Trying to fix a Spectrum Analyzer but have no clue - Page 1
Forum section: electronics / rf-microwave

author: tombi
author_rank: Regular Contributor
timestamp: March 01, 2017, 01:26:14 pm
content: So I bought a broken SA (GW INSTEK GSP-827) - self-diagnostics say both LOs are good and everything appears in order except there is no signal.Input protection diodes look ok.I can see a signal at the output of the MMIC just before the first mixer. Have heard that overloading the input can destroy the first mixer.Mixer is a SKY-60 (markings are MCL 0 503 SKY-60-1). I don't have any gear to check the signal after the mixer so I thought I would take a shot in the dark and replace it. I think the part is thishttps://www.minicircuits.com/pdfs/SKY-60.pdfProblem is I have no idea where to get it. I found a SKY-60MH on ebay but I'm not sure what trouble this might cause.https://www.minicircuits.com/pdfs/SKY-60MH+.pdfShould I just go for it?Is there a place where I can get the SKY-60?Interestingly when I run the RF board out of its can (so I can probe it) some weak signals appear and 1.97GHz and one other frequency. I presume this is the LO leaking through but it gives me hope that the damage is close to the front-end.Thanks,Tom

author: chrisl
author_rank: Regular Contributor
timestamp: March 01, 2017, 06:09:41 pm
content: It is a minicircuits mixer and you should be able to get one from them:http://www.minicircuits.com/products/fm_sm_level_7.shtml

author: hendorog
author_rank: Super Contributor
timestamp: March 01, 2017, 11:39:27 pm
content: Quote from: chrisl on March 01, 2017, 06:09:41 pmNope, but he could get 10 from them. At $13 each.MC are a bit of a pain like that as their parts aren't available at the usual places like DK or Mouser etc.They don't have an eval board with the mixer populated either.Kynix sell MC stuff in single units, but they don't have that particular (7dBm) mixer:http://www.kynix.com/Search/SKY-60.html

author: tombi
author_rank: Regular Contributor
timestamp: March 02, 2017, 01:01:07 am
content: Thanks for the replies.I contacted Clarke and Severn (the local Mini circuits supplier). The part wasn't in their website but a guy named John came back and said he added it to the catalog and there was stock in the US.So now I can order one but the cost (with shipping) was an eye-watering $AUD38Tom

author: IconicPCB
author_rank: Super Contributor
timestamp: March 02, 2017, 11:04:57 am
content: Clark and Severn are OK.You sshould also contacty can helpwww.minikits.com.auand see if they can help

author: tombi
author_rank: Regular Contributor
timestamp: March 03, 2017, 02:37:54 pm
content: Thanks for the tip - mini-kits has some interesting stuff.Tom

author: tombi
author_rank: Regular Contributor
timestamp: March 06, 2017, 03:47:50 am
content: So a few amusing things happened.1) GW INSTEK came back to my support request and gave me a service manual! It doesn't have schematics but has pretty good block diagrams and the circuits aren't hard to follow. Even better they were willing to help (advice wise) with board level repair. GW INSTEK are awesome!2) The 452MHz IF is outside (by over two times) the bandwidth of my scope but I thought I'd see what is coming out anyway. The frequency was 100MHz and it didn't seem to matter what I did with the controls or added to the input but it stayed 100MHz.The coax cables are mostly labelled but some are a ambiguous. I found a photo in this old thread that showed the internalshttps://www.eevblog.com/forum/repair/instek-spectrum-analyzer-gsp-827-6-dbm-down-than-the-real-value/msg630880/#msg630880In mine the two cables at the front of the RF board (bottom board) were connected incorrectly. The one on the left connected to the TG and the second from the left to the LO board. They were one board over basically. The machine was originally owned by TAFE and there was a note describing a motherboard repair. Looks like they didn't assemble it correctly after the repair.Moving these, rebooting and voila! It all works. Power levels even within a db or two of what I expect.3) I cancelled the order with Clarke and Severn. They happily refunded me. Awesome!Tom