Studer Strangenesses(sss)

[bristolBobby]

Hello to all!

I know it's not good form to make my first post a plea for help, so I can also let you know that I have a lot of experience with 70's/80's CADAC mixing desks including A, B, G, E, J, M, 'Concert' and some custom stuff they made. I also have full schematics for the Soundcraft Magnetics range of Multitrack tape machines, and have spent A LOT of time fixing my SCM381-8 1" 8-Track machine, and have customised it quite a bit. So if anyone needs help with these things please let me know.

My own problem is with my Studer B67 MkII, which I have owned and used happily for about four years. The trouble started when I made a varispeed card for it, and although I checked and rechecked it for errors I clearly got something very wrong as when I started up the machine with the card connected the capstan started to spin extremely fast, and then slowed to a halt as the +12V line's fuse popped.

With the varispeed card removed and the fuse replaced the state of affairs is this:

1) When the machine is switched on the Capstan motor spins momentarily and slows to a stop, it is as if a single pulse of DC has been applied, and it stops rotating under its own momentum. It's just a twitch but it shows that the motor is alive at least.

2) none of the three speed select switches illuminate, except ever so briefly when the machine is switched off.

3) all of the voltages as measured on the psu test points are a long way from where they should be;
-20V = -28V
+24V = +36V
-12V = -16V
+5V = +6.8V
+12V = +16V
+20V = +29.5
This is with the voltage selector set to 240V 50Hz as it should be ..surely.

3) I can only test the transport without a tape on it as the pinch roller will trap the tape against the stationary capstan otherwise. Without a tape on it behaves as expected, motors spinning to take up tension which is not there to be taken up.

My first suspect was the capstan motor, but that seems at least to be in one piece as indicated by its little bit of movement. The capstan control card, then would surely be the culprit, although this does contain a rather hard to find tda1000 chip. But the PSU readings really suprised me. Could it be that I simply have a higher mains voltage where I live in Bristol? I wonder because even the +24 +20/-20 rails are way off the mark, and they come pretty much straight off the transformer.

I wish I knew what the voltages were before when the machine was working fine.

Does anybody out there recognise this oddness at all?

cheers
Bobby (of Bristol).

[Radio Wrangler]

Do all those power rails use a single voltage reference?

Otherwise I'd be checking my voltmeter very carefully.

David

[Vicboduk]

Hi Bobby, welcome to the forum.
I’ve just looked at the PSU circuit and as you say, very straight forward. I agree you need to make sure that the voltages are correct before you go any further. The only common reference is 0v.
Do as David suggests – check your meter first. If it is OK, I would check for continuity of the 0v line between each of the supply rails, they should all be linked. It’s possible that when you had your fault a 0v PCB track blew open thus losing a reference to 0v. You could then be measuring the difference between supplies which would read high.

Your concern with a high mains input voltage would only affect the non regulated rails.
The regulated +5v, +12v & -12v should still be correct but the regulators will be running hotter as they would be dissipating more power.

Vic

[bristolBobby]

Hi David,

The 0V rails associated with each of these power rails are tied together and to the chassis so I measured all the voltages in relation to the 0V tag on the PSU test point.

However I will get a new 9V battery today with my groceries and test again.

thanks,
Bobby.

[bristolBobby]

And so with head screwed on properly this time , and a fresh battery in the meter, the voltages now read:
-20V = -22V
+24V = +27.6V
-12V = -12.2V
+5V = +5.2V
+12V = +12.4V
+20V = +22.3

...which is pretty acceptable at this stage. absolute symmetry on the +/-12V should be attained by the stabaliser card in the audio rack. But playing the tape has to be the priority

Later on I will pull out the capstan speed card leaving it connected and bolt it to the outside of the chassis so I can access the test point on it and try to work out what is not working. The manual does go very briefly over what the various stages do, but it will take some re-reading and a lot of testing to get my head around it...

Being as the +12V fuse was the one to go originally, would it be a good idea to look at parts of the card powered from this rail?

cheers
Bobby.

[Michael Maurice]

The only parts on the speed control card powered from the 12V rail are the RC4558 IC's and the dual fet.

What we really need to know is where you got the varyspeed card from or perhaps its circuit to see it would interface with the B67's capstan control circuit.

