Hi all,
so on a bit of a whim I bought a known broken Rotel 931 (mk1) amplifier - much to my surprise everything worked after replacing the two output fuses with no additional drama.. or so I thought.
so after maybe 10 mins or so of listening the channels aren't balanced very well if using the balance dial selecting the left speaker the sound is weak in bass, harsh and to my somewhat unlearned ears marginally distorted.
The right channel sounds full with suitable amounts of base and warm in comparison
What I have done so far
Opened it up and given it a visual once over,
- No leaking caps
- one slightly discolored driver transistor on the left channel side (see photo- the right transistor) - Thoughts are if its silicon it either works or its totally non functional 99.9% of the time? please correct me if I am wrong
- Using a Diode test function on my multi meter on all main drive transistors when compared to their mirror channel read equivalent values. and no C-E junctions shorted on the main drive transistors.
- Vbias checked and measured OK @ ~4.0mV (albeit unloaded)
Image links
IMG-20190213-204645 — imgbb.com
IMG-20190213-204736 — imgbb.com
So my thoughts are to replace all the electrolytic and coupling capacitors but this is where my (admittedly very limited) knowledge runs out.
What are the key caps to replace? does lack of bass indicate one of the main 6800uf output caps is near dead (I'd think not given its unbalanced at even low volume)
So what is most likely to be at fault the large(r) electrolytic caps or the film capacitors, or is it unrelated to the filtering at all? could it be the breakdown of a key feedback function?
schematic & circuit layout can be found here:
Rotel RA-931 - Manual - Stereo Integrated Amplifier - HiFi Engine
Many thanks in advance!
Aaron
so on a bit of a whim I bought a known broken Rotel 931 (mk1) amplifier - much to my surprise everything worked after replacing the two output fuses with no additional drama.. or so I thought.
so after maybe 10 mins or so of listening the channels aren't balanced very well if using the balance dial selecting the left speaker the sound is weak in bass, harsh and to my somewhat unlearned ears marginally distorted.
The right channel sounds full with suitable amounts of base and warm in comparison
What I have done so far
Opened it up and given it a visual once over,
- No leaking caps
- one slightly discolored driver transistor on the left channel side (see photo- the right transistor) - Thoughts are if its silicon it either works or its totally non functional 99.9% of the time? please correct me if I am wrong
- Using a Diode test function on my multi meter on all main drive transistors when compared to their mirror channel read equivalent values. and no C-E junctions shorted on the main drive transistors.
- Vbias checked and measured OK @ ~4.0mV (albeit unloaded)
Image links
IMG-20190213-204645 — imgbb.com
IMG-20190213-204736 — imgbb.com
So my thoughts are to replace all the electrolytic and coupling capacitors but this is where my (admittedly very limited) knowledge runs out.
What are the key caps to replace? does lack of bass indicate one of the main 6800uf output caps is near dead (I'd think not given its unbalanced at even low volume)
So what is most likely to be at fault the large(r) electrolytic caps or the film capacitors, or is it unrelated to the filtering at all? could it be the breakdown of a key feedback function?
schematic & circuit layout can be found here:
Rotel RA-931 - Manual - Stereo Integrated Amplifier - HiFi Engine
Many thanks in advance!
Aaron
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A great thing about stereo and multichannel amplifiers is that you usually have a working reference(s) to compare many things that point to fault areas and even individual components. All you need sometimes, is a decent multimeter to measure and compare the DC voltages at various circuit nodes and at the Base, Emitter and Collector terminals of the semis.
To get on with it, start with the output and driver transistors of both channels. Write down the circuit reference numbers (Qxxx etc) of the 4 transistors in each channel or print out the schematic and write them there, listing B,C,E voltages measured for each, with respect to power ground, which is the same as the speaker ground terminal if you can't be sure where to to look. Finally, check and record the Vbe of each transistor to confirm the other measurements are realistic. That's the voltage between the Base and Emitter terminals which makes a quick check of any bipolar transistor's functionality. Typically this about 0.65V, less for power transistors and will be reverse polarity for PNP V NPN types.
