Astron RS-35M Back in Action

In my prior post (Astron RS-35M RIP), I shared how my 16-year-old workhorse linear power supply failed and my intent to bring it back to life by repairing it myself.  I studied basic electronics in college and have completed many electronic projects over the years, so I felt confident that repairing the power supply was within my capability.  I also enjoy a good challenge, keeping electronic gear out of landfills, and saving money.  I saw no reason to buy a brand-new linear power supply.

Dead Astron RS-35M Power Supply

What is a linear power supply?

The main purpose of a power supply is to convert electric current from a source to the correct voltage, current, and frequency to the load. 

A linear power supply provides regulated output voltage (in this case 13.8VDC) to power an electrical load (amateur radio equipment in this case) without the use of switching or digital components.  A key component of linear power supplies are large transformers.

A linear power supply has many outstanding characteristics as compared to switching power supplies including low noise, low ripple, simplicity, robustness, and ease of repair. This comes at a cost of being relatively heavy, large in size, and requiring heatsinking.  Linear power supplies are generally preferred by hams for amateur radio applications.

Diagnosis

Over the years, the linear power supply had slowly acquired a noticeable hum.  The 60-Hz hum seemed to have grown more loudly in the last year or two.  Hum in a linear power supply can be an indication of bridge rectifier and filter capacitor degradation.  Electrolytic filter capacitors are notorious for breaking down with time.  The power supply's original filter capacitor was 16 years old and possibly in need of replacement.  My hypothesis was that the power supply blew its fuse at power-on due to a bridge rectifier with a dead-short and this may have also taken out the electrolytic filter capacitor.

To verify my hypothesis, I started by unplugging the power supply from house power and the attached equipment and put it on the bench.  The first step was to remove the chassis cover.  This was accomplished by removing three screws on the top of the case cover and four screws on the bottom and then sliding off the cover.

The next step was to discharge the electrolytic filter capacitor.  This is an important safety step as the filter capacitor is physically large and can store potentially dangerous electrical energy.  To discharge the capacitor, I placed a power resister across the filter capacitor terminals in place.  I then confirmed the capacitor was fully discharged with my voltage meter.

I then loosened the screw on the bracket that holds the capacitor in place.  I also removed the positive and negative terminal screws from the filter capacitor.  This permitted the filter capacitor to be removed from the power supply.  I placed the discharge resister across the capacitor terminals again as a safety precaution.

The next step was to remove the bridge rectifier gang from the power supply.  I decided the easiest way to remove the bridge rectifier was to simply snip the two AC (yellow) and the positive DC (red) leads and then unbolt the bridge rectifier gang assembly from the chassis.

Original Diotec DB2501 dual bridge rectifier gang 

Once the bridge rectifier gang assembly was out of the chassis, I used my multimeter's diode test feature to check the bridge rectifier gang.  The test showed that indeed the rectifier had a dead short (between positive DC and one of the AC terminals in both directions) and needed to be replaced.  It is likely that only one of the rectifiers had a dead short.  I decided to replace both DB2501 rectifiers as the cost is low and not really worth trying to salvage one rectifier.  It is also important to match the two rectifiers, so they work together.

Original DB2501 bridge rectifier assembly testing found a dead short

I checked various RS-35M schematics, and they indicated that the correct bridge rectifier was supposed to be a pair of DB3501.  For some unexplained reason, my particular RS-35M power supply had a pair of DB2501 bridge rectifiers installed.  A pair of DB2501 rectifiers would have to work much harder and thus not last as long as a pair of DB3501.  Having the incorrect underrated components was likely a root cause issue.

Original (undersized) DB2501 removed from the Astron RS-35M

Replacement Parts

I bought the replacement parts directly from Astron rather than risk getting it wrong with other sources.  Buying from Astron made it very easy to purchase parts that were both electronically and mechanically suitable.

I decided to simply replace the original electrolytic filter capacitor rather than try to determine if it was still serviceable.  This was an easy decision.

Replacement Electrolytic Filter Capacitor (25 VDC, 100,000 µF)

For the bridge rectifiers, I could have replaced the original Diotec DB2501 (100V, 25A) rectifiers with the same or returned to the specified DB3501 (100V, 35A).  However, for a nominal additional cost, I found I could replace the original DB2501 rectifiers with two heavier duty DB5001 (100V, 50A) components.  This seemed like a good choice.

Replacement DB5001 bridge rectifiers

To reconstruct the rectifier gang assembly, I upgraded the wire to 10 gauge.  The heavier gauge wire would more easily handle the electrical power while also providing a little more heat dissipation.

10-gauge wire ready for assembling the bridge rectifier gang

The bridge rectifiers are mounted directly to the power supply chassis.  The metal chassis acts as a heatsink for the rectifiers.  When I removed the original rectifiers, I noticed that they were installed with white heat sink compound.  For the repair, I used GC Electronics Type Z9 silicone heat sink compound.

Type Z9 Heat Sink Compound

I replaced the linear power supply's fuse with the same standard AGC-8A fuse.

Original AGC-A8 blown fuse (left) and new replacement AGC-8A fuse (right)

Repair

Step 1: Assemble the bridge rectifier gang.

The DB5001 bridge rectifier connector pins need to be twisted 90 degrees so that the connector holes line up.  I used a pair of needle nose pliers to gently twist the connectors.

Next, I oriented the DB5001 bridge rectifiers, so they matched the original DB2501 orientation.

