Tandberg 64X woes

[ricard]

In my quest for bringing back old machines to life, I recently landed a Tandberg 64X in my collection. Never having worked on these it is interesting to see an upmarket version of the classic Tandberg design.

There are a couple of things which partly stump me though. First of all, I must admit that I have not (yet) replaced the coupling caps, and they are leaking quite a bit (i.e. the grids of the valves are typically at 0.7V instead of ground, and the cathodes are also a couple of hundred mV too high to keep up with the increased grid voltage. The third playback stage has a plate voltage of 30V rather than 50V due to this), so this could be (part of) the cause of my problems.

First of all, when adjusting the bias head, the service instructions say to record a 1 kc tone on tape, then adjust the sideways position of the bias head for maximum output on both tracks. Here it seems that the maximum occurs in two quite different positions. Odd. Secondly, after the physical positioning of the head has been completed, the instructions say to adjust the bias trimpots, again for maximum output from a recorded 1 kc tone. For the right-hand channel this seems to work fine, there is a clear level peak somewhere in the middle of the pot range. For the left-hand channel there is no maxium, the output level continues to rise as the bias is increased, eventually ending up with a bias voltage (measured across the bias head) of twice the value of the right hand channel. Checking the frequency response at 1 kc vs. 10 kc, for the left channel, which is now overbiased, this results in a drop of about 4 dB at 10 kc compared to 1 kc, whereas the right hand channel has a difference in level of 1 dB or less.

I've adjusted the hight according to the instructions (set in record mode, and set the height for maximum bias appearing at recording head).

So, something seems odd here. Listening to a recording though, the channels sound pretty similar, but that brings us to the third point: the recording performance is very low, with quite a bit of distortion at all signal levels. Furthermore the bass is lacking, and to a certain point the treble too, there's a slight 'telephone quality' about it all. Again, the same at maximum recording level as at -10 dB or -20 dB. Both channels are the same.

One thing that I have thought of is that this machine, being all valve based, has a fairly high output impedence; the output stage is a cathode follower with an output impedence of 2k, but the minimum recommended load is 10k to avoid distorsion. I don't know for certain what the input impedence of my amplifier is, it is a (transistorized) Tandberg Huldra 10 in this case, can't find any data on it, but its predecessor the Huldra 9 had an input impedence of 30k, so I would expect this one is similar. At least not below 10k.

So, anyone with any experiences to share? I could imagine the distortion to be higher than expected due to leaky coupling caps, but I wouldn't expect the frequency response to be so off. And the bias thing is odd too (this machine has a transistorized erase/bias oscillator, although most of the rest of the machine is valve based).

[DOFFERY]

As an old hand Ricard yourself it would be a start to replace the leaky caps. of which there are many, not only would the distortion be cured but possibly other problems. As for the bias settings, this as always is a balancing act with cross field machines, there are so many variables. There should be no problem matching with your Huldra 10 & the 6X cathode follower will match most amps. Not an easy machine to replace caps. nor set up from the manual ,bearing in mind this was for machines in excellent condition.

Regards.

Colin.

[ricard]

I agree with the cap replacement, it looks daunting primarily from the fact that not only do the two amplifier boards need to be removed, the eq switch linkage must be disconnected, it also looks like 6 wires or so need to be desoldered in order to fold the boards up - at least most of the wires are emanate from the other side of the board and can remain connected. Once that has been done it should be fairly easy from what I can see now.

Regarding setting up, I'm in the lucky position of having a set of Tandberg test tapes, meaning that I can actually set the playback level as per the manual (using Tandberg test tape #4), and work from there. I went through the procedure last night and I thought it was fairly easy. I had a similar experience when setting up a model 2000. They key was in setting the playback level properly which of course is impossible if you don't have a reference tape. As you say, a couple of compromises to be made given that the machine is not up to spec right now, I basically just wanted to see how well it responded to setting up to see if it was worth while going on with the cap replacement.

I was surprised to see a different set-up procedure for the 6X compared to the 1200X and 1600X. For the latter it basically says something like measure the induced bias voltage at the record/playback head, find the maximum by sliding the bias head back and forth, and then slide it to the left to drop the level by 5-10%. For the 6X the manual says record a 1kc signal and slide the head till a maximum is found in the played back signal. I suppose that procedure would have been too exhausting on a machine with combined record/playback head (i.e. the 1200X and 1600X), because one would have to make a recording, wind back, play back, re-set the head, lather, rince, repeat, hence the different instructions.

[bobbyball]

Hello, replacing the caps in the machine is a must -but isn't impossible, just give yourself plenty of time and space to do it. I have done several of these machines and the performance afterwards has been simply superb so it's well worth the task.

