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

http://speakyssb.blogspot.ie/2017/04/more-filters-iii-telrad-board.html.        Also there is information about the filters


8 thoughts on “KVN 3 Filter PCB. AM and SSB Receiver. Telrad.

  1. Hello, we have been working with this IF strip for a couple months and are designing a SSB transceiver around it! Several people have it working as an HF receiver. My own design / build is in progress. This IF was used in telecommunications equipment. Someone said they saw a photo of a bank of 10 of these boards. Not all of the circuitry on the board is useful, and I plan to strip some off it off in order to make room for other stuff. The purpose of the FET followed by the 741 is not known. I plan to repurpose this 741 as an audio amplifier to supply audio to U2 reconfigured as a balance modulator in transmit. Anyway, for more details visit us at Yahoo Groups. https://groups.yahoo.com/neo/groups/ssb-avenger/info


    1. Hi Chuck,
      I intend to build at least a good receiver around this board. I am more than pleased to find that there are others with a similar interest. I will get around to your group as soon as I can.

      Will (Derwyn)


  2. One thing, the 2n4416 is definitely an RF transistor. The other thing is your schematic is beautiful, but the resolution is so low that when I blow it up to a good size, I still can not read most of the lettering, just a blur. Could you post a high resolution schematic please? Thanks, Chuck K8LBH


  3. Goodmorning everyone,
    congratulations to ZL4SAE for the drafting of the schema.
    I purchased two of these cards, I removed the input filter (bandpass 1.5 to 4.2 MHz).
    Supply + -12V: working perfectly. Only problem with the AGC, has a very high threshold and comes into operation only for very strong signals.
    I am evaluating the AGC circuit diagram adopted on Elekraft K2.
    See you soon, Luigi Giuffrè, IT9BYB.


    1. Ciao Luigi,
      I hope the schematic is useful for you.
      I have the board working now. Also removed the filter.
      SL1621 AGC chip needs 4mv audio to start operating. Maybe need to make the 8k2 input resistor smaller. Good luck with the K2 circuit.
      Will ZL4SAE


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s