The AM Forum

I am out of ideas here..  I had a DBX compressor inline for a year.  No RFI issues on any band. Unplugged it, installed the DAP310. The second I key the Rig, The meters on the dap peg and there is a massive tone on the signal. I can pass a bit of audio but when I try to increase gain, The oscilation takes over.

The audio rack and DAP are working normaly in and out with out the Rig being keyed up. 

1. Tried brute force ac line filters on dap. No change.
2. Grounded Dap case to 8 ft rod, Problem is worse.
3. Chokes on input and output audio lines. no change.
4. Grounded dap to transmitter case, Problem is worse.
5. Someone suggested putting 200 to 500 puff across audio leads. oscilation changed tone. Not fixed.
6. Put field strength meter ontop of dap. Pegs the needle in transmit.
7. Unplug dap power cord, FS meter goes down half way. RF is comming into DAP mainly on the AC line cord which is three prong.

Normaly, I am very good at solving problems like this. IN fact, I help alot of people and donate time to issues like this. This time, I am out of ideas. Hours spent trying to get the dap to stop acting like this and nothing works.  Take the dap out of line and the station is back to normal.

Inside, the dap I can see there are two micas from the power lines to Ground. Maybe I should try more or different kind of cap?

This thing is acting like a magnet for RF for some reason. Maybe it was intended to be installed far from the transmitter?

Should I order audio line isolation transformers? Is this common practice in BC world? 

Thanks for any help guys..  I am sure I am overlooking something here..  I keep focusing on the dap itself as the second I remove this item from the chain, The problem is gone.

Clark

Strange problem. I've never had one RF problem with my DAP.

How are you making the input and output connections (balanced/unbalanced, shield cable, ground connected/not connected)?

What is the input for the DAP and what does the DAP output feed?

What is the setting for the audio input level (0 or -20)?

Have you reseated all the cards or cleaned the card contacts?

Does the DAP work normally when no RF is present?

If it is coming in on the AC cord, can you add a little RF PI filter right outside the chassis? They make chassis mount ones, that you can plug your power cable in, and then attach the cut off end of a powr cable to the filter, very short, and plug that in to the DAP. Grounding the new filter to the rack. Just something to try that is cheap and involves no hole-drilling.

Thanks for the help steve.

Audio input to DAP comes from a small mixing board using a 1/4 jack on the right channel to the DAP. I cut the cord and went to pos and neg terminals. This is NOT a balanced input. I believe the behringer can take a Three conductor balanced 1/4 inch jack..I dont have one, I could go buy a cheap cable, Cut it and try this if you think my problem lies here...

The output of the DAP is a balanced mic cable.  High quality with XLR on one end. Its wired right to the DAPs screw terminals.

This XLR plugs into an audio technica hi to low converter. This converter plugs into the GK500 phone jack on the modulator deck. This jack Drives the 6C5 directly. Only the gain control is in line.

Keep in mind this mixer and cord arangment has been in use for 2 years. Sometimes I run the DBX compressor but recently I have had it out of line and just run the GK directly off the mixer.

Could it be that non balanced cable from the behringer to the DAP?

Clark

FIXED.  Thanks steve... You got me thinking about the cabling and the problem was that unbalanced cable. I found a TRS cable with an XLR on the end.  I plugged that into the behringer and then wired it directly to the dap.  I can now hit 100% without the RF feedback.  Not to adjust this thing..

Is the goal to have the meters go into compression when you are talking? 

C

Cool. Glad you got it fixed. IIRC, you want the meters to go about mid-way into compression on average. Of course, not each meter will read the same all the time since they cover the various bands.

Thats what I have now Steve.  Sometimes heavy mid if I talk loud.  Mike D is said he would have time to call me this evening to help me fine tune it.   I am really happy with the added punch.

C

I've modified mine to change the crossover freq between the mid and high bands. The stock setting is rather high for voice-only use.

Cool.  Mine was changed also. I dont know what to though. i have not measured the caps and resistors. I have the V2 peak limiter with brightness control and low pass switch.

C

I don't use the limiter in mine. I have another box that does the limiting. The V1 limiter was pretty simplistic. I had a V2 board but never tried it. I probably should have before I sold it.

It seems to work great.. Very soft brick wall limit and the pre-emphisis really woke up the top end.  Also. This V2 has a wide range of neg peak to pos peak adjustment.

Mine just needs some fine tuning now.  I have the volume but it sounds busy and over compressed.

C

Steve, I was wondering what values you changed in your 310 to make it more voice friendly.  I was thinking along the same lines with mine as it will never need to pass music . Patrick

I have a modified noted on the schematic somewhere. Let me see if I can find it.

Hi Clark,

I am making this suggestion to help and it's based on experience. What you do with it doesn't matter to me but I hope it helps.

Stop trying to listen to yourself on a local receiver. Find a way to listen to detector audio without coloration from large speakers and receiver amplifiers meant to fill a room with sound.

I didn't look back to see if the mod monitor your using has detector output. Even if it dose I'm almost sure it isn't amplified for headphones but headphones hooked up to low level detector audio is what your after. You might need a headphone preamp.

