TS-870s 4Kc Inrad Mod

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TS-870s 4Kc Inrad Mod

Post by Voodoo Guru » Wed May 30, 2007 9:25 pm

Allot has been asked about the 4Kc Inrad filter swap in the TS-870s.
Does it help?
Does it make the Receiver Wider?
Does it make the Transmitter wider?
Does it do anything at all? :?

Well, we have delved into it here to answer your questions......

And the answer is No. :(

There is no difference in bandwidth on Transmit or Receive at the wider
bandwidth settings.
What, How, Where, Why you ask?
Well, here are the reasons...

We'll have to tell you a little about the TS-870s Analog and DSP IF
Filter circuitry first.
The Receiver will be discussed first, then the Transmitter.

Receiver:
Upon looking at the Service Manual for the 870, We noticed that the 3ea
8.83MHz and the 3ea 455kHz Analog IF Filters are switched in and out and
are 'interlocked' with the 'WIDTH' controls on the Front of the 870.
The 'Width' controls are what ultimately chooses which analog filters
are chosen.
This is for 2 reasons.

Reason 1 - The 870 uses the DSP for filtering and demodulation on
Receive. If there is a high level signal outside the DSP filter band,
the Analog to Digital Converter (ADC) gets saturated and the DSP gets
overloaded. Then you get awful digital distortion.
By inserting the Analog filters before the DSP, this will help to smooth
out the signal and keep the interfering signals from saturating the ADC.

Reason 2 - The 870 uses the Analog filters for Slope Tuning. The
different bandwidth analog filters are selected and the Local
Frequencies of the IF's are tweaked in frequency to narrow or widen the
pass frequency bandwidth thru the IF to the DSP.

The 8.83MHz Filters consist of:
1 - a 3Kc Crystal filter, (this is the one that is changed with the 4Kc INRAD)
2 - a 6kc Dual Monolithic Crystal Filter, (looks like CB crystals)
3 - a 'Thru' Filter (which is actually no filter at all).

The 455kHz filters consist of:
1 - a 3Kc Ceramic Murata Filter, (a little black box)
2 - a 6Kc Ceramic Murata Filter,
3 - a 15Kc Ceramic Murata filter.

The Filters are switched simultaneously by the CPU Logic depending where
the 'Width' controls are set for the Slope Tuning.
These means that when CPU Logic states for the 8.83MHz IF 3Kc Filters to
be used, the 455kHz 3Kc filter is also chosen.
When the 6Kc 8.83MHz filter is engaged, so is the 6Kc 455kHz filter.
When the 'Thru' 8.83MHz filter is picked, so is the 15Kc 455kHz filter.
They are never different.

Now...all that was said to say this,
There is a chart on page 30 of the Service Manual that shows the
relationship of the 'Width' controls to the Analog Filter selection.
If you look at it, you will see the correlation between the frequencies
of interest for ESSB, and the analog filters that are chosen.

now listen close....

When you select the TS-870s 'Width' Filter controls for a Receive
Bandwidth of 4.6Kc,
the CPU Logic selects the Analog 6Kc Dual Monolithic Crystal Filter &
the 6Kc Murata Ceramic Filter.

When you select the 'Width' Filter controls for a Receive Bandwidth of 6Kc,
the CPU Logic selects the 'Thru' Filter & the 15Kc Murata.

What this means is:
Changing the 3kc Crystal filter with the INRAD 4Kc Filter will do
absolutely nothing for your ESSB Receiving bandwidth.
The 3Kc filter is NOT used on receive until the 'Width' controls are
reduced to 2.4kc.
This swap will also have a detrimental effect to the slope tuning and
bandwidth at the narrower bandwidth settings.

Transmitter:

OK, now for some more bad news.
On Transmit, the DSP sets your ultimate TX Bandwidth according to Menus
29 & 30.
The 870 DSP TX Audio bandwidth at its maximum, can go from '0' & '3000'.
(The 870 can of course be pushed past that upper 3Kc limit with proper
Audio EQ'ing. He He!)
After the DSP is finished doing what it does, the result is a 455kHz,
SSB Signal.
This signal is mixed up to 8.83MHz, and then sent thru the Analog
8.83MHz IF, 6Kc Dual Monolithic Crystal Filter. This is for filtering
components outside of the band.
This 6Kc Filter is ALWAYS used for transmitting on SSB.
The 3Kc Crystal filter is Never used on TX...ever.

How do We know this?
The Service Manual tells you this on page 18, paragraph 5.
But wait you say, The Fig. 7 diagram on page 20 shows the 8.83MHz IF,
3kc filter as the one being used!
The diagram is in error.
We measured which filter was actually being used on TX with our own TS-870s.
It is the 6Kc Dual Monolithic Crystal Filters.

So there you see,
The 4Kc INRAD filter swap in the TS-870s for ESSB is for nothing.
It does nothing to improve bandwidth on either TX or RX.
It is not used when the 'Width' Controls are wider then 2.4Kc on Receive.
It is not ever used on Transmit.
The 870 switching arrangement does not allow it to be used in the wider
'Width' Control settings.

The Contesters CAN benefit from this as they are using the narrower
'Width' control settings which would engage these filters.
This would give them the benefit of the INRADS flatter bandpass, and
steeper skirt edges....
But for ESSB......useless. :cry:
Last edited by Voodoo Guru on Mon Apr 29, 2013 6:02 am, edited 4 times in total.
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Post by !KANT » Wed Jul 25, 2007 9:48 am

Thank the Heavens of Audio that this was investigated and concluded.

As I have always stated, the '870S is the [UNMODIFIED-stand-alone] standard for ESSB. And I still love mine [since summer 2000]. 8)

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