Tektronix calibration set (TM503, PG506A, TG501A, SG503) repair - Page 1
Forum section: electronics / repair

author: Pinörkel
author_rank: Regular Contributor
timestamp: April 29, 2024, 05:36:27 pm
content: Hello,due to having been infected by Zenwizard and NFM with a medium severe vintage-Tek-syndrome, I tried to get my hands on a Tektronix calibration set, consisting of a PG506A, TG501A and an SG503, which are quite hard to get here in Germany. Recently five years of waiting payed off and I was finally able to acquire a decommissioned set from an equipment rental service via Ebay in a TM503 in surprisingly well condition. However, a few repairs are needed and I have some questions that I hope will find some knowledgeable answers here. Also, documenting this repair here might help others with getting their units repaired. Since the units look like they are in factory condition(apart from some funny bodges), I will take some high quality pictures of the insides to put on Tekwiki later.At first, it seemed to be impossible to remove the modules from the frame. They were stuck in the slots. To not break the pull tabs, I removed the TM503 top cover and cautiously levered the modules out by pushing from the back side. Is it normal, that the modules fit so tight into the connectors, that they require large forces to remove them?Unfortunately, with the modules removed, one of the edge connector slots in the TM503 revealed to have a damaged contact spring, which was heavily bent and twisted and also damaged part of the green housing. Apparently someone had incautiously inserted a module, smashing the barrier key into the contact spring and killing the spring and the barrier key in the process. With much luck, I managed to bend the contact spring back using some dentist tools(see attached before and after image). Since I could not reach the lower inner part of the spring safely, something in there is still not correct, causing the spring to have way less spring force that the other contacts. I do not think, I can get in there to correct that without further damaging the contact spring. Does anyone know, if those spring contacts can be pulled out of the connector (when de-soldered) or are those injection molded into the socket and cannot be removed? That way it might be possible to correct its shape. Alternatively: is there a known source for replacement sockets, apart from killing another TM500 Frame to salvage one?Apart from that, I had to replace the missing barrier key with a 3D-printed one, which came out quite nice on an FDM printer with a 0.25mm nozzle. If anyone needs the 3D-model, just drop me a note.During trying to figure out, how the slot disaster could have happened, I noticed that the TM501A module in the respective slot did not lock in with a click. This was due to the hook of the locking mechanism being installed backwards(see image). Reversing the hook required disassembling the latch mechanism and loosening the front panel to get it right. Since the hook did not look like a replacement part, I am suspecting a factory fault here. So it looks like, the slightly raised module front in the inserted position due to the reversed hook may have caused the module to damage the connector.After verifying that the pass elements in the frame were not shorted, I checked all voltages in the TM503 and they looked about right. Does anyone know what the expected ripple currents are on those rails? I could not find that in the specifications in the manual.After the frame was checked, I ran a basic test on the three modules to discover that they were mostly working. The PG506A, however, showed some strange readings in standard and high amplitude mode. In standard amplitude mode, the DC voltages were only a little over half as high as expected and the high amplitude mode maxed out at about 30V. So I first checked the power rails in the module (5V, -16V, 16V, 120V, -72V) which were all spot on. I could trace back the strange readings in standard amplitude mode to a bad switch (S2020) in the unit. S2020 switches between a square wave and DC for calibration purposes and just needed some basic cleaning to get standard amplitude working again.To find the fault in high amplitude mode, I searched the respective circuit, which I could not fully understand. My first clue was that the output resistance measured differently, depending on the polarity of the DC resistance measurement (604Ω in one direction, 502Ω the other way). Can anyone verify with another unit that this is indeed not normal? As far as I could see, the -72V rail was fine and setting the amplitude regulated the rail. So the voltage sensing in the high amplitude circuitry should be doing something useful. I put the amplitude on max and measured along the path on which, according to my understanding, -72V rail should feed the output. As can be seen in the attached image nothing too suspicious popped up, except that the drain of Q758 JFET dropped about 30V with respect to the source. Together with the output resistance measurement, I suspected a diode or transistor loading down the output and after some more measurements, the CR810 diode seemed to be the only candidate (and also the only diode in the circuit to show a diode drop in both directions in-circuit). Seeing the location and the voltage rating of the diode, it looks to me like a protection diode which prevents the -72V rail to exceed -80V. Is that for protecting the PG506A from outside threats or for protecting the outside from too much voltage in the PG506A? Unfortunately, the manual only states that CR810 is a silicon diode DA2737 with 80V and 75mA. The PG506 manual mentions an FDH1986 (possibly Fairchild) diode, and I could not find any known replacement types on the net. Does anyone know if this could be replaced with a 1N5282 general purpose diode (80V 200mA)? Does my lousy fault diagnosis sound plausible?Thank you for reading this and for any helpful advice and comments.

author: fmashockie
author_rank: Frequent Contributor
timestamp: April 30, 2024, 08:06:36 pm
content: The PARD (which is periodic and random deviation. that includes ripple) is listed as <= 2.5Vpp in the manual for the TM503.

author: Pinörkel
author_rank: Regular Contributor
timestamp: April 30, 2024, 09:07:07 pm
content: Thank you for the reply and for pointing that out. Shame on me: It seems I downloaded the only version of the manual on Tekwiki (070-1305-00) that did not include that information.

author: Pinörkel
author_rank: Regular Contributor
timestamp: May 01, 2024, 07:57:11 pm
content: Just an update to my issue with the high amplitude mode. I de-soldered the CR810 diode, only to find that it measured OK and was not shorted. Also the asymmetric resistance measurement at the output disappeared, which means that this behavior is most likely normal. The reduced amplitude went away after putting the diode in again and retouching a suspicious looking solder joint at Q758.As a side note: The solder mask does not seem to be very resistant to either heat or flux and gets damaged really quick, even at 280°C. I think, it is mostly a heat issue, since I could observe damaged solder mask around other factory soldered components like the cam switch contacts.