| Electronics > Repair |
| Datron 1271 repair |
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| rigrunner:
A while ago a picked up a faulty Datron 1271 and have been meaning to post repair details for a while. I recently built an LTZ KX reference and measuring this was enough to be reasonably certain that the 1271 is working as it should. Most of this is from notes i've made during the repair but some is from memory. The 1271 was offered on the usual auction site as "Powers up but unresponsive front panel" by what appeared to be an individual seller. Once i'd won the auction I found out that the seller was an employee of a calibration house that offered repairs. If I had know this before hand I probably would have bid less, but hindsight is a wonderful thing. I'm under no illusion with used electronics from auction sites, I treat pretty much everything I buy from ebay as a parts only purchase. Initial inspection The meter powered up as expected with the defaults: DCV, 1KV range 6.5 digits resolution. Applying 10V to the terminals showed a sensible reading and a short showed close to 0V. Other than the power switch there was no response from any of the front panel buttons. After a few minutes there was an interesting smell that was reminiscent of schaffner AC filter so it was powered off quickly and the covers removed for a closer look. The first thing I noticed was the that mains earth connectors were all disconnected due to a missing nut and the rear panel was loose because the retaining screws weren't fully tight. A sure sign that someone has already been inside previously :( The IEC mains filter and plastic shroud were covered in thick ooze. The Waycom WF 120-3/05 filter appears to have been leaking into its shroud for some time. This was replaced with a Roxburgh RID-0642-H. The earth connection between the filter and the chassis is by exposed braid so I placed shrink wrap over the live and neutral terminals as an extra safety precaution. The earth connections were placed on to their post and secured with a new nut. |
| rigrunner:
Fault finding The Digital assembly power supplies were measured and found to be within specification. (I can't find my notes for these measurements :( ) Test points are: TP501 = -14V TL502 = 5V TP502 = 45V TP504 = -14V (unregulated) TP503 = 0V reference My first check for the keyboard fault was to remove the fascia and check the front panel buttons were actually connected and were contacting when pushed. The buttons were all fine, the repair wasn't going to be that easy. The signal from the keys input the display PCB through ribbon cables via SK22 & SK23 and are fed to a 74C922 key encoder. The keyboard signal leaves the display panel through PL21 and enters the digital assembly via PL2. Line KB5 acts as a clock signal to flip-flop U302 and signals U203 CLA3721 ASIC that a key is pressed by pulling FP_INT_L low. All 6 keyboard lines KB0-KB5 are fed through buffer U301 74HCT244 and placed on the 8 bit BUFFERED_DATA(0:7) bus when latched by RD_FP_L signal generated by PAL U111. I traced these signals and the keyboard input was being read and placed on the lower 8 bit bus but still no response to a key press from the system. At this point i thought the ROM might be the issue. The meter is quite old and bit rot is not unheard of. I pulled the ROMs U103 and U104 and read them several times checking the read data for differences between reads. One ROM produced bit errors at 5V but read ok at 5.5V. I burned a couple of new ROMs from the data and tested the 1271 again. No change, it still ignored keyboard input. I hooked up the GPIB port to see if that worked. Sending commands to the 1271 caused Rem to illuminate on the main front display and subsequent commands caused a 68000 bus error displaying error "9002 - Unexpected Exception" on the secondary display and locked up the 1271. The only thing i hadn't checked at this point was the SRAM. Swapping U105 and U112 caused the 1271 to run for a little while and then crash, but whilst it was running it did respond to the keyboard! After replacing U105 and U112 with a pair of NOS M5M5256BP SRAMs the 1271 powered up with everything functioning – success. |
| rigrunner:
