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
I recently spent a large amount of time troubleshooting a beautiful bombaster 2400. Here's the problem and the solution found:
-At each startup, when an input was selected, the left channel would start, then gradually fade out until it became distorted and muffled. The problem remained after a complete recap, including lamps.
-I suspected that the no-load current was too low: a quick check revealed about 1mV between R256. Turning the pot would not change this too low voltage at all.
-I then checked the signal path for a voltage loss: The collector of TR212 revealed only 22mV (1.7V should be present) because of that, only a few mV were present at the base of each output transistor: probably not enough current to turn them on.
-I thought that TR212 was the culprit: it was not. I went on to the other transistors present on the signal path, including the output one. They were not the cause.
-Culprit was resistor R247 (15K) that was completely open. It did not showed, however, any exterior signs of burning… Put a brand new resistor. Voltages were now back and I was able to adjust the no load current to the prescribed setting (25mA collector current or 12mV between R256)
-One more thing: the bass pot didn't work. I followed the instruction provided here on the archive forum:
http://archivedarchivedforum2.beoworld.org/forums/t/2323.aspx
(A huge thanks to all the people involved in this old thread!)
And Voilà!: another one saved...
I really love working in these small machine. It's my second successful attempt at restoring them. Their styling is beautiful.
By the way, I have an amateurish question: When checking on the output transistors, I found out that they were attached to the heatsink without Mica in between. Since the heatsinks seemed to be conductive, isn't it strange? Why touching them does not provide an electric shock? Or it's the thermal compound that isolates them?
Guillaume
Good job !Beautiful pieces !
Unfortunately, long time testing has revealed that no glue (none that I could find anyway)will grip the quite oil-rich type of plastic used in the sliding bridges of the tonecontrolpotentiometers and hold on for very long.I've glued many and they have started to fall apart again, regardless ofthe type of glue used.
I have therefore reproduced the sliding bridges, and also improved them so theywon't break, which means that replacement parts that lasts are available now.Actually also makes for a faster repair time, since you don't have to clean and clean andyou don't have to wait for the glue to set.
You will not get an electric shock from voltages of about 30V.
Martin
Thanks Martin,
good to know about the slider pot. The actual bridge seems to be a design flaw: it's hard to imagine how they expected that such a tiny bit of plastic could hold that copper piece, which is under constant pressure. However, the 2 other pots plus the three in my other 2400 did not sufferered from that problem. I'll see how it goes and when it fails, I might change all three of them.
On a side note: I'm still wondering why and when the suffix "-2" came for these Beomasters. My other 2400 is the canadian type (2908), which sold from 1979 to 1983. It has the new display, 7 pins inputs for the turntable and the 128 steps volume control. Still it's not a 2400-2
I was sure that the one I just repaired was also a canadian type since it shares exactly the same characteristics. But it's the US type 2902, from 1976 to 1979.
The only difference I noted on both machines was the tint of the back of the PCB board.
Could it be that the "2400-2" form never existed in the north American market? Why a Canadian type since both North American countries shares the same residential voltage?
That "-2" is a big mystery...
Oh and also, about the electric shock: it's just that some time ago I had the brilliant idea to touch the casing of one of the transistors in my beocenter 3500 to see if it was hot: It did surprise me and it's just 50V! So I learned this way that touching the collectors of an output transistor was not a good idea..