Tektronix 7904 rear AC mains range selector switch missing. - Page 1
Forum section: electronics / repair

author: BlownUpCapacitor
author_rank: Frequent Contributor
timestamp: February 11, 2024, 08:00:04 pm
content: Hi, I wanted to know the connections of the AC mains range selector switch of the 7904. This isn't the A model. I know the AC mains range selector switch is similar to the Tek 465.I don't have the scope with me as it is at a recycling center. So I can't supply more images. But the guy who owns the shop said I can have it for free if I manage to get it to power up. I've bought from this guy numerous times before so I trust this claim.The scope is in pretty rough condition, with dirt everywhere inside. I think it might have been rained on. But I think the main thing that is preventing power-up is the missing AC range selector switch.The AC range selector switch uses two switches. One that determines if it runs on 220v or 110v. The other determines the Hi-Med-Low setting. I'm missing the one for the Hi-Med-Low.I have provided a drawing I made in MS paint that resembles the range switch construction. The 3 columns of dots on the left side are the ones that are for Hi-Med-Low, An image of the 7904 I took is also supplied. (ignore the 3 timebases)I drew that from memory though so it may not be correct.I want to know the wiring inside the little white block that should go there. Are they all shorted? Are the top ones shorted together, and the bottom ones shorted together?Thanks.

author: med6753
author_rank: Super Contributor
timestamp: February 12, 2024, 05:55:39 pm
content: I assume you are referring to this assembly. On my 7904 nothing is plugged into the L-M-H as shown. As we are both in the USA I think it's a safe bet that nothing was ever plugged into the switch in that 7904 either. As far as the backside wiring I really don't want to pull the PSU and check, sorry. But I think if you study the schematic I suspect that you may not even need that L-M-H assembly and just wire without it.And be aware that if you do take on the repair I can guarantee the PSU will not power up because it's loaded with beaded tantalums that are infamous for shorting out. I speak from experience.

author: BlownUpCapacitor
author_rank: Frequent Contributor
timestamp: February 12, 2024, 06:48:19 pm
content: Thanks for the clarification! At the recycling center I did try using some copper wires to try to get it to work.Maybe Tek was just using some parts that were already in manufacture to use, and ended up with a function that wasn't needed.When I plugged the unit in, I didn't hear any pulse mode, and the fuse was intact. The scope wasn't showing any signs of life. What does this indicate?If it was just shorted tants, it would just go into pulse mode yes?

author: med6753
author_rank: Super Contributor
timestamp: February 12, 2024, 07:27:06 pm
content: Typically a shorted tant will cause the PSU to go into tick mode but of course it's not an absolute. If the scope is completely dead then obviously the PSU should be checked first for correct voltages.

author: BlownUpCapacitor
author_rank: Frequent Contributor
timestamp: February 12, 2024, 11:43:46 pm
content: Looking at the rear fuse interface more, is there a fuse in the bottom fuse compartment?I'm "my" 7904, that fuse is absent. I thought it was a fuse socket for the 230v line, and so was unused. But I'm not so sure anymore.Perhaps that single missing fuse was the main fault?

author: med6753
author_rank: Super Contributor
timestamp: February 12, 2024, 11:51:10 pm
content: Yes, 2 fuses. One for the line and the other for the inverter.

author: BlownUpCapacitor
author_rank: Frequent Contributor
timestamp: February 13, 2024, 01:06:11 am
content: Thank you! I will have to go back to the recycling center again next week when school is off for me because of President's Day.I know it's common for people to wash the PCBs of old gear with soap and water, and I was thinking of doing that if it doesn't cause harm to the boards.I cracked the scope open at the shop and there was mud everywhere inside the unit. I mean actual literal dirt from the ground. Would washing the boards be recommended? If not, how would I go about cleaning it?

author: donlisms
author_rank: Frequent Contributor
timestamp: February 13, 2024, 04:00:02 am
content: I'd also assume a strong possibility of bad capacitors in there; it's very likely not going to power up, and will need some investigating. Maybe you can renegotiate with the current owner? I'm not sure I understand his offer - y you can have it if you can power it up? Maybe he just wants to be sure you're not going to waste it, but actually do something useful with it?On my 7904A, the bit that made it more painful for me is that the supplies take feedback from the rest of the scope to make sure the current is okay and such. So you can't just pull the supply out onto the bench and fire it up in isolation. I used some female/male ribbon cables (readily available) to extend the ribbon connections between the power supply and the rest of the scope, so I could indeed get the PS out on the bench while trying to make sense of it. (Yes... bad tants.)The other bit is that the power supply module seemed to me to be put together like a 3D puzzle, with interlocking bits and a general sense of structural complexity. Sigh. I just bit the bullet and started taking things off, one at a time, because I wanted full access.(You do have all the docs and info from TekWiki, yes?)

author: BlownUpCapacitor
author_rank: Frequent Contributor
timestamp: February 13, 2024, 04:55:49 am
content: Quote from: donlisms on February 13, 2024, 04:00:02 amI don't think this will be a problem for me as I don't think the PSU on the 7904 non-A version uses such a feedback circuit. The service manual from Tekwiki mentions isolating the PSU from the actual mainframe.Looking at the block diagram for the PSU, the only feedback I see is from the CRT circuit, presumably for safety reasons.But this is just at a quick glance though. I may be missing something.Quote from: donlisms on February 13, 2024, 04:00:02 amYes, I do. I'm actually the one who added the service manual to the tek wiki.Quote from: donlisms on February 13, 2024, 04:00:02 am...I can see what you mean... Looking at images on tekwiki, the PSU just looks like a brick full of PCBs, capacitors, coils, heat sinks, and transformers.Luckily, Tekwiki has shared a document explaining how to service the power supply in the 7904. I predict this will be very useful as the service manual doesn't provide many details about the power supply.