Tektronix 492 Spectrum Analyzer--multiple PSU issues. - Page 1
Forum section: electronics / repair

author: N2IXK
author_rank: Frequent Contributor
timestamp: October 23, 2018, 04:30:29 pm
content: Have been going over a recently purchased Tek 492 SA, and the unit was found to have multiple issues related to the PSU, which is common to other units in this series, including the 496 and /P and /AP variants. Thought I would share what I found in case anyone else is working on one of these beasts.The unit was somewhat working as delivered (it made a trace and most controls responded appropriately) , but WAY out of calibration in both center frequency and amplitude. Checking all of the supply rails at the test points on the Z-axis board showed that the +300V rail was almost 75 volts too low, and most of the lower voltage rails were also out of spec and loaded with high frequency noise. The unit would also trip the GFCI protecting the workbench at random intervals, whether it was turned on or not.Opening up the PSU showed several electrolytic caps that had obviously overheated and melted their plastic sleeves. I went ahead and replaced EVERY aluminum electrolytic cap on the board with low ESR Nichicons, rated at 105C. For the sake of completeness, I also replaced the 2 primary side reservoir caps (750 uf/200V). These were originally the "twistlock" type, but I found some Nichicon 820 uf snap-in types that were able to fit the existing holes in the board. There were also 5 of the infamous RIFA film capacitors and 3 RIFA "snubber networks" (0.1uF cap and 100 ohm resistor in series) in the AC line circuitry, several of which were showing the typical cracking of the plastic case. So they were all replaced with modern equivalents.The original Tek cooling fan was a brushless 3 phase type, operating from the -17V rail with a small driver board providing the phase switching and temperature sensing/speed control. A previous owner had replaced it with a cheap 12VDC fan which was operating VERY slowly and hardly moving any air. They had it wired into the original fan control board, which wasn't providing sufficient voltage to run it properly. Because a proper OEM fan was unobtanium at this point, I found a 24VDC EBM/Papst unit of the same physical size which provided equivalent airflow (~30 CFM), and powered it from the +9V and -17V rails that fed the now unused fan control circuitry. Some online documents that talk about suitable fan replacements for these units show the fan control board being completely removed, but a look at the schematic showed that the board also housed the OVP circuitry for the +5V rail, so it was kept in place, with just the fan related circuitry being disconnected.After all this work was done, the unit was reassembled and powered up, and the calibration appears just fine!But the intermittent GFCI trips were still happening. A look at the schematic showed that the only possible cause left was the IEC input filter module, which has a couple caps inside it which go from the line to earth ground. The original unit was made by Sprague, but I replaced it with a physically equivalent Corcom unit, and the GFCI trips seem to have stopped.The only other issues I see are a loose optical encoder for the center frequency adjustment (2 of the 3 attaching screws appear to be missing) and a front panel power LED that doesn't light. Both of these will require removal of the CRT and front panel from the unit, then removal of the PCB from the panel to gain access.

author: sbvr4
author_rank: Contributor
timestamp: October 24, 2018, 08:16:01 pm
content: I'd highly advise on replacing all of the tantalum caps as well. I just repaired a unit that was inoperable due to one cap that shorted the +15v rail. Looked totally fine otherwise.

author: shakalnokturn
author_rank: Super Contributor
timestamp: October 24, 2018, 10:42:53 pm
content: I have a long term tantalum issue on my Tek 2756P, mainly due to laziness, I know the right answer would be to flog it and buy a HP S.A.Does anyone know if the 1st L.O. can contains a tant with series resistor? Or should I start looking for a replacement YTO assembly?

author: KE5FX
author_rank: Super Contributor
timestamp: October 24, 2018, 11:31:24 pm
content: Quote from: shakalnokturn on October 24, 2018, 10:42:53 pmThe 2756Ps are nice units, well worth fixing up. It will be expensive to replace with a newer analyzer from HP (or anyone else) that will do the same things. I still use the benchtop version (494AP) as a selective frequency counter, myself.The YTO itself is a sealed assembly that just contains the hybrid oscillator components and the YIG tuning coils. As long as it puts out a clean signal, it won't need any intervention. There are caps on the external PCB that's attached to the oscillator, though. There's a filter cap that may or may not be tantalum (the photo athttp://w140.com/tekwiki/images/0/0a/Tek_2756_internal3.jpgseems to show an aluminum one). The other electrolytics are paralleled to form a low-ESR LPF for the tuning current, and while they should be checked while you're in there, they will not be responsible for shorting the power supply directly.Quote from: N2IXKRecapping the whole PSU was probably a good idea given that it had been running hot for an indeterminate period of time. Assuming you used new parts with (e.g.) DigiKey/Mouser part #s, if you'll post those I'll update the service notes document. Good point on the +5V protection circuit as well, I never noticed that myself.

author: VK2IPD
author_rank: Newbie
timestamp: January 10, 2020, 10:14:56 pm
content: Hi sbvr4, My Tek-492 has exactly the same problem. I isolated the power supply and checked, all voltage outputs are fine. When plugged in, the +15v rail is shorted. I've removed all the add-on boards and left just the Power supply, Motherboard and the Front Panel Board in place. I can confirm that the +15v rail is shorted and mostly from a fault (cap?) on the Front Panel Board. It's going to be very hard to remove this board.Do you remember which capacitor you replaced in which board to rectify the fault (+15v shorted rail)?

author: F5EFD
author_rank: Newbie
timestamp: March 09, 2021, 02:58:00 pm
content: Hello, My TEK-492 does not run completely its start sequence.Only some lights on the front panel,but no reactivityand no switching noise at powering on.No short circuit detected on power supply capacitors . Voltages on test point next to the CRT are good but it seems that the cards are not supplied .Any help please.Thanks