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Old 14th Sep 2017, 12:56 am   #1
Techman
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Default The Arbiter Soundimension

I've just been up in the loft and found one of my old echo chambers, made by Arbiter, as shown in the last two photos below. The mains lead has gone a bit rotten since I last looked at it. I seem to remember that it developed a bit of a fault after it had been on for a while so I stopped using it, but it's been decades since it was last fired up. I've just opened it up for the last picture below, and I seem to remember being told a while ago that some of the transistors in it were likely to be quite desirable, whether that's true or not, I don't know. Probably time to give it a bit of an overhaul and find that fault that made it become unreliable all those years ago.
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Old 14th Sep 2017, 8:09 am   #2
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Default Re: Transistors for Fuzz Pedals.

The Arbiter unit is a bit of a beast apparently with a classic tape loop and 4 heads. If you can get it working I would say it was quite desirable and valuable.

The original was the Watkins Copycat, originally made in 1958 and still made in the latest iteration, three years after Charlie Watkins death (at 91) for about two grand new. Just checking on the auction site, and the Watkins units sell for about £350, but they are really common. Charlie churned hundreds of thousands of them, which depresses the value.

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Old 15th Sep 2017, 5:49 pm   #3
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Default Re: Transistors for Fuzz Pedals.

The Arbiter Soundimension actually uses a disc with a magnetic coating the width of a standard tape and has six heads, but you’re correct in a way saying four heads as the other two are for record and erase. This having proper erase rather than the permanent magnet as used in those early Watkins units. There is a precision mounted disk with a very fine air gap, probably the thickness of a fag paper, between the magnetic coating and the heads, so no tape to wear out and no wear on the heads. I do have one of those Watkins units with a tape loop somewhere. It uses valve technology and has Hunts paper capacitors and dates from around 1961. Looking on-line there’s very little, to no information on these Arbiter Soundimension units, and what there is, is either incorrect or scant. When searching, you find that the same incorrect information has just been copied and repeated on different forums and sites. There’s no service information or circuits available anywhere as I can see.

It’s lucky I opened the Arbiter before finding another mains lead with a Bulgin connector to power it up as the disc was hanging in cobwebs. Having given it a blow out with air and a fine paint brush and freeing off the belt which was stuck to the pulleys (replacing it without disturbing all the heads looks like a bit of a game unless the disc spindle can be removed from its bearing), power was applied and it ran and worked as a pre-amp, but there was no echo. There was a lot of mains hum and the board was alive with crackle and noise everywhere you tapped it and I started to think ‘tin whiskers’ in some of the transistors. The transistors are an NKT274, a few NKT275 (AC128 then) and a couple of NKT221. However, removal and cleaning of the edge connector on the board and cleaning of the three control pots got rid of all the crackles, but still no echo. The mains hum has also now gone down to a normal level, so smoothers have probably now re-formed themselves.

I next removed the circuit board, which is not too difficult, and did a quick check of all the transistor junctions in situ, and they all gave correct looking readings. I did measure a little erroneous leakage here and there, but this could well be other components in the circuit – removing each transistor and testing individually would give a definitive result.

With no circuit diagram to work from, this unit is going to be hard work to fault find. There are many small electrolytics that are likely to be past their best. One thing I did notice was that it has been worked on before. I bought this unit in the early 70s, but I think it dates from mid to late 60s. Someone has done a lot of fault finding on this unit at some point before I got it and most of the transistors look like they’ve been removed and replaced at some time in the very distant past, but definitely not by me. I also noticed that when I plugged a foot switch into the socket that the ’crunch’ of the connection and of the actual switch being switched gave a single echoed repeat, the duration of which was dependent on the head selected. Also, with a strong audio note fed in (guitar chord), a faint single repeat could just be heard, but it was only just audible. So, it’s probably capable of working if I spent some time on it. It seems that it's quite an unusual device these days and that perhaps not many were made. I’ve put it back together for the time being, but have taken a few pictures for the record.

I’m a little mindful that going into detail of the workings of this unit rather than just its transistor types may take the thread ‘off topic’, so perhaps it ought to have a new thread of its own if discussion on it are going to continue. It’s neither tape nor disc, but probably nearer to tape technology. The tape recorder section would probably be the nearest, I don’t know, it’s a bit of a difficult one and I’ll leave it up to the mods to decide if and where they want to move it and delete this last paragraph if necessary.
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Old 16th Sep 2017, 1:45 pm   #4
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Default Re: The Arbiter Soundimension

I had (and lost) an all valve Schaller Echo unit from early/mid 60's which had a similar rotating head rather than tape for the delays. They were much more sophisticated but also more difficult to fix. Mine had liquid damage to the oxide coating which caused a noise as the heads passed that area. I tried in vain to get the head re-coated, which wasn't and probably still isn't possible? In the end a buddy of mine took it to one of his supposed 'electronic wizard' pals and I never saw it again

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Old 16th Sep 2017, 4:05 pm   #5
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Default Re: The Arbiter Soundimension

If the coating on the cylinder was damaged, might it have been possible to bond on a length of regular tape with a splice at the crossover? Might have needed some adjustment of the gap between the head and the cylinder.

