RTB-2004 - Page 1
Forum section: electronics / repair

author: Joel_l
author_rank: Frequent Contributor
timestamp: March 16, 2025, 08:39:13 pm
content: Hi,I was working on a transmitter today and making some measurements with my scope. During one of of them I forgot to switch the TX to service mode ( tube finals ). Well, it was not kind to the input of that channel ( 1 ). The channel only sort of works now. Scale is wrong, how bad it's wrong seems to change with scale selection, wave form not good, sine waves better than square.So I figure some components are damaged along the input path.I got it apart and will take the main board into work where I have a microscope to better see things.Has anyone already drawn up the input path on this scope?ThanksJoel

author: electronix27
author_rank: Contributor
timestamp: March 17, 2025, 06:39:42 pm
content: Hi JoelDon't have what you're looking for unfortunately, but very interested in the progress of your repair if you'd like to update. Have one of these myself and if the need ever arises, I'll crack it open before sending it back.Russell

author: Joel_l
author_rank: Frequent Contributor
timestamp: March 17, 2025, 08:30:37 pm
content: I did some poking around. So far all I have found is the input resistor is slightly damaged, should be 68.1 Ohms, I'm guessing 1% or better. On my damaged channel it was 75 Ohms. I replaced it and now they are all 68.3 ( in tolerance ). I did not expect this to be the cause and it was not,I also found that the scope has two HW voltage ranges, at least what is switched through a relay, lets say high and low. Only the low range is affected, this is helpful as it now gives me some direction on what to look for.Using 10:1 that switch point is 2V, at 1V the relay switches and the wave form is broken. I'm thinking there is an op-amp buffer that now is working as a integrator.I think the good news so far is that it does not seem like that channel is severely broken.In the attached shots, both probes are connected to the cal signal. The yellow trace is the broken channel, at 2V it looks fine ( I offset it so you can see both), at 1V/div, it's not right.

author: Joel_l
author_rank: Frequent Contributor
timestamp: March 17, 2025, 09:48:57 pm
content: now that I had something to look for, I got the scope up and running. I found a resistor that measured 2M Ohs, should have been 62 Ohms. Should have been an 0402, didn't have one of those, but an 0603 slid in just fine.

author: EEVblog
author_rank: Unknown
timestamp: March 18, 2025, 12:23:05 pm
content: The great thing about multiple channels is you have a handy direct comparison reference.Even just randomly probing resistances can often pinpoint the problem.Nice find and fix. I'm surprised nothing else is busted.

author: Joel_l
author_rank: Frequent Contributor
timestamp: March 18, 2025, 08:22:37 pm
content: Quote from: EEVblog on March 18, 2025, 12:23:05 pmEach channel was layed out exactly the same which did indeed make things much simpler. I was worried that I might have damaged the ADC, sooo glad I didn't. As it is, I got lucky. I'm sure a repair at R&S would have been a board swap for a flat way too high of a price.

author: pdenisowski
author_rank: Super Contributor
timestamp: March 19, 2025, 10:22:19 pm
content: Quote from: Joel_l on March 18, 2025, 08:22:37 pmI can't comment on our repair costs off the top of my head, but most RTB repairs are in fact board swaps. People sometimes ask me for the RTB service manual thinking it contains component-level schematics, etc., but most of it is just "replace the board".