ARCHIVED FORUM -- March 2012 to February 2022READ ONLY FORUM
This is the second Archived Forum which was active between 1st March 2012 and 23rd February 2022
Good evening,
this is my first post here on Beoworld and I'm happy to have some knowledgeable minds and eyes reading it as I attempt to solve this issue!
Story goes as follows. (I apologize for the length but I believe the explanantion might be of help.)
I purchased 2 (!!!) BeoCenter 7000 units several months ago from a local owner who was moving away to Europe and could not transport the systems safely.
He had the turntable portion of the first unit serviced by B&O here in NYC and said that it functioned properly, but that some time later after the repair, he turned the unit on and saw and smelled smoke coming out of the back of the machine. He purchased a 2nd BC7000 for parts, and he told me that he thought there might have been a short where the wiring harness meets the speaker outputs.
This month I've finally had the chance to open them up and take a look. What follows are my discoveries thus far in chronological order.
NOTE: I have the DIN connectors on order for the speaker outputs, so the audio monitoring in the following descriptions is through the front panel headphone jack.
2nd NOTE: Visual inspection of the exterior reveals a dent in the bottom metal cover on the L corner, when viewed from the rear of the unit. I suspect it was dropped in it's history at some point (it is a deceptively heavy bugger!)
1. The unit powers on and there is a red colon : symbol on the LED readout .When pressing any function button, a temporary click is heard from the back of the unit (which I discovered is a relay with a plastic cover.), a yellow exclamation ! symbol appears, and immediately goes back to the : . As a happy mistake I found that if you hold one of the phono/tape/radio preset buttons down, those functions will work temporarily as long as the button as held, for a maximum of 10 seconds. Unfortunately at the end of this temporary state there is a painfully loud static howl in the R channel, and function ceases.
2. I inspected the original output harness and indeed found two breaks in the wires, but the way in which they were broken made it seem that just replacing the entire harness itself with the secondary units might be easier (the 2nd harness was already detached as the previous owner had planned on making this fix himself.)
It didn't take too long to replace the harness once I discovered how to open the Beocenter. The new harness on visual inspection had no wire breaks. In the process of replacing it I also noticed that the RCA plug coming from beneath the turntable section was severely frayed so I replaced that as well.
Same original issue occurred after harness replacement.
3. I reopened the unit and did some further visual inspection. I thought that perhaps there was a signal malfunction from one of the wiring bundles coming from behind the keypad, but all were securely fastened and I saw no loose or broken wires. I did notice a metal shield that adheres to the back of the PCB board of the display unit, which has several tabs that solder to the board itself. 3 of these locations had broken joints and I resoldered them.
Same result.
4. Further inspection revealed three blown and badly burnt resistors beside the plastic relay, directly in front of the R output transistors (IC 201/200.)
a. R220 270 ohm
b. R 226 and 227 (2 watt 0.39 ohm metal oxide resistors.)
The striped wires that sit between R226 and 227 and their respective ICs were also badly singed by the resistor failures.
Curiously, 226/227 had the wrong values inserted: they were 22 ohm resistors which I found odd. I took the bottom cover off of the unit and immediately noticed that the solder joints differed from those of the factory, and surmised that they had been replaced.
Luckily! I had the 2nd Beocenter to work with. I opened that up and removed the proper components from it, and replaced them in the faulty 1st unit. (R226/227 in the 2nd unit were indeed the correct 0.39 ohm rating).
5. I reattached the bottom cover ( a bit tricky fishing the ground wires through it) and plugged the Beocenter back in. I pressed the Phono button, the display read PH, showed a time signature, and also the ! symbol. The tonearm began to move and the platter spun (I had a record on it of course).
After 7-10 seconds smoke began to emanate out of the back of the unit and I quickly unplugged it. You know a blown resistor when you've smelled one and that is exactly what happened. Both R226 and 227 had exploded (R 226 worse, which is the same state I found originally) and the new R220 had slightly bulged. I also found that R221 was also burned, but could not confirm that it had not previously been that way.
6. I did some reading on the unit and found some past threads here with reference to the fact that the ICs and their heatsick mounting could be problematic. I discovered that on my spare Beocenter (which had good resistors) the output ICs have some visual differences with my problem child.
a. there is a generous amount of white paste between the metal back of the IC and the small plastic rectangle; for that matter there is a thin film between the rectangle and the heatsink itself.
b. each IC has insulation surrounding it's legs.
This is not the case on the primary Beocenter I am working on. There is no leg insulation, and the ICs linked to the problematic 2 watt resistors have comparatively little paste on them (what paste is on them seems brittle.)
NOTE: My primary unit has Philips transistors on the R output and Texas Instruments transistors on the L output. The TI transistors have generous paste applied. I observed no visual damage to the transistors in question.
I have also read about trimmers that control idle current neding adjustment, possibly faulty capacitors, etc. being probable causes of shutoff. I suspect next I will need to remove transistors and check for continuity, but perhaps my lengthy explanation will spark another thought from one of you.
Any thoughts would be much appreciated as I continue on the journey to having this unit function again!
--- John