Rotel 820BX4 driver transistors hotter on one channel - diyAudio
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Old 28th March 2015, 09:40 AM   #1
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Default Rotel 820BX4 driver transistors hotter on one channel

Hi all

I got an Rotel 820BX4 off ebay recently which had one channel not working.

After i got it I had a good look on the parts and for shorts did and quickly found that fuse F601 was blown but couldn't find any other issues.

I've replaced the fuse and adjusted the bias to 2.5mv and dc offset to about +/-5mv (best could get close to 0) and so now the amp works.

However I've discovered another issue. Driver transistors Q623 & Q625 run much hotter on one channel. I can only barely keep my finger on the heatsinks whereas on the other channel the heatsinks feel stone cold.

Any idea what I should be looking at next? I still don't feel I got to the bottom of why this fuse was blown and somehow feel this is related

Cheers Evan
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Last edited by chatziva; 28th March 2015 at 09:49 AM.
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Old 28th March 2015, 10:58 AM   #2
wwenze is offline wwenze  Singapore
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Well if the blown fuse and the hot transistors are on the same channel you may be onto something.

Strange that Q623 and Q625 can run hot without something else glowing with heat. Check if the the base of the transistors are at the +-0.53V as stated in that picture, and the emitters at 0V.

Check the resistance value of R641 and 643 too. If the values in the picture are true the designers used 2W resistors in a location that would emit 1.85W of heat...

If the voltages are correct and the resistance values are correct, then most likely one or more of the output transistors are dead.
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Old 28th March 2015, 11:15 AM   #3
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Quote:
Originally Posted by chatziva View Post
...I've replaced the fuse and adjusted the bias to 2.5mv and dc offset to about +/-5mv (best could get close to 0) and so now the amp works.

However I've discovered another issue. Driver transistors Q623 & Q625 run much hotter on one channel. I can only barely keep my finger on the heatsinks whereas on the other channel the heatsinks feel stone cold.
If you reduce the bias (VR601), do these transistors run cooler?
Is Q621 in thermal contact with the heatsink?
Are Q627, Q629, Q631 & Q633 running hot? Are they good?
Are the voltages across R645, R647, R649 & R651 the same?

Good Luck!
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Old 28th March 2015, 01:11 PM   #4
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I read +/-1.1V at the bases of the 4 driver transistors, so it would appear there is variation from the schematic, but could be normal... not sure...

The driver transistors are thermally connected to their heatsinks, when I touch the transistors themselves they are hot/cold.

I can't get a good resistance reading for all the 2w resistors, but I do read 35.7V across all 4 of them. Interesting enough I can get a ~0.9kR reading across the resistors in the hot channel but I read in the kilo/mega range for the cold channel. All 4 resistors have slight cracks on them so it could be that they are beginning to go open? I might as well replace them anyway as I don't like the look of them.

All power transistors read well and don't get hot, just lukewarm.

It's difficult to tell but reducing bias via VR601 seems to make the drivers warm up slower, but they seem to reach the same temperature in the end.

I've attached some pics, first one is the cold channel, second is the warm one.

Strange thing is the amp seems to work okay... I wouldn't say it sounds amazing but not bad
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File Type: jpg IMG_0682.jpg (736.0 KB, 64 views)
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Last edited by chatziva; 28th March 2015 at 01:24 PM.
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Old 28th March 2015, 02:06 PM   #5
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Quote:
Originally Posted by chatziva View Post
I read +/-1.1V at the bases of the 4 driver transistors...

All power transistors read well and don't get hot, just lukewarm.
What voltage do you read at the emitters of Q623 & Q625?
Are the voltages across R645, R647, R649 & R651 the same?
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Old 28th March 2015, 03:57 PM   #6
wwenze is offline wwenze  Singapore
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Quote:
Originally Posted by chatziva View Post
I read +/-1.1V at the bases of the 4 driver transistors, so it would appear there is variation from the schematic, but could be normal... not sure...

1.1V with respect to ground or to each other? Actually I'm not surprised if it is higher than 0.5V from ground because after all, the voltage there needs to be at least 2 VBE for the output stage to be biased class AB.

