The Workbench

Re: Resurrecting a Beomaster 8000

tournedos — Mon, 19 Apr 2010 02:27:59 GMT

To me, the only RF critical part seems the front end IC (TDA 1062). The IF part runs at about 10 MHz and the stereo decoder at audio frequencies, so they should have no problem sitting in sockets. Same for the various op-amps.

I don't think sockets are necessary though if you check the supply voltages, and other semiconductors for shorts before installing the ICs; they shouldn't be in any danger. Use good quality sockets anyway if you end up installing them.

Re: Resurrecting a Beomaster 8000

betapictoris — Mon, 19 Apr 2010 00:36:53 GMT

Hi Martin,

Thanks for your advice - it looks like the screw at the front is short enough. It doesn't quite protrude beyond the plastic.

Once I recieve the replacement ICs, I've been thinking about putting them in sockets. Could this cause any problems considering it's an RF circuit?

Ian

Re: Resurrecting a Beomaster 8000

Dillen — Sun, 18 Apr 2010 13:29:40 GMT

I agree that it sounds like a lightning strike or at least some kind
of strong voltage burst and I also agree that you shouldn't touch any adjustments
but instead focus primarily on semiconductors.
The IC's being a good place to start.

Regarding the tuner module, check also that none of the Beomasters bottom plate screws
protrude inside.
There is one particular screw right at the front edge which is too long in some
Beomasters, long enough to cause intermittent grounding of a PCB track in the tuner.
Most often experienced as intermittent (or constant) muting of the tuner.

Martin

Re: Resurrecting a Beomaster 8000

betapictoris — Sun, 18 Apr 2010 10:42:48 GMT

It sounds like wise advice to not fiddle with adjustments right away. I'll probably try measuring the voltages that the tuner ICs are seeing with them in and out of circuit - that might shed some light on whether there are any shorted transistors in the circuit. I already made sure the tuner boards wouldn't deliver any dangerous voltages to the rest of the receiver before connecting it up.

Ian

Re: Resurrecting a Beomaster 8000

tournedos — Sun, 18 Apr 2010 05:13:27 GMT

Great work on the volume control ICs - we need more stuff like this to convince people to not simply junk the kit that needs "unobtainable" spare parts!

Considering the extent of the damage to the unit, I think it might have been caused by a lightning. Unfortunately that kind of damage defies most regular fault finding, you could be looking at just about anything. The best thing to do with the tuner board is NOT to touch any adjustments, as getting it re-aligned will need experience and special equipment. Check supply voltages and all the other circuitry you can before installing the new ICs, so that they won't get damaged right away. With some luck, the circuitry that controls the tuner is OK and it will just begin to work after that.

Our BM8000 experts will probably chip in soon - I believe that in addition to what you have already done, there will be many typical capacitors to replace and bad solder joints to fix.

Resurrecting a Beomaster 8000

betapictoris — Sat, 17 Apr 2010 11:32:26 GMT

Hi Everyone, I'm new to this site and have just spent the past day or so reading many interesting posts in the Workbench forum. Looks like there are lots of passionate and talented members here :-)

The Beomaster 8000 was given to me for free, but dead. I decided to take on the challenge of saving it because I became enamored with the design and the fact that it represented the pinnacle of B&O receiver engineering in its day.

I started with the power supply, and then proceeded through the various circuit boards such as preamp, tone control, power amp, etc. So far, I've replaced a handful of discrete semiconductors and two linear voltage regulators. I'm not sure what caused the system to die in the first place, but I'm finding blown components all over the place (and also one blown ground trace on the FM interface board)

After replacing a few semis, I was still unable to get the unit to power up out of standby mode. It was then when I discovered that there were two "chassis ground" points in the unit that were not electrically connected to each other ....after running a ground wire between the two, the DC offset on the speaker output disappeared, which cleared the fault signal that the power amp was sending to the power supply board, thereby allowing the unit to power up normally. (Still haven't figured out what the"proper" electrical connection between these two ground points should be.)

Now that the unit powered up, I was able to start testing the signal path. Input switching on the preamp board seemed to work, but only one channel had a signal at the speaker output and even so, the electronic volume control would not attenuate the signal all the way down to zero. It turned out that two digital volume attenuators were bad - they were Analog Devices AD7110's. Unfortunately I wasn't able to find any replacements, so I simulated them by building a daughter board with an AD7111, a microcontroller and a 4 channel CMOS analog switch - a project that took some time to complete:

Having overcome the volume control problem, everything seems to be working except the tuner. As I mentioned above, there's a blown ground trace on the FM interface board. After repairing this trace and applying power, I get erratic tuning needle movement and the stereo decoder chip gets slightly warm to the touch. In light of the blown trace, I'm suspicious that the tuner section will need lots of replacement parts. I've already placed an order for the three ICs on the FM board, but otherwise, I'm not sure where to start because I'm not familiar with RF circuitry.

Does anyone have any thoughts or suggestions?

Sincerely,

Ian