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Old 28th January 2009, 01:15 PM   #1
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Join Date: Nov 2005
Location: Greenwich, London, UK
Question RB250 Rewiring - accidental mess up?

Hi,

A long time ago, a friend gave me a Rega P2 with RB250 tonearm. It's sat in the loft for years.

My wife got me a CA Azur 640P for Christmas, so down it came. The friend had given me the P2 as "faulty" and it had an AT95M cartridge and only one channel played.

Checked leads of tonarm for breaks with multimeter, all seemed fine. Was fiddly, in hindsight I have gone back and checked and left channel is out on both L and LG.

Seen an old thread on here where people looked at after-market mod rewiring from Origin Live, AudioMods and so forth. Many of the instructional links on rewiring seem to have vanished of the virtual face of the internet.

While I had thought it was the cartridge initially, I imported a new AT33PTG cartridge which was more costly than I wanted to spend to get the other channel working, but I figured if it was as good as hyped I would be pleased. Now, I don't want to spend more to rewire with something like an Incognito kit, but would rather get to grips with rewiring the tonearm myself.

So, here are my questions to any experienced rewirers/experts out there as it now needs rewiring as all other sources for channel loss are rules out and I also have a new cartridge and tried that:

1) I removed the base grommet after loosening the allen bolt that locks it in place. I found a little PCB board for 5 connectors (L, LG, R, RG, Earth). It looks like the brass chamber is earthed as there is a foil lead wrapped around the grommet to contact the inside of the chamber. This then leads to one of the 5 connector points, and interestingly, a second point - is that correct or do I have something that's been bodged?

It got me to thinking?....

2) Do the LG and RG common onto the main earth, OR do they just represent the return half of a circuit and ONLY connect to the outer ground/negative point of RCA jack?

3) Is there a nice metal replacement that anybody knows of for the plastic grommet that will act as a better ground contact for the brass chamber?

4) I can see up into the chamber and turning the tonearm shows some movement. I am NOT going further without good advice as I can see the bearings will be up there and previous forum posts said the RB250 bearings should not be tampered with, yet there is still a way to rewire without ruining bearings. Any guidance anyone with clear photos?

5) Inner cables up in brass base chamber are coming through a rubber grommet. I suppose this will need removing for rewiring? Wife commented on soldering any replacement wire to the existing wire in the chamber so I can pull it through formthe headshell end. I gather she is right?

6) How do I get the counterweight stub off to view into the tonearm wand without damaging it or anything else?


Thanks for any advice in advance. I'll try and answer any questions, but basically I now have inner leads exposed at headshell end, base chamber grommet off and PCB detached from wires. I'd like to get some cheap Cardas, or other good wire to rewire with and make a single rewire from cartridge to phonos with improved earth if possible. I cannot outlay for the Icognito one right now as I have just been laid off because of the screwy economy and need another job!
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Old 29th January 2009, 05:57 PM   #2
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Join Date: Oct 2008
Hi there,

1) the two terminals on the PCB are indeed connected with the standard wiring scheme. The black wire is attached to the back of the arm tube, just in front of the counterweight stub, and then connected to one of the terminals on the PCB. This is then linked to the LG (blue) terminal, because there is no separate ground wire, so the arm is grounded via the left channel screen of the external cable, leading to the ground side within the CA 640P (good choice, BTW). The little metal strap on the plastic grommet links the arm tube ground to the brass base. This answers question (2) as well, I think.

Some claim an improvement simply by breaking the LG/ground link and adding a separate ground wire to the cable, connecting this to the ground terminal on the PCB (and the ground terminal on your 640P at the other end). However, this won't solve your problem with lost continuity.

3) My own first rewiring efforts involved re-use of the black grommet, and just replacing the skimpy external cable with something a little better, with a separate ground wire. I simply drilled the grommet out, after detaching the little PCB. I didn't disturb the internal wiring, though soldering it back onto the terminals was a challenge! I don't know of a commercial replacement for the grommet, though one could probably make something up, perhaps using DIN plug/socket components. I use the Incognito kit now.

4/5) I removed the rubber grommets when rewiring the Incognito kit, as per instructions. Your wife is right that new internal wires can be dragged through by soldering them to the old ones. If you work carefully, and the wire is not too stiff, the bearings should be fine afterwards. I felt the main factor was ensuring the wire didn't bind in the brass pillar after assembly; careful "dressing" helps.

