Dillen wrote:

You will never run out of work with a Beomaster 6000/4.
Isn't it wonderful ?  😂

Thanks for taking the time to do this thread, it's great reading and will, no doubt, come in handy for many owners and DIY'ers.

Martin

I learn from every step

Hi Everyone

Thanks for all of the recent feedback, it has all been examined and I continue to find your comments very helpful and supportive.

The next phase was to try and improve the sensitivity of the keyboard because some of those switches were really hard work at times.  The harder you have to press, the more damage you do to the keyboard appearance as well as putting a permanent kink in the key itself.  Some of the switches were worse than others and it is no surprise to find out that the commonly used switches such as volume, stand by, FM and AUX2 were particularly bad.

Olly was right when he mentioned that the first thing to do is improve the contact surfaces.  Each switch is made up of two parts.  The first part is the copper bridge that is fixed to the PCB and the second part is the spring contact plate.  I chose not to dismantle any of these parts and wanted to fix them in situ so as to avoid any unforeseen damage.

I decided to clean the contacts by cutting a thin strip of very fine emery paper and running this across both sides of the contacts.  Afterwards, the switches were vacuumed and wiped over.

You can feel an improvement in the switching when the contacts are clean but it doesn’t provide a solution to how far the key has to be pressed.  I used a small feeler gauge to test the gap in these switches.  In the good switches, a 0.15mm gauge was just starting to find a snug fit.  In the bad switches, this gauge just floated around all over the place and the gap could be more than 0.5mm.  When I examined the different switches, I started to appreciate that the bridges were all the same but the spring plate had lost its shape in many cases.  When I tried to bend these springs, nothing really happened.  To be successful, one would probably have to remove the spring plates and reshape them externally.

I decided that the simplest solution was to make some shims to fit between the button at the back of the flexible key and the spring plate.  I found some old photograph negative which was quite strong and about 0.25mm thick.  A few layers of this would close up the gap significantly.

The dimensions of the shim are 10 x 15mm and on each side, a 4 x 4mm slot is cut around the centre.  They don’t have to be that precise but should fit nicely over the two plastic lugs that support the switch.

After these had been cut, I found that they were very difficult to slide into the switch and they would snag around the plastic button.  The solution was to cut a strip of negative about 5mm x 50mm and used this to wiggle into place over the plastic push button.  Next cut a strip 10mm x 50mm and slide this over the top.  Then pull out the first strip.  It is now easy to slide in the shims over the top of the strip.

After putting in one shim, I connected a beep meter and pressed the switch to judge its sensitivity.  In some cases, I had to put in three or four shims to make the switch acceptable.  All switches now have a clearance of 0.10 to 0.15mm and the switching functionality has improved no end.  This is a photo of the volume control panel with all of the shims in place.

I am very satisfied with this solution.  Five problems are now fixed and there are two to go.  Some would say that it’s not quite a B&O replica but it doesn’t worry me when these parts are not visible to the user.  The main objective remains to have a piece of equipment that is a pleasure to operate and ultimately a pleasure to listen to.

Regards

Geoff

What can I say, AMAZING

Hi Everyone

The latest update on the quad has both good news and bad news.  First the good news.

After fixing the control panel, I reassembled the unit to give it a serious test run.  The first problem I encountered was related to the improved key sensitivity because after the panel was pressed into position, program P1 was permanently engaged.  Removing one of the shims under the P1 key quickly fixed that.

The control panel was now a pleasure to use – it was very sensitive and getting close to the touch panels on the BM2400.  What a difference from the earlier setup.  I settled down to listen to some music on the radio and connected a tape for better quality.  My first impressions were that the sound quality was very good – I have heard it described as a “full” sound and I liked it.  On many other machines, I tend to use the loudness function to get a fuller sound but this wasn’t necessary on the quad.  As I said earlier, I was very impressed and from an operating perspective I can now give it very high marks.  🤒

So what is the bad news?  Well, if you can remember, my two outstanding problem areas were the switch on “thud” and general box cosmetics.  It is the “thud” that I simply haven’t been able to find a solution to.  I have tested lots of things and tried a number of fixes but so far, the solution eludes me.  So, what have I done to date?

All of the electrolytics on the main amplifier have all been changed.

All 20 tantalums on the tone board have been replaced.  This actually changed the sound of the thud from the horrible rasping sound of a lightning bolt to a lower volume “boom”.  I was grateful for that improvement because the former noise made my hair stand on end.

