Samson Rubicon Studio Monitor Repair
I own a pair of these monitors. They have 5 inch bass speakers and s ribbon tweeter. The crossover frequency is about 3.2 khz (revised from 2khz since receiving circuit diagram). They are well regarded, having received good reviews, see Sound on Sound and they have given me good service and over perhaps 7 years. However, I have had a couple of faults which I suspect are related. The first one was caused D.C. to appear on a bass speaker, threatening to throw the cone across the room. It was under guarantee so I sent it back. It was returned, "No Fault Found". The fault reoccurred within a short time. The second, a tweeter open circuit. I suspect this may also have been caused by D.C. but I have little proof. The first fault righted itself after removing and refitting the power amplifier board and has reoccurred. The tweeter required replacement.
Update Feb 2019, 2-years later
The fault described above reoccurred with increasing frequency. It tried on numerous occasions to trace the source but it always cleared when I opened the box. Until one day it didn't. I measured the supply voltages to the pre-amp board and the -ve was missing. A tap with a screwdriver handle on the power-amp board would restore it an another would see it go off again. So, I had an inkling of its source. I took the main amp board out (again) and looked at the soldering of the components in the -ve regulator through a magnifying glass and I spotted one that was suspect. It was on the series resistor to the emitter follower regulator transistor. I re-soldered it refitted the board. I tested it using a hit from the screwdriver and it was OK. It was fixed.
Here is the ribbon tweeter, this is the good one but there was little external evidence of the fault on the other one. It was revealed by the ohmmeter.
This is the enclosure with all the components removed. A substantial construction. The bass port extends almost to the back of the cabinet.
Below is the electronics assembly. The top board is the crossover and the bottom, the power amplifier and power supply. There are two pairs of power rails, one for each amplifier. The amplifiers themselves are integrated devices. Th treble amp is a TDA2052. I could not read the number on the bass amp component. The mains transformer is at the bottom. It is fairly small for the power of the amplifier and it runs fairly warm even in quiescent state.
In trying to find a possible reason for d.c. appearing on the output, I traced the input and feedback circuits of the treble amplifier. I was surprised to find that, with the input cable unplugged, there was no route to earth on the inverting and non-inverting inputs. The earth reference is derived from the crossover board. I assume that it was done thus to prevent any noise on the power amp board from being injected into the amplifier. All well and good but if the connection to the crossover board were to go open circuit, the amplifier output would drift to one rail or the other. This connection is made through a plug and socket. This is a possible cause of the two faults I have encountered, but I have no real proof that this was the case. On both occasions the faults were transitory.
I was unable to find a direct replacement for the tweeter so I looked for an alternative. I investigated ribbon tweeters but I did not find any that were suitable for the crossover frequency employed. There are plenty of dome tweeters that can cope with 2khz. I chose the Seas 27TFFC H0881-06. It has a free-air resonance of 550 hz and can handle 200w with a crossover frequency of 2.5khz and 12 dB/octave filters. I figured that it would be fine at 2.0 khz with 24 dB/octave and a 20w amplifier. Fitting it (them) would be quite a job. I made a plate to adapt the cabinet to the new tweeter.
I made an adaptor plate to mount the new tweeter, Here it is. It is MDF faced with 3mm of hardwood - wenge I think. Here it is glued in place. I sawed and routed the unwanted bits the cabinet and dropped in the new tweeter.
The Completed Repair
Here are some images on the finished job. I am fairly pleased with the look, good enough anyway. As for the sound, I like it. It is different to the original but certainly not worse. After I repaired the first monitor and before I changed the tweeter in the second, I listened to the two side by side, switching between the two. The one with the new tweeter sounded to had a similar level of treble, so I judged it OK. I tried measuring the frequency response with a condenser microphone knowing that the resultant graph would be of little value because of room resonances. It was of no value. So, the listening test has it.
I feared that d.c. may wreck my new tweeters so I provided some protection in the form of series capacitors. I wired 47 microfarad non-polarised components in series with the tweeters. They do add a further high-pass response to the tweeter at around 500 Hz. The purists would not like it!