Kenwood DSP-100 Modifications

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Kenwood DSP-100 Modifications:

The Kenwood DSP-100 is what makes the 850 such a flexible transceiver and is key in producing Extended-Band SSB (ESSB) Hi-fi quality audio. You could simply use the DSP-100 as is and still get excellent results. The usual way the signal is routed is as follows:

Mic > 850 Mic input / 850 ACC1 output > DSP-100 Controller Input / DSP-100 DSP3 output > 850 DSP3 Input

However, the Kenwood 850 internal mic amp has a severe low frequency response roll-off below about 150 Hz. Additionally, DSP-100 Mods

there are some minor inherent noise and distortion properties associated with the 850 preamp section.

If you are a purist and want to circumvent these problems, it was determined that bypassing the 850 mic preamp by feeding the audio chain's Line-Level signal directly to the DSP-100 input would be advantageous in the following three ways;

1) Eliminate the low frequency roll-off inherent in the 850's internal mic amp, lowering the frequency response down to 20Hz
2) Eliminate any distortion properties inherent in the internal 850 mic preamp
3) Eliminate several signal cut/gain stages through noisy and lossy components

This page describes in detail how to perform the mods mentioned above.

(Click here to see an after mod KW0AM on-air spectral analysis)

DSP-100 Modifications Instructions:

You will need to purchase a good 4:1 Audio Isolation Transformer. (Preferably the Jensen JT-10KB-D)

Remove the cover from the DSP-100 and face the front of the DSP-100 toward you.

Drill a hole in the rear of the DSP100 to accommodate either an XLR or 1/4" TRS plug.
Orient the hole just to the right of the power cord but to the left of the boundary between the Power supply and signal compartments. (See Photo 1)

Wire the transformer input to the jack you just installed and place the transformer in the far right corner of the power supply compartment. Jensen suggests that the transformer casing be isolated from the chassis, although others have used mounting brackets bonded to the chassis with no ill effects. (Wrap electrical tape around it if you prefer or mound it via mounting hardware as shown in Photo 1.) (See Photo 1)

Locate the BLACK , WHITE and PINK wires near the rear that goes into the Molex connector.

Cut the PINK wire all the way to the right of this connector. (Leave about 1/4" of the wire on the Molex)
(See Photo 2)

Replace R13 with a 20k ohm resistor for +15.56dB of gain. R13 is located directly between IC5 and the ground point as seen in Photo 2. (See Photo 2) and (See Photo 2b)
Part description is: 605 Package - Mouser Part # 660RK73H1JLTD2002F

Place a 1200 ohm resistor to the PINK wire that will be fed from the RED output wire from the Jensen 4:1 transformer.

Combine the Black, White and Brown wires from the Jensen Transformer and connect them to a suitable SIGNAL ground inside the DSP100. The best location for this signal ground is located directly in front of the left pin of the Molex connector where the black wire is connected. There is a tiny little hole on the board, and a soldier point just in front of this hole. (See photo 2) (See photo 2b)

Click here to view "Before/After" schematic circuit modification details:

DSP-100 Notes:

Do NOT remove the J3 DSP connector ! ... It must stay to accommodate other connections in the circuit.

Set DSP-100 Rear Panel Dip Switches 1, 8 and 9 to OFF for full RX/TX Fidelity.

For AM operations it's best to just simply turn OFF the DSP-100. It's not needed.

If you will NOT be performing the Balanced Modulator input mod or using the DSP-100, you will have to feed your audio the conventional way into the 850 front panel 8-pin mic connector.
(For AM this is better because of the bandwidth limitations being governed by the DSP-100 in the AM mode)