Hi everyone.
As I mentioned in another thread, one of my acquaintances has a BeoMaster 5500 and a MCP5500 that are acting up a bit, so he asked me, if I wanted to take a look at it. I told him about the usual problems with dried out electrolytic caps and bad trimmers, so we agreed that it was probably best to give it a complete overhaul.
He had told me, that there was a humming sound in the speakers and that the BM would turn off by itself quite often. After he had dropped the BM and MCP off at my place, I hooked a pair of speakers up to the BM and fired it up. There was clearly some humming, and it sounded like the humming from the BM was being amplified through the speakers. I unplugged the speakers to listen to the BM, and it was indeed humming by itself. I turned the BM off again and unplugged it from the mains. It was here I realised how hot the heatsink had gotten. I only had the BM turned on for a couple of minutes, but the heatsink was very hot, so something was clearly wrong!
I had made some initial research on the BM5500, and at some point I found link on Beocentral describing that high frequency noise in the preamplifier could make the power amplifier overheat badly, so that was definitely something I wanted to look at, as I had a feeling the overheating wasn’t just caused by a pair of trimmers that had gone out of spec.
I got the BM on the workbench today to make some measurements and tests to give me an idea of what was going on. I took the cover off first, as I wanted to measure the idle current. I noticed that the screws for the tuner board were loose, so it could seem someone has been in here before.
I slowly increased the voltage on the variac until it reached 220V. Everything was looking fine, so I turned the BM on. Idle current and power consumption raised quite a bit, but since the idle current must be measured with the volume at 00, I turned it down to that. With the volume at 00 the I got a measuring of 21 mV between TP200 and TP201, which is 10mV more than specified which isn't all too bad.
Power consumption at this point was 220V x 0.17A ~ 40W. Heatsink was staying cold.
I then increased the volume to 02 which more than doubled the power consumption (220V x 0.42A ~ 90W)!
I then looked at the idle current. I had to press the '1000' button to get a useful reading. The idle current had tenfolded so it was 200mV now!
The heatsink was getting hot quite fast when the volume was more than 00. I noticed a strange thing when playing with the volume. At 08 it would use 120W but at 10 it would drop to 80W. This was in fact the case for the whole volume scale; power consumption would increase and then drop again for every few steps. I could hear a slight change in the hum when the power consumption dropped. I turned the BM off again and cut the power.
Next step was to try with 3P22 disconnected. Power consumption dropped to 35W with that disconnected and idle current was measured to 18mV. Naturally changing the volume wouldn't change anything now, as the power amp wasn't getting any signal from the preamp.
Since the power amp seems to be OK, the next step will be to see, if adding a 100nF 63V cap to the B-E junction of 2TR8 as described on Beocentral will cure this problem.
/Dennis
