Well I finally got the time to look over this and figure out the low volume issue.
Here is a bit of a breakdown.
There was no doubt about it, the volume was low, clear and working but I would say roughly less than half the volume it should be beating out.
I tried the usual things to see if I could affect the volume, poking around and generally being a bit of a nuisance in the vain attempt of making the volume alter a little.
It worked, because I noticed that the SNRS switch on the front made the volume go even quieter when off than when on.
I did check the connections mentioned above but found them to be in good working order, one is ground and the other is the power rail.
With this information I set about downloading a beautifully clear service manual from Norm's site instead of using the crappy scanned versions doing the rounds.
I then focussed my attention on the power line and looking for a general idea of the flow of signal by looking at the block diagram.
After checking the power amp chips and establishing that those were working fine I then started looking for the single common components.
Back to the block diagram I could see some single components common to both channels.
This is what I started to focus my attention on:
These two IC's are located on the power board, the same board where the power amps are located.
See the noise reduction (SNRS) SW202?
At this stage I think I'm onto something so I go about checking voltages and find that IC551 checks out voltage wise, apart form the pins being fed from the peak detector IC552.
After a few more checks I focussed my attentions to the circuit around IC552.
Checkin the circuit diagram for this section, I could see that the SW202 was directly involved here in feeding the input signal to the IC552 pin 6.
Voltage checks on IC552 were not looking right but I was being careful to note that SW202 affects the voltage on IC552 so I checked with the switch on or off and found that there was no difference... and there most certainly should have been...
In order to prove that the IC was faulty I checked the resistors and noted that D554 was showing as though it could be shorted by allowing resistance readings in both directions so I lifted that diode out of the board to check it to be 100%. The diode checked out fine.
I did a quick search online for a replacement to LM385N and to my amazement found one in our local supply store Maplin for 0.39p. Picked one of these up the next day on lunch from work.
I figured that for the price I couldn't go wrong and if this gave the same results as before at least I knew I was in the right area and it could be another component such as a bad cap or resistor affecting the flow in the circuit.
That night I replaced the IC in 2 minutes and powered it up.
Hey Presto! volume had returned and was much louder. The kind of loudness I would expect.
IC552 is a very simple little chip.
In and amongst the above I checked the diagram of that chip since I couldn't see it in the service manual.
When I looked at the diagram and saw the voltages I was getting I became more and more convinced that this IC was the fault.
Hope this helps with others experiencing this issue in the future.
Norm, I hope you don't mind the snippets of the diagrams I have posted. If so let me know and I will remove them.
