Picked up 2 Rotel RB-970BX MKII (and a Rotel RC-970BX MKII to feed them). They have been sitting in a basement for most of their life until I got them. One of them played music once I briefly tested them on a shity bookshelf speaker while the other made a loud thumping sound on startup and a loud buzzing sound from the amp while sounding all distorted. I've replaced all the small electrolytic caps on both amps and the power caps on the one that was buzzing when turned on but both of them still sounds heavily distorted when playing anything and I'm measuring some power between the speaker terminals even with no volume.
What should I do/replace? Should I replace some more non-electrolytic caps and/or some transistors and in that case, where would I begin looking for causes?
I'm really new to this and would appreciate some much-needed help
What should I do/replace? Should I replace some more non-electrolytic caps and/or some transistors and in that case, where would I begin looking for causes?
I'm really new to this and would appreciate some much-needed help
On old amps it is usually the electrolytics that go first.
Then its pots, switches and connectors that become worn/corroded.
I would apply a sine wave and follow it through the amp and see where distortion happens. Could be as simple as the dc offset and bias need adjusting after changing components.
Then its pots, switches and connectors that become worn/corroded.
I would apply a sine wave and follow it through the amp and see where distortion happens. Could be as simple as the dc offset and bias need adjusting after changing components.
Aha, I've replaced all the electrolytic caps (just the power caps on one amp left) and I'm currently going through all transistors by hand (still left on the PCB and with a multimeter). The problem is that the only equipment except for a soldering kit I have is a multimeter so I'm pretty much limited to testing every part one by one by hand and just replacing all electrolytic caps. I've so far found 1 transistor that I've found to read as faulty (an A1208).
I'll read up on how I should test the pots and switches (I've never done that before).
What do you think about the non-electrolytic caps? I don't know much about other types of caps. Do they usually fail and is it possible to test them with just a multimeter?
Not likely. Most likely the problem is a shorted (or less common open) semi.
Semis can usually be screened enough with the meter's diode function,
if done on all possible pairs of terminals of the transistor. The C-E should be
open for either polarity of probes. The B-E or B-C should be diodes for one polarity.
Make sure the bias pots are set properly for power up.
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Okay. Yes, I've found 2 failing transistors, an C2910 and an A1208 (beeps when tested) and I'm trying to find new one or replacements for them. Not sure how to move on from here though.
Test as many of the parts as you can. You can safely ignore fim caps,
they seldom fail, but electrolytic and tantalum caps often fail. Buy some
replacement parts and see if that fixes the problem(s). If not, set it aside
for a week or two, and then try another round of troubleshooting. The more
of this you do, the better you'll be at it. Look at this as a learning experience,
not as a chore.
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