Checked for continuity around the opamp - all fine.
Checked voltages on opamp and input selector - all fine.
Checked voltage to regulators - fine 24v
Checked decoupling caps to ground continuity - all fine.
Checked resistors around opamp - all reding correct.
Im a bit lost really, never done this before. Im not sure what to do next and i'm uncertain if ive checked the continuity properly.
Any advice please?
Checked voltages on opamp and input selector - all fine.
Checked voltage to regulators - fine 24v
Checked decoupling caps to ground continuity - all fine.
Checked resistors around opamp - all reding correct.
Im a bit lost really, never done this before. Im not sure what to do next and i'm uncertain if ive checked the continuity properly.
Any advice please?
Here's a report so far:
I haven't tried a new pot yet but I do have a 20K Alps Blue which has the same lead pitch. I have checked the ground pads on the Vol and balance pots to spkr ground and its ok.
Checked the soldering and continuity around the op-amp and I think its ok.
+/- inputs to op-amp read 15v DC respectively, .003v AC.
0.003 AND 0.005v DC ON OUTPUT OF OPAMP, 0v AC.
Spkr terminals read:
DC: WITH VOL AT MAX READS 0.010 ON RIGHT AND 0.016 ON LEFT (no input)
AC: WITH VOL AT MAX READS ZERO ON RIGHT AND -0.001 ON LEFT (no input)
Diodes seem ok though I'm not 100% certain. They read a resistance greater in 1 direction than the other.
Does any of that indicate anything?
Im all out of ideas now, any suggestions?
I haven't tried a new pot yet but I do have a 20K Alps Blue which has the same lead pitch. I have checked the ground pads on the Vol and balance pots to spkr ground and its ok.
Checked the soldering and continuity around the op-amp and I think its ok.
+/- inputs to op-amp read 15v DC respectively, .003v AC.
0.003 AND 0.005v DC ON OUTPUT OF OPAMP, 0v AC.
Spkr terminals read:
DC: WITH VOL AT MAX READS 0.010 ON RIGHT AND 0.016 ON LEFT (no input)
AC: WITH VOL AT MAX READS ZERO ON RIGHT AND -0.001 ON LEFT (no input)
Diodes seem ok though I'm not 100% certain. They read a resistance greater in 1 direction than the other.
Does any of that indicate anything?
Im all out of ideas now, any suggestions?
No pre out on this sucker AFAIK.
Have you tried with direct on by-passing the tone controls as that circuit is nearest the transformer?
Have you tried running the speakers off speaker B instead of A?
What are the 2 Mundorf caps closest to the transformer for? Tone circuit / direct?
How close are the Mundorf values to the original cap values?
You may have to go as far as re-installing the orginal caps in 1 section at a time and the best one to start with is that pair near the transformer.
Have you tried with direct on by-passing the tone controls as that circuit is nearest the transformer?
Have you tried running the speakers off speaker B instead of A?
What are the 2 Mundorf caps closest to the transformer for? Tone circuit / direct?
How close are the Mundorf values to the original cap values?
You may have to go as far as re-installing the orginal caps in 1 section at a time and the best one to start with is that pair near the transformer.
Have you tried with direct on by-passing the tone controls as that circuit is nearest the transformer?
No but Im thinking of running good quality coax direct from the input buffer to the vol pot and then to the power amps as an upgrade.. First I will try it with some normal wire to see if it solves the issue.
Have you tried running the speakers off speaker B instead of A?
Yes, the buzz remains on both outputs.
What are the 2 Mundorf caps closest to the transformer for? Tone circuit / direct?
They are dc blocking caps in the tone bypass circuit.
How close are the Mundorf values to the original cap values?
The original caps were 10uF and the Mundorfs are 2.2uF. This isnt an issue as the buzz was there before I installed them.
No but Im thinking of running good quality coax direct from the input buffer to the vol pot and then to the power amps as an upgrade.. First I will try it with some normal wire to see if it solves the issue.
Have you tried running the speakers off speaker B instead of A?
Yes, the buzz remains on both outputs.
What are the 2 Mundorf caps closest to the transformer for? Tone circuit / direct?
They are dc blocking caps in the tone bypass circuit.
How close are the Mundorf values to the original cap values?
The original caps were 10uF and the Mundorfs are 2.2uF. This isnt an issue as the buzz was there before I installed them.
I just insalled the ALPS pot and the buzz remains. Its not the pot then.
Time to start lifting signal caps to try and isolate the problem. But first Im going to bypass the Nichion muse with the 47n plastic caps that were originally there. If problem remains I will hopefully find out where it is by lifting signal caps until it goes.....
Time to start lifting signal caps to try and isolate the problem. But first Im going to bypass the Nichion muse with the 47n plastic caps that were originally there. If problem remains I will hopefully find out where it is by lifting signal caps until it goes.....
An externally hosted image should be here but it was not working when we last tested it.
Thre image is not very clear so I will try to describe where R33 and R34 are. R33 is the resistor with the 22p cap bypass. R34 is the resistor before it).
In the schematic above both R33 and R34 measure roughly 8k.
They should be both 10k and measure corectly when I remove them from the circuit.
For some reason they read 8k when In the circuit. If I remove R33 - then R34 measures correctly.