I realise that is no help to ITAM805, alas. I've lost audio gear a similar way - lending them to a "friend", never to see them again.
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Old 16th Sep 2017, 5:07 pm   #6
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Default Re: The Arbiter Soundimension

Oh for a time machine Craig! I actually did try splicing some 1/2" tape around the drum but it didn't work too well IIRC.

Incredibly it appears that someone did a successful repair on their Schaller echo rotor - here, although it's unclear if he used tape or coated it himself?
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Old 17th Sep 2017, 1:56 am   #7
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Default Re: The Arbiter Soundimension

That link is an interesting read despite the rather broken English. It seems from what I can make out that all the heads were open circuit which I find difficult to believe. I suppose that the “Handyman”, as he calls him, may have put some current through them and blown them. From what I gather, he didn’t re-coat the drum with oxide, but bought another scrap unit where someone had cleaned the drum of oxide thinking it was rust, thus ruining it, but he was able to use the heads from this unit on his original one.

I’ve done quite a lot of internet searching and there really is very little information on these units other than a few pictures and early adverts. There’s certainly no internal pictures other than those that I’ve posed of mine on this thread. There’s various reports of people finding these units and none of those found are working. It seems that the usual fault other than a broken drive belt, is no echo, the same as mine, and no one as far as I can find has ever reported fixing one or found a circuit diagram for one. I’ve got a couple of valve amplifiers made by Arbiter and all the circuit diagrams for these are available. There was a smaller version of this unit known as the Soundette, and it seems that circuit information has been available for this unit at some point.

As far as I can gather, the Soundimension dates from 1966. It’s interesting to note the extra holes in the chassis under the top cover. It looks like a prototype may have originally had another control, perhaps a motor speed control on the original design. Also, a square hole for what would likely be a VU meter and another two holes perhaps for indicator lamps, but perhaps the bean counters knocked all this on the head before the unit went into production. The square hole for a possible meter is even cut out of the top cover, but is covered over by that strip of trim.

During my search, I came across an old picture of The Beatles in a studio rehearsal session showing them using the smaller Soundette unit. A discussion on the picture indicated that it must have been a rehearsal session as they normally used the Soundimension unit for live recording and the ‘roof top performance’ was mentioned. So I looked on YouTube at the 1969 performance of ‘Get Back’, and sure enough the Arbiter Soundimension can be seen at 1:45 with leads plugged into it on the far left, being partially obscured by the head of Paul's bass moving in front of it. I suppose this gives the unit at least some sort of credibility – link to video below:-
https://www.youtube.com/watch?v=qCIsfZkRi3Q
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Old 17th Sep 2017, 8:51 pm   #8
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Default Re: The Arbiter Soundimension

Hi Techman, yes I think you're right with the Schaller link, the rotor drum looks like the same chocolate brown as my one did, lucky guy to find a spare!

Regarding your Arbiter, it really is ultra rare and probably worth a fair bit I have searched and can't find a schematic either.

You'll need to establish if the bias oscillator is running, trace the signal from the record head back to the board. Then, see if the p/b heads are intact. I did this on an old Copicat using a signal driven 'in ear' magnetic headphone held against the head and scoping the anode signal, or the tranny collectors in your machine. In your case, hold the earpiece as near the pick up as possible, there should be enough flux to get past the drum rotor, which is rather in the way! Then it's a case of methodically tracing the signal path - easy for me to say I realise!
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Old 17th Sep 2017, 11:03 pm   #9
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Default Re: The Arbiter Soundimension

Hi, Yes, I scoped the bias oscillator at the erase and record heads and it's a good 40 volts p/p at the erase head and a bit under 30 volts at the record head and around 30KHz in frequency, give or take, and very stable. However, there's no audio at the record head. This is where a circuit diagram would be useful.
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Old 18th Sep 2017, 5:07 am   #10
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Default Re: The Arbiter Soundimension

The amount of audio voltage at the recording head will be very small compared to the bias voltage.

It's the current in any tape recorder which does the work in a recording head, so the audio is applied as a drive current, usually by using a dominant series resistor. The impedance of the head falls proportionately to frequency so a 1kHz test signal of the same current as the bias would give you 1/30th of the bias voltage. But the signal current will be smaller than the bias current, so the max signal voltage is even smaller.