Before proceeding, the measurements are when powered but unloaded and in a steady state so remove all speakers and inputs before testing. Also a warning about using multimeters on live circuits and the usual probes they come with: Apart from personal safety concerns with mains voltages and using cheap, potentially unsafe meters on them, it's very easy to slip and short things, causing worse damage in just the blink of an eye with not even a spark to show for it. Use a hook probe or clip, clip leads etc to connect to ground whilst you wander about, concentrating on where to place the + probe. Otherwise, the insulating sleeves supplied with probes these days can help reduce shorts and really sharp probe tips that wont slip on solder joints help too. Take care, look at those signs of heat damage with suspicion and return with what you find. There are plenty of folk here who tinker with Rotel and similar products and others can help too when you can share a little more about the symptoms and hence get closer to where the fault is likely to be.
To get on with it, start with the output and driver transistors of both channels. Write down the circuit reference numbers (Qxxx etc) of the 4 transistors in each channel or print out the schematic and write them there, listing B,C,E voltages measured for each, with respect to power ground, which is the same as the speaker ground terminal if you can't be sure where to to look. Finally, check and record the Vbe of each transistor to confirm the other measurements are realistic. That's the voltage between the Base and Emitter terminals which makes a quick check of any bipolar transistor's functionality. Typically this about 0.65V, less for power transistors and will be reverse polarity for PNP V NPN types.
Before proceeding, the measurements are when powered but unloaded and in a steady state so remove all speakers and inputs before testing. Also a warning about using multimeters on live circuits and the usual probes they come with: Apart from personal safety concerns with mains voltages and using cheap, potentially unsafe meters on them, it's very easy to slip and short things, causing worse damage in just the blink of an eye with not even a spark to show for it. Use a hook probe or clip, clip leads etc to connect to ground whilst you wander about, concentrating on where to place the + probe. Otherwise, the insulating sleeves supplied with probes these days can help reduce shorts and really sharp probe tips that wont slip on solder joints help too. Take care, look at those signs of heat damage with suspicion and return with what you find. There are plenty of folk here who tinker with Rotel and similar products and others can help too when you can share a little more about the symptoms and hence get closer to where the fault is likely to be.
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Thanks Ian,
that sounds like a sensible place to start, a hook probe for my multi meter might be in order. I'll report back when I accumulate some more data
regards
that sounds like a sensible place to start, a hook probe for my multi meter might be in order. I'll report back when I accumulate some more data
regards
Just to close this out, I check all voltage across the transistors and everything matched up,
after some more digging it ended up being the speakers, one of the coils on the main driver must have partially failed (they are fairly old mordaunt short MS10's) as one is 13 ohm and the other 16ohm which explains the lack of bass on one side only.
thanks for the input anyway, time for a new pair of speakers!
after some more digging it ended up being the speakers, one of the coils on the main driver must have partially failed (they are fairly old mordaunt short MS10's) as one is 13 ohm and the other 16ohm which explains the lack of bass on one side only.
thanks for the input anyway, time for a new pair of speakers!
No problem - 'good that you have a simpler resolution for the present. I haven't come across MS10 speakers but my limited experience with MS20 is that they last quite well, if not being particularly robust or living up to the hype surrounding the brand introduction long ago. Did they come with the amplifier or were they a separate acquisition? If they came with amp, I guess that might explain the blown fuses. Otherwise, that's still a loose end that needs sorting out to avoid a repeat performance.
Both fuses and both speakers blowing would be pretty rare - usually one fuse blows and the worried owner immediately panics and becomes protective about their pride and joy. This situation seems a bit different so perhaps the owner wasn't around when the volume was turned up loud and the first bit of bass simply overpowered such little speakers before anyone thought to turn the volume down or off 😉
Both fuses and both speakers blowing would be pretty rare - usually one fuse blows and the worried owner immediately panics and becomes protective about their pride and joy. This situation seems a bit different so perhaps the owner wasn't around when the volume was turned up loud and the first bit of bass simply overpowered such little speakers before anyone thought to turn the volume down or off 😉
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