Page 1 of 1

TS870 - C131 Cap Mod

Posted: Mon Jul 30, 2007 2:09 pm
by ruisgc
Hi Guys, how are you.
Can anyone explain the c131 mod to the 870s :D

Take care

C131 Mod for TS870

Posted: Tue Jul 31, 2007 2:26 am
by Voodoo Guru
Howdy Rui!
Just want to welcome you to the Voodoo Forum again!
Glad to have you here!!
Can anyone explain the c131 mod to the 870s
C131 is a .1uf cap that is in between the ACC2 input connector and the Mic Audio circuitry of the Transmitter.
A cap this low in value will roll off all the low frequencies.
The reason Kenwood put it there was because that is a Digital Audio input.
In order for us to use that for Voodoo Audio, it has to be increased in value.
A 1uf chip cap is a good value to change it too.
The reason to even use this ACC2 Mic Audio input is that it bypasses the Mic Amplifier inside the rig and allows us to be able to input Line level Audio.
The second reason is that since we don't have to go thru this first op amp, we can boost the high frequencies allot more to increase the bandwidth without getting the dreaded '870 SPLAT'.
It is a win win situation.
Here is a website that has pictures and some more information about it.
We've also made some High Resolution Pictures of it to help you locate it.
These pictures are 1meg in size, but you can blow them up to see whats around C131.



Hope this helped Rui!

Posted: Tue Jul 31, 2007 5:28 am
by ruisgc
Ok. Thanks afain for the help.
Back to the micro surgery hihi. yabadabaVoodooooo :twisted:

Posted: Sat Jul 18, 2009 7:32 am
Question about the C131 mod. I just preformed the micro-surgery. From monitoring from another radio, it sounds better. Of course the radio I am monitoring on does not have the great receive that the 870 does, and that is the one I am doing the transmitting on and I can't afford 2 870s!

OK, now on to the question...

Now that I am coming in from ACC2 jack, my understanding is that I am bypassing the mic pre-amp and all of its limitations. But I notice that if I have the meter in the ALC mode, it is still bouncing around all over the place with my audio. The mic gain control of course does not do anything now, so can someone give me a good skool'n on the whole ALC thing and explain what I am seeing?

Thanks in advance for the ALC 101 class that I am about to receive :-)


Posted: Sat Jul 18, 2009 8:11 am
by Voodoo Guru
Howdy Steve,
Well basically, the ALC (Automatic Limiting Control) is there to limit the amount of RF drive sent to the main RF Power Amplifier to prevent overdrive, distortion, and the resulting IMD (InterModulation Distortion).
This is done by monitoring the Amplifier Output Power.
Once it has reached its Maximum RF Output, the ALC circuitry starts to cut back the amount of drive to the amplifier by reducing the RF Drive Level in the IF stages(Intermediate Frequency).
This amount of Drive reduction is shown as an indication on the ALC Meter.
Of course nothing is instantaneous, so there is some ALC overshoot and other problems with it.
Needless to say, the best ALC is actually NO ALC.
If you can keep the Drive Level at a point were you don't see any ALC indication at all, that would be perfect.
On the 870, some ALC has to show or the transmitter just doesn't work right.
The TS-950sdx and the TS-850/DSP-100 combo run perfect with No ALC showing or just an occasional light.

Now, why your seeing so much ALC action...
More then likely you dont have the Menu 20 set correctly.
It should be on 0 with the correct amount of Audio level coming from the Rack.
Thinking back, the audio level is set about right when the ALC meter is showing from half to 3/4 scale.
Anybody else with the 870 got some input on this?
Take Care,

Posted: Sat Jul 18, 2009 8:32 am
With meun 20 set to 0, my output meter show only 50 watts on peaks, but with no ALC meter movement. If I set it to 1, I peak at 100 watts +/-, but I sometimes peg the ALC meter.

For the record, the rack is as follows:
Heil PR40
W2IHY iPlus
(in that order)

It is as if I am overdriving at menu 20 = 1 and underdriving at menu 20 = 0.

