Testing removal of spurs with additional 45MHz filter

The photo above shows an additional 45MHz filter (15khz passband) inserted in place of R27 (you can’t see the centre wire on the filter, which is attached to the ground end of R13).

Early indications are that this removes all of the offending spurs.   This will make it  a uBITx.net ESSENTIAL MOD.  The mod has, however, yet to be tested by uBITX.net.

Warren WA8TOD has completed spectrum analysis plots for each band, and these have been reproduced below.  The plots show removal of all unwanted spurs.

Conditions for the test:

  •  eBay filter in place of R27. No other changes.
  • Audio input: 100 mVrms, 1.5 kHz tone. RV1 adjusted in each case for 2 watts output.

Yet to be verified:

  • 100 mV audio drive, without the filter in place, gave very unacceptable IMD performance.  It may well be in the case of the added filter that the stages preceding the filter have enough dynamic range to work at that level and it is simply compensating for the insertion loss of the filter itself. That can and will be confirmed with two tone IMD testing.
  • Listening to the recovered voice quality and decide if it is adequate.

Adding the filter has introduced low frequency rolloff for LSB and high frequency rolloff for USB. The change is less than 6 dB and may not be objectionable but that will be a subjective judgement.

28MHz results

24.9 MHz results

18 MHz

And a wider scan …

And finally, here is a wider span showing 15 through 10 meters harmonic performance.  Warren’s unit has the onboard filters completely removed so this scan was made with an external 30 MHz LPF plus the new 45MHz R27 filter.

Comparison of CW and SSB power out using the added 45 MHz filter

The chart was made by adjusting RV1 to maximum key down CW power, and then keying PTT with an input tone at the specified level. There are a couple of caveats here:

1) 120 mVrms is far above the audio level that caused unacceptable IMD before the filter mod. IMD must be checked and the audio levels adjusted to make it acceptable.

2) 120 mVrms is also far above the output level of most microphones, at least without shouting.

If IMD is bad at this level then the audio level must be reduced. Before the mod the radio showed terrible IMD at any input level higher that about 25 mV and, at that level, the radio produced less than 2 watts.

If it turns out the filter is a ‘magic bullet’ and the radio can actually sustain this level of input with acceptable IMD, then the input audio stages need more gain.

Comparison of CW and SSB power out using the added 45 MHz filter

The chart was made by adjusting RV1 to maximum key down CW power, and then keying PTT with an input tone at the specified level. There are a couple of caveats here:

1) 120 mVrms is far above the audio level that caused unacceptable IMD before the filter mod. IMD must be checked and the audio levels adjusted to make it acceptable.

2) 120 mVrms is also far above the output level of most microphones, at least without shouting. If IMD is bad at this level then the audio level must be reduced. Before the mod the radio showed terrible IMD at any input level higher that about 25 mV and, at that level, the radio produced less than 2 watts.

If it turns out the filter is a ‘magic bullet’ and the radio can actually sustain this level of input with acceptable IMD, then the input audio stages need more gain.

Reference

10dB spur reduction mod

Raj, VU2ZAP has come up with a fix that reduces the spurs by up to 10 db  and requires ONLY ONE part to be added.   Farhan VU2ESE has come up with an alternative modification.

These mods result in a significant change in the level of spurs above 10MHz with some improvement below this frequency as well.

With Raj’s mod CW may not work anymore and will need some more mods.  With Farhan’s modification CW will still work.

Raj VU2ZAP Instructions

  1. T2 – desolder the transformer wires that go to pin 3 and 5. Pin 1 has a square pad.
  2. Bring out the two wires above board and join them together and solder.
  3. Take a 45Mhz filter- 45M15 or  similar 2 pole  (one crystal only) and solder one end of the filter to the wires of T2 pulled out. The centre filter wire to ground at one end of R26. Check which end of the resistor is grounded.
  4. Solder the third wire of the filter to C10/R27 junction.

This mod prevents the leaked TX signal that gets amplified by the 1st bidirectional amplifier from getting into the first mixer and creating havoc.

Farhan VU2ESE Instructions

  1. Remove R27
  2. Solder the 45Mhz filter two extreme ends to the pads of the resistor.
  3. Solder the center lead of the filter to the nearest ground. R13 is very near with a ground via.

Using the first method (Raj’s solution) the extra filter will work in RX mode as well as TX, but CW is disabled.  In  the second approach, the filter is only used in the TX path.

Folks with DSA815 or better please share your feed back. The filter may work better properly terminated.

Reference

VU2ZAP experiments with TDA2822 Audio Chip

Raj VU2ZAP has done some experimenting with TDA2822 chip failures.

FCI chips were sourced locally and plugged in. No failure was observed when using the chip with normal volume.  However, when he raised the volume to a high level the chip failed immediately!

Solutions

1: Add a resistor in series with the 12V line maybe 10-15 ohms to drop the voltage.

2. Simple solution is to use only a 16 Ohm speaker or add a series 8-10 Ohm resistor with the 8 ohm speakers.

3. Cut the power track to the TDA and insert a 78L08 or 78L09 to reduce the voltage. There is a convenient track which can be cut.  Solder either an SMD or DIL regulator onto the board at this point. This track leads directly to the chip and filter cap (see photo below).

The center lead looks soldered, but is in fact floating.  However, the tab is soldered to the copper groundplane below. The white wire is part of Raj’s fix to prevent the filter relays from clicking during PTT.  Raj suggests the board as illustrated now needs a brush cleaning!

Raj observes that with this mod, audio is fine but distorts at high volume. The 9V voltage out does not drop at maximum volume.

Feedback on the solution from Jerry KE7ER

Jerry KE7ER says “This looks like a fine solution!   The fact that the 78L09 inherently limits current to something reasonable is a bonus.  And it fits in there perfectly!

Jerry suggest that it might be a little bit easier to just rip that trace out entirely. You could place the part up against the through-hole on one side and run a short wire to the other through-hole.

Gain should remain as it was at 12v.   Just that when you turn the volume up it distorts earlier.

If what you have on hand is the TO220 LM7809 (or any voltage from 5v to 9v), it will fit in there nicely as well.  Glue it face down to the board with pins hovering over  the trace that Raj has cut.   Or, if you don’t like glue, solder a short wire from tab to ground plane.

ST makes the 78L** parts in a SOT-89 package.   This is the same package as U2, the 78L05 for the IRF510 gate bias.

TI and Fairchild LM78L** parts only come in the SOIC-8 and TO-90 (also some dinky BGA that we don’t want to talk about).  Digikey and Mouser don’t sell ST’s SOT-89, so a bit harder to obtain. The SOIC-8 should do fine, solder the ground pins to the ground plane for heat transfer.   The TO92 could work, although it would likely heat up faster because the heat has nowhere to go.

The maximum available current draw from a 78L09 would limit the maximum audio level.    Apparently this provides enough audio for Raj, and limiting that current does protect the TDA2822.   If it gets to be too much current for the 78L09, it will shut down till it cools off.   Just what we want, if it’s enough audio power for your situation.

If you want more power and less protection, use the 1 Amp TO220 LM7809 (or 08 or 06 or 05) face down, flat on the board.   ST sells the 1 Amp L78** in the Dpak/TO252 package, about the same as the TO220.  TI sells the 500ma LM78M** in the Dpak/TO252 package, might be an ideal solution for moderate levels of current.

Any of the above could be made to work, and in fact any regulator providing between 5 and 9 volts.   This assumes you don’t have one of the clones made from factory floor sweepings that smokes at 7v.

Reference