The Left channel resistors measure correctly when in the circuit.
What does this indicate?
Mike.
I checked and found that with the opamp removed the resistors R33,34 measure correctly (checked the 22p ceramic and it wasnt the problem). I replaced the original NE5532 op-amp and R33,34 resistors measured correctly, 10K. So I fired her up expecting no buz as the 'bad op-amp' was replaced with a good one......
Still buzzzin! ARRGHHHHHHH!
Im all out of ideas now. Ready to give up.
Theres so much info on this and alot of possible problems have been eliminated. Surley someone out there knows whats up?
Still buzzzin! ARRGHHHHHHH!
Im all out of ideas now. Ready to give up.
Theres so much info on this and alot of possible problems have been eliminated. Surley someone out there knows whats up?
Its not the input selector (switcher) as with this disconnected from the supply makes no difference. Same applies to the phono stage and tone circuit. (Both checked with their op-amps supply diconnected)
Resistance of resistors around 'input buffer' opamp while powered up are:
R38 = 100K.
R37 = 88.4K (one way, and 115.1K the other?)
R36 = 47 Ohm
R34 = 47 Ohm
R39 = 87.7k (one way, 112k the other?)
R40 = 100k.
R33 = 11.14k one way, 8.86k t'other?
R55 = 11.12k and 8.91k the other way.
With no power all resistors check out ok while in the circuit.
Does this mean anything?
Resistance of resistors around 'input buffer' opamp while powered up are:
R38 = 100K.
R37 = 88.4K (one way, and 115.1K the other?)
R36 = 47 Ohm
R34 = 47 Ohm
R39 = 87.7k (one way, 112k the other?)
R40 = 100k.
R33 = 11.14k one way, 8.86k t'other?
R55 = 11.12k and 8.91k the other way.
With no power all resistors check out ok while in the circuit.
Does this mean anything?
I would reiterate Stream's point. If the amp is on your workbench, are you replacing the lid each time you test?
I had a very frustrating problem with my first chip amp buzzing that turned out to be because my soldering iron was turned on on the same desk as the amp and is seemingly a great source of EMI, causing a lot of buzzing.
Make sure all other equipment is turned off on the testbench to eliminate that possibility
I had a very frustrating problem with my first chip amp buzzing that turned out to be because my soldering iron was turned on on the same desk as the amp and is seemingly a great source of EMI, causing a lot of buzzing.
Make sure all other equipment is turned off on the testbench to eliminate that possibility
rabbitz said:
Going back a few months......
I removed the NE5532 opamp, didnt do a good job and caused minor damage to the PCB.
Fitted a OPA2107 op-amp.
Fitted Blackgate STD supply decoupling caps (in place of the stock plastic caps).
Replaced binding posts with chunky WBT stlye ones.
Fired up the amp and got a loud 'full range' buzzing so I quickly turned it off. Tried again with vol at zero and same thing hapopened. On the 3rd attempt it seemed to work ok so I played it for the rest of the day and all seemed fine. That night I noticed the very faint buzzing from one channel, so I:
Opened the amp and noticed the Blackgate cap on the +VE side had swollen. This was due to the cap on the -VE side having incorrect orientation. Replaced original decoupling caps.
Still buzzing,
Replaced original binding posts.
Still buzzing,
Assumend it was the opamp but gave upo and left it as it was. So at that point the only change from standard was the OPA2107 op-amp.
mikesnowdon said:
mikesnowdon said:
For one thing you should never measure resistor values while the amp is powered up. Resistor measurement means that the meter puts a DC voltage across the resistor via an internal one and measures the voltage drop. The DC can become dangerous for your amp and speakers, if injected at the wrong place.
The different resistor values you measure powered up or down are a consequence of that measuring principle. When powered up the internal voltages of the amplifier add up to the meter's voltage and produce a wrong measurement.
A reliable resistor measurement is only possible with the resistor singled out of the circuit. If it is in the circuit you measure the combined resistance of the circuit and not the resistor anymore.
Did you try to replace the Mundorfs with the original capacitors? The Mundorf capacitors are quite likely to pick up noise. Their sheer size makes them very good antennas.
Thanks for the data.
The buzz was there before I installed the Mundorfs which rules out the possibility of them oicking up RF transformer noise etc.
The buzz is so faqint you can only hear it if you put your ear upto the tweeter on the right speaker. Obviously it cant be heard when music is playing. A freind who aparrently knows what he's talking about said 'dont worry about it, its normal'. I suppose if I had a 840 I'd expect it to have dead silent noise floor, but this is a £200 'budget' amplifier. I'd still expect it to be quieter than it is, a little hiss IS normal but buzz im not so sure. I give up.
I had a 840A V2 for a few days and it buzzed like crazy due to a bad transformer.... it went back to the distributor very rapidly. The buzz came from the case as well as the speakers so was a real POS.
The 840C CD player is fine with a slight bit of transformer noise but none comes through the speakers.... very quiet.
The 840C CD player is fine with a slight bit of transformer noise but none comes through the speakers.... very quiet.
Im borrowing a 840A from a freind and initially I was really impressed. However, thers a distinct lack of dynamics IMO which is a shame because otherwise it has great sound quality. I havent listened to it that much so I reserve my verdict until Iver tried some other music. I love the features though.
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