If the internal wires are broken they will need to be replaced; there are several sources on UK E-bay, for a start. You could run these continuously to the phono plugs, like the Incognito, but would need to run them externally through some screened cable with the inner core removed (e.g. from an old phono lead or decent aerial cable), and reinforce at the plug end.

One option would be to get Moth Marketing, aka
British Audio Products online to rewire the arm. They can re-fit a standard Rega wiring harness for much less than an Incognito rewire.

6) Safest way to remove the stub is to grasp the arm tube (NOT the base or the bias housing) in front of the bearings, and twist the stub off using a piece of rubber for grip - though I found it came off very easily with both my RB250s using just my fingers. You can remove the arm from the deck to do this if you are really worried.

HTH,

Mark
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Old 29th January 2009, 11:12 PM   #3
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Join Date: Nov 2005
Location: Greenwich, London, UK
Mark, thanks for the time for all that. I feel much more confident now.

I did look at making a Cardas 4x33 tonearm with added ground wire (the 5th wire) and buying decent phono plugs and stuff. I could only find a Can-Am source for the Cardas, who also did plugs and copper braid for shielding etc. In all, it worked out a bit cheaper than the Incognito, but once shipping, customs duty and VAT goes on top it would be the same or more expensive.

So I've had to bite the bullet and order the Incognito. Given it's cheaper than I would be able to do on a DIY basis, it looks like great value for money.

Lastly, you said you took the tiny grommet up in the brass chamber (not the biggy required to get into it) and re-used it? How? I am still unsure if I could get the thing out without breaking the inner tonearm wires. I need those to do the pull-through with the new wires. I taped the wires at the headshell end to the tonearm so they don't accidentally get pulled up the arm preventing me from using them for the pull through. If I pull the little grommet out, will it not pull on the inner wires? Or do I just pick away at the grommet destroying it carefully?

The standard Rega grounding on blue (Left Ground) as you described seems really odd to me. Surely wiring left and right ground as normal RCA returns and use a ground from the arm casing on its own, or common all grounds, are the two sensible options? This Rega one-channel shares proper ground the other doesn't has me confused. Do you know why it's done this way? I would have thought we wanted both channels to share the same reference to ground with resepct to the 'hot' L and R signal wires?

Any way, many, many thanks for taking time to answer. I was beginning to think everybody with experience of rewiring the RB250 had long gone.
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Old 30th January 2009, 10:13 AM   #4
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Hi,

The Incognito will be very good, so you haven't wasted your money. In my experience, improving the wiring over the standard Rega item is especially important with a good MC like the Audio Technica you've bought. The gains are more subtle with an MM, unless it's of very high quality.

How to deal with the little grommets is explained in the thorough instructions that come with the kit, but basically you tease them out with an allen key. The only reason for removing them is to make the rewire easier, since the wires then pass through more smoothly. In fact, you don't have to solder the new wire to the old with the Incognito kit, but you'll see how to proceed when you read the instruction sheet.

The kit is very easy to fit, with the only difficult part being soldering the cartridge clips to the end of the wire. As I've said before, a little attention to the arrangement of the wire inside the pillar will prevent it from impeding free movement. It's a good idea to balance the arm once the kit has been fitted, and see how easily it moves, especially horizontally: does it glide smoothly across when gently pushed, or want to stop at any point in the arc? Loosening the allen bolt and rotating the gold-plated "plug" a little often cures any slight problems.

Rega maintain that their wiring scheme minimizes noise and hum, and I've certainly never had a problem in this respect with any of their arms fitted with the stock wire. Everybody else seems to disagree with Rega, however, and feels that the arm ground and signal return(s) should be separate!

Mark
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Old 30th January 2009, 09:52 PM   #5
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Join Date: Nov 2005
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Thanks Mark you've been very helpful. We'll see how I get on, as the kit will arrive tomorrow of early next week. Maybe I should post some pics of the rewire after in case anybody wants to see the Incognito rewire on here.
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Old 30th January 2009, 10:11 PM   #6
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I'm sure photos would interest people. There are none in the instructions, though here's a link to an article showing the kit being fitted to an RB300 (where it's safe to remove the arm tube from the base, unlike the RB250) TNT-Audio article

Looking at the instructions I have, I remembered that a special piece of slightly thicker wire is included, to which the 4 ends of the Incognito harness are soldered, and this wire is used to draw them through the arm. The original Rega wire is removed entirely, and it is then that you can push the little grommets out. Having a single piece of wire to draw the harness through is easier than the 4 fragile Rega wires, one of which might detach or break

Mark
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