Oscope readout problem, help me understanding this comparator circuit. - Page 1
Forum section: electronics / repair

author: pitpat
author_rank: Contributor
timestamp: May 30, 2021, 06:10:54 pm
content: Hi! thanks for passing by.I have this Tektronix 2230 that has an incorrect sweep readout that shows 2s sec/div but only on a few of the sweep settings in B hor. Mode.In A hor. mode it's just stuck, also to 2s.Accordigly to the schematics the readout logic works by some comparators connected to a MUX.I've checked the voltages (as stated by table 6-4) and found some incorect values that correspond to the settings with a non functioning readout, i'll post the schematics, some photo and the table i'm referring to.Now the question is, how can i confirm that the comparators are fine?I'm quite stuck...Thanks for any help.FedericoPS In table 6-4 the yellow highlited voltages are the ones that tested ok.Schematicshttps://www.tek.com/manual/2230-service-manual

author: shakalnokturn
author_rank: Super Contributor
timestamp: May 30, 2021, 07:27:08 pm
content: The sweeping speed is OK and only readout wrong?You don't give your actual measured values for the bad measurements.I'd start by checking: -8.6V supply, "RES REF", what ever supplies the comparators themselves, that no connectors are misplaced or corroded.Checking the comparators themselves should be easy enough by disconnecting W1304 and applying a test voltage to the inverting inputs checking switching on corresponding output.

author: pitpat
author_rank: Contributor
timestamp: May 30, 2021, 08:12:10 pm
content: Hi thanks for the reply, yes the analog sweeping is ok, the digital one instead use the values from the comparators to set himself to the correct time-base setting.I honestly din't check the -8.6v and RES REF yet, i'll try... I've aready had a look for disconnected and corroded connectors and found any.The comparators should be powered by the 5v supplied by the psu.PS for the B REF all the wrong voltages were almost 5v (4.98) for the A RES 1-2 I'm not quite sure but around 4.01v I'll check again

author: pitpat
author_rank: Contributor
timestamp: June 01, 2021, 09:14:35 am
content: Update, something strange happened! I was sending some basic commands to the gpio expansion of the scope when, with the corner of my eye, I've noticed some changes. Toh! The readout changed from 2s to the correct value...Im now suspecting the power lines...Any opinions would be appreciatedThanksFederico

author: shakalnokturn
author_rank: Super Contributor
timestamp: June 01, 2021, 02:30:18 pm
content: As previously suggested check -8.6V and comparator supply if you haven't done.I don't have the schematic at hand right now but recall seeing a 100nF capacitor either on -8.6V decoupling before RES REF divider or filtering RES REF itself. If it's on RES REF and a tantalum drop it's a good suspect.

author: pitpat
author_rank: Contributor
timestamp: June 01, 2021, 05:13:19 pm
content: Hi! You mean C768 and C767?I've already tested and replaced C766 but not because it didn't test fine just because it was already out of circuit.They doesn't looks like tantalum cap to me though, they are like the old oil-paper ones but small as a grain of rice and black.I'll post a photo in the next few hours.Thanks againFederico

author: shakalnokturn
author_rank: Super Contributor
timestamp: June 01, 2021, 05:41:47 pm
content: C766 was the one I had in mind.

author: pitpat
author_rank: Contributor
timestamp: June 03, 2021, 10:21:16 am
content: I've replaced c768 and c767 but still nothing.I've also did quick ripple test on the -8.6v RES REFIt doesn't look so bad to me though.

author: David Hess
author_rank: Super Contributor
timestamp: June 05, 2021, 02:26:04 am
content: My first guess is that the mechanical switches used to read the time/div control are dirty.

author: shakalnokturn
author_rank: Super Contributor
timestamp: June 07, 2021, 01:40:48 pm
content: Quote from: David Hess on June 05, 2021, 02:26:04 amThat would have been my first guess too, but wouldn't the actual sweep speed also be affected in that case?Here the analog timebase is said to be OK doesn't that rule the timebase switches out?

author: David Hess
author_rank: Super Contributor
timestamp: June 07, 2021, 06:40:20 pm
content: Quote from: shakalnokturn on June 07, 2021, 01:40:48 pmThe readout switches are separate from switches which control the analog timebase.