[snipped]

I can't get a good resistance reading for all the 2w resistors, but I do read 35.7V across all 4 of them. Interesting enough I can get a ~0.9kR reading across the resistors in the hot channel but I read in the kilo/mega range for the cold channel. All 4 resistors have slight cracks on them so it could be that they are beginning to go open? I might as well replace them anyway as I don't like the look of them.

Ahh, so it's the cold channel that's the spoilt one then.

The voltage across those resistor would always be the same regardless of resistance value, because of the beauty of semiconductors. The difference is the bias current that results. 910R would be following the schematic and kilo/mega not. Higher resistance results in lower (or none) bias current and hence the transistor not getting as hot as normal. "Cold" channel will still make sound but performance will be degraded.


[snipped]
So yea the resistors probably died from the heat. I would suggest changing them to 5W type.
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Old 28th March 2015, 03:59 PM   #7
mjona is offline mjona  New Zealand
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Quote:
Originally Posted by chatziva View Post
I read +/-1.1V at the bases of the 4 driver transistors, so it would appear there is variation from the schematic, but could be normal... not sure...

The driver transistors are thermally connected to their heatsinks, when I touch the transistors themselves they are hot/cold.

I can't get a good resistance reading for all the 2w resistors, but I do read 35.7V across all 4 of them. Interesting enough I can get a ~0.9kR reading across the resistors in the hot channel but I read in the kilo/mega range for the cold channel. All 4 resistors have slight cracks on them so it could be that they are beginning to go open? I might as well replace them anyway as I don't like the look of them.

All power transistors read well and don't get hot, just lukewarm.

It's difficult to tell but reducing bias via VR601 seems to make the drivers warm up slower, but they seem to reach the same temperature in the end.

I've attached some pics, first one is the cold channel, second is the warm one.

Strange thing is the amp seems to work okay... I wouldn't say it sounds amazing but not bad
The problem is in the channel that is running cool since the 910R resistors have "gone high".

The driver transistors in Class AB amplifiers switch off at some point on opposite cycle voltage swings. This happens if the transistor emitters drive the load through a resistor or if the emitters are cross-coupled by a resistor.

The driver transistors in this amplifier are resistively connected to the opposite voltage rail which breaks with the normal conventions and hotter running is to be expected.

You are on the right track to replace the 910R resistors.
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Old 28th March 2015, 04:29 PM   #8
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I've ran into a problem. I was probing the voltages across the emitter resistors when I touched one of the bus bars with my probe. It would appear I have shorted something and now I have one dead channel... school boy error...!!

Now the previously cold channel reads open on the speaker terminals and the main heatsink seems to heat up very rapidly on the side of this channel.

Whoops
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Old 28th March 2015, 06:11 PM   #9
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Right then so verdict is not as bad as I thought. Turns out I fried a couple of output and one driver transistor, Q628, Q630 (2SD1047) and Q624 (2SD600K) which coincidentally are all on the cold channel. All remaining transistors appear to read ok at diode test.

Might order a full set of drivers/output transistors for the bad channel, it deserves it after all that trouble!

Emitter resistors seem to be in good knick, all 4 read the same 0.34 ohm resistance but not 0.22 ohm as per schematic. Hmm should I worry about that and change them too?

The 2W910 resistors will also get changed with higher wattage ones.

Anything else I should be changing?
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Last edited by chatziva; 28th March 2015 at 06:16 PM.
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Old 29th March 2015, 12:26 AM   #10
wwenze is offline wwenze  Singapore
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A multimeter with transistor test is useful, but also
https://www.youtube.com/watch?v=b0jCUsenNsY

As well as temperature measurement, because "hot according to fingers" is seldom accurate when "too hot" can be 50-60 degrees when feeling metal but the silicon is feeling pretty okay. Have you touched the heatsink of a graphics card while playing?

If all 4 of resistors read the same... well for resistors the value is printed on them so...
If they all measure the same and different from what's printed on them, I would question the multimeter. After all, 0.1ohm can easily come from connectors and probe and stuff. I just measured my probes and each (with the wires and stuff) is 0.2ohm.
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