Original DB2501 bridge rectifier assembly

I then placed the three 10-gauge wires through the connector holes AC to AC, positive DC to positive DC, and AC to AC.

Old DB2501 bridge rectifier assembly with oriented DB5001 rectifiers and 10-guage wires

Next, I used original DB2501 bridge rectifier gang assembly as a jig for the next DB5001 assembly.  I pinned the two assemblies together by placing a hex key and a punch through the center holes.  This made it easy to solder the connectors and wires while maintaining the proper orientation and hole spacing.  Correct hole spacing is necessary to install the assembly in the chassis.

The next step was to solder the 10-gauge wires.  I used a dab of electrical solder flux to prep the wires and connectors for soldering.  Flux cleans the metal surface which helps the solder to flow.  After soldering the wires and connectors, I also tinned the middle section of each of the three 10-gauge wires.  Having tinned wires helps when soldering the power leads.

The next step was to solder the two AC input transformer leads (yellow) and the DC positive output lead (red).  I removed a small length of the plastic insulation from each wire.  I then wrapped the 10-gauge solid wire with a lead wire and soldered it.  The heavy gauge of these wires required me to turn up my soldering iron temperature to high (750F) to properly flow the solder throughout the connection.

Step 2: Prep the chassis for installation of the bridge rectifier gang assembly.

I carefully removed all traces of the original white heat sink compound from the chassis using a paper towel and wiped down the chassis interior to remove dust and debris that had accumulated over the years.  It is important to have a clean chassis surface free of debris.  I then applied a liberal coat of the fresh white heat sink compound to bottom surface of each of the DB5001 bridge rectifiers.

Step 3: Install the bridge rectifier gang assembly.

I placed the chassis on its side to make it easy to position the bridge rectifier gang assembly while simultaneously pushing the rectifier screws through the bottom chassis holes.  I used a 1/4" drive 9/32" deep well socket to hold the nut while using a short-handle Philips head screwdriver to tighten the rectifier screw.  The space is a little tight, so this technique made it easy. After tightening the screws and nuts holding the rectifiers to the chassis, I used a paper towel to wipe up the excess white heat sink compound that was squeezed out.

Short handle Philips screwdriver and a 9/32" deep well socket

Step 4: Install the replacement electrolytic filter capacitor.

I first discharged the new replacement filter capacitor with a resistor to make sure the capacitor was safe to handle.  I then mounted the new replacement filter capacitor in its bracket making sure the capacitor polarity was oriented the same as the polarity as indicated on the circuit board.   I then tightened the negative (black) and positive (red) wires to the circuit board and capacitor.  I then tightened the capacitor bracket so that the assembly was secure to the chassis.

New replacement electrolytic filter capacitor installed.

Step 5: Replace the AGC-8A fuse.

I removed the old burnt out AGC-8A fuse and replaced it with a new AGC-8A fuse.  

Step 6: Test the power supply and measure the voltage output.

I connected my multimeter to the power supply's output terminals.  I plugged in the power supply and turned it on using the switch on the front of the chassis.  To my delight, the volage output measured at 13.74 volts.  The power supply was working again!

Power supply output voltage test (no load)

Step 7: Install the power supply chassis cover.

I turned off the power supply and then disconnected the AC power input.  I then placed a resistor across the filter capacitor terminals to discharge the capacitor as a safety measure.  Then placed the power supply on its side and carefully slid the cover onto the chassis.  Once in position, I screwed in three screws on the top and four screws on the bottom. 

Step 8: Test the power supply with a load (radio).

I returned the power supply to its operating position and reconnected my main radio.  I powered up the power supply and then the radio.  The voltage was consistent with minimal load (receive only) and with a full load (transmit).  It was wonderful to hear the power supply function silently both idle and under load.  The hum is gone!

Repaired Astron RS-35M power supply under load

Conclusion

Repairing my Astron RS-35M was a very satisfying project.  The process was thoroughly enjoyable - figuring out what to do, ordering parts, performing the repair with the sweet smell of solder smoke, and testing to find the issue and repair outcome.  The repair cost about $40 for replacement parts and $13 for a tube of heat sink compound (with 99.9% of the tube left over).  The repair represents value as compared to buying a brand-new power supply.

I am delighted to be back on the air and making contacts again with a nice silent power supply.  A big thank you to Astron for making such a great power supply.  My Astron RS-35M has been an outstanding performer for many years.  I appreciate its design, years of reliable service, and serviceability.  In spite of Astron originally installing the incorrect and underpowered DB2501 rectifiers 16 years ago, I plan to stick with Astron for all my future power supply needs and I would recommend Astron to my fellow hams should anyone ask.

Have you repaired an Astron power supply before?  How do it go?  Would love to hear from you.  Please leave a comment.

For those who appreciate a bit of electronics trivia, Astron designates standard desktop power supplies with the "RS-" prefix meaning Regulated Supply.  The "M" suffix denotes power supplies with front panel Meters.

Good DX and 73, NJ2X

December 3, 2023 Update

I discovered today that the original failure also caused both meter lamps to burn out.  I didn't catch this at first due to working with the linear power supply on a well-lit bench.  The meters seem to also be registering a little higher voltage and current values than before the failure which may be due to the absence of the load from the incandescent lamps.  I am considering replacing the lamps with LEDs as a future project.  For now, the meters are perfectly readable in their usual operating position.