I replaced the power supply electrolytic capacitors as a matter of course. The replacements I used are in the table below. Note that if you choose to replace with different parts that space is limited and you won't be able to install anything taller than 25mm. ID Original New Farnell # C501 1000uf 40v Panasonic ECA-1HHG102 1000uF 63V 9693009 C502 470uf 63V Panasonic ECA-1JHG471 470uF 63V 9693068 C510 220uf 40V Rubycon 100YXJ220M12.5X25 220uF 100V 2346277 C513, C514 2200uf 16V Panasonic ECA1EHG222 2200uF 25V 9692851 C515 470uF 25V Panasonic ECA-1JHG471 470uF 63V 9693068 C520 330uF 100V Nichicon UVZ2A331MHD1TO 330uF 100V 1822727 The digital assembly withdraws from the main chassis complete with the rear panel. To remove: Unplug the pcb cable plugs from their sockets Remove the two plastic edge covers Remove the 4 countersunk screws located at the corners of the rear panel Remove the 3 securing screws along the centre of the rear panel taking note that one of the screws is shorter than the others. Withdraw the digital assembly from the chassis, keeping the terminal release bar above the components as you pull the assembly. After the capacitor replacement the voltages were measured as: TP501 = -13.92V TL502 = 5.00V (3.3mV RMS) TP502 = 47.03V (107mV RMS) TP504 = -14.164V |
| rigrunner:
With the digital assembly now fully working i could run a full self test. This failed with one error 2863 which points to the Ohms assembly. The service manual says that 2863 is a Gain Drift error - Digital comparison of the present Gain Magnitude against that recorded at the most recent Internal Source Cal 0.999,000 < Drift Ration < 1.001,000. Mickle.T suggested that this might be sign of long term drift and performing an internal source cal might be the solution. The internal source cal had an effect in that it caused the self test to pass for a short period after warm up but the self test failed when cold and also after a long power on time. The service manual shows pathways against each test condition and associated errors. Switching in pathway 53 and monitoring the displayed value showed an upward creep over time that didn't seem to stabilise no matter how long i left the 1271 powered on. I monitored a few points on the Ohms assembly around the constant current sink and found the voltage on the gate of Q207 was drifting upward slowly. I replaced R216 10M? 1% 0.12W 100PPM and D203 1N458A which stabilised the voltage at Q207 gate. After another internal source cal was performed the 1271 passed self tests continuously once warmed up. |
| rigrunner:
With everything seemingly working it was time to inspect the other boards. A pair of FETs, Q114 & Q119, on the AC assembly looked a little crusty with flux. I removed these, cleaned the board and reinstalled them. I also replaced C133 100uF 40V whilst the board was out. The DC assembly also had a pair of crusty looking FETs, Q206 & Q207. The flux around the leads of these seems to have eaten into the top of the PCB a little. I removed them both and cleaned the board around them before reinstalling. The final task was to replace the electrolytics on the Digital assembly. I think i was just in time with this as the large electrolytics had started to leak. C004 was the worst but all of the large caps showed some electrolyte leakage. The replacements I used are in the table below. Note that if you choose to replace with different parts that space is also limited here, but not as limited as with the Digital assembly. I stuck with 25mm capacitors as per the digital replacement parts, but you can probably just about squeeze in 30mm. ID Original New Farnell # C001, C004 1000uf 40V Panasonic ECA-1HHG102 1000uf 63V 9693009 C007, C012 220uF 63V Rubycon 100YXJ220M12.5X25 220uF 100V 2346277 C015 2200uF 16V Panasonic ECA1EHG222 2200uF 25V 9692851 C912 470uF 25V Panasonic ECA-1JHG471 470uF 63V 9693068 C002, C003, C005, C006, C009, C010, 1uF 63V Wurth WCAP-ATLL 860160672002 1uF 50V 2495115 C011, C014, C016, C017 C309, C313, C314 10uF 50V Wurth WCAP-ATLL 860160672009 10uF 50V 2495117 After the capacitor replacement the voltages were measured as: TL001 = +15.215 (0.4mV RMS) TL002 = -15.090 (0.4mV RMS) TL003 = +35.542 (0.4mV RMS) TL004 = -35.093 (0.4mV RMS) TL005 = +5.038 (0.9mV RMS) 0V from U002 centre pin or heat sink. With a little help from ManateeMafia I determined that the calibration key is this one from RS “Key for Operated Switch” RS Stock no.: 319-792 All that is left to do is find a way of dumping the calram and convert to f-ram with one of ManateeMafia's converter boards. Finally a big thank you to Mickle.T and ManateeMafia for answering the questions i had whilst working on this :) |
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