There were several things going on with this "restore". Below are the details.
This PS-7 power supply was very professionally modified to have a power switch on the front and a 120 volt power light. The cable for the control from the TR-7 is missing. None of which were causing issues. After working on the thing, my guess is that the power supply was hit with a rather large voltage spike, and/or the over voltage SCR fired and was not reset in a reasonable amount of time. This was acquired at Findlay Hamfest. Symptoms were that it blew fuses upon power on. I was told that the transformer was good. When I ask how he knew it was good, he backed off that claim and accepted my offer. Found the following for sure. I am not fully done testing the supply. All 4 power transistors were new/replaced (when I got it), 4 power transistors were missing their plastic covers (don't run the PS long term w/o those, that is just asking for trouble), the bridge rectifier was shorted with both sides of the transformer to ground, and the AC line bypass capacitors were in some kind of high resistance short mode. I disconnected the transformer from the bridge rectifier. I have seen the bridge rectifiers go shorted like this when the AC mains get onto the chassis. I was thinking the transformer had a primary to frame short. I tested the transformer by connecting a 1 Amp 12 Volt AC transformer to the secondary of the PS-7 transformer and was getting roughly 75 volts on the primary. Although a bit lower than I wanted to see, at least not dead shorted. It also did not seem to be putting voltage on the chassis. Next I put a 5 Amp fuse in the power supply and connected the power plug to 120 Volts. Turned on the switch (with an insulated rod) and got power indicator light and ~18 VAC on the secondary. Still not convinced that I didn't have a primary side short of some sort, I put the system on a GFCI. I have one set up for testing because I have seem too many bad things on older radios. Didn't pop the GFCI at first, but did after a while. Being somewhat intermittent I worked with that for a long time. Figured out that the GFCI popped with only the line cord and voltage adjustment PCB attached. So it about had to be the AC line bypass capacitors. Garey, K4OAH, gave me a heads up on "X and Y bypass capacitors". I Googled and found tons of good data. Here is a very easy to understand URL http://www.justradios.com/safetytips.html Changed out those AC line bypass capacitors with "Y caps" taken from old PC power supplies, and things seemed stable. I replaced the bridge rectifier with a 50 Amp I had in the junk box. I added back the plastic insulator caps on the 4 power transistors. I currently have the power supply running with 13.6 Volts output, without the over voltage SCR in the circuit. So I am in need of load testing and over voltage testing. I appreciate the load testing ideas so far, and look forward to any others. I will likely use a small tail light bulb in series with the over voltage SCR to test its turn on voltage. Really don't want to try to short the power supply with the SCR. The 4 pass transistors that are there are rated slightly less total wattage dissipation, but otherwise exactly the same as the originals. I was also thinking of a mod for both my the PS7s. I was going to put a 25 Amp fuse at the filter caps as the 2 wires leave for the pass transistors. Figured it would blow the fuse on over voltage short as opposed to the other devices. It would however leave the 723 powered up as that power is from another rectifier path. Any thoughts? I also have an idea for the missing control cable. Will post when/if I get that done. I think I may also replace the filter caps. They are 30ish year old now, about time for a fresh set. Sorry for such a long thread. I told Garey I would give a report to the list when I e-mailed him questions off list. I do some minor off list traffic, but prefer on list because I learn a lot from the list. I appreciate reading a good troubleshooting report. Hopefully this report may help someone else to troubleshoot their PS-7. TNX es 73, Ron WD8SBB --- On Mon, 9/19/11, Michael & Sue Trussell <[email protected]> wrote: > From: Michael & Sue Trussell <[email protected]> > Subject: [Drakelist] FW: Drake PS-7 test ideas > To: [email protected] > Date: Monday, September 19, 2011, 8:00 PM > In regards to the PS-7 power supply, > I was wondering what was broken. I also > have a PS-7 power supply which I think is getting tired. > It's working ok > right now but there have been a few times when I can hear > the cage > vibrating! What needed to be replaced on these guys? I > would not want mine > to break down when it feels like it. Is there a schematic, > operating > instruction manual etc to be found? How about a parts > source? > > > Thank You in Advance > > Michael J Trussell KA8ASN > > -----Original Message----- > From: [email protected] > [mailto:[email protected]] > On Behalf Of Ron > Sent: Monday, September 19, 2011 6:45 PM > To: [email protected] > Subject: [Drakelist] Drake PS-7 test ideas > > I acquired a broken PS-7 which I now have working. > > I am wondering if anyone has ideas for a temporary testing > setup for this > beefy 13.6 Volt, 25 Amp power supply? > > I really don't want my TR-7 to be the first test load. > > TNX es 73, > Ron WD8SBB > > _______________________________________________ > Drakelist mailing list > [email protected] > http://mailman.zerobeat.net/mailman/listinfo/drakelist > > > _______________________________________________ > Drakelist mailing list > [email protected] > http://mailman.zerobeat.net/mailman/listinfo/drakelist > _______________________________________________ Drakelist mailing list [email protected] http://mailman.zerobeat.net/mailman/listinfo/drakelist