So it's easy to not see the audio amongst the bias. If you trace the recording signal back from the head, you should find a capacitor which injects the bias current going off to the erase oscillator, and then in series an inductor used to stop bias current going to the recording amplifier, and then a resistor used to set the current.

Some variants omitted the inductor and relied on the resistor to limit the loss of bias current. Some variants had amplifiers configured as active current sources

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Old 18th Sep 2017, 5:38 pm   #11
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Default Re: The Arbiter Soundimension

I was looking hard to see if I could see any audio superimposed on the bias frequency at the record head and there was none.

The inductor you mention can be seen in the pictures below. There's also a transformer. Waveforms are only measurable on the tag shown, scope set at 5v/cm. The first waveform is the actual bias oscillator. The second waveform is with the timebase of the scope wound down to measure audio frequency and shown is 1KHz superimposed on that bias waveform. The Audio waveform level had been set to a level just below where it started clipping when measured as it passed through the audio amplifier stages that preceded. The next waveform is on the tag of the inductor, scope set at 0.5v/cm. The other tag of this inductor has the full bias waveform on it at a higher level and no audio, so this is working as it should.

I don't know the reason for that transformer and you may have noted that there are now fixing nuts and bolts holding it to the circuit board, which you can see were missing in the original pictures. It's definite that I'm following in the path of someone who's had this thing apart many years ago and removed and then replaced a lot of parts, probably over some time on a bench, struggling to find a fault, hence a number of missing nuts and bolts - I'd be having a word with Mr Capaldi who I bought this from if he were still around, although it did originally work when I first bought it. The fault it developed was that when it had been on for about an hour, the echo would start to decline and become distorted. When it was turned off for a while it would recover and work for a while until the fault gradually started to develop again. I seem to remember that the time it took for the fault to develop gradually became shorter and you could only get about twenty minutes use out of it before it started to struggle. I expected to find a thermal runaway type fault with a leaky transistor, but find it is as it is now all these years later.
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Old 18th Sep 2017, 6:30 pm   #12
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Default Re: The Arbiter Soundimension

It looks like the oscillator section uses those two NKT221 transistors with the heatsink fins on them. I don't think those two silver looking capacitors that are mounted sort of crossing against each other are original. I think the original ones may have been mounted upright and may have been electrolytic types that blew their ends out at some time in the very distant past as there's corresponding marks on the casing in line with them - as shown in the first picture below.

I'll take back what I said in an earlier post about holes for indicator lamps as you can see in the second picture that those holes are there to accommodate two of the bolts that hold the trim on. The trim can be seen covering the hole for a possible meter though.

There's another couple of holes which have been quite roughly drilled in the chassis. These holes seem to have been hand drilled during manufacture as they've been drilled from a direction which would be impossible with the unit assembled as it is. If they weren't there it would be impossible to get a screwdriver to the top of two of the bolts holding the unit together.
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Old 18th Sep 2017, 6:58 pm   #13
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Default Re: The Arbiter Soundimension

Would those two capacitors be cross couplers for the bias oscillator?

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Old 20th Sep 2017, 1:19 am   #14
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I didn’t mention it previously, but that was a good tip in post #8 with regards to picking up output from the play back heads using a magnetic ear piece. I haven’t personally ever used this method and on this occasion the fault we have is that no actual audio is getting to the record head, although bias is normal.

I’ve had to completely remove the circuit board from the unit (again) so that I can get a proper look at where all the track runs go, as the part of the circuit we need to look at is partially obscured behind the front chassis. Although the fixing bolts were originally missing from the transformer, it doesn't actually look like the connections to it have ever been un-soldered. I’ve added pictures of both sides of the board below.

I’ve also pictured that roughly drilled hole in the chassis rail to get a screwdriver onto that screw head, drilled from behind, so must have been during manufacture. There’s two of these holes, one at each top corner of the chassis.

The last picture is truly shocking! The mechanical deck is resiliently mounted on four bonded rubber mounts. There’s only actually three, as one of the bottom ones has become un-bonded, but the deck is still held by the remaining three. The picture shows the right hand top mount. These top mounts are made from ‘L’ shaped plates which have been doubled up on each side to give the required thickness. It’s fairly obvious that the designers specified the ‘L’ shape for a particular reason. Unfortunately, the component part makers drilled one of the holes in the wrong place on both sets of plates. You can see that the hole for the bolt with a nut with orange paint seal on it should have been drilled at the end of the bottom leg of the ‘L’ and the plates mounted the other way up, but the assemblers didn’t bother to check and used the plates as they were. As you can see, with the plates fitted in this way, one corner is only a mm or so from the live terminal of the mains on / off switch. You can see that the solder terminals have been bent flat against the body of the switch so as to just avoid contact with that plate. However, contact has been made at some point in time, as you can see the black burn mark on the edge of the plate where it shorted out – that must have been a fuse blowing moment, but at least it must have showed that the earthing was all good, although this must have been before my ownership as I never had any fuse blowing problems with it.

