Last week I showed the Beomaster 1000 with a faulty right channel that i bought. After a bit of drama finding the rather rare transistors that needed replacing (2N5034, kindly provided by by Gapco, also Rick offered), it was time to sort it out.
Open up:
Eish! 45 years of dust in there....
And how cool is this - that little envelope in the lid (see first photo) has the circuit diagram! It's so nice to think that all those years ago somebody slipped it into the envelope just for this moment.
So first strip it completely and clean the inside:
Before
After:
Then put in the new transistors:
and in - back view, mounted on exterior on cabinet, which acts as heat sink
I fired it up, and hooray! Right channel working. So now i thought i was very clever. But after a few minutes, the dreaded smell that any elec eng knows - hot components! So i checked, and both the right channel transistors were extremely hot - much hotter than they should be - their left channel brothers were completely cool. I switched off, disconnected the speakers and turned the volume down to zero. Switched on again, and they very soon got hot. So i now knew that it was something further back in the circuit that caused the transistor to pop in the first place. Then the real fun started. I haven't had much to do with analogue electronics since 2nd/3rd year electronics, so my brain is very rusty with this stuff. My first guess was that something was wrong with the biasing, but adjusting the trim pot made no difference. Then I spent about 3 hours measuring, thinking, swearing, checking and nothing could explain to me what was causing such high current in the right channel power transistors. The quiescent voltage across those big 0.39ohm resistors was 10s of times higher than it should have been according to the biasing instructions in the service manual. Because the circuit diagram was done all deurmekaar, I even redrew it in desperation: (it didn't help that the "ground" is 30V - kept having to reorientate)
I was on the point of giving up, but scratched around on the interwebs. I read somewhere that those kind of open trimpots can oxidise with age, so as a last resort I gave the bias trim pot about ten full turns from one side to the other hoping that the wiper would rub off the possible oxidation, it being 1am and no contact cleaner at hand. As i did this, a little piece of something peered out! It was this little bloody piece of metal (about 3mm) that had somehow got into the trim pot and was shorting it!
And guess what - that was it! Jeez you have no idea how relieved I was. So now two happy little channels:
Yes there seems to be an issue with the gain (no the trim pot doesn't change it) but it sounds fine so that's a problem for another day. Just relieved to have it working. I still need to clean out the selector switches - sometimes they don't work too well when switching between tape and FM for example.
So a long story. Now for the Beogram 1000.... I'll post my progress over the next few days
Open up:
Eish! 45 years of dust in there....
And how cool is this - that little envelope in the lid (see first photo) has the circuit diagram! It's so nice to think that all those years ago somebody slipped it into the envelope just for this moment.
So first strip it completely and clean the inside:
Before
After:
Then put in the new transistors:
and in - back view, mounted on exterior on cabinet, which acts as heat sink
I fired it up, and hooray! Right channel working. So now i thought i was very clever. But after a few minutes, the dreaded smell that any elec eng knows - hot components! So i checked, and both the right channel transistors were extremely hot - much hotter than they should be - their left channel brothers were completely cool. I switched off, disconnected the speakers and turned the volume down to zero. Switched on again, and they very soon got hot. So i now knew that it was something further back in the circuit that caused the transistor to pop in the first place. Then the real fun started. I haven't had much to do with analogue electronics since 2nd/3rd year electronics, so my brain is very rusty with this stuff. My first guess was that something was wrong with the biasing, but adjusting the trim pot made no difference. Then I spent about 3 hours measuring, thinking, swearing, checking and nothing could explain to me what was causing such high current in the right channel power transistors. The quiescent voltage across those big 0.39ohm resistors was 10s of times higher than it should have been according to the biasing instructions in the service manual. Because the circuit diagram was done all deurmekaar, I even redrew it in desperation: (it didn't help that the "ground" is 30V - kept having to reorientate)
I was on the point of giving up, but scratched around on the interwebs. I read somewhere that those kind of open trimpots can oxidise with age, so as a last resort I gave the bias trim pot about ten full turns from one side to the other hoping that the wiper would rub off the possible oxidation, it being 1am and no contact cleaner at hand. As i did this, a little piece of something peered out! It was this little bloody piece of metal (about 3mm) that had somehow got into the trim pot and was shorting it!
And guess what - that was it! Jeez you have no idea how relieved I was. So now two happy little channels:
Yes there seems to be an issue with the gain (no the trim pot doesn't change it) but it sounds fine so that's a problem for another day. Just relieved to have it working. I still need to clean out the selector switches - sometimes they don't work too well when switching between tape and FM for example.
So a long story. Now for the Beogram 1000.... I'll post my progress over the next few days