I found a faulty transistor 2TR8 on the 20V supply and replaced the BC143 with an NTE129.  This fixed the local voltages but didn’t help my thud.

Martin mentioned that there might be a problem with some parts of the ground wiring and I did loose a connection on one occasion.  The earthing is now good but it hasn’t improved anything.

I lost 12IC1 which was the volume controller and replaced the MPSA65 with a BC516.  The volume control is now good but I was starting to wonder if the “thud” was now blowing up some of the components.  That was a worry because I always needed to hear the thud to check if it had been fixed or not.

On one occasion when the amplifier board had been lifted and the tone board was in its service position, I switched on to find that there was no thud.  With a smile on my face, I replaced the boards only to find that it had returned.  I was never able to reproduce that condition and wondered whether there were loose joints or tracks around.  That was going to be an awful job to undertake and I never started it.

I did extensive tests on the power characteristics and was generally happy with the power supply.  However, I did decide to replace the 3000uF cap on the 18V supply.  That change didn’t alter anything and the original cap must still be good after all these years.  To date I haven’t swapped the 10000uF cap but the 60V supply seems to look pretty good.  I did replace the small 47uF cap which filters part of the 60V supply but still no joy.

So there we have it – I am now the owner of an acoustically disadvantaged quad.  I will leave the cosmetic repairs until a much later date because I know that this machine will be taken apart again sometime.  I would have loved to end this saga on a note of perfection but that is not meant to be.  This project is now in the “too hard” basket but at least I was able to obtain a good sounding, nice working unit and it has all been good, hard, honest fun.

Thanks for all of your support.  It has been great to read your comments and I think that we have all learned a lot.  Now to find my next project - hopefully easier than this one ???? 🙂

Regards

Geoff

This project has been a superb read.

Well done Geoff.

Hi Geoff I currently have a 6000 which is fine in every respect but you are welcomed with a thud at standby 'on'. I cannot see how this can be reduced, due to the fact we are dealing with a single rail supply whose mid operating point (at the amplifier output, about +30v to ground) is isolated via those large capacitors.

Those capacitors on initial power up will, no matter what you do let current flow through to the speakers. Now unless I have missed something I cannot see how this initial 'surge' is controlled, and how your thud can be the result of a fault, especially if it is the same on all channels!

One thing that could result in a quieter thud is output capacitors that are faulty and as a result have a higher ESR...?

Maybe this has more to do with speaker sensitivity and / or impedance? Also when you had the boards in the service position are you sure it wasn't a loose wire that resulted in the silence, not the other way round!?

BTW your switch mod really is superb 😁 Laminated film can be made up with different numbers of layers for various thickness's.. Use a guillotine & scalpel to cut up and it takes very little time... Am going to try this on my Begrams I think!

If there was an award or a payment I would give it. What a great technical resource.

Hopefully you will be inclined to do another at some point.  This thread needs to be archived safely.

Graham and Eugene

Thanks for your positive response.  I too like reading this type of thread.  When the more experienced people contribute their wealth of knowledge, these documents can become invaluable sources of information and ideas.  Glad you enjoyed.

Regards

Geoff

Hello Olly

I definitely agree with your thoughts on this type of amplifier and have been wondering myself how the design can ever work without causing a “thud” on startup.  I have used the scope to have a look at the voltage changes and noted the following points. 

The DC voltage on the output capacitors builds up very slowly, taking up to 10 seconds to ramp up to 30V.  This in itself should not cause a “thud”.

During a one second period immediately after the relay activates, the voltage rises sharply and drops away.  This voltage peak is hard to pick up but could be in the range of 5 to 10V before it melts away into the normal DC ramp.

This machine immediately goes to “Standby ON” when the power is applied.  Hence the relay is activated while the 60V supply is being established.  I thought that the 60V might be causing the problem but then found that the same noise occurred when the unit has been in “Standby OFF” for a period of time.  During this time, the 10000uF cap is kept fully charged so it is unlikely to surge when the unit is being switched back to “Standby ON”.

As I said, I have now run out of ideas.  Some days it seems that the intensity of the thud is becoming less but I think that is an illusion caused by gradually becoming accustomed to it.

Glad to hear that you are going to try some shims on your beograms – I hope they work as well as these ones have done.  A very simple solution isn’t it.

Regards

Geoff

Hi Geoff, how are things doing with your 6000? I designed and built the circuit I mentioned works very well and fits neatly between capacitors and pre-amp board, I will pm with details if you want. I also tested the switch mod on a 4004 with lovely results, very happy!

Anyway, give us an update :)