Posted: Sat Jul 18, 2009 8:49 am
by Voodoo Guru
It sounds and looks like you don't have enough audio drive coming into the radio ACC2 jack.
You should be on Menu 20 set to '0' then have enough Drive going into the ACC2 jack to adjust it.
Looking at your rack equipment, there is a problem as there are no input and output drive level controls on those last pieces of gear to set the gain correctly on your rack.
You should be able to have a level of -6 to -3db on peaks going into and coming out of each piece of rack gear.

The EX3200, DSP2024p do not have any way to adjust this gain.
The DEQ2496 does in its menu and is called Global Gain, but it is tricky as it is not a gain control so to say.

Your MIC2200 does and can be set to drive the MDX2200 correctly.
The MDX has an output control which can be used to set the DEQ2496 input gain correctly. You must look at the Input meters on the DEQ2496 and adjust the MDX output control accordingly.

That's were the easy stops.
Look at the output meter of the DEQ2496 and see what the peaks are.
It will be less because of the PEQ is cutting allot of gain, so you have to make that up somehow to drive the EX3200 and the rest of the gear down the line.
Try upping the Global Gain setting and see if that helps.
You want a maximum of -3db coming out of the DEQ2496.
This should drive the EX3200 and the DSP2024p correctly.

Don't know anything about the W2IHY i Plus.
so no help there.

What you want is a way to control the gain to the ACC2 jack on the rig.
You need from about -10db to -3db to drive the 870 correctly with the Menu 20 at '0'.

Hope that helps ya.
Take Care,

Posted: Sat Jul 18, 2009 9:19 am
#1. Thanks for taking the time to help a newbie.

#2. I forgot to mention that between the Ex3200 and the W2IHY iPlus and the 870 I have a MX1604A mixer. The output of the mixer is set to 0 db output and the peak light hits occationaly.

I am now wondering if the iPlus box is taking my rack (out of the mixer) output and <b>reducing</b> it to mic level. Thought I needed it as an interface.

I am guessing I might have to make a new cable for the ACC2 jack and instead of coming out of the iPlus using a 5 pin DIN, come directly out of the mixer and into the ACC2.

Think I am headed in the right direction?


Posted: Sat Jul 18, 2009 9:21 am
by Voodoo Guru
Yup! :wink:

Got to try things till it works!
Take Care,

Posted: Tue Jul 10, 2012 3:12 pm
by ECC82
It looks like the link is dead:

Use of ACC2 for original purpose

Posted: Mon Jan 16, 2017 5:58 pm
Does the cap mod prevent the ACC2 connection from being used for data modes?
My assumption is that the change in audio characteristics should have no effect on data modes therefore not requiring any de-mod or switching.

Re: Use of ACC2 for original purpose

Posted: Tue Jan 17, 2017 5:38 am
by Voodoo Guru
KA5BBC wrote:Does the cap mod prevent the ACC2 connection from being used for data modes?
My assumption is that the change in audio characteristics should have no effect on data modes therefore not requiring any de-mod or switching.
Howdy and welcome to the Forum!

Unfortunately due to a QTH move and other life ordeals, our participation in this forum has been minimal to say the least. Hopefully that will change this year.

The ACC2 Connection is still used for the Data Modes after the cap mod. Your assumption is correct, the only thing the cap mod accomplishes is the ability to pass more low frequencies. Nothing else is affected.

Re: TS870 - C131 Cap Mod

Posted: Thu Feb 28, 2019 8:14 pm
I have seen the information in a dozen different places with almost as many variables of cap value from 1uF to 4.7uF...
So which one is correct and why, how does the value and effect the results?

Re: TS870 - C131 Cap Mod

Posted: Fri Mar 01, 2019 1:33 pm
I should have also asked... Is there any need for a device such as a W2IHY iBox of passive DI/Ground lift between the audio gear and the rear audio input?