Studer B67 follies

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I just acquired one of these (7.5-15-30ips) through ebay for pretty cheap, but it is missing some pieces which I found replacements for, except no back or top or bottom panels which I still would want to find original parts for. (if somebody knows a person with some spare B67 back/top/bottom panels cluttering up their storage space, I would like to know) The heads in the unit look surprisingly unworn, so I wonder what its history was - did something major give out early in its life and so it was put in storage early? IAC, I should get some idea tonight when I power it up for the first time. Unfortunately, it has no pinch roller at the moment so I can't actually run a tape on it yet (I've got one on order from Germany, though.)

One thing I was considering was converting it to vacuum tube audio, so I pulled its tech manual. I was shocked to see that this high level mastering machine merely used around 2K Ohm resistance at the output of a NE5532 type amplifier (TDA1034B) to drive the music signal to the record head and that the playback amplifier first stage has a 330 ohm cathode, er...emitter resistor. The good news about that is that it should make conversion to tube audio much more straightforward than feared. I was afraid that these 'solid state' heads might be much harder to get good signal in and out of using tubes, but that may not be so much the case, given this.

Might get the vacuum tubes in the original box if I move the bias oscillator board over to the end - it was apparently set up to run with a separate bridge panel in a cart (which is why no back panel, etc), so I can punch out the existing bottom front panel for pots/meters for a more portable arrangement. I plan to keep it as a pretty basic line level deck, although adding two tube mic pres is a possibility - space and heat are probably already fairly marginal without tube mic pre's but I can work on that possibility.

I plan to add a couple playback features also including a tube phase correction circuit and a tube peak unlimit circuit that should extract 2-3 more headroom from tapes, potentially including those recorded on other machines. I've successfully prototyped both of these circuits a long time ago (need to find any notes), and was able to restore the peaks on a triangle waveform recorded 3 db into saturation with the peak unlimit and to get linear phase response to 21 Khz at 7.5 ips with the phase correction circuit (have an old scope photo of a 7 khz square wave with correctly phased 3rd harmonic with a record/playback signal). I also acquired a half worn set of EIA type 'butterfly' heads 'cheap' for it for another db of S/N so might be able to get overall SN equivalent to 2 track half inch with this quarter inch machine.
 
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Some kinda bad news after power up. I noticed the left reel rotor always ran at full torque when enabled, and a few minutes into operation, a part on that side let its magic smoke out, whereupon the left reel rotor began running at full torque whenever power was on.

My initial response will be to replace all the more susceptible components for that reel motor torque control schematic (which are all located on a fairly small pcb next to that reel motor where there was a failure, except for a TO-3 case power transistor, which I may also replace) and hope the damage is limited just to that location.

Everything else appeared to come up normally except for a couple sticky tape lifters and the counter pushbutton - no fuses have popped so far.
 
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Still waiting on some parts to fix the left reel motor drive, as well as the pinch roller. I came across a Modern Recording and Music review of the B67 where they measured an A-weighted SN ration of 77 db with presumably a NAB eq'd machine using an Ampex 456 equivalent tape.

With EIA heads and my peak unlimiting circuit and a hotter tape than Ampex 456, SN's exceeding 80 db seem quite possible. Actually, I found one way to gainseverl db beyond this. If a tape is bulk erased, and then the record head is turned off during recording, that seems to drop the background noise by several db when recording. Anybody else notice this? Possibly mid-80's then if the electronics are quiet enough.
 
Ooops. In my last post, I meant 'turn off the erase head', not the record head for lower noise on a bulk erased tape. This makes me wonder if an erase head with a more slowly diminishing field might result in quieter tapes since it would approximate the slow field decrease of a bulk eraser a bit better?

Well, this evening, I got some of the replacement parts for the left reel driver board. When I pulled the board out to test it, a chunk of filter capacitor fell out, providing a clue. It turns out that one cap (0.47uf, 150 VAC) had gone short. There were three of them, so I replaced all three with 275VAC Grade B 0.47uF, and did a diode check of the power transistor and diode bridge which appeared ok. So I put it all back together and powered it up. It now appears both reel motors are working properly. So, all I need is to receive the pinch roller to start rolling tapes, it appears.

Of course there are still a few problems with the transport. It appeared to have been dropped at one time since there is a little distortion of the lower frame which shouldn't be too hard to deal with. The digital time counter has a couple of dead segments which I attribute to the display LEDs - need to troubleshoot that, plus its binding reset switch. And an anti-magnetic shield that is supposed to pop up in front of the playback head isn't working. However, the practical headstack I ordered also has one, so that might be a simple swap.

So, before the end of this week, I might be able to try running a few tapes through this machine.
 
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got the pinch roller last night, was able to roll a reel of tape, and checked the playback audio path. I see that since the unit was previously a horizontal case mount, it presumably once had an external VU bridge and some control switches that are now missing, so there was a break in the audio path for playback going through an external DB connector.

I found the point previous to that where the playback audio was routed to this external connector and tonight will try wiring that up to a pair of rca jacks and try playing through my HT initially. Assuming that works out, I can then try recording, but will have to wire an external potentiometer to the input circuit for that.
 
