Repaired a 60dsm. It had obvious leaky caps, and overheated and ejected one of the 150ohm 2w resistors on the speaker outputs. Also replaced the LM833 opamps on the preamp riser since I had it out and had past experience of them damaged by the leaky caps. Nothing else seemed damaged in the output stage.
Amp seemingly works fine and drives above rated power, but bias pots ended up at about 10:00 and 12:00 setting them via 5mA draw. Usually they are closer to the same position. Found that one 2k bias pot read 2120ohm and the other 1842ohm. Swapped the 1842ohm for one that was about 2150ohm with no real change in the resulting position.
Should I be concerned about the difference?
Amp seemingly works fine and drives above rated power, but bias pots ended up at about 10:00 and 12:00 setting them via 5mA draw. Usually they are closer to the same position. Found that one 2k bias pot read 2120ohm and the other 1842ohm. Swapped the 1842ohm for one that was about 2150ohm with no real change in the resulting position.
Should I be concerned about the difference?
Any semiconductors (from one channel to the other) in the power amplifier section (differential amplifier to the outputs) that are not from the same batch can make the bias requirements different. That's why they have bias pots. Most people who have replaced output transistors see that that alone requires re-biasing the channel.
Sometimes, the semiconductor properties can drift a bit and that alone can cause the bias to be off. I saw this in a lot of Rockford amps where an amp would come in drawing 8-10 amps at idle and the only problem was biasing.
Sometimes, the semiconductor properties can drift a bit and that alone can cause the bias to be off. I saw this in a lot of Rockford amps where an amp would come in drawing 8-10 amps at idle and the only problem was biasing.
I'd guess I've done around 15-20 output repairs and I don't recall any being this different, so it caught my attention. The vertical boards without part number designations make these a little hard to trace, and the schematic I have doesn't include them either. There are a couple components I can heat and cause the idle current to drop
The Hfe of the transistor changes with temperature. Virtually any transistor with current flowing through it will make the idle current change if heated/cooled.
Are the corresponding outputs in both channel from the same batch?
Did you run the amp up to clipping with a sine sweep to see if both channels were distortion free?
What diagram are you using?
Are the corresponding outputs in both channel from the same batch?
Did you run the amp up to clipping with a sine sweep to see if both channels were distortion free?
What diagram are you using?
I'm using this diagram I believe you posted in another thread.
Both channels go up to about 16.6v with 4ohm dummy loads before clipping, using some test tones ranging 50hz-70hz.
All outputs have same date code
Both channels go up to about 16.6v with 4ohm dummy loads before clipping, using some test tones ranging 50hz-70hz.
All outputs have same date code
You can generate any tones you need with Audacity. The attached file is a sweep. Check it on your scope out of the signal source to confirm that it's clean.
http://www.bcae1.com/temp/sweep20hzto20000hzin30sec.mp3
There is likely a signal generator for your phone as well.
http://www.bcae1.com/temp/sweep20hzto20000hzin30sec.mp3
There is likely a signal generator for your phone as well.
Sweep seems to be clean. For whatever reason the amp induces some noise to my source (phone at the moment...not sure where I buried my head unit). So a clean input source like this:
becomes this when I power on the amp with scope attached to source RCA, completely disconnected from the amp:
Captures from sweep (not shown at clipping, but I did run it to clipping):
becomes this when I power on the amp with scope attached to source RCA, completely disconnected from the amp:
Captures from sweep (not shown at clipping, but I did run it to clipping):
There don't seem to be any serious problems but the resolution of the LCD display isn't good enough to be definitive. There may be some noise problems that appear on some parts of the waveforms. Those may clean up if you either ground the shield or use a source with a grounded shield.