7L12 - No trace/readout - Page 1
Forum section: electronics / repair

author: Taelonious
author_rank: Contributor
timestamp: May 13, 2020, 07:28:41 am
content: Howdy,I stumbled upon this spectrum analyzer along with a 7603 mainframe. Sadly, it shows no trace or readout at all.Other plugins work in the 7603.Absolutely no background on it (supposedly from an estate sale).Followed the manual for operation and had no luck.Looks happy and clean from what I can tell. Lights up and doesn't smoke. Brief look at the boards show no signs of explosion or anything else anomalous (I didn't look too deep though).The frequency span knob does look like it took a whack, as the pull out bit is slightly bent.I'm only a lowly EE student and have no experience with such a repair, but thought it would be fun to try. Any help/advice is greatly appreciated!

author: wn1fju
author_rank: Frequent Contributor
timestamp: May 13, 2020, 12:01:54 pm
content: I wish I had better news for you, but you have stumbled across one of the most complex and densely packed 7000-series plug-ins Tektronix ever produced. I have the 7L13 which is even worse (it is three bays wide) and it was not easy to work on.It would be nice if you had a set of extender cables so you could work on the thing outside of the scope chassis. Otherwise, you need to pull of both side panelsof the 7603 (and the 7L12) and work your way through to the left and right sides of the 7L12.There are a couple of power supplies inside the 7L12, so check the voltages first. I would also be suspicious of mechanical damage as you have indicated with thefrequency span knob. These knobs usually turn a large cam switch inside and I've experienced a case where the knob took such a hit that it actually dislodged the cam and moved it out of position. You can dissect and inspect the cam with the unit out of the scope and on the bench. Always look for mechanical problems before attacking electrical problems. Clean the edge connectors and reseat. The fact that you aren't even getting a readout hopefully points to a more global problem.If you don't already have the schematics, download them. You might also consult the Tektronix newsgroups (search for Tektronix groups.io).To accurately diagnose and calibrate such a beast probably requires a full suite of test equipment (scope, voltmeter, spectrum analyzer, counter, etc.). Maybe you can borrow some items from your school? Attacking the RF portions usually requires disconnecting the various SMB cables and applying them to your test equipment. The signal flow follows the schematic and is fairly easy to understand.As I implied, this is not a good piece for the novice. Difficult, yes. Impossible, no. Good luck!

author: Taelonious
author_rank: Contributor
timestamp: May 14, 2020, 07:12:41 am
content: Oh, I knew I was way over my head before even getting the thing. Still had a good laugh when I first looked inside. It'll be fun. I had just picked up a OS-245(P)/U when I found this and so began the deep dark hole of owning a 7000 series.I did notice some unpopulated pins at P1050 (1 and 2) and P1035 (1 and 2). They appear to go to a variable resistor that would be on the end of the frequency span/div cams. I also noticed a capacitor (C1215) that looks like it was dinged by the part being removed.I've been going over schematics from TekWiki and searching around for other information. One of the first posts I read (but now can't find) mentioned the 7603 not displaying anything if the mainframe wasn't getting any horizontal input. Although, before testing the power supplies or horizontal outputs I should figure out the story of this seemingly missing part.For the extender cable, I'm hoping I can build one or find one I could borrow. I live near the the Tek museum and was considering bothering those folks for help as well as going to my school, but given the times I'm on my own for now. My ability to read schematics is at the "see BJT run" level. It's going to be a long project for sure.I attached a pic of the suspect area.Thanks for the input!

author: Haenk
author_rank: Super Contributor
timestamp: May 14, 2020, 11:58:16 am
content: defect part bottom left in picture (white part with two stripes, I assume it's a cap) - but somehow does look out of place with no visible solder pads, gone missing somewhere else?

author: wn1fju
author_rank: Frequent Contributor
timestamp: May 14, 2020, 12:30:21 pm
content: Looks like you are zeroing in on the problem. From the pictures on the Tek Wiki,http://w140.com/tekwiki/wiki/7L12,there is a missing pot (variable) on the rear of the span/div cam switch. Did you remove it or was it already gone? And the capacitor at the end of where the pot would be is definitely cracked. The Tek Wiki pictures show a big black cylinder at the end of the pot - not sure exactly if that is part of the unlocking mechanism. But I can see the cap getting dislodged by removing the pot.Similarly, the missing P1035 and P1050 lines can't be good. In particular, I looked very briefly at the schematic and if P1035 is disconnected, then the op-amp U1260 won't act right and that ultimately goes through the span/div resistors to the YIG driver. The YIG is the basic oscillator in the whole piece, so you will get nowhere fast without that working properly.Looking at the schematic, you might be able to tack in a small pot and see what happens. Probably should fix that broken capacitor too.

author: wn1fju
author_rank: Frequent Contributor
timestamp: May 14, 2020, 01:07:48 pm
content: Actually, the more I look at it, that missing pot doesn't seem that crucial since the thing normally is in CAL mode, not Variable mode. So you might be able to hook things up (if they aren't already) so that the U1260 pin 3 goes to P1035 pin 3.

author: Taelonious
author_rank: Contributor
timestamp: May 14, 2020, 04:43:03 pm
content: Quote from: Haenk on May 14, 2020, 11:58:16 amYeah, the cap was connected in parallel directly to the resistor below it. They apparently belong to an op amp labeled U1210C. Some sort of LPF(I think?) on the sweep generator & holdoff board. It's output is A3 and B3, which I believe belong to the A and B sweep connections. I'm not familiar with these type of caps, I was guessing it wasn't happy.Quote from: wn1fju on May 14, 2020, 12:30:21 pmHa, yeah. How it looks is exactly as I got it. I looked at that picture when I started getting suspicious of the unconnected pins and noticed the missing pot. Wasn't sure what it was for or why it would be missing.P1050 seems to go to S109B (Var freq span). Not a clue what that's about.If I'm understanding the pinout correctly, pin 3 is connected on P1035 to P1037.The missing pot is interesting. The cap looking bashed really makes it seem like the pot was pulled for some reason. Although when looking at the picture from TekWiki, the connector is for 3 pins/wires, but in mine it's only a single pin connector for pin 3 instead of the 3 pin connection. Strange.Very cool beans. Some things to poke at. Again, thanks for the input!

author: texaspyro
author_rank: Super Contributor
timestamp: May 15, 2020, 01:21:39 am
content: The number 1 thing to check in old Tek modular instruments is all the dipped tantalum capacitors. I've fixed several 7L12's with shorted tantalum caps. And one with a bad YIG oscillator.

author: Taelonious
author_rank: Contributor
timestamp: May 26, 2020, 01:54:48 am
content: It was indeed a shorted tantalum. C1382 to be exact.I didn't find it though. I ended up stopping by the Vintage Tektronix Museum to speak with a guy there about it. He ended up taking it home and found the culprit. Now it's going to star in a repair video they are making. Kind of took the fun out of it, but probably for the best. I have lots of other stuff to try and fix anyways.It seems the missing pot is a mod done in later models. Not too sure, but it works fine.Thanks again for all your advice!