TS-930S repair notes (17 April 2010)

April 17th, 2010 by k8gu Leave a reply »

This is just a quick note summarizing my repair of the 5-million S/N TS-930S.  Both the fragile MRF-485 drivers and the hardy MRF-422 finals were toast.  The power supply pass transistors (2N5886s) were also dead.  Two electrolytic capacitors in the PA bias network over-voltaged and exploded.  Original cause unknown, but certainly exacerbated by a slipped probe when setting the driver bias previously.

I almost wrote this one off and parted-out the goodies from this radio to finance a new radio.  But, when I saw the price of a used Orion or properly-configured K3/100, I realized I could repair this one a lot of times and save my money for my newly-acquired VHF+ habit and the eventual, mythical, first tower.

To begin with, I got the power supply working again with my last set of spare 2N5886s.  Should buy a half-dozen more of these to keep on-hand.  With the PA disconnected, I set the 28B line at 28.5 volts.

Now for the big job:  I tore-down the entire PA board, cleaned it, and cleaned the heatsink.  Replaced all of the bias resistors with new carbon composition types from Mouser.  Replaced the MC1723 regulator IC in the PA bias with LM723.  Replaced all electrolytic capacitors in PA.  Replaced drivers with MRF-485MP-HB (matched-pair, high-beta) from RF Parts.  Used all-new mounting hardware and mica insulators.  Replaced finals with 2SC2510A-MP, also from RF Parts.  This part is essentially the Toshiba version of the Motorola MRF-422, but at about 1/2 of the price and a little higher beta.  Yes, I know the higher beta may cause instability if run at full output or poorer IM if run at lower power.  But, I was in a thrifty and experimental state of mind.  Performed the PA bias alignment according to the Service Manual.

I fired the radio up into a dummy load on my bench (which is in the basement, two floors away from the shack—note to self:  this was a dumb idea).  No output on CW.  Check ALC—pegged.  Check Ic meter (PA collector current)—pegged.  The radio wasn’t groaning (or popping/stinking) like a circuit drawing too much current.  So, I figured it was in the current transducer circuit.  Righto!  This circuit contains a beefy 0.5-ohm cement resistor bolted to the power supply heatsink—basically they use the voltage drop across this resistor to estimate the current.  One of the sensor leads from this resistor read 0 volts.  After a good deal of tracing and troubleshooting, including inductors on the power supply board, I tracked it down to a bad trace near the connector on the power supply board, which was easily shorted with a small piece of wire.

That fixed it right up.  The radio made slightly more than 100 watts on CW and about 55 in TUNE mode, just as it ought to.  Next, I noted that the PA and P/S fans did not come on while I was transmitting, even though the heatsinks had become appreciably warm to the touch.  So, I hard-wired the P/S fan to the 8-volt line in the fan case (see the W6NL mods for more details).  In the case of the PA fan, I hard-wired a 7812 regulator (with an appropriate capacitor–0.68 uF/50 volts) to the 28B line on the power supply and brought 12 volts out to the fan.  So, both of the fans run all of the time now, which is a better arrangement.  Plus, I now have a regulated 12-volt source easily accessible inside the top portion of the radio.

I put the radio back together and took it up to the shack.  Made a few QSOs in the Michigan QSO Party and it seemed to be working great.  However, I did hear a little popping on the second radio.  I don’t know if this is a symptom of poor isolation in my station (need to follow the K9YC directives for SO2R bonding) or a power spike or IM problem on the radio.  Probably should make some measurements on those things.