Tektronix 1502 TDR - Page 1
Forum section: electronics / general

author: Jon Chandler
author_rank: Frequent Contributor
timestamp: September 20, 2013, 04:50:04 am
content: The local thrift store has a Tektronix 1502 Time Domain Reflectometer on sale for $100. It's clean and in beatiful condition and I would have snatched it up in a minute, but there's no function at all when it's plugged in and switched on.I was thinking about this. It's designed to run from an internal battery pack. I wonder if totally dead batteries would keep it from running from line power.I'm hesitant to invest a hundred bucks with no indication that it's working. The fuses look good. Maybe no-power is a sign that any needed repairs could be simple.Does anybody here have experience with this unit? I have seen a few comments that the tunnel diode used to generate the pulses is easily damaged. Is there any replacement if this is the case?Should I be rushing back to buy this?

author: TerraHertz
author_rank: Super Contributor
timestamp: September 21, 2013, 05:45:39 am
content: http://w140.com/tekwiki/wiki/1502has good info on the various unobtainiums in the 1502.Yes, dead battery does indeed stop it from running.

author: tekfan
author_rank: Frequent Contributor
timestamp: September 21, 2013, 06:39:54 am
content: The only way to operate it without modifying the power supply is to connect a external power supply directly to the battery terminals inside the case. If you have a variable supply just connect it to the terminals and turn the TDR on, then increase the power supply voltage until the small battery indicator on the front panel is in the green range.As for the tunnel diodes, I've successfully replaced the two faulty diodes with russian diodes from a seller on ebay.I used a 1I308E as the 20mA and AI201V for the 10mA diode. I managed to solder the 20mA russian diode to the small heatsink of the original diode. This will provide better heat dissipation and a lower inductance connection to ground. It is very tricky to do this as you must apply as little heat to the tunnel diode as possible to prevent it from failing. Testing it without the heatsink produced almost the same pulse response, but you don't know how much that thing's heating up. I also found the spring loaded contact in need of a good cleaning. The oxide on it prevents good contact with the diode and causes some very strange waveforms. The 10mA diode is easy to solder in.If you look into the BNC connector you will see a small shorting link which shorts the center pin to ground if there is nothing connected to it. This is the reason the TDR will show its port shorted rather than open. The BNC is supposed to discharge the cable and prevent the charge stored in the capacitive cable from damaging the tunnel diode. This works, but if there's a signal of more than a few volts on the other side of the cable it will fry the tunnel diodes.There are also two pickup diodes in a single ceramic package located somewhere between the output and the tunnel diodes. These are the part of the sampling circuit that captures the very fast risetime (<140ps) of the outgoing and incoming pulses. I don't have any information on these but it seems that they should also be replaceable with new fast signal diodes of the correct type.Links to the ebay diodes:http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=200801163008&ssPageName=ADME:L:OU:US:3160http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=200788034167&ssPageName=ADME:L:OU:US:3160

author: Jon Chandler
author_rank: Frequent Contributor
timestamp: September 21, 2013, 08:27:32 am
content: Thanks for the info! I don't know that Ineeda time domain reflectometer, but it seems like a nice thing to have. I think I'll see if it's still there.

author: Fraser
author_rank: Super Contributor
timestamp: September 21, 2013, 09:29:17 am
content: I own one of these. My battery pack has failed and sure enough the unit will not function. It is otherwise in perfect working order. My unit is sat in my lab, unused for years. They are interesting but I have never had a use for it. They are very specialist in their application, unlike some other weird test equipment that may be used for alternative tasks with some modification. Be certain that it is truly worth $100 to you. These do sell for reasonable sums when working so you may turn a profit selling it in with a rebuilt battery pack ?

author: SeanB
author_rank: Super Contributor
timestamp: September 21, 2013, 09:49:08 am
content: I could use a TDR meter for cable fault finding, very useful in doing network repairs and phone fault finding. Currently the tone and probe works well, but being able to tell where the break is with ease will make finding the faults a lot easier.

author: Fraser
author_rank: Super Contributor
timestamp: September 21, 2013, 11:37:06 am
content: I have a fluke networks DSP series commissioning meter kit that includes a TDR for CAT5/6 cable. I think its a little different to the Tektronix 1502 in terms of capabilities, and far superior for such tasks. Never used it in angerJust another impulse buy. For telephony TDR work the UK is awash with Ex British Telecom cable testing TDRs at give away prices. Again, these are far more advanced than the 1502. Sadly the 1502 is now a bit of a museum piece.

author: Jon Chandler
author_rank: Frequent Contributor
timestamp: September 21, 2013, 04:15:13 pm
content: Thanks for all the great info! I suspect that it would work fine if I connected an external power supply. But like Aurora said, even if the works perfectly, I can't see any applications where I'd use it.I think this can be a project for somebody else. I'll print out a note with some of these links to help out whoever does buy it.

author: woodchips
author_rank: Frequent Contributor
timestamp: September 22, 2013, 11:01:47 am
content: Umm, aren't the ex-British Telecomm TDRs more like the 1503? The 1502 is a close range unit, metres, the 1503, and telephone cables, require hundreds of metres. Having said that the Cossor TDRs seem to imply a distance resolution similar to the 1502, about 8mm.Useful for finding shorts and opens on complex PCBs, and measuring impedance changes at 90 degree track changes, see other thread. Most of all as a spare tunnel diode for your favourite Tek scope.

author: w2aew
author_rank: Super Contributor
timestamp: September 22, 2013, 04:29:41 pm
content: I'm not sure if the 1503 power supply is similar to the 1401 spectrum analyzer module supply/charger, but.... In that case, a bad/shorted battery pack would prevent operation from the external line cord connection. In looking at the schematic I found that replacing the faulty pack with a large electrolytic cap was sufficient to allow me to use line power.