Date: Thu, 10 Nov 94 00:35 CST Status: Read From: John Bednar To: netsig@tapr.org Subject: [NETSIG:259] Re: TEKK radio mods Tim, Here is the scoop on the Tekk mods. I am the author of the mods and original postee. John, WB3ESS PS: Does anyone want to work on a Tekk (19.2 k or 38.2 k) mod? I haven't done much work but having a buddy or two might keep the project moving. My internet address is aljkbe@attme.att.com. =================================================================== The older style radio has a black case that only has a top cover. If you make these mods to the radio you will find an improvement in 9600 bps operation. Yes, the radio works right out of the box, but you *will* see an improvement by adding two caps and one resistor. It's a very simple mod I can't understand why some readers are grumping. The modification was developed because this radio was not developed for 9600 bps service and I wasn't happy with the stock performance. TEKK KS-900L Receiver Modification (black case with only a top cover) The KS900 recovered audio has a high frequency (2.2MHz) signal superimposed on the recovered audio. This high frequency signal causes jitter in the sliced output. To eliminate the unwanted signal, install a lowpass RC filter in the audio signal line with a corner frequency well above the frequencies of interest. The modification: At the 9-pin interface connector add a 2.7k ohm resistor in series with the wire going to connector pin 5. Next, solder a 0.001uf capacitor between pin 2 and pin 5 of the same connector. TEKK KS-900L Transmitter Modification (black case with only a top cover) The stock KS900 transmit modulation response is not flat from "dc to 7.2 Khz". To correct this deficiency, the input coupling capacitor must be increased in value. I have found it easier to solder in the additional capacitor without removing the board from the case. The modification: Solder a 3.3uf tantalum capacitor (observe the polarity) in parallel with capacitor C31. The transmit requirements are not changed so re-adjusting the transmit level will not be required. =================================================================== =================================================================== The newer style radio has a chrome case. It uses chip components and the case has a top and bottom cover. The model numbering of this radio is very confusing. When these radio's were originally shipped, the thruput at 9600 bps was poor but they also worked out of the box (much worse than the older style radio). If you make these mods to the radio you will find an improvement in 9600 bps operation. This modification is a little more detailed but don't let the quanity of text prevent you from performing the change. Now here is the interesting part. I released the modification to Tekk around early January 1994. Naturally, it has found it's way to the other suppliers of Tekk radio's. Based on coorespondence I bsome suppliers are *not* making all the modifications. If you want the maximum performance at 9600 bps and near identical frequency response to the older style radio, I contend that all modifications must be completed. It is possible to complete some of the modifications and see a dramatic improvement in performance *but* with all the changes, performance will improve. I would expect by now that Tekk is shipping radio's that work *well* out of the box. There is no need to stay away from this radio on a frequency response standpoint. Someone mentioned RF transistor concerns a few days ago. So please factor this into your purchase. TEKK KS-960L Receiver Modification The un-modified KS960 recovered audio is attenuated by a single pole RC filter on the receiver output. The roll- off begins around 1200hz in an unmodified radio. Please note that a simple low-pass filter is required to eliminate a high frequency (2.2MHz) conversion product. The solution is to move the corner frequency of this RC filter well above the frequencies of interest. The modification: Remove chip capacitor C18 and replace it with a 0.001uf leaded capacitor or chip capacitor. TEKK KS-960L Transmitter Modification The un-modified KS960 transmitter suffers from several problems. First, this radio contains an input amplifier, a limiter, and a reconstruction filter in the input of the transmit section. Secondly, the modulator frequency response is not not flat from "dc to 7.2 Khz". The solution is to bypass and ground the input to the existing limiter circuit and to modify the modulator to achive a flat frequency response. The modification: 1) Remove the wire attached to pin 4 of the 9-pin connector and solder it to ground. 2) Solder the (+) lead of a 10uf tantalum capacitor with 0.2" leads to the 9-pin connector - pin 4. 3) Prepare a 1.8" piece of wire with 0.2" ends and solder one end of the wire to the (-) lead of the capacitor added in step 2. Tuck the other end of the wire around the circuit board in the corner. Don't use the corner where the BNC is mounted. 4) Locate the junction of C43, C92, and R41. Form the wire, trim the end to 0.1" and solder it carefully to this junction. 5) Locate the junction of R43 and C47. Carefully remove the solder from that solder pad. With a sharp knife cut the trace diagonally between R43 and C47. Check your cut with an ohm meter to insure that the trace is open. If you break R43, C47, C48 or C49 don't worry because these parts are no longer used. 6) Locate potentiometer VR1 and turn it fully CW. 7) Re-adjust the transmit frequency and the transmit deviation.