The Workbench

Re: Repairing BM8000 LEDs.

GDSammet — Thu, 12 Mar 2009 13:47:57 GMT

I just finished replacing the R226 Trim Pots on the Amplifier boards. I found 100ohm Trim Pots with the same footprint as the stock ones. I used Digi-Key (DigiKey.com) part number 3352E-101LF-ND ($1.24 USD each). The legs fit perfectly in the factory holes without bending them or making any modifications. Once I replaced them and reset the milivolts to 18mv everything worked perfectly. It was easy to get exactly 18mv also, since the new Trim Pots were clean.

Re: Repairing BM8000 LEDs.

GDSammet — Mon, 09 Feb 2009 14:51:20 GMT

Derek

auric:

Gerhard:

I successfully redid the entire display on my BM 8000 2 weeks ago replacing all segments with surface mount LEDs.

You really need to replace all of them because there the voltage between the old and new leds don't match and more importantly because the display is strobed. The new ones are much more efficient and are too bright.

I replaced all the current dropping resistors with 820 R in order to match the brightness of the original display. I prefered mine to be a bit brighter than original, but if you are picky you can go with a slightly higher value dim it a bit.

As you dim the LED the color gets closer to the original, but it isn't exactly the same.

I've taken pictures of the entire process which I'd like to share with the group but haven't figured out how to attach them.

Regards,

Derek

I am curious which LEDs you replaced, for which display, and what value. I have ordered several which might work, but I have not received them yet. Which resistors did you replace? My volume display and balance displays are completely dead. Only one or two segments work. The tuning display has 3-4 segments which are dead or dying. In looking at the Service Manual diagram, the volume and balance "share" resistors which might be good in my case, however, the tuning display might have to be completely rebuilt also since all of the segments are related to the same set of resistors.

Re: Heat from BM8000 heat sinks

tournedos — Mon, 09 Feb 2009 09:19:18 GMT

GDSammet:

Here is a close up image of one of the segments with 1.7v power applied. I used a 105mm macro lens with a 36mm extension tube. The "silicone chip" is less than .5mm in diameter.

Yes, this is indeed the LED type I was referring to. Although it was still smaller in the early calculators, and the displays actually had a plastic "magnifying lens" over each digit to make them readable. I think I have one in my junk box somewhere... as well as some larger early LED 7-segment displays, which internally had LEDs pretty much like in your picture and a "light box" above to diffuse the light.

Well done with the picture BTW

Re: Repairing BM8000 LEDs.

GDSammet — Mon, 09 Feb 2009 08:25:13 GMT

I am wondering what value and physical size SMD LEDs you used. I have ordered two sizes with a 2.1Vf. I would like to see your picture too. To attach a pic to a message, just reply, not quick reply, and click the "Insert Media" button.

Gerhard

Re: Heat from BM8000 heat sinks

GDSammet — Mon, 09 Feb 2009 08:23:26 GMT

Here is a close up image of one of the segments with 1.7v power applied. I used a 105mm macro lens with a 36mm extension tube. The "silicone chip" is less than .5mm in diameter. The difference between the Beo LEDs and traditional LEDs is the LEDs we use now are encapsulated to prevent damage from vibration etc. The filament wire you see is a gold strand and is very delicate. This is the piece that breaks from vibration.

Re: Heat from BM8000 heat sinks

tournedos — Mon, 09 Feb 2009 03:57:47 GMT

GDSammet:

I removed the display for volume (two seven segment displays). I used a high power magnifying glass to see the existing "LEDS" on the board. These belong in a museum!

So what are they actually? Chips bonded in place like in 70's pocket calculators?

@auric, I'd love to see those photos. When composing a post, click on the "Options" tab and you'll see an add/update option. Only one attachment per post, though...

Repairing BM8000 LEDs.

auric — Sun, 08 Feb 2009 19:00:17 GMT

Gerhard:

I successfully redid the entire display on my BM 8000 2 weeks ago replacing all segments with surface mount LEDs.

You really need to replace all of them because there the voltage between the old and new leds don't match and more importantly because the display is strobed. The new ones are much more efficient and are too bright.

I replaced all the current dropping resistors with 820 R in order to match the brightness of the original display. I prefered mine to be a bit brighter than original, but if you are picky you can go with a slightly higher value dim it a bit.

As you dim the LED the color gets closer to the original, but it isn't exactly the same.

I've taken pictures of the entire process which I'd like to share with the group but haven't figured out how to attach them.

Regards,

Derek

Re: Heat from BM8000 heat sinks

GDSammet — Fri, 06 Feb 2009 11:23:19 GMT

Martin,

I removed the display for volume (two seven segment displays). I used a high power magnifying glass to see the existing "LEDS" on the board. These belong in a museum! I can't believe they even used these, but now it makes sense why they are so vulnerable to vibration etc. I am going to replace with smd LEDs that fit into the display's case. I am not sure about the Vf on the LEDs but I am going to try a couple different ones. I will also mount a socket header strip to the pc board, with a matching male header on the display. This will make for much easier future removal/replacement.

