FET Audio | Hi-End Audio & Electronics DIY Electronics & Repair

I won this lot of 5 pcs Tektronix P6247 differential probes about 2 years ago from Seagate Singapore sales. 

1. Out of 5, only two probe tip heads are working and thus I am able to recover two probes in full working conditions after repairs and adjustments.  Thus it is a real bargain for me!

2. One box is with two burnt logic ICs inside and no way I can repair it!  What I can do is just repair the connection cable.

3. Another box is working but with X10 not functioning.  Guess there is some logic defective inside.

4. The other box is with unknown conditions as I have not tested it with a working probe tip head.  Believe it is working conditions as I check the voltage at probe tip socket is normal conditions.

5. All the 5 connection wires were repaired.  The usual defective point is the probe head joining point loose and control wires may be broken or intermittent connection.  I peel off more plastic of the coaxial cable ground and wrap it with some ground shielding cable and then solder the ground wire onto the frame of the tip socket end.  After that wire connection is checked one by one by multimeters.

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This P6247 differential probe is very good for testing high frequency signal as it only has below 1pF capacitance value. It also has x1 and x10 selection, AC and DC mode plus the bandwith limit of 200Mhz.  I use that with a tek 1103 probe power supply and then I can connect it to any measurement equipment with 50 ohm termination like oscilloscope, frequency meter, spectrum analyzer etc…

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Here I put up some photos which will be very easy to understand more about the repair:

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I owned this equipment for few years and got that from ebay Singapore for less than US$500.  The equipment label indicated that this machine was from AMD Singapore lab.  The condition is excellent as it was seldom used in a smoke free environment.  In fact, I want to own a curve tracer long ago because my first job is using Tektronix 576 for discrete part quality control.  Tek 577 fit to my needs for its size, weight, price and higher supply voltages till 1600V!

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Here I use 577 to demo the measurement of jfets 2SK363BL (or 2SK369BL) that I am still selling right now.  The horizontal is 5v/div and thus 5 division is about 25V to the middle of the screen.  The vertical scale is 2mA/div.  Thus you can see the idss value by multiple the div to 2mA easily.  In my experience, due to its scale is on the screen with 0.2div resolution, thus the accuracy of the measurement is about 0.2/8 (max number of division) = 2.5%.  The top line is Idss, the line below top line is Idss at – 1 div or -0.1V and so on…  From the measurement, the Yfs is more than 50mS at about 8mA and this show that the jfet is well within specification.

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If you observe the curve carefully, and calculate the Yfs using the -0.1V and -0.2V line, you will get a lower Yfs.  That means the Yfs decrease with working current (or bias point).

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Another idea is use this machine to measurement vacumn tube (valve) but a fixture with safety precautions must be built as the it has very high voltage and power up to 100W!!! Take you own risk as I am not liable for that!

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HP 1660EP Logic analyzer

I recently acquired this machine for a reasonable price in Singapore.  The set did not bootup properly when received due to hard disk failure.  After replacing the hard disk, the set is able to power up, passed all performance verification tests and function normally.  The model of IBM drive I use is Travelstar DJSA-210 6.0GB.  I also tried one Toshiba 6G harddisk but the 1660EP refuse to detect the harddisk. Thus I strongly recomment only IBM brand hard disk should be used.

There are few difficulties in using this old equipment.  Firstly is to replace a old hard disk with about 2.1G to 4.3G size.  So the only place is to look into old computer shops (local junk yards or ebay).  The machine is able to use only 2GB of the hard disk space even if the original size of HD is larger than 2GB (may be OS/firmware limitation).  Secondly, the software from hp website has to be loaded into the hard disk from the floopy drive.  It is not easy to get diskette and copy the download files onto it.  I manage to get a used IBM USB floopy drive for cheap price and NOS box of floopy diskette to complete the job easily.  Thirdly, the ROM update has to be done by floopy drive and it is not possible to boot up the machine once the ROM data is erased.  Thus do not erase the ROM content if you are not sure if the floopy or floopy drive is working perfectly.  Lastly, if the floopy dirve need to be replaced, the source is from some old notebook drive which can be very cheap at scrap cost.  Model is Teac FD-05HF and the same model is used in those TDS744 series digital oscilloscopes.

Some photos below:

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After a bit study, I install a X-server cygwin on my PC and then I can link the 1660EP X-window to my computer screen.  It is not that reliable due to some wild character transfer and breaks the connection. 

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I have owned this equipment for few years and recently I find that the Digital input from BNC for channel I and II is not working properly.  The symptoms are summary as below:

1. Channel I level cannot detect and thus refuse to do any measurement.  Channel II can detect signal and do measurment.

2. The 75 ohm and Hi-Z switch is not functioning.  For XLR input, the 110 ohm or Hi-Z is able to be selected and hear relay “click”.

3. When Gen Mon is selected, no measurement for both channels.

After checking the signal path of all digital signals using the generator as the digital source, it seems that the circuit is all working fine.  On the front panel connector pcb, there is a MAX310 multiplexter to select different input connector and signals.  Pin 15, 16 and 17 are address input A0, A1 and A2.  In XLR mode, the voltages are solidly at ground level but at BNC mode, the pin A0 is like floating. 

After spending hours of tracing the path (It is a multi layer pcb and thus the trace is not on top or bottom and I do not have a service manual of this equipment!) from the connectors of the main board, it was found that the address bus A0 to A3 is connected to an IC 74FCT16374AT – Pin 2, 3 and 5.  Since XLR mode is working, thus there is no connection issue for this three pins. Pin 1 of 74FCT16374AT is the Enable pin of these three output address and thus I use a scope probe pin to check on this voltage.  When I touch the pin 1, I hear a relay “click” sound and then the measurement of input digital signal is back to normal.  When I remove the probe, the issue is back again!   After I check connection of pin 1 using multimeter, the pin seems to be not connected to the pad due to dry joint (visually I cannot see any issue!).  So the problem is removed after touching up the pin 1 of this IC with some solder paste.  Here I attached a photo for those people who may have similar problem as my equipment.

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I owned this Tek scope since 2007 but the scope was burnt fuse in 2009 April when I moved it to a new rack for usage.  Since then I tried to repair the power supply but was not successful without a schematic.  Later in 2010, I was able to find a TDS520B component service manual in which there is a LV supply schematic.  The schematic is not exactly same as my scope’s supply revision but is 85% similar.  Thus I am able to trouble shoot and located the faulty part Q5 – 2SK1018 booster mosfet.  In fact Q5 and Q6 are running in parallel and thus I try to replace Q5 with IRFP460 but it was burnt again after power up.  This inducated that some more parts are faulty!  Thus later I found out that R1 and R2 gate resistor of Q5 is at high resistance and thus I have to replace it with dale 1/2 W resistors 10 ohm.  I ordered some IRFP450 from RS component and is able to delivery in same day.  After replacement, the supply never burnt fuse and works again.  See some internal photo for the TDS744A.

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