Products > Test Equipment
Accurate Low Amp Current Probe Advice
KungFuJosh:
I removed and tested the 3 caps from the PSU, and...it seems pointless to replace them. They look new, I don't know if they're original or not. I assume not original, because there was some flux residue on the PCB around them.
Also, the ESR is super low. I can't get anything lower, so what's the point if they're not bad.
Does anybody else have a TM502A they can check the PSU caps in? I'm curious what's normally in there.
Thanks,
Josh
jonpaul:
KunfFu: In the lab since 1990s, we have many TM50x MF , AM503A, AM503B, AM503 and probes, manuals.
Can you say what AC/DC measurements you make and what accuracy/resolution you expect?
Happy to have contact via PM.
Have an absolutely fantastic day,
Jon
KungFuJosh:
--- Quote from: jonpaul on November 11, 2024, 04:32:21 am ---KunfFu: In the lab since 1990s, we have many TM50x MF , AM503A, AM503B, AM503 and probes, manuals.
Can you say what AC/DC measurements you make and what accuracy/resolution you expect?
--- End quote ---
Hey Jon!
Accuracy of around 1% is fine. Resolution isn't that important; 3 or 4 digits is plenty.
Messing with this right now is mostly education/entertainment. I was going after different caps to see about improving noise and stability. Stability is pretty decent now.
I took a look at the PSU electrolytics to see if any further improvements could be made with those, but as I posted above, it doesn't look like those caps are a problem. They actually seem quite nice.
Thanks,
Josh
MarkL:
I noticed on the AM503A schematic there was a manufacturing test serial port. On the AM503B, this serial port is connected to the GPIB option connector. Using the infinite monkeys method, I did some poking at this port and was able to figure out most of the internal commands to control the AM503B, as would the GPIB controller card.
My goal was to find a way to read and write the cal constants without having to modify the board or start pulling chips. If GPIB could do it, then it must be happening through this serial port. (Hint: It's the "CC" command). Along the way I also found most of the internal commands used by the GPIB option card to control the AM503B and it's twin, the AM5030.
There's probably other things I haven't discovered, but I found what I needed so I'm not going to invest much more time in this. At most I will probably write a Linux bash script to make it easier to extract the cal constants for backup, and also allow a user to update the cal constants based on the procedure found in the service manual.
Below are my notes. Comments/corrections are welcome from anyone who wants to give this a try. I'll also post this to the TekScopes group as it may be of interest there too.
AM503B Internal Serial Port Notes
---------------------------------
11/11/2024 - MarkL @ eevblog
The serial port on the AM503B's internal processor (a P87C528),
communicates with the optional GPIB controller card, and this is where
we can connect and issue text commands and receive text responses.
It's not GPIB commands at this interface, but is closely related in
functionality, parameters, and naming.
The interface could be the same as "MFG_DATA_IN" and "MFG_DATA_OUT" on
the AM503A schematic, but I don't have a AM503A to test if the same
command set is in use there.
Some of the remaining mysteries below could probably be figured out by
eavesdropping on the internal GPIB controller header on an AM5030. An
AM503B with the GPIB option card is the same as an AM5030.
References (all are on https://w140.com/tekwiki):
- AM 503B & AM 5030 AC/DC Current Probe Amplifiers Instruction Manual,
publication #070-8766-05
- AM 503B, AM 5030 & A6300 Series, 067-0271-00 Verification and
Adjustment Kit, publication #070-9352-01
- AM 503A Current Probe Amplifier Service Manual, publication
#070-8174-01
- AM503A_Schematics.pdf, unknown
Internal GPIB Controller Connector
----------------------------------
- 2x10 header directly to the right of the processor
- Serial port is 5V TTL level (not 3.3V!)
- Header pins to serial adapter:
16: Rx (yellow, data coming from AM503B)
14: Tx (orange, data going to AM503B)
12: GND (black, there are other GND pins: 2, 3, 11, 20)
Notes: Header square pad is pin 1 (lower right)
Colors refer to FDTI TTL-232R adapter cable
- 4800 baud, 8 bits, no parity, idle high (the usual)
Commands and Queries
--------------------
- A command is two characters followed by optional arguments.
