

Bri

 

[man-bot]

Hi Brian,

On the good channel while playing the MRL on the 1khz tone I get a reading that fluctuates but is between 950 hz and 1.2 kHz, at 10k it is about 9.96khz both are a nice sine wav.

On the funky channel when the 1khz tone is played it is just a fuzzy line jumping rapidly and randomly between 3 and 20mhz (definitely mhz is what the meter shows), at the 10 k tone it is about 9.97 kHz and steady with a nice sine.

FYI - I'm use a fluke scope meter to probe.

Mike

 

[Brian Roth]

I am now suspicious of the relay K1.  Unfortunately, the levels there might be  too low to see with your particular scope.  You MIGHT be able to see a signal at the end of R1 that connects to the relay in a good channel.  If so, compare that to a funky channel.

I've had problems with the relays in both Tascam and Otari decks.  Many of the relays were not properly sealed and the contacts go bad.

I have a hunch you might be seeing the 10K on a funky channel because of capacitive coupling/leakage through the relay.

Bri

 

[man-bot]

Hi Brian,

I have swapped out the relays in these channels with new Omron relays. I have swapped relays between good and funky channels as well and the new relays operate fine in the good channels.

All relays are socketed with mill-max sockets...  I guess I could reflow all the relay solder joints and report back.

Mike

UPDATE: Reflowing relay sockets and changing relays did not do anything unfortunately.

 

[radardoug]

Do you have an extender card? Check with the electronics card removed whether you have continuity from the reproduce head to the appropriate contacts on the extender card.
Extreme lack of l.f. is one of two things, head only connected by one leg or faulty caps.

 

[man-bot]

Hi radardoug - I built an "extender" with ribbon cable and molex connectors.  I do not believe it is a continuity issue as you describe as known good cards reproduce and record properly when loaded in those slots.

Thanks

 

[Brian Roth]

I presume you weren't able to see signal with your scope at R1 on a good channel or a funky channel?  The signal level there will be very small.

Just for grins, try scoping the signal at the junction of R18, R19, and R20 on both a good and bad channel.  I would think it should be the same as the signal at U1/pin 3, but I'm a bit unclear on the function of U1 pins5/6/7 (looks like a DC servo??).

If the same bad results are at that R18/19/20 node and the sync relay K1 is "known good", there is a mysterious problem in the Q1/3/4/5/6/7/8 block.

Stumped,

Bri

 

[moamps]

.... but I'm a bit unclear on the function of U1 pins5/6/7 (looks like a DC servo??).

Hi,
the DC servo is needed because of high DC gain of the discrete repro amplifier.
Man-bot,  please also measure DC voltage on TP1 and the junction R18-R19-R20.
Regards,
Milan

 

[Brian Roth]

.... but I'm a bit unclear on the function of U1 pins5/6/7 (looks like a DC servo??).

Hi,
the DC servo is needed because of high DC gain of the discrete repro amplifier.
Man-bot,  please also measure DC voltage on TP1 and the junction R18-R19-R20.
Regards,
Milan

Well, what was confusing to me is the fact that servo is wrapped around the two opamp stages that do the EQ adjustments, and the servo appears to have NOTHING to do with the DC operation of the discrete preamp front end.

Bri