[ICOM] IC PS-35 Problem

[Jaime Gomez]

Hi Ted, thank you very much. This information is really very helpfull. I
will start taking out IC-1 tomorrow and will check it. How much voltage did
you apply in the neon side to test it? I wonder, if the CDS LSR is ok, if it
is better to replace the neon with a LED, using the proper resistor to limit
the voltage. I think they use the neon - CDS isolator in prevention of a
possible spike damaging Q1 if it is connected directly with a resistor.
Now I have very good information to go ahead. All other components are OK,
as I checked each of them out of the board. (Has not been easy !!!!)

73, Jim, HK1N

On Tue, Jan 11, 2011 at 1:42 PM, James T Wassell <k3jt at westco.net> wrote:

> Jim,   HK1N:PS-35 repair.  I've put in a good bit of time repairing my
> PS-35 from my IC-765 and have discovered somethings that may help you.
>  First, (as suggested in the ICOM service bulletin) I replaced electrolytic
> caps, C18 and C19 (33uF, 250V) that had blown, I replaced R5 and some other
> resistors (R3, R4, R28-R31, R32, R33) and I replaced power transistors Q2,
> Q3, Q4 & Q5 (Q4 and Q5 were bad shorted), all with parts from Mouser
> electronics. I could not get the circuit to produce 13.8 VDC. In fact, first
> time I powered up the unit with 110 VAC, R5 failed again. I measured
> voltages as I could and checked components D1, Q1, D2, even L4 (thinking it
> was open because I could not measure any voltage on IC2 pin 15).    IC1
> seemed to be a problem and I don't think it's easy to find a replacement.
>  IC1 looks like an electrolytic capacitor, in a black can, with 4 wire
> leads.  Two of the leads go to a neon lamp and the other two leads are from
> a CDS light sensitive resistor.  As noted below (credit to ZL4WHO)  (
> http://forums.qrz.com/archive/index.php/t-223836.html), IC1 is a simple
> opto-isolator, driven by a neon lamp bulb that illuminates a CDS light
> sensitive resistor.  The CDS resistor is intended to drop to about 1K ohm
> when illuminated and the lower resistance turns on Q1 that drives the
> regulator chip IC2.      I took IC1 apart in my PS-35.  The neon bulb was
> not working (a curious failure as the glass envelope was intact and there is
> no filament to burn out?).  The CDS cell would drop to about a 1K ohm when I
> shined a high intensity LED flashlight on it, but with (a substitute,
> working) neon bulb, it would only drop to about 10k ohm.  I tried to
> "manually" activate the IC1 circuit that drives Q1 by powering on the
> circuit and then touching a lead to a 1K ohm resistor in place of the CDS
> cell.  (I also soldered in a  working NE-2 bulb on the other side of the
> circuit - not sure if this is needed, I may remove it).   I powered up the
> circuit with 110VAC and measured 13.8 VDC output.  It worked!  Then I tried,
> just powering up with a 1k ohm resistor in place of the CDS cell.  Powering
> up the PS-35 with a 1K ohm resistor in place of the CDS cell works also.  I
> can't smell, hear or see any destructive consequences.   I am not sure why
> the opto-isolator is needed for the circuit.  I think R5 (the so-called
> ballast resistor) provides enough time delay.   I am now to the point where
> I will simply replace the cds cell with a 1K ohm resistor and carry on.
> I suppose there may be some hidden problem with replacing the opto-isolator
> with a 1K ohm resistor, but it works for me.  It is possible that this
> places some additional surge or stress on other components.  I have seen
> references to developing a new modern opto-isolator that will take 140 VDC
> to activate Q1 by switching 1K ohm to ground.   I have also thought about
> rebuilding IC1 as some places will still sell CDS cells (
> http://www.goldmine-elec-products.com/prodinfo.asp?number=G17170) and neon
> bulbs, but, for now I'm inclined to simply use the 1K ohm resistor.   I
> would welcome thoughts that others may have on my "substitution" solution.
> When re-assembling the PS-35, ZL4WHO made these suggestions:  "Remember to
> be careful of the insulated copper plate UNDER the two power transistors.
> Make sure it stays ISOLATED from both the case AND the transistors. Also its
> a good idea to replace the two nylon screws holding Q2&3 with insulated
> washers and 6mm long steel screws."  Note that you can also power the IC-765
> (IC-751A) with an external DC power supply, which has the advantages of a
> cooler and lighter radio.Good luck getting your PS-35 working again.   73,
> Terry    K3JT.    email:   k3jt at arrl dot net
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-- 
Jim, HK1N