[ICOM] IC PS-35 Problem

[James T Wassell]

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