BWD504 horizontal issue  - Page 1
Forum section: electronics / repair

author: gkmaia
author_rank: Frequent Contributor
timestamp: March 17, 2019, 08:14:46 am
content: I am struggling to get an old BWD504 working.All I can see is a vertical line that changes according to the input voltage I probe. If no voltage input I see only a dot.I cannot get the horizontal (time) to change.If you look at the image bellow all the green dots are components & rails I tested so far. Caps are old bu they are all within spec, no leaks or shorts. All transistors and diodes are fine.BUT, when probing with an oscilloscope I could not get any oscillation at test points as suggested by the manual. That makes me thing if that is my issue, but as all components seem fine not sure why it does not oscillate.So I am really stuck here... any help would be appreciated

author: tautech
author_rank: Super Contributor
timestamp: March 17, 2019, 09:04:42 am
content: Sounds like zero sweep and I don't see the waveform listed for the sweep generator.Can you post up a link for the full manual.BTW, a BWD I once owned had a shorted tantalum, it was used in parallel with an electrolytic on a rail for remote bulk capacitance and somewhat a bitch to diagnose.

author: vk6zgo
author_rank: Super Contributor
timestamp: March 17, 2019, 10:04:26 am
content: Does it work in X-Y?How about in "external trigger"?If so, you are probably not getting a high enough amplitude pulse back from the vertical output.BWD use a network of resistor back to the "internal trigger" circuit, one of which is around 220k or so( maybe higher or lower in some models).These have a bad habit of going high resistance.

author: Chris56000
author_rank: Frequent Contributor
timestamp: March 17, 2019, 04:42:30 pm
content: Hi!http://bama.edebris.com/manuals/bwd/504I'll study this and come back on this with fault-finding advice presently!Chris Williams

author: gkmaia
author_rank: Frequent Contributor
timestamp: March 17, 2019, 06:55:10 pm
content: Quote from: tautech on March 17, 2019, 09:04:42 amI did probe all electrolytics positive sides for shorts and found none... but did not went all the way to test the ceramics and tantalums. I will try that today when I am back home.Quote from: Chris56000 on March 17, 2019, 04:42:30 pmAwesome, thanks Chris!Quote from: vk6zgo on March 17, 2019, 10:04:26 amAttached the schematic of the internal trigger section. I will scope and see what signals I am getting from it.Resistor wise, many of them matched the specification on the board, the ones that did not I removed to test. All greens are fine. Oranges did not match so removing to test later.

author: tautech
author_rank: Super Contributor
timestamp: March 17, 2019, 07:10:23 pm
content: If you're into BWD stuff Kevin Chant has lots on his Aussie site:https://www.kevinchant.com/bwd.html

author: gkmaia
author_rank: Frequent Contributor
timestamp: March 18, 2019, 07:25:42 pm
content: I think I may be onto something... the hex inverter.The original DM74L04N... none of the gates were working but none were short. Failed open.Just for testing I got a spare M4069ubp I had on the inventory so I could test it.BUT...The 4069 at 5V needs input HIGH 4v, LOW 1v, 60nsThe 74L04 at 5v needs input HIGH 2v, LOW 0.7v, 60nsIf you look at the schematic, at some points the input threshold is pretty near 4v. So maybe my choice of replacement is what is making it fail to oscillate.The one I can get my hands on here in NZ is the SN7404n. The 74L04 is obsolete. I could get on ebay but will take ages.What you guys think about the SN7404? Better choice?

author: tautech
author_rank: Super Contributor
timestamp: March 19, 2019, 05:01:31 am
content: I have some 74LS04N in junk pile PCB's.Check the datasheet to see if they'll do you.I can pull them or cut the PCB where there's 2 next to each other.

author: gkmaia
author_rank: Frequent Contributor
timestamp: March 19, 2019, 07:24:29 pm
content: Quote from: tautech on March 19, 2019, 05:01:31 amI just bought a SN7404N must arrive this week. If does not work we can maybe try the LS.But, I would be interested on buying some junk PCBs with some variety of TTL ICs on them. If you are keen to sell maybe 2 or 3 boards please let me know.

author: tautech
author_rank: Super Contributor
timestamp: March 19, 2019, 07:33:03 pm
content: I have a good few, some are plugins from mainframe computers and some are old 186/286 PC MB's.Maybe you can visit if you are in/near AK.

author: gkmaia
author_rank: Frequent Contributor
timestamp: March 20, 2019, 07:51:44 am
content: I've got some news! The horizontal now oscillates as if it did not trigger.So the HC7404N fixed half of the problem.I believe I got a secondary problem around the internal trigger circuit. I am not getting any oscillation into Q40 base. That stops the signal to flow through the auto circuit, to the time base circuit. I think it's final stop is PIN 3 of the 7404. At least that is what flows to the amplifier.I will try to identify if the problem is in the output amplifier tomorrow.