Do you have a service manual? if not I suggest you dont do anything until you have access to one.

Michael

[bristolBobby]

Hi Michael,

Yes I have the manual as a pdf. I made the varispeed card myself on veroboard, from the schematic in the manual.
Perhaps the thing to do then would be to dig the card out again and inspect it until I can see what the error may have been. That seems a very good idea.

After a bit more testing of the machine, with a meter measuring the voltage on the Test Point on the speed card I see a momentary 3-5 volts (coinciding with the small movement of the motor) falling immediately to 0.4V and remaining there until switch off when it falls very slowly to zero.

Also the slight motor twitch does not always occur on startup. It seems to depend upon how long the machine has been left switched on the previous time, and how long between.

...anyway yes, great idea thanks about checking the card i made again. I will do that later today/tomorrow morning (its 2:51am here atm)

cheers
Bobby.

[Michael Maurice]

I've looked at the circuits for both the capstan speed control card and the varyspeed card.

The main interface between the two is Y refext and S refext, both go to the TDA1000. i'f you've somehow got more than a couple of volts on these points and especially if its exceeded 5V then you may have damaged that chip.

I would start by seeing what comes out of Pin 7 of the TDA1000.

I would concentrate on getting the capstan running then try and find yourself a s/h varyspeed card. They are available from various sources.

PS: Do you know that a) the capstan wont turn unless a speed is selected? b) the speed lamps wont light until the capstan is locked to that speed?

[bristolBobby]

Hi Michael

Yes thanks I read the section in the service manual about the workings of the capstan speed card. As noted in my previous post I measured the voltage at TP1, which is the voltage at pin 7 of TDA1000. What I should see on a working machine is for this to go high until the motor reaches the correct speed, then the voltage should become a pulse waveform to maintain this speed. What I see ATM is the voltage going high momentarily and the motor twitching, then the voltage falls to 0.4V and the motor slows to a halt.

The Y refext and S refext from my varispeed came from a SN74LS132 and a BC108B both operating from the +5V rail. However as previously noted the +5V rail on my machine is actually +5.2V. So therefore during operation the input to the TDA1000 would more than likely have exceeded 5V. If this was enough to mangle the TDA chip it maybe could have latched it's output high, thus driving all of the filter, multiplicator and output amp circuitry unnecessarily hard, speeding up the motor out of all proportion until the +12V fuse blew.
Maybe. - does this sound feasible?

If so I will need to get a new TDA1000.
I see that LittleDiode.com carries TDA1000 chips for £23, which yes is pricey but really the only other option would be to program a PIC or similar to behave in the same way, or build a little board with CMOS logic to replicate the actions of the TDA1000. Both of these options are skills that I once had but have now sadly lost.

cheers
Bobby.

[Michael Maurice]

What supplies does the varyspeed card use?

I can't see even if the op amps were driven to the extreme why it would cause the 12V rail to fuse.

[bristolBobby]

The varispeed uses +24V for the two bulbs, +/-12V (and 0V) for the 555 timers and associated transistor circuitry, and for the op amps. The +5V is used for the logic chip and bc108b supplying S-refext and Y-refext, the higher level voltages having been bled down before reaching this stage by 3.9V Zeners.

A problem has arisen though, as I was looking again at the varispeed board it triggered a vague memory of correcting an accidental solder bridge on the board when I first re-inspected it after the motor stopped working. This was quite some time ago as I have moved my studio across the country from London since then. I had completely forgotten this, and I cannot remember /where/ the bridge was.

This being the case it is going to be impossible to deduce what was wrong with the varispeed board back then. It appears fine now on visual inspection and there are no shorts between rails.

Things being as they are I am feeling inclined to order a TDA1000 when they come back in stock. If it makes no difference then it will be a very valuable spare to have for the future. The existing chip certainly is not behaving as it should. It plays such a major part that surely the current behavior must be because of it's failure.

as far as a future varispeed is concerned there is a much simpler method described in the manual where a square wave (say from a bench oscillator) +/- 12% of the frequency of the internal crystal, oscillating between 0-0.5V and 2-5V can be applied across S-refext and Y-refext via the 25 pin D-connector on the machine. There is also a switch and a bulb involved for switching between quartz and external reference, it's explained in a diagram on page 17 of the pdf manual.