- A query is two characters followed by a "?" and then optional arguments.
- Not all commands have a query format and visa-versa, but if they do
the query form generally shows what was last set via the command
form.
- All numbers are ALWAYS HEXADECIMAL, including error responses.
- Everything is ALWAYS UPPERCASE.
- Commands/queries are terminated with a single carriage return (0x0D).
- Responses are terminated with a single carriage return (0x0D).
- Query forms (xx?) below show an example response, current settings
will vary the response.
AD?: AD (unknown meaning, appears to be constant)
AMx: Set amps per division x=[A-L] (range steps 1mA/div to 5A/div)
AM?: AMB
BWx: Set BW limiter x=[0,1], toggle if x omitted
BW?: BW1
CCrr+xxxx: Set gain cal constant rr=[00,01,02] to xxxx.
CC02?: CC02+61A8 (0x61A8 == 25000, which is the default)
COx: Set coupling x=[A,D,R] (AC, DC, Ref), toggle if x omitted
CO?: COR
DAxxx: OK (unknown effect, xxx is optional and any length)
DC+xxxxx: Set DC level to xxxxx
DC?: DC-00172 (was set to -2.3 from front panel)
DC?: DC-00162 (was set to -2.2 from front panel)
DC?: DC-00012 (was set to -0.1 from front panel)
DC?: DC+00002 (was set to 0.0 from front panel)
DC?: DC+00012 (was set to 0.1 from front panel)
DC?: DC+00142 (was set to 2.0 from front panel)
DC?: DC+00000 (after initialize)
DEx: Perform degauss operation x=(0, 1, 2):
0 or omitted: Just like pressing front panel button
1: Forces gain calibration (see manual)
2: Gain calibration without balancing Hall device (see manual)
EX: Exit error mode (e.g., 266 being displayed on front panel)
FPx: Front panel lock x=[0,1], lock if x omitted
GP?: GP050 (unknown, appears to be constant)
ID?: ID3.0 (returns firmware version)
IN: Initialize
KYxx: Front panel button/knob simulation(?) xx=key to press, 00=release
(not too sure how this is used)
KY?: KY00
OV?: OV0 (status of front panel OVERLOAD light)
PO?: PO0 (status of front panel PROBE OPEN light, returns "p" when
open, huh?)
PT?: PTA6302 (returns probe type)
SNxxxxxxx: Set serial number to xxxxxxx, set to null if xxxxxxx omitted
SN?: SNB012345
TE?: TE000 (first error from self test, 000=no error)
TE?: TE165 (0x165 == 355 dead battery, NVRAM reset)
TMxx: Run test xx, see AM503A service manual, examples:
00 = blink all LEDs
02 = display firmware version on front panel
TRxxx: Set probe trim to xxx
TR?: TR42D (was set to 069 from front panel, 0x42D == 1069)
UN?: UNA (returns output units, A=amps, V=volts, see manual)
Some Responses Seen
-------------------
- Many of the error message appear to be analogous to the ones listed
in the GPIB section in the manual (not surprising).
OK: Command executed successfully
KP: A front panel button was pushed or knob turned
"KP" and other key press events appear to have the 8th bit set
PW: Power up (8th bit set)
ER065: 101: Unknown command
ER0FC: 252: AM argument out of range
ER105: 261: DE argument out of range
ER10A: 266: Auto-balance not 50 ohm terminated
jonpaul:
Best amp is TEK AM503B.
Look at epay, ham fleas, auctions.
TM50x MF not critical, of the dozens we have just a few were bad, mostly blown pass trans, the lytics hold up.
The AM503 are not stable and low res.
AM503A is not grat but better than AM503.
Lots of bad probes ....dropping a TEK P6302 a few cm on concrete/metal may hariline crack the ferrite jaw.
Symptom is poor LF/DC or unstable...Beware.
j
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