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 day fellow Beofans
I purchased a Beomaster 5500 that didn't work through right channel. The history of this amplifier is a repair and overhaul after it blew up its right channel power transistors. After 2 years of faithful service it blew again on right channel.
I took the gamble that the new problem would be simple and bought it. In fact, it blew again the right channel power transistors. I noted that the previous repair placed bridges to connect transistors, as board damaged in these areas. The 0,22 ohm resistors were also replaced.. I placed new Philips BDV 64B and -65B and the rest seemed all right. In moving the BM around I heard some noise from under the other board where the input/output are. After slight removal I found a pretty big screw. I imagined it getting under the section of relay and shorting right channel, but pure speculation.
After power up I measured and found right channel sticking to positive rail voltage. I opened again and found R220 reading low and ID201 and TR206 suspect. I removed TR206 (BC547B) and it is bad. I also noted that it is different and already replaced in previous repair. Now R220 and ID201 read good. Other components in neighborhood seem OK. I will order new TR206.
Since I am not so good in this, is there something else to look out for?
Thanks for your reading time
I'm no expert but I did manage to blow some transitors in my BM2000 by not correctly setting the idle current. Not sure if there are any variable resistors on your board nearby, but maybe the need replacing/adjusting. Capacitors with internal short could cause undesirable conditions so maybe something else to look at. A photo or 3 might help.
Sorry for my late answer. I recently replaced TR206. I hooked it up with a bulb in series (luckily) and after power up a flash of light and it goes into stand-by. I will thus have to look further and might have blown again the transistors.
3 pictures of how it arrived:
They also replaced the 2 big caps, too bad, they usually are still OK
One can see the big resistors placed where they already replaced the transistors before. They were again cooked.
The underside of the board shows that the previous time the damage was massive
The 2 replacement transistors which were again dead
The other 2 look to be originals.
The board after replacement of output transistors and removal of TR206 which was bad.
The board after putting a new TR206 in place. But unfortunately it does not turn on and I guess the power transistors are dead (with TR206 dead there was 40V on output). Could that have damaged the output transistors ? I did not have the unit hooked up to speakers. I will look into that. I guess this is going to keep my busy for some while.
I'll assume you have a mulitimeter with continuity testing...
Put probes on each of the legs of your output transistors - if you get shorts (beeps) between legs, then you'll need to pull it out and replace most likely. From memory the short is usually from the middle to an outer leg, but easy enough to test all combinations. Once bad transistors are removed, I think you should be able to power it on safely - at least I could on my BM2000. Flash of light sounds like a fuse blowing...? I'm not familiar with that board sorry.
Dillen might be able to provide more insight if he pops in.
In fact, I was wrong in placing a lighbulb (90W) in series with the amplifier. I tested this with a working Beomaster 5500 and it will not power up. This method worked with a big (in size, but also about 60W of output power) Technics amplifier (SA 5460), but not here.
As I had already opened this amplifier I decided to take the time to measure the output transistors. They measure OK (no shorts), but as they were not reading the same values as the good channel I kept looking and found TR208 and TR209 (BF858 types, see previous pictures) suspect. Indeed, TR208 is dead and TR209 had a lifted leg (broken trace). I repaired the trace of TR209 and put it back and will now order a new TR208 transistor and repeat measurements to see if all looks OK before powering it up (without a lightbulb in series).
To be continued.
I finally got my new BF858 transistor, put it in, replaced the big 5W 0.22 ohm resistors with smaller 1W versions (as original specification) and it powered up and plays :)
So, to resume :
-New Philips BDV 64B and -65B output transistors
-New TR206 (BC547B) and TR208 (BF858)transistors
-Repair of lifted trace ofTR209
-New R228 and R229 (0.22 ohm) resistors. The previous big ones left little room for mounting the cooling flow guide and forced the resistors against the TR208/9 transistors. The trace of TR209 probably lifted after my multiple assemblies
I checked output voltage with volume at zero. On a 15V scale the needle did not move on either channel and so I touched nothing. I will have to get myself a better meeter. After running some time it is Luke warm on the fins, so it should be in the ballpark.
I now am looking into a problem of the according MCP5500: turning the dial does not wake up the remote and it is not the rotary contacts, the resistance drops to zero when turning. I also have about 5V on each leg before turning, so I assume the problem is downstream.
But the amplifier story has a happy end :)
You should also set the idle current!Ideally replace the trimmers in both channels and adjust both.
Martin
BeoCD!
Both Philips BDV are shorted to the ground by the retaining clips! As you will see, both clips are shorting the transistors from the heat sink to the transistor BASE.
You must move them to the plastic case instead.
And by the way, you applied way too much silicon paste.
Good luck! :-)
Charles
To Beo_Jean, I Had moved the clips for the picture so one could see the original and replaced transistors.
I finally came around to set idle current. The left channel was a bit high at 19mV. The right channel a bit low at 9mV. But adjusting the right channel showed that the variable resistor very sensitive to pressure, so it seems bad. I assumed that since the amp had an overhaul, that they would have changed these as well. So I will order new ones to have that right as well.
I used a vintage multimeter as well :)