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Soldered a pair of RCA jacks to the playback amplifier and ran some tape I had prerecorded from the early '90's. Overall, the operation is pretty sweet, and the machine is not too noisy.

I just found somebody in GB who is selling me the external panels that were missing with the original machine. Problem is, the price he is charging me for the panels actually exceeds the price of the original machine I paid (out of a rollaround rack) which is killing me, but I really felt like I had to bite the bullet here since several inquiries in the US failed to find anybody with spare external panels. I am still going to make my own side panels out of wood, though, since on the market, people are charging $300 and up for a pair.
 
Well, I pulled out most of my old tapes I did on 10.5" reels, and I had forgot that I did most of them on Maxell UDXLII at 3 3/4 ips (mostly recorded from turntable). Of course the Studer won't play that speed since it is 7.5 - 30 ips, so I guess I will stay with my modified Ampex 351 for these tapes for my listening room.

Of course, some people might be wondering how I could have run Maxell UDXLII on a tube Ampex. The answer is that in the early '90's, I bought some Saki glass ferrite heads for it and reworked the bias oscillator to run at about 500 khz into the record head with a tube version of Dolby HX. I believe I was getting decent highs up to 15khz at 3 3/4 ips this way - UDXLII was supposed to be an iron oxide equivalent to 'chrome' or 'metal' cassette tapes back in the day, and really did feature super hot highs at low speeds compared to standard open reel tapes.

Anybody remember Saki glass ferrite heads? They were supposed to be pretty good quality back then, but now I don't see any discussion of them.
 
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Never heard of Saki heads until you mentioned it in your previous post, perhaps they were available only for a short period of time.

Sounds like the Studer is coming along pretty well. I spent a lot of time and effort, and some money on the much less worthy G36 a few years ago. I should have started with a better machine.. :D
 
I know there was some debate at the time about whether glass-ferrites sounded as good as some other head construction types, or could take enough bias, but they seemed ok there to me, although I never found out exactly what this debate was about, and I really liked the idea of their not wearing out quickly since I could detect noticeable wear on the original Ampex heads from my use within a couple of years.
 
Well, I really did it this time. I won another busted B67 on Ebay - this one a slow speed model that is supposed to 'not make tape move'. I figured if I could get it going and add tube HX (which I've been working on on paper and believe I've simplified quite a bit) instead of a mic preamp, it would make a good machine for my stereo. I could add a 4-track playback head perhaps (switchable) in place of the stock tape scissors (that make me nervous) & switch at top of head block in that opening, and play tapes recorded that way at speeds down to 3 3/4 ips, as well as my Maxell EE tapes, if I replace the existing PB head with a B77 or PR99 or A700 type which is supposed to have a narrower gap.

I had a fairly easy time bringing up the first B67, so I am hoping for good success with this one also. One area of concern is that the transport operation is supposed to be controlled by a special IC, and if this is blown, I could see some issues/delays in getting it repaired. The person who put it up for auction claims that the motor control transistors could be blown, and that is certainly possible, but to see both reel motors *and* the capstan motor all out at once due to that raises a couple of questions. IAC, I still have some spare transistors and capacitors I didn't need to fix the first machine, so who knows?

It could also be a fuse, but I would assume that he would have ruled that out. However, you never know with the lateral fuse clips. I remember a long time ago that some guy at work wanted to sell me his Hafler DH220 cheap because he said one channel was out. Since we worked in electronics, I opened the amp up right at work and began poking around the fuse holders for each channel. All of a sudden the 'bad' channel came up just fine, and this guy decided he didn't want to sell after all. So, basically, I fixed it for him for free. What a ripoff.
 
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Got the second B67 last night. There's both good and bad news regarding it. I was able to get it to 'run' a tape in play, rewind and fast forward - perhaps its journey from Canada remade a connection? The bad news is that someone had modified it to run at one speed only regardless of speed selection and had added a switch to reverse the tape direction. Also, due to this mod, the speed feedback control is disabled, so the speed is hardly constant - I presume it was used as some sort of special effects machine.

So what I need to do is to figure out what I need to do to restore the original 3 speed operation. Hopefully, it will just be rewiring, but I better check the capstan control board to see if there are any cuts or jumps on it to avoid surprises.

It is also a mono machine, not stereo as claimed in the ebay ad, but that, for me, is not a major obstacle since I have the set of stereo IEC heads (needing relapping) that I can retrofit to its head block. Then I would swap the head block to the other 30ips machine and use the unworn head block for this machine. Perhaps I can also find a buyer for the mono heads - they look almost unworn. It was also apparently in a heavily cigarette smoke filled environment for much of its life - it has a definite dead tobacco smell that hopefully I can minimize.
 
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Well, I removed the reverse speed mod and now the capstan only turns in the proper direction. However, I also found that an IC (TDA1000) had been removed from the capstan control board which would pretty much disable any capstan speed control. So I now have one of these ICs on order and a bunch of capacitors to replace the original lytics and tants, starting with this board.

