Moddeed Cambridge Audio 340A SE buzz problem

[mikesnowdon]

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?

[mikesnowdon]

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?

[jaycee]

Originally posted by mikesnowdon

At least I have eliminated a few possibilitys. I used the headphones to see if the buzz was still there on my BOSE TP-1's and it was. Im prettey sure this means the problem dosent lie in the power stages. I think its got to be either the pot's or someting wrong with the input buffer circuit.

You will probably find that the headphone output is driven by the power amp stage, using a potential divider to bring the voltage down for headphones.

The best thing to do is check the signal at the pre-out sockets, if the amp has one.

[rabbitz]

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.

[mikesnowdon]

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.

[mikesnowdon]

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.....

[Stream]

So, buzzing started when AMP was in stock condition?

P.s. maibe it seems like joke, but have You tried to move Amp far from any EMI-generating tools? I have not buzzing, but something close to it, when placed Wi-Fi modem on the top of Amp

[rabbitz]

If the buzzing started before the Mundorfs went in then my idea is kaput.

At what point did the buzzing start?

Walk us through the sequence of changes prior to the buzz.

[mikesnowdon]

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.

[mikesnowdon]

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?