Another safety issue and bad design is that the live terminals of the mains transformer are dangerously close to the rear case cover. There was a chunk of very sticky doubled up tape floating around and stuck partially to the motor and the cover which I identified as having been originally stuck over the transformer connections by the residue of stickiness thereon. I don’t know if this tape had been added at manufacture or by someone later. It’s even possible that I may have added it myself many years ago, as I did open this unit up in the 70s to have a look inside and may have noticed that if the case took a bash on the back that it could easily come into contact with the live terminals, I just can’t remember, it was a long time ago.
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Old 20th Sep 2017, 7:53 am   #15
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Default Re: The Arbiter Soundimension

Are the mounting plates handed?
I'm just wondering if they ran out of the correct type, and made do with the 'other' sort while awaiting supplies.
Re the access holes - I thought that as what prototypes were for!
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Old 22nd Sep 2017, 1:19 am   #16
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Default Re: The Arbiter Soundimension

No, as far as I can see the plates aren't handed.

Prototype? I think what I have here IS the actual prototype!

There's holes cut and drillings that don't do anything all over the place!

It gets worse! On closer inspection of those plates, it's just the top ones that are 'L' shaped, the underneath ones that have the actual bonded part attached are just a rectangle. Below is a picture showing how the rubber mountings are fitted out of line and are under permanent stress. It looks like a problem was identified at the prototyping stage as regards to the alignment of the mountings, so the 'L' shaped plates were specified with a hole drilled at the end of the bottom leg of the 'L' and the bonding attached to these new plates. This obviously didn't happen and the old rectangular plates were used with the assemblers not knowing what to do with the new plates but were told they needed to be used, so they just stuck them on top and bolted it all up. There's actually no need for a double thickness of metal plates, it just looks like a complete lack of communication between designers, component part makers and assemblers - well that's my take on it! Perhaps someone who used to work there will come out of the woodwork and tell us the story?

Those two capacitors that are 'crossed over' are labeled as 10000pf 630 volts +/- 5%. If I get time, some time, I'll have to try to draw out the circuit of this thing to try to work out what's going on. That transformer is a step down transformer with DC readings of around 184 ohms on the primary and around 13 ohms on the secondary - the primary is helpfully marked as such on the board. With this design having an air gap between heads and recording media, it's going to require a lot more energy to impart and remove recorded information from the media than it would from traditional tape in direct contact with heads - then there's the loss of higher frequency response to deal with. I read somewhere, perhaps on one of those adverts, that five years of research had gone into this unit, It may have seemed a good idea at the time to have no more head wear or worn and broken tapes, but I think this design turned out to be a bit of a flop. While doing my internet researching I read a comment by someone that few to none of these disc/drum units are now running, but many of the older Watkins tape units are still spinning away and echoing to this day - and I think there's probably some truth in that statement.

I removed the transformer from the board and did some tests on it, as a single shorted turn would kill it dead, but it tested OK as far as I could determine. I removed an associated NKT274 transistor and it showed some leakage from collector to emitter and hfe around 165. I didn't think that leakage would be having any particular effect and a quick sub of another similar random PNP transistor made no difference. There's actually 2 x NKT274 transistors and 8 x NKT275 transistors - enough for 4 x 'Fuzz-Faces' Pehaps I'll start a strong rumor that this is the actual unit that The Beatles used in their live performances and put it up for sale Or rather put it back into cold storage again for another forty plus years!

One last thing that shows this unit has had the attention of the 'Handyman' in its early life is shown in a couple of pictures below. Someone has tried to remove the record and erase heads, but two of the screws are underneath the motor. The 'Handyman' managed to pick the screws undone so far, but couldn't screw them back up. The motor mounting screws don't look to have been disturbed and one is beneath the recording disc, so that would have to be removed to get to it. I haven't checked, but I suspect that there's some sort of circlip holding the disc spindle beneath the bearing bottom cover.
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Old 22nd Sep 2017, 8:02 am   #17
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Default Re: The Arbiter Soundimension

Here's a suggestion. Roger Mayer was (and still is) active in effects pedals used by rock musicians, starting with Jimmy Page, Jeff Beck and Jimi Hendrix back in the 60's. He seemed to quite involved in improving the Arbiter Fuzz Face, and probably at least knows of the Soundimesion.

Might be worth contacting him and seeing if he has anything lurking in his mind or archives.

https://www.roger-mayer.co.uk/
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