For the person wanting to know about replacement heating elements, ebay: A1321 (next time, please keep the sheet that comes with the replacement heater!)
As for the 150ohm resistor on the power supply, its supposed to be an "active resistor" to reduce power supply noise. I dont like it either but left it there (these things are in my old oscilloscope also). they also help when the device on the power supply tries to put power back into the supply.
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Now, I have my own little review, and after this much I am still happy with it. i guess if it quits on me and i cant fix it then i wont be happy, but i will probably continue trying to fix it - probably with new guts.
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Here is a list of improvements i had to do to my 853D+.
Required tools:
another soldering iron.
dremel or the like (otherwise, a saw and chisel/screwdriver)
callipers (optional)
axial resistor for LED (220 ohm i think)
Drill and one bit (10mm or whatever)
spare thick and thin wire.
First impressions:
I noticed the soldering iron was not made quite right. But i could not determine the fix i needed to do.
I did take a peek inside and i did notice how underutilised the earth connection was.
The power supply would beep if i applied power to anything connected to earth.
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It was not until after the soldering iron heating element did snap did i try to solve this, the ultimate solution was to shorten the metal collar that the tip rests against (around the heating element) so the sleeve that holds down the tip sits firmly on the threaded section that the nut screws onto. (i also extended the channel inside the tips with a drill bit, but that was a previous attempt at repair and i cant remember enough to say if it is necessary.) But do this first: the other tips may need some cutting so they extend the same length inside the sleeve (perhaps use calipers here, sticking the back end down the sleeve with the tip inside to determine the required trimming, and trim to the shortest one - i didn't do that, and mines not correct but will do)
recently,
the meter reading began to jump around and the RF meter would flash. I decided that it was time to go fix all the problems i noticed inside.
First i grounded the negatives of the rework, soldering and power supply. the reason why the thing beeped when i connected the power supply to ground was because the insulator under one of the transistors was breached by a burr from one of the holes. I used a drill to remove all the burrs and replaced the TO-3's. Good. For good measure i connected dedicated wires to each one and bypassed the header. couldn't get rid of the 0.1ohm resistor as my new wiring didnt total that. (they have the resistors there because not all transistors are made the same. i should measure to see how equal the current gets distributed. Although 0.1ohm was also recommended by the guy that told me about it, it should work as well with less. At 3 amps, the voltage will drop 0.15V and they will consume 0.45W together)
Next, i didnt like how high voltage was so close to my grounded low voltage. i cut the tracks surrounding the HV area and connected these to earth and used wires to replace the tracks I cut. I also removed the tracks around one of the optoisolators for better isolation and used the resistor here. i could have completely removed these tracks that i have made earths for a nice dielectric gap (as seen on SMPS power supplies) but didnt want lead shavings (i would probably say i dont know enough to determine how much safer it is with the gap). Because the mains goes into the heater with the other wires that go into low voltage areas of the circuit, i couldnt think of any solution other than weaving the earth wire among these before it goes to the chassis.
All done! so i thought.
When i reassembled, the heater fan would stay on. then it went off but at the same time the meter began jumping around again. So there was lots of thought and wasted time over this co-incidence.
after trial and fail over these two problems, i got the pen and paper out and traced the fan circuit. seemed to be Q1. but there was nothing that could cause that to blow, oh well replace it. well it turned out that the vibration loosened it as it was not soldered properly.
That did leave the meter's intermittant fault. I had found during the fan episode that when the meter figures began jumping, U3 (s3e9454bzz, but for the datasheet search for: samsung 9454) would become HOT. The 5V regulator output would drop to 4.8V. When it was quiet again, I loaded the 5V heavily myself, the meter would jump but U3 would not heat. I considered making a special cable for U3 to measure the pin current but not having the parts i was trying to reseat the IC without different pins to remove the trigger. gave up on that. i then decided to measure the current on Vss and Vdd and routed dedicated wires from the unseated legs to the regulator and to the now useful ground. it behaved itself. i gave up on waiting for it to malfunction and soldered permanent connections to the regulator. Its been behaving itself for some days now with no repeat, so i guess i can say i accidentally fixed it.
the soldering iron funciton remains untested because its been misplaced at this moment, but with what i can test, there doesnt seem to be any incompatibility with my earthing fix.
i have passed on a summary of this to Yihua, who knows if they will acknowledge it.
Charlie.
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