As you correctly deduce, the equalisation switch linkage needs to be removed and several wires unsoldered in order to "hinge" the boards up to get at the print side. Also check the valveholders as well, they can get very badly tarnished and cause all sorts of problems. In two of the machines I worked on I actually replaced all the valveholders, even those on the EAM86 magic eye valves and the PCB for these is especially hard to get at! This meant drilling larger holes for the new valveholders (90 in total!!) as the new ones had larger pins. However again, well worth the effort.

The manual is not easy to follow "to the letter" and working on these machines is definately a challenge but when corretly set up they are up amongst the best. I don't have the Tandberg test tapes sadly but eventually got it right. I sat recording a tape of the Beatles and switching between "before" and "after" recording - I could not tell the difference!

Robert

[ricard]

Thanks Robert, it is enouraging just to know what can be acheived. Doing an A-B test last night (on BASF LH35 tape) there was definitely a very large difference between the original signal and the signal from the tape.

It struck me that I have never worked on this old a three-head machine before, and wasn't really sure what performance was obtainable. I know my Tandberg 9100X sounds great even at 3 3/4, but of course it was made a decade later so it's hard to know if the same level of performance can in fact be had by the older ones. A machine that's closer in vintage is my Beocord 2000 which does sound very good.

A couple of the valve holders seem to have oxidized slightly, but just moving the valves around a bit in the holders seems to have cured that to a large extent, so I think some contact cleaner will be my first resort. They seem fairly high grade to me anyway.

Did you replace all caps in your machine? I thought I'd start with the coupling caps, as that's where leakege could cause problems in terms of incorrect valve biasing or scratchy pots.

[bobbyball]

HI Ricard, yes I replaced all the caps in the machines but with the exception of the HT smoothing ones as these seemed OK. The electrolytic capacitors on the two PCB's were a strange 4 legged design with the positive plate in the middle. The designers clearly made use of the three negative pins as I had to make some wire links to maintin earth paths so watch out there! It may have not been necessary to replace these electrolytics but since the machine is such a fiddle to dismantle I didn't want to have to take it to pieces again and the capacitors I used, although of high quality, were not excessively expensive.

I think I might have just been very unlucky with my bad valveholders as the third machine I did was OK in this respect and I just cleaned them very well.

I read again your comment about how "awful" the recording sounded when you first tried the machine and it was just the same for me - how could such a supposedly "great" machine sound so bad? It was then that I realised that capacitor trouble would probably be the issue and sure enough many of the coupling capaditors (branded "Rifa") were visibly cracked and must have allowed air/mositure to get in and ruin them. They were all replaced, without exception, and ample proof of their "guilt" was correct deflection of the magic eyes with the new parts fitted. It's quite a fiddle setting up the levels as the preset pots are not easy to get to, however I'm sure you will do fine!

Robert

[ricard]

The electrolytics which I think are the most suspect are the ones decoupling the valve cathodes. They have got less than a volt across them and I've seen from other designs that electrolytics with that low a voltage across them tend to degrade over time, I think because the polarizing voltage is not large enough to properly maintain the isolating oxide layer inside.

Thanks for the heads-up regarding the four-legged cap design.

The thing that has me a bit stumped is that the machine sounds fine in "A-test" i.e. monitoring the input directly to the output. However, not all the parts of the amplifier are in that signal chain, only two valve sections (per channel) in the recording amplifier, and one (the cathode follower) in the playback amplifier.

One thing I noticed now is that if the coupling capacitor from the final recording amp stage starts leaking, you'll get DC bias on the recording, which will certainly raise the noise floor if nothing else.

The pots are not easy to get at but I've seen worse. I used a long plastic tool really intended for trimming coils, to avoid any risk of short circuits. For the oscillator trimpots and trimmers I have to prop the machine up on its back though to get at the oscillator board. Thankfully the pots on the amp pcbs can be reached with the machine just lying flat.

It's certainly easier to get at the things than on another 'corpse' I've got lying around, a model 12, where the record/playback switch needs a thurough cleaning, and I'll probably have to remove the complete PCB in order to dismantle so I can give the parts a good scrub with contact cleaner and possibly polish them. And even then they don't always respond favorably and continue to snap, cracke and pop. Really a bad design.

[bobbyball]

I think with your "A-test" results you are just lucky. If you have the time and patience, a "blanket" recap and subsequent set up will give you years of trustworthy service.

The machines are indeed awkward to work on and are very compact for what has been squeezed into them. The mechanics are well worth adjusting - the turntables on the machines I have all had too much vertical "play" and as a consequence the actions of the various clutches were not to specification. It's a clever design, not having to run the capstan during forward and reverse fast wind, but depends on the integrity of the main drive belt. I'm sure you know all this already anyway.