After passing the blush moment of listening to yourself up close and personal and getting use to any latency your DAP might produce it's time to tailor your processing to match your voice. Try Not to create a voice you want. Enhance your voice. You can color it a bit to fill in the hollow areas in the 5 or 8 KHz with an eq if you use one but the processing is just to focus energy in an envelope. You need to be able to listen to this up close and personally while tuning up the processor and/or EQ.

It (your audio) might sound a bit harsh to you. Watching the scope and mod monitor will help to make sure your envelope is within bounds. After that, you NEED recordings to fine tune. Ask your friends for recordings... NOT opinions. My last recomendation is set it up and STOP adjusting it for a few weeks or months. You might be supprised with the unsolisited audio reports.

If every QSO has a question about audio than it might focus your friends ears on audio and away from free thinking. My two cents worth.

I forgot one point. You have your overshoot in hand but have you checked your tilt? It's an easy test and both should be checked and set right (with an audio generator and not your voice) before the carrier hits the air after any processor or EQ adjustment.

Best Regards,
Mike

Mike. I use a Gates modulation monitor. It has an amplified output and is full range.  I also use the cleanRF audio detector which is base band audio with headphones.

What I need to do now is to get this HISSS noise out when I speak. I think its to much compression. I am going to back down the input to the dap and boost its output to see if its minimised.

c

This is from another post you made


I never heard you mention using the mod monitor, or any other, detector output but I have read several times about you listening on your receiver. No matter.

Your input to the processor should be no more than zero db on peaks and if you can manage it -20 db or better with no audio measured on your mixer vu meter. Your on the right track using the processor gain after that to match those gain measurements into the transmitter (0 db on peaks). From that point the transmitter takes over.

Mike

I also listen on an SX62 and Sp600 on the other side of the house with small antenna.

0 db.. I will have to try that.. Thanks!

C

Thanks Steve,any info you can come up with would be of interest to me.Was also wondering what limiter you use with your 310.  Patrick

Hello had the same problem with a DAP and mixer, after I tried all you have done cut the ground off of the 3 prong plug and RFI is gone, also later I added a input line filter conditioner , but still left the ground plug off of the plug into the wall , W5DUD

I might try this. I had a chance to mount the DAP into the rack. RFI city.  I have it eliminated now using Clip leads from the mixing board input shield to Ground.  The entire system worked perfectly for 2 years. Then adding the dap caused me major headaches.  It sure works great on the air though!

C

You thought I forgot, didn't you? Well I did. And then I remembered but was busy. I finally dug out the schematic last night.

Stock the crossover freqs are 120 Hz and 6.5 kHz. I changed the freqs to roughly 300 Hz and 1.6 kHz. Whether these are better or not is debatable. Considering the filters have a 3 dB/octave response, moving the low-to-mid crossover from 120 to 300 Hz may not have that much effect. But, if you are only looking at doing 6 dB or so of compression, then 3 dB at 300 Hz may be significant.

Changing the mid-to-high freq makes a little more sense to me now. In my case, I usually lowpass filter at 5 kHz (sometimes at 4 kHz), so having the high band working mostly at 6.5 kHz and above seemed like a waste of a band.

The only way to tell would be to do some really good A/B comparisons. I never did that. I made the changes and off I went. Kinda JS, but I do notice much more high band meter action now. And I get tons of SWL reports on how my audio is the best on the band.  ::)

Only four caps need to be changed. So, you can easily experiment or go back to stock. All changes are on the equalizer board.

Change C3 to a 0.05 uF  (Changes the low-to-mid freq)
Change C7 to a 0.01 uF   (Changes the mid-to-high freq)
Change C12 to a 0.5 uF    (Changes the low-to-mid freq)
Change C16 to a 0.1uF    (Changes the mid-to-high freq)
 

That's it. Have fun.

THanks Steve. I will pull the board out and have a look at what my unit has in it. I have a feeling they have been changed. My low compresses alot, My mid boosts and my high really boosts. .

C

I had a DAP once and I thought you could adjust each 'band' / meter for the amount of action/compression you wanted. Have you tried these adjustments before modifying the circuits?
BTW there is no latency from the DAP it's all analog. What you're hearing is what's happening on the air.

Fred

Fred, I think He was talking about listening on a monitor.  The gates monitor is Wide banded. So when you narrow up and use pre-emphisis it tends to sound like hell on a monitor. On the air its fine.

The dap is adjusted for compression and expansion for each band. You can vary that all you want. What steve is talking about is where those bands are split.

C

Got some trouble with the DAP this weekend.  Looking for some help.  I noticed a high pitched noise that appears on the scope.

I noticed that the Middle Compressor needled BANGS back hard on its own and sticks..  Looks like its the old issue of the boards not making contact all the time. A swift tap to the case gets the meters back.

What is the suggested solution?  I thought of trying to build up the contacts on the boards using solder or even that "solder paste" stuff that was posted a while back. I dont want to hard wire the boards in as I cant pull them out for service work.

C

Try a new, clean #2 pencil eraser to burnish the gold contacts on the board. Sometimes a careful cleaning will help. Don't touch the contacts with your fingers after cleaning. Refit the board dry.

ITs not dirty contacts. Its a design flaw of a 35 year old piece of gear. The tabs get loose and dont fit snug. Alot of people soldered them in with wires.  I wanted to see if anyone had any other suggestions before I try to build up the tabs with solder. thanks for the tip

C