According to an internet search, the TDA1000 which is used in the A700, A67 and B67 is notorious for not having the best reliability. I have a suspicion that this may be because it connects directly with other boards and the pushbutton panel in this design without any local protection, so I also ordered some 5V and 12V TVS diodes to clamp its external I/O ports.
 
Well, I traced three outputs and three inputs of the TDA1000. The outputs are switched by the speed control switches and then run all over the back of the audio "masterboard" and the digital inputs go straight to the external speed control input at the top of the chassis with little or no protection.

Not too surprising given this that maybe the TDA1000's got an unreliable reputation. I could imagine somebody hot inserting an audio card or a speed control, and zzzzz-aaaa-pppp! - the capstan stops or spins out of control.

I think I will add 100 ohm series resistors to all that have no resistance and then add the TVS's I mentioned in the last post.
 
I cleaned and recapped the capstan speed control circuit board, including replacing the 'lytics with fresh ones at higher voltage values as well as all tantalums with ceramic styles (except for one value I forgot to order), and added 47 ohm resistors in series with all off board I/O coming to and from the TDA1000. I decided to not put the TVS's on yet due to the space issues I already had cutting the traces and adding the 47 ohm resistors (which are 1/16 watt types, physically very small) on the pcb. I figure the 47 ohm resistors by themselves will offer some protection to the TDA1000. I can add some sort of TVS protection later, perhaps. Like if it pops this TDA1000.

Well, I put it all together, and now I have all three speeds that the B67 was designed for. Of course, I will never know whether the original TDA1000 was removed because it failed, or whether it was removed to allow the reversing speed option.

So, after this, I put on a reel of tape and put the transport through its paces - it seemed to do everything it is supposed to do properly, including run at 3 3/4, 7 1/2 and 15 ips, fast forward, rewind, and perform its special functions as far as I could check them.

I still appear to have a problem with the timing counter on this machine - it only runs for the most part when the machine is rewinding, so something new to troubleshoot, but at least this problem doesn't affect any basic transport function, and I can begin to modify this machine with tube electronics, etc.

I'm thinking of adding unbalanced line I/O through RCA jacks for the most direct electronic access 'stereo' operation of this machine accessible from near the bottom of the back panel and retaining the original XLR I/O to be used with the original transformers that Studer had applied in its original design for the B67. However, since this B67 turned out to be mono, I have only one set of audio transformers, so ideally, I should try to find a second set cheap or use substitutes....
 
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Was able to fix the timing counter by reseating the chips on the counter circuit board. However, the board should be cleaned up since some of the IC connections to the sockets seem to be iffy. It's interesting with this B67 that all the ICs are socketed instead of being soldered into the PCBs. At least nothing is fundamentally wrong with the circuit.


I also 'filled in' the holes the previous owner drilled for the switch and LED and even tried to anodize the new aluminum with lye and a home power supply. It kind of worked, but doesn't match the original clear anodized finish that closely, I think partly because the surface smoothness is different. Also, the aluminum I used to fill in the gaps anodized darker than the rest, so I'll probably find some matte silver dye or paint and kind of cover it up:)
 
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Now that both transports appear functional, I am starting to work on the tube electronics. I will start with the mono machine which needs to be converted to stereo anyway, beginning with the HX circuit which needs the most work to optimize. I have sketched out a schematic version of something to test here that might be doable with a single compactron for each channel's HX bias control. I figure compactrons are good packages here because they combine more elements and have wider pin arrangements and lower heights for some types than standard miniature tubes so should be more stable mechanically. Plus, if the one I want to use works out, it'll still be quite cheap:)
 
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Hi -

I can draw a schematic representation of what I'm thinking of. One interesting thing is that Studer is using a ferrite transformer to add the record bias as a series voltage summation added to the record signal, which also seems like a good approach to be implemented with tubes. I have a couple candidates for the transformer (which interestingly enough are basically 'pulse' types typically used to drive MOSFETs and IGBTs) but have contacted their manufacturers regarding a couple of technical questions re this application (however without telling them it's for a tube HX circuit!).
 
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I've attached a .gif of the tube HX concept circuit. If I can make this work ok, then I can manage it with a single compactron (dual diode, dual triode). There may be an issue with getting the output stage to provide sufficient drive, but in that case, I can probably add componentry and drive with a pentode (but then would need a second tube). The interstage transformer is an old UTD 0-7 "Ouncer" which I have a few of and which is designed with up to 2mA DC capability on the primary. I actually successfully made an HX circuit in the past using this part.

The exact point at which the bias trim starts to become effective is set by the pot which is shown with the wiper between the DC filament supply limits (but not showing the additional filtering to keep ripple out of this circuit).

To keep circuit complexity down, I'm actually driving the cathode of the second stage with a signal derived from a solid state circuit into a parallel resonant network (this may need some tweaking). This is where the bias level would be adjusted also (not shown to keep displayed circuit simple). A nice thing about this is that the bias is added *after* the record signal is already generated, so except for fairly simple filtering (as Studer employed), the two can be kept separate.

I look forward to any feedback on this.
 

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