Gerhard

Re: Heat from BM8000 heat sinks

Dillen — Fri, 06 Feb 2009 02:14:09 GMT

Welcome to Beoworld and thanks for signing up as a silver member, your support is greatly appreciated !

I've developed my own techniques for repairing the original displays and it's not an easy task but please keep us informed as you progress.

Martin

Re: Heat from BM8000 heat sinks

GDSammet — Thu, 05 Feb 2009 10:51:56 GMT

I am new to the BM world and was lucky enough to have been able to acquire a BM8000 in pretty good condition. It has similar issues to many that have posted here, hot heat sinks, burned out LEDs. I want to thank everyone for their posts, because if it had not been for you, I would have never known the "quick" fix to the hot heatsinks.

I am starting to replace the LEDs on the displays, as they are all bad. I will keep you posted.

Thanks!

Re: Heat from BM8000 heat sinks

geearr — Tue, 11 Nov 2008 22:24:10 GMT

Thanks Martin. This morning I managed to find a supplier of a suitable mains capacitor, class x2 and have fitted it. The BM8000 now seems to be working well but no doubt something else will crop up soon - such is the nature of these old amps. As for the trimmers, I should be able to source some locally but they might look a bit different to the existing ones. At the moment, I can only source 200ohm items in the original style so I might have to wait. Anyway, I am in no hurry to take the Beomaster apart again right now.

Thanks for the offers and the help

Geoff

Re: Heat from BM8000 heat sinks

Dillen — Tue, 11 Nov 2008 02:24:39 GMT

Hi Geoff,

The trimmer should be 100 Ohms. I have some in stock if you need.

That capacitor should be a Class X component. I don't know where to buy a new cap of this value but I may have a good used component in stock, I will have to check.

Martin

Re: Heat from BM8000 heat sinks

geearr — Mon, 10 Nov 2008 23:47:13 GMT

Thanks Martin, I always appreciate your advice. I will be replacing the trimmers as soon as I can source some horizontal, 100ohm items which are pretty hard to get over here. Can you use a higher rating such as 200 or 250 ohm if the 100's prove too hard to get. They will be less sensitive but should work??

One item that has cropped up since the 8000 has cooled down is a faulty mains capacitor on board 7, rated 330nF and 275Volts. This has been slowly burning in the background but wasn't apparent when everything else was smelling hot. From local information, this item is going to be hard to get. Can you recommend some alternative values or combinations that I can try if such a capacitor is no longer available.

Thanks

Geoff

Re: Heat from BM8000 heat sinks

Dillen — Mon, 10 Nov 2008 02:52:04 GMT

Yes, those idle current trimmers are often past their exp. date. and will easily affect your electric bill.

They don't go out of adjustment, they go bad. Sometimes you can adjust back and forth, eventually breaking through oxidation, duse etc. to find a spot where it looks fine but that's just until the Beomaster has been moved around or gets warm. The trimmers oxidize and develop intermittent faults, sometimes even fall apart physically.

They are cheap components, replace them and readjust. Do one channel at a time and do the DC offset ones too while you are at it. You will be surprised at the difference, the adjustments will be very smooth.

The Beomaster 6000 has exactly the same problems, actually all 1970's and 80's Beomasters with discretely built output stages have.

The Beomaster 6000 will have a very warm spot on the bottom plate, that's normal. It's cooling fins will also feel slightly hotter than the BM8000 but that's mainly because of the power supply regulators heat sink at the right side. Similarly, the BM8000 will feel warm at the top of the glass panel where heat escapes through a grill. The BM8000 will keep it's cooling fins at room temperature even if playing at "normal levels" and definitely if idle. The Beomaster 6000 likewise, apart from the right side as mentioned, where it will become "pleasantly handwarm".

Martin

Re: Heat from BM8000 heat sinks

geearr — Mon, 10 Nov 2008 02:01:26 GMT

Just an update

Did two things today, first tested the BM6000 and found that the heat sink temperature only went up about 15°C. I could put my hand on it quite comfortably.

Second, dismantled the BM8000 to get access to the amplifier boards to set the no load currents. These were way off mark, the worst being around 250mV, so I reset them as close as I could get to 18mV. Next time round, I'll take the amp boards out and replace the pots because they are now very unstable. The end result is that the heat sinks are now running with barely a 5 to 10°C increase and the power draw has gone down from 80W to 42W. It cetainly does feel cooler than the BM6000 so your advice was correct.

Took out a lot of dust in the process and she's looking much better all ready. The dismantling and reassembly wasn't anywhere near as complex as I expected so my confidence is high to do a proper job after I source some spare parts. By the way, it looks as if someone has had a good go at the caps already.

Thanks again for putting me on the right track, it looks as if another Beomaster has been saved from the scrap yard. I'll definitely be keeping this one.

Regards

Geoff