Negative voltage generator using 555

Negative voltage generator.
I originally designed this circuit in order to repair an Icom 701. It is a fairly standard application of the ubiquitous 555 chip. The output is dependant on the supply voltage and the load, you will not get -9v out with 10v in for example. There is no such thing as a free lunch, and the diodes drop some volts. Replacing silicon diodes with Schottky types will improve things a little.  With a bit of imagination it can be constructed in a very small volume. 

The available current is quite small, about 20mA maximum into a 500R load. Efficiency around 30%. If I recall correctly, the 701 needed just a few uA, so well within the capability of this circuit.

Having recently purchased an SSB i/f PCB that needs -ve volts on the mixers and op-amps, I searched my notebook for
the schematic and made an electronic copy on my Samsung tablet, using the amazing app, SCHEMATIC.

KVN 3 Filter PCB. AM and SSB Receiver. Telrad.

This is a work in progress., and some progress has been made

Due to information from the Yahoo group and others (see below) I have updated some capacitor values, and added a capacitor that I omitted. There was a resistor missing from the original drawing that I worked from, this has been corrected. I have added the test-point numbers, and a couple of notes about the audio filter.

I purchased this board from eBay. The vendor was based in Israel. At $37 NZ I considered it to be good value, as the circuit included 3 Crystal filters and 3 mixers. Searching Google produced so little information I was concerned that I would have to reverse engineer the board in order to produce a schematic. I eventually found a German language site ( forgotten the address ) with a link to a drawing. It appears to have been done in Eagle Lite. There were no component numbers or values. Some reverse engineering was going to be needed after all.


I spent some considerable time working my way through the circuit and identifying the component values. I sincerely hope that there are not too many mistakes. The drawing would be very cluttered if the part numbers were included, so I have omitted them, except for the values around the LPF. A couple of the transistors were oriented wrongly in the drawing, I have corrected them in my schematic. I have also separated the AGC and the AUDIO modules from the rest of the circuit, along with the voltage regulator. This makes it a little less confusing to my eye. I hope this works for you too.

Telrad SSB PCB

Download the PDF    Israel Board Schematic Ver 1.2 ZL4SAE

The signal input is on pin 16, where it passes through a couple of resistors before being split to the FET and the Low Pass Filter. The FET circuit buffers the signal, rectifies it and routes the resulting DC to pin 13. Purpose ? Unknown.

Following the LPF, the signal is coupled into the mixer by a 22uF capacitor. Yes 22uF. This is a Tantalum part, as are most of the polarised capacitors on the board. Top left corner of the pic. Something tells me this is not an RF signal. The output of the mixer is undoubtedly DSB at 9MHz. The following ( roofing ? ) filter is AM bandwidth at 9MHz. So the local oscillator input at pin 10 could well be a carrier, and the signal could be audio. The DC at pin 13 then maybe acts like a VOX or squelch circuit. 

Following the AM filter the 9MHz I/F is amplified in the MC1350. It then passes to both SSB filters, only one is selected, then to the second mixer.

The stages following the second mixer are at audio frequencies. The required sideband is selected by the voltage level on pin 7 which gates the 4016. So only one of the 2 sideband mixers or filters, is actually used at any one time. The CIO or BFO is applied via pin 9. This must surely be at 9.000MHz in order to demodulate the SSB signal from the filter.

The audio is passed to an Op Amp filter circuit, it looks like bandpass. Comprised of high pass / lowpass. Then the signal is routed to the 600 Ohm output transformer at pins 5 and 6. Pins 1,2,3 and 4 carry the same signal at different impedances along with a couple of grounds. A sample of the audio is also fed to the Plessey SL1621 AGC generator. The capacitor values around this device are approx 50% of those in the data sheet. This would make the AGC response quite fast I believe.

The overall circuit had me confused for a while. The Yahoo group has certainly helped by proving that the board receives SSB. Not so sure about AM. AGC is applied to the MC1350. So the signal level must be expected to vary, as we expect in a receiver. The switches in the AGC circuit appear to be able to select normal, delayed or off. I would have expected the AGC to also drive an S meter, but there is nowhere in the circuit to sample the AGC for indicating purposes, that has been brought out to a connection point. Test point three looks like a good candidate.

I have had the board powered up, but no signals yet. The negative voltage generator measured 9.5v off load and 6.5 when connected to the PCB. Not enough current available I assume. A better solution is needed.

Ultimately some modifications around signal pin 16, changing the 22uF capacitor and removing the LPF would enable the first mixer to accept RF. The Carrier input would then be the LO. It might make an interesting receiver.

If anyone has made any more progress than this I would be more than happy to hear from you. gw4sae at AOL dot com

Thanks to Chuck K8LBH for the link to his YAHOO group, they are actively building radio’s using the PCB.

Ricardo / CT2GQV. Also has some information        Also there is information about the filters