The 62/64x series are probably my favourite tape recorder of all time and I had to stop myself buying another one at a collectors fair (NVCF to those in England!) - I have four already!

Robert

[TIMTAPE]

Ricard,

Re the bias settings on the bad channel it's possible the two heads are not parallel in the vertical plane, (tilt forward and back) with the result that the bias head's gap is too far away from the tape, resulting in underbias that cant be compensated with by the trim pot.

The same could apply in the horizontal plane re the azimuth angle. You could have the angles right for one track but not the other.
The bias head needs to accurately bias the tape right at the point where the record head imprints its signal.

Cheers Tim

[ricard]

Thanks Tim, I hadn't really thought of that, I'll carefully check the mechanical alignment of the record and bias heads. In fact, the record head height was off when I got the machine, so I think someone has tried to fiddle with it before, always a bad starting point.

Having once gotten a 1200X going, when parts of the bias head mechanism was missing, leaving me to figure out how it all was supposed to work, makes me feel vaguely hopeful I'll sort it out in the end...

[ricard]

Just a small update, in case anyone reads this thread in the future for reference, spent the better part of the evening replacing coupling capacitors. I went for all coupling capacitors from the valve anodes, as well as two 0.1µF caps not in the schematics, feeding the record head, and also the two 0.022µF caps in the signal line from the playback heads. I did not go for the 16µF power line decoupling caps (the large cans with four legs) or the 22µF cathode decoupling ones. All in all about 20 or so, mostly 0.047µF, some 0.022µF. I figured if I don't remount all the screws for the PCB's or the complete linkage for the equalization switch for the test run, it wouldn't be too much of a chore to remove the PCB again and replace some more caps.

Only had time for a quick test recording at low (speaker) volume - it was getting late! Initial impression is that the machine sounds the way it should now. Have not measured valve voltages yet, but I really should, especially the cathodes, as I'm still a bit suspicious of the cathode decoupling caps.

Yeah, I could have replaceed all the caps while I was at it, but I really don't like replacing components that are still servicable (which is why I left a few Rifas in there which didn't appear to have any appreciable amount of DC across them, as well as the electrolytics). Figure out what's wrong, and fix that, sort of. Besides, it would change the appearence rather a lot if I'd replaced the large 16µF/350V cans (not that that matters as much as with a radio; the electronics is inside the case and inside a metal cage to boot). And once you've replaced all the caps, why stop there, why not replace all resistors with modern low tolerance low noise types too ... and the valves while you're at it, and then you could redo the PCB so it's made of glass fiber rather than phenolic, new wiring etc etc ... for me it quickly gets over the top once I've replaced the components that obviously are causing problems.

[bobbyball]

Well done, glad it works better now...

Robert

[ricard]

Yeah, next step is to set it up correctly and see how it does. Well, it wouldn't have been if I hadn't zapped one of the output transistors on the oscillator board. It was on the right channel which was behaving suspect anyway, so it could have been that it was failing anyway. New ones ordered, should get to me today, so see if I can install them tonight.

Interestingly enough, the transistor in that failed was a TIS60M, which is sub spec compared to the 2N3705 that's listed in the schematics (most notably regarding maximum collector current), but as the it doesn't look like any replacement has been made previously in the board, I guess the 2N3705 was overspeced and subsequently replaced by the (possibly cheaper) TI transistor during production. I in turn substituted a BC337/327 pair which was readily available for me.

[bobbyball]

Never had any problems with the bias oscillator, just changed the caps while I was doing the other bits...

Another nice thing on these machines is that the heads seem to last, none of mine really look worn. The auto stop spring is a bit of a fiddle but usually works ok...

Keep at it!

Robert

[ricard]

It could have been my own fault that the output transistor blew, I found a piece of desoldering braid lying in the machine when I turned it over, so it could have damaged the output transistor. Anyway, a pair of BC327/337s installed, seems to work fine.

I set the machine up tonight. As someone mentioned it can be tricky, but the real tricky part is to do the things in the right order! In the service manual, a rough bias setting is made, and then the bias head adjusted mechanically. I found that if the bias was way off on one of the two channels, moving the bias head would yield different maxima depending on which channel was being tested. With the bias set correctly, the maximum point was the same for both channels.

Setting the bias is different on this recorder than on ones with conventional biasing. The service manual says to aim for the maximum output level from a recorded 1 kHz signal and that's it. Normally I would have checked the frequency response at 1 kHz and 10 kHz and set the machine slightly overbiased to get a dB or two of drop at 10 kHz. Having used the set-up procedure in the service manual, I checked the frequency response and it was fine. Even went up to 15 kHz, and it still looks fine. Impressive for its age. And an unimaginable difference compared to the machine before I'd replaced anything in it. There were DC bias 'plops', no bass, no treble, and distortion all over the place. I've never come across a machine which such an enormous difference in performance after cap replacement.

