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Author Topic: IC-471A Receiver modifications  (Read 6171 times)
WW2L
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« on: September 04, 2010, 06:05:40 PM »

Hi Folks,

I am modifying one of  IC-471A for weak signal work. I replaced the all caps with the ceramic ones and 3SK48 on the receiver chain is replaced with the BF981, 2SK125's are also replaced with J310s.  When I was checking the circuit I find 2 capacitive diode on the gate of the first amplifier (3SK48) as far as I know and this radio does not re-tune the input and it is directly getting it's voltage (which is fix) from R8V point. These two cap-diodes are 1S2090 which very well known with high noise characteristics. Is any one work around one of these old ICOM UHF all modes? Or can any one has a good explanation for this diode pair.

Thanks and 73
WW2L

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KE3WD
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« Reply #1 on: September 04, 2010, 06:54:17 PM »

Likely placed there to protect the gate of the FET from hole punches due to static, lightning pulse damage. 
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WW2L
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« Reply #2 on: September 04, 2010, 07:05:53 PM »

Thanks old man,

I believe there are plenty of diodes (1n4148, 1n914, etc) you can use for the static protection and they are in a parallel configuration not head to head. What is puzzling me here these are capacitive diodes and there are in a center voltage feed configuration!!! I wish you can find the schematic for this old rig and check.

73,
WW2L
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G3RZP
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« Reply #3 on: September 05, 2010, 12:54:18 AM »

D3 will provide a high resistance (10kohm) clamp to any positive going spike exceeding 12 volts. It doesn't do anything for negative going spikes, and D4 is reverse biased to the tune of 12volts. So I don't see the point, either. Unless maybe it's to provide a low Q capacitor to make the first tuned circuit more broadband - if the Q is 20, then the gain will be down 3dB at the band edges. For weak signal work, I would look for a higher Q input circuit, although it quite possibly needs a stability criteria analysis doing to make sure that it's stable under such conditions.

Now I don't have the board layout: is it actually built like this? It is not unknown for circuit diagrams and board layouts to differ....

Related to this, does anyone know if the basic RAM contents i.e. enough to make the rig work but without memory, are available? This is one of those rigs where the memory battery dies and the RAM data is lost.
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G3RZP
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« Reply #4 on: September 05, 2010, 03:10:38 AM »

One other thought that struck me. Is it possible that the original idea was to use the synthesiser VCO control line to tune that tuned circuit according to the operating frequency, and it was found not worth the bother?

Be definitely interesting to know if the junction of those two diodes really does go to a fixed 12volts. I've too many things on the bench right now to go find out - plus getting ready for W9DXCC next weekend.
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KE3WD
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« Reply #5 on: September 05, 2010, 08:20:00 AM »

If you can point us to an online schematic url, please do so. 

This still may be a protection situation, if the 12V is fixed, think about the current flow in the opposite direction.  Sometimes the offending overvoltage pulse is bled off to a regulated supply like that.  It may also combine the protection with a set of biased diodes for tuning, as well.  But that is all conjecture without the schematic to peruse. 

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G3RZP
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« Reply #6 on: September 06, 2010, 12:03:19 AM »

I have an IC471H, so I have the circuit diagram.

It's a parallel tuned  circuit: the input is tapped onto the coil with a small capacitor, and the top end of the coil conects to g1 of the FET. Across the coil are the two diodes, back to back, with the cathodes connected together and then fed via 10k to the 12volt line. So both diodes are reversed biased to the tune of 12volts.

I could understand the junction being fed from the VCO control line, as covering 420 to 440MHz requires either a low Q tuned input or retuning, if sensitivity isn't to be lost somewhere. What I don't know is if the diode junction really is connected to 12 volts as the circuit diagram says, or to somewhere else when you look at the actual board. It is not unknown for circuit diagrams to be somewhat different to the real equipment - it even happens with QRH (Quick Reference Handbook) on airliners at times!
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WW2L
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« Reply #7 on: September 06, 2010, 09:32:14 PM »

I am thinking that it is only design to un tune the first parallel circuit. This will prevent any un necessary osilations when TX is active.  Normally we will tune (peak) the this circuit when Receive voltage is on when TX will become active RX voltage will be off then the tune of the first tuned circuit will be some where out of the band.  May be this is the way ICOM engineers think to prevent high voltage on the MOSFET?? What you folks think about this theory?

73,
Levent - WW2L

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G3RZP
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Posts: 1218




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« Reply #8 on: September 07, 2010, 01:04:49 AM »

In that case, the diodes would still get enough volts to make D2 conduct. But, as you say, it would detune the circuit. Do you know what the capacitance of the diode at Vr = 12V is? And at say 0.5V?

When Mutek (G4DGU) was in business making replacement front end boads for VHF xcvrs, he went over to relays for antenna switching to win a dB or so at 2m. Might be a better approach at 70cm than the existing switching.

I doubt that a working Q for the input tuned circuit as low as is the existing case is really as conducive to low NF as desirable. As I recall, the NF of the stock rx is around 7 or 8dB - hardly a 'weak' signal rx by today's standards!
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WW2L
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« Reply #9 on: September 07, 2010, 04:13:09 PM »

7-8 db is really high NF that's why I try to change all critical components on the RX chain. Now I need to find a easy way to measure NF.  I am open for the suggestions here.
If it is not in my acceptable limits I have one more card to play. Replacing the first RF amp with pHEMT device or put the pHEMT pre amp in the box.  My entire objective is to utilize this oldie for Radio Astronomy or EME Smiley

73,
WW2L
 
« Last Edit: September 07, 2010, 04:18:46 PM by Levent Sasmazel » Logged
G3RZP
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Posts: 1218




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« Reply #10 on: September 08, 2010, 12:17:53 AM »

In that case, I'd look for using a low loss relay for the antenna switching, a GaAsFET for the RF amp, possibly a MOSFET following, and then  a good DBM as a mixer, followed by a diplexer and a BF981 at 70MHz before the second mixer.

That's pretty well a complete rebuild of the front end, and I'd use helical resonators between the 1st and 2nd RF stages and the 2nd RF stage and the mixer. Then a helical resonator at the input, and sacrifice performance outside about 432 - 438 - it will still be reasonable enough for local FM work.

Carefully done, that should give you a NF of around the 1dB mark, together with reasonable signal handling.

I picked up a professional 10MHz to 4 GHz noise head at Dayton some years back for the princely sum of $20: worth looking round the local hamfests - and Dayton flea market. The only danger is that some jerk has burnt it out, but at one time, there was a company in the US who would sell you a noise diode and calibrate your noise generator.

Thermionic diode noise generators can have the noise calculated from basic principles. However, unless the diode has had very few hours use, they can be wildly out, due to gas and to filament poisoning.
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