The Workbench

Re: Beocenter 3500 restoration

chartz — Sun, 23 Oct 2011 09:41:45 GMT

Today, I can't hear the transformer hum at all.

Why? I don't know really.

Anyway, I'm keeping the Beocenter. It is now playing the Round Midnight (Bertrand Tavernier's film) LP.

This thing just plays LPs so well!

Re: Beocenter 3500 restoration

solderon29 — Sun, 21 Aug 2011 11:38:17 GMT

Yes indeed,it would have to be two transformers I think.Don't think there would need to be any difference between 3/4000 in practice,as long as the rating was suitable for the 4000?

The second supply is not crucial,you could probably stabilise it too,and be better than the original?

I remember now of course that the turntable motor is a different voltage in the BC,to that in the BG3000,will have to get cracking on this project now!!

The other main problem with all these Beomasters/Beocentre's of course is the function select switch assembly.

Cleaning the switch contacts can make a dramatic difference to performance!!

Again,back in the day,we used to just replace the assembly complete,as we were told it was wear n tear.I now realise that the switches were very well made,and simply subject to oxidation.

I've rebuilt several since,and found the contacts to be perfect after cleaning off the sometimes black oxidation.

A tedious but rewarding job in terms of sound improvement.

Regards,

Nick

Re: Beocenter 3500 restoration

chartz — Sat, 20 Aug 2011 12:05:33 GMT

Thanks Nick for your specialist's input!

I agree the rectifier looks small-ish! This was actually taken from an old NAD 3020... A KBL02 (4 amp/200V, see datasheet here). The original B80C5000 was given for 3.3 amp/80V with 5000µF max!

I have been thinking of a toroïdal too.

And a separate coil will be needed for the tuner and preamp! Or maybe a second smaller transformer... The ideal would be to put those two inside the old transformer box!

The annoying question is: will the 110V half-primary (thus acting as an auto-transformer) take the turntable motor draw?

Re: Beocenter 3500 restoration

tournedos — Sat, 20 Aug 2011 12:03:43 GMT

solderon29:

I've often wondered about replacing the transformer with a toroidal type,as they are quite cheap nowaday's.

A good idea, it seems there would be plenty of room for a toroidal of similar power rating... unfortunately the voltages are a bit odd and I don't think it will be easy to find a suitable generic substitute. For example BM4000 (I have a humming one) seems to need a 22+47V secondary. 3000 is 22 + something slightly less than BM4000 for the power amps. Perhaps the 22V circuit wouldn't need much power.

Years ago I had some custom toroidals made for a company project and they weren't that expensive per piece in series of 20, but I'm not sure if it would be feasible for this...

Re: Beocenter 3500 restoration

solderon29 — Sat, 20 Aug 2011 11:38:43 GMT

Nice unit,and well worth repair indeed.I think your replacement bridge recccy is too small,and may not last long!!The original type,or very similar are still available though.

The humming transformer was indeed an "achilles heel" with this and other similar designs.The idea seemed to be to encase the C core type transformer in resin,inside a steel box,to achieve very low mechanical and electrical noise.

Sadly,in time it had the opposite effect.

When we were repairing these units,back in the day,we would compete in how quickly we could change the transformer assembly,it was such a common cause of complaint with owners.

I've often wondered about replacing the transformer with a toroidal type,as they are quite cheap nowaday's.

I have 2xBC3500,a BM30002 and a BM4000,all needing this work!!!

Regards,

Nick

Re: Beocenter 3500 restoration

chartz — Fri, 19 Aug 2011 16:08:48 GMT

Squash? What's that?

Thanks Peter.

Re: Beocenter 3500 restoration

Peter — Fri, 19 Aug 2011 10:22:46 GMT

I used two squash balls cut in half under a Beomaster 3000 with the same problem - seemed to work reasonably. I used yellow spot ones!

Re: Beocenter 3500 restoration

chartz — Fri, 19 Aug 2011 08:50:15 GMT

By the way, the humming from the transformer couldn't be cured, even by inserting rubber grommets. Its vibrations resonate into the chassis (I use the 240V setting) and on to the shelf which amplifies it... It is almost inaudible on the carpet !

Any ideas please?

Re: Beocenter 3500 restoration

chartz — Thu, 18 Aug 2011 11:45:13 GMT

Update!

The noise I had on the phono left channel turned out to be a bad pol. 270k resistor, at the input stage (base of TR34).

I'm glad to report that this gorgeous Beocenter now fully works (it has been hard work), and boy, what a sweet and sophisticated sound! Just plain enjoyable, especially with LPs, which do sound fabulous!

A wonderful machine, as Martin would put it!

Re: Beocenter 3500 restoration

tournedos — Sat, 07 May 2011 08:23:14 GMT

Thanks for the pointer Jacques, I'll remember that when / if I ever get to go through my 5000.

I'll keep my fingers crossed for the Mrs.! Other problems always seem very small when something like that happens.

Re: Beocenter 3500 restoration

chartz — Sat, 07 May 2011 07:40:22 GMT

Hi everyone!

Tim Jarman has published a notebook article about a possible dull sound in the phono input of the Beolab 5000 amp.

Now my Beocenter has similar RIAA circuitry and I asked him whether the Beocenter 3500/Beomaster 3000 series could benefit the same mod.

The answer is yes! I replaced the 1 nF caps (ceramic, marked 1k) between the base and emitters of the first transistors with 120 pF ones, to good effect. It brightens things up, and the sound is more modern although still very refined (SP12 cartridge, which was given to me, yes really!).

Re: Beocenter 3500 restoration

chartz — Wed, 02 Mar 2011 02:56:23 GMT

Yes, rubber grommets, that's a classic fix!

The transformer hums, but it doesn't transmit any humming to the circuits, which is already something I suppose.

Thanks.

Re: Beocenter 3500 restoration

tournedos — Tue, 01 Mar 2011 13:23:22 GMT

chartz:

Otherwise, after adjusting everything (quiescent current, FM settings) there is no noise at all, but the transformer itself hums quite a bit!

The sound is good. I haven't replaced the three big caps. Would that make a difference?

Nice piece of kit!

I'm afraid the transformer hum will not go away with new caps. It's a bit of a problem with all of these series, including my BM4000. I think somebody (geearr?) mentioned getting pretty good results by fixing the transformer box in some rubber grommets (not enough space for much suspension, though).

Re: Beocenter 3500 restoration

chartz — Mon, 28 Feb 2011 06:10:59 GMT

I am now testing the phono. A nice person on this forum sent me an SP14 for testing purposes, as seen on the pictures.

The problem is that there is a constant faint but audible "grating" noise (both L and R), especially obvious on lead-in grooves and silent musical passages... which is a shame because the cartridge tracks very well. Even the end of side distortion seems absent!

I can't make a close-up of the tip, but there is a rondel and the tip (black in colour) is attached to this rondel.

It does seem to excite the arm resonance, because it is the same frequency as the arm when you touch it.

Any ideas?

Edit: an o-ring around the cartridge body attenuates this noise strongly... Knackered stylus?

Re: Beocenter 3500 restoration

chartz — Sun, 27 Feb 2011 10:56:20 GMT

Hi,

The bouncing problem is now fixed. The idler wheel was not properly aligned. I took the opportunity to re-lubricate the shaft properly and deglaze the idler itself. It is still noisy though...