The acid test of course is recording something more humanly enjoyable than test tones and playing it back. Quite frankly I could not hear the difference between the recorded signal and the original even with headphones. Again impressive given that the machine is closing in on the 50 year mark. Even sounded good with my now-worn test tape.

Setting up the rest is really a breeze: play a Tandberg test tape, trim the playback output to 1.3V with volume controls at max, send in a 400 Hz signal in record mode, adjust input levels to 1.5V at the output in A-test, start tape, adjust record current trimpot for 1.5V playback, then adjust the meter trimpot so the magic eye just closes. Done. Three adjustments per channel.

[bobbyball]

Well done again and you echo how I felt when I had just recapped my first machine: I couldn't believe the difference and indeed couldn't tell whether the signal was "before" or "after" recording. Just goes to show how well these machines were designed, they even look nice with the wooden cabinets. Bit harder for me as I don't have the luxury of Tandberg test tapes though I did make a "0 dB" reference tape on a friend's Studer machine that I use...

Robert

[ricard]

The really weird thing about this machine was that the left and right channels of the playback head were swapped when I received it. Of course the head could have been replaced and someone connected it wrong, but wouldn't they have noticed it when setting it up? Wear grooves on the head show that it has been in operation for some time. So was it originally a manufacturing defect that got unnoticed, because the previous owner recorded in stereo at all times and never noticed that the playback channels were swapped?

Another Tandberg I had had a 470k pot instead of a 47k in the meter adjustment circuit, making adjustment tricky, as well as a motor wired for 110V rather than 220V. The motor ran hot, giving the machine an odd smell, but strangely enough (this one being a model 14 with an asynchronous motor) the motor speed was not off. It didn't look like it was an aftermarket repair so I'm supposing it was that way from the factory and went unnoticed through quality control.

And then there's the meter illumination circuitry in the 9100 series were the driver transistor does not saturate properly due to insufficient drive, causing it to run hot, slowly failing over the course of 20 years or so. Not to mention the logic circuits in said machine where capacitors on the inputs of some logic gates cause voltage spikes above Vcc on the gate inputs, which I think was the reason one of the logic chips in mine failed.

Tandbergs are very well designed, but operating the machines way past their intended lifespan shows up interesting design faults. (And I'm not even mentioning the DC switching in the 12 and 1200X series which is bound to cause pops and cracks at the slightest sign of dirt on the contacts. Oops, I just did. ).

[bobbyball]

I have to agree with you that there are certainly some design "quirks" in these machines and quality control may well have been a bit lax. I don't actually like the later machines and have got rid of most of them (except for a couple of 6000 machines which were the "modern" all transistor version of the 6X) - the clutches for example don't use the tried and tested felt rings of the deck used on the 6x and similar, using instead some sort of waxy type material which just disintegrates. The long switch banks on the 12 and 1200x are a disaster and virtually impossible to service, especially one of the designs, later "improved" a little (or was it the other way round). I guess my love of Tandberg tape recorders has to end with the deck used in the 6x!

Robert

[ricard]

Agreed, the early 70s machines such as the 6000X don't have the appeal of the earlier ones - although I have a 4000X mostly for completeness. Also agree on those clutches, although in honesty replacements can be had which work fine, if nothing else one can make ones own out of cork. But it's always nice when the machines are built with long lasting materials like felt and metal rather than various decomposing plastics.

Yes the long switches on the 12 and 1200X are a real pain, not to mention the plastic bits that that guide the rec-pb-amp switches, and at least the earlier ones tend to come apart with age. The 1200X is a bit better in that there are only a couple of large connectors on the PCB, in contrast to the 12 where every wire has its own separate terminal. I have a 12 though that for some reason actually works the way it should, a prized pearl if there ever was one.

I have a 9100X though and I feel that over the years it has proven itself to be a real tried-and-tested design, despite that it was was a new design at the time with many new (for Tandberg) design features. 3 motor design, logic control, brakes use felt-lined brake bands, no pressure pads on the heads, powerful capstan drive train. Earlier ones with Papst winding motors tend to wind slowly when cold, on the other hand the other motors used in later versions create a very loose wind due to the extremely fast winding speed. The biggest problem I've had on mine is getting the right damping in the tension arms so they catch tape loops efficiently. And it has a headphone output so one can actually listen directly to the recorder without an external amp unlike the 6.

But I must admit that I have a fondness for the 60s gray-top machines, and the 6/6X does have an unquestionable charm.