Quote:
Originally Posted by Radio Wrangler
Service info should give you normal audio voltages at various places along the chain. Speech isn't a steady enough level nor is whistling. There are apps for smartphones which will make audio tones, but the headphone jack levels will be too high for a microphone input, so you'll need an attenuator (three resistors)
Then you'll need something to measure the level with... oscilloscope, audio millivoltmeter etc.
The likelihood is that some component along the chain is sulking there is a steady failure rate of transistors, capacitors and resistors. These things were built with cost-competitive components... mind you, some of the o ce premium components didn't fare much better. Electrolytic capacitors use water-based conductive fluid in them as one of their electrodes. Over time water leaves as vapour past the seals. It just needs a slightly iffy rubber seal or a bit of muck and they fail without needing high temperatures to accelerate them.
So it's simple spadework. Apply a signal and follow it along, checking levels, to get the fault down to an area, then start focusing in on the problem.
The audio path may have some AGC to correct for microphone levels, there may be a compressor or speech processor. So it may be a bit more complex than just a couple of amplifiers and a gain pot.
One quick check would be to substitute a known good microphone. You never know.
David
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thanks for this excellent advise i will be on the look out for a second hand osciliscope and a few other peices of test equipment, but for now ive managed to order a working board of ebay which im awaiting receipt of. once ive got the test equipment i will continue to figure out the problem, i beleive i may well have destroyed a transistor or ic LD3001 which is obsolete with the soldering iron when removing and replacing the electrolytic capacitors.