Boatanchors

MY REPAIR AND MOD

So I have removed the clipper and meter driver from the circuit and only use the 2 transistor audio preamp. The clipper is formed by the base to emitter junction in the last transistor Q3 and the "?" diode. These diodes are back to back in the circuit and clip off all audio signals greater than +/- 0.6volts that reach the base of the Q3 transistor which also serves as the meter amplifier. To do this modification remove the 3.9K resistor and swing the negative end of the 1 ufd coupling cap to the top of the gain control where one end of the 3.9K was soldered. (Note the X on the schematic) That's it. Of course the meter wont work now and the two controls both act like gain adjustments. I moved the white wire on the negative terminal of the meter to the collector of the Q2 transistor and now the meter swings with voice peaks. You could just remove the amplifier totally but what's the fun in that...?

Note that all of the electrolytic caps in the amplifier were bad, and that the .01 mfd cap across the gain pot had failed and shorted. The transistors have some "house" number on them. I think any general purpose NPN would work here but none of mine were bad. So the proof of the pudding is in the final results and I have been getting great audio reports as long as I can resist turning up the gain and over driving my SB102. If you see one of these microphones at a swap meet they usually come at a good price because the amplifiers are either dead or the clipping makes the audio sound horrible the owner wants nothing to do with it, but underneath all those bad vibes is a great ElecroVoice mike element and a simple 2 transistor amplifier, so don't hesitate to buy it.


The ElectroVoice 619TR Microphone
	This is one of my favorite styles of Vintage desk microphones. These dynamic high Z microphones are ideal for heathkits and other boat anchor transmitters. So when I saw one at my local radio swap meet last Summer I just had to have it. I had never seen the 619 model with a built in amplifier/compressor. I parted with 40 bucks and the prize was all mine. Needless to say it didn't work when I got it home and a search of the Internet didn't provide much information of the compressor circuit board either. The mike sat on my to-do shelf until just a few days ago. I did another search on the Internet for a schematic of the preamp/compressor board and still no luck. I decided to take the board out of the mic and trace out the circuit myself. Below in Figure 1 is what I discovered the board to contain. I also repaired the amplifier and made some minor mods. Update (3-5-2019) Mike K9LSH provided this link to a German webpage with a 4 Transistor version of the 619R amplifier board schematic. dk4sx.darc.de
Figure 1
Well, as you can see from the schematic this is not a "real" compressor but a clipper. In a "real"compression type microphone amplifier the voice signal is set at a constant level before it enters the radio. This is done by amplifying the weaker voice elements in relation to the stronger ones, by using a closed loop gain control much like the AVC or ALC circuits we are all familiar with. This microphone uses a clipping action to limit the stronger voice peaks. So by amplifying all of the audio elements in a linear fashion and clipping off the strongest elements the overall audio "energy" reaching the radio is increased. This comes with a cost of some unwanted distortion. A bandpass filter following the clipper is required to reduce unwanted harmonic distortion. This circuit has only a low pass filter made up of 3.9K and .01 cap. This "clipping" method doesn't amplify the weak sounds in relation to the strong ones, but rather reduces the strong sounds relative to the weak ones. The use of these circuits has been around for at least 40 years and I have seen many construction articles using this same method of increasing "talk power". However, I find these microphones hard to adjust and not very useful in "rag chew" QSOs. Also the distortions can be down right horrible.

MY REPAIR AND MOD So I have removed the clipper and meter driver from the circuit and only use the 2 transistor audio preamp. The clipper is formed by the base to emitter junction in the last transistor Q3 and the "?" diode. These diodes are back to back in the circuit and clip off all audio signals greater than +/- 0.6volts that reach the base of the Q3 transistor which also serves as the meter amplifier. To do this modification remove the 3.9K resistor and swing the negative end of the 1 ufd coupling cap to the top of the gain control where one end of the 3.9K was soldered. (Note the X on the schematic) That's it. Of course the meter wont work now and the two controls both act like gain adjustments. I moved the white wire on the negative terminal of the meter to the collector of the Q2 transistor and now the meter swings with voice peaks. You could just remove the amplifier totally but what's the fun in that...? Note that all of the electrolytic caps in the amplifier were bad, and that the .01 mfd cap across the gain pot had failed and shorted. The transistors have some "house" number on them. I think any general purpose NPN would work here but none of mine were bad. So the proof of the pudding is in the final results and I have been getting great audio reports as long as I can resist turning up the gain and over driving my SB102. If you see one of these microphones at a swap meet they usually come at a good price because the amplifiers are either dead or the clipping makes the audio sound horrible the owner wants nothing to do with it, but underneath all those bad vibes is a great ElecroVoice mike element and a simple 2 transistor amplifier, so don't hesitate to buy it.
Oh yes, don't miss this little grommet, it falls out on the floor and rolls into never-never land. It is required to keep the board in place.