Tek 2445A repair help - Page 1
Forum section: electronics / repair

author: okney1lz
author_rank: Contributor
timestamp: September 10, 2014, 01:43:51 pm
content: Hi all,I'm a lurker, but finally decided to get in the proverbial game.I recently acquired a 2445A for a song.I had to just about power wash the grime off of it, but it does power up and show a trace. I replaced the seized fan with new unit from Mouser that I had laying around. Part # AD0812HB-C73-LFHowever, after about 2 minutes the trace and readout starts jittering a lot.Here's a video of what I'm talking about:I also noticed a buzzing or high speed clicking sound. Maybe an arcing sound?I did the requisite voltage check at J119, and obviously there's a problem with the +15v going on.Here are the results:I used a couple references for ground in order to double check myself.I have the 2445 and the 2465B manual. The board layouts are a bit different, but close. I'll keep looking for a 2445A specific manual.Here is the 15v regulator circuit from the 2465B manual, a close approximation of what I'm dealing with.Where to go from here?I'll try to keep this on topic and post pictures to help others.Thanks,Lawrence

author: mij59
author_rank: Frequent Contributor
timestamp: September 10, 2014, 03:53:28 pm
content: Hi,Looks the same as the in the service manual of the 2445a.Check the voltages of U1260-317.

author: okney1lz
author_rank: Contributor
timestamp: September 10, 2014, 05:35:51 pm
content: So, I tested the voltage regulator, and it looks good to me.So, still open to suggestions.Here is another video, better showing the symptoms it's experiencing:I don't think turning channel 2 on had anything to do with it, as it happens after a bit of time with no probes attached. Just coincidence. Also, it will sometimes clear up for a bit, then go crazy again.Also, you can here the crt buzzing and carrying on a little better in this video.Thanks for any input,Lawrence

author: tautech
author_rank: Super Contributor
timestamp: September 10, 2014, 08:05:54 pm
content: Get the Tek scope troubleshooting pdf from this post:https://www.eevblog.com/forum/repair/repair-documents-and-links-sticky-me-please-mods/msg477909/#msg477909There are some tips in it for troubleshooting the SMPSQuoteThat symptom is described.Failing E-Caps?The 317 readings, can you monitor them to see if they are stable?Do you have another scope to monitor PSU rails for ripple and when it misbehaves?

author: okney1lz
author_rank: Contributor
timestamp: September 10, 2014, 09:28:56 pm
content: So,Will read recommended repair manual tonight, so forgive anything that may overlap. Also, it's really annoying that a lot of these manuals are password protected, allowing print only. I have a great OCR program that would be awesome to make these searchable. Oh wellHave another scope on the way, but it's probably wasted too. lol I'm a glutton for punishment. I have 10 8640B's waiting in line, that need functional checks and gone through.With the Vreg ruled out for the most part, it's output seems to be stable, I focused on where the +15 was coming from before it reaches the J119 header.It looks like the DLY REF 1 headed from U2521 through U2620 (High Speed OpAmp).This is the schematic I'm working with:A quick check of supporting components in the path turned up a bad resistor, R2720. Bottom of the red circle. I replaced that, and things have improved. It still freaks out, just not as bad and it takes a bit longer. Even a blind squirrel gets a nut once in a while.However, the sound is definitely coming from the back of the crt, so maybe all of this is moot, if that's garbage. Maybe a problem with HV side of things?Again, any suggestions are welcome.Lawrence

author: tautech
author_rank: Super Contributor
timestamp: September 10, 2014, 09:41:08 pm
content: QuoteQuite likely.The Tek pdf has plenty in it to guide you through faults in the CRT HV area.Some study required.

author: christopher iles
author_rank: Regular Contributor
timestamp: September 11, 2014, 06:40:05 am
content: Hello,When I was a TV engineer years ago, it was not unknown for arcing to occur around a dirty anode cap on a CRT. Put the 'scope on in a darkened room and look for arcing/sparking around the cap; it will be clear to see if that is the problem.Should that be the case, removal of the cap and cleaning the cap and CRT normally effects a complete cure.Just a thought, hope it's helpful.Regards,Chris.

author: mij59
author_rank: Frequent Contributor
timestamp: September 11, 2014, 08:43:36 am
content: Quote from: okney1lz on September 10, 2014, 05:35:51 pmPlease check the +15V on J119 at start up and when the problem occurs.It seems that the character generator and the triggering are affected,m must be some kind of power supply problem.

author: okney1lz
author_rank: Contributor
timestamp: September 11, 2014, 06:56:16 pm
content: Alright, I've rechecked everything, with the right schematic(Almost 2465B) this time! And a better bench top meter.Oh, and I almost forgot, a bunch of reading material.It seems the -15 volt leg is dropping (or is it adding) to -14.5 Volts.As the HV circuit turns this -15 to the proper A/C levels for the crt, I have to believe this is the root of the display issue and noise.I'm scanning the schematics now to localize the source of the -15v, and trace back to it.See, I just needed some prodding in the right direction, and not to get ahead of myself.Will update over the next few days.

author: okney1lz
author_rank: Contributor
timestamp: September 11, 2014, 08:15:48 pm
content: So, here goes. I need someone to check my math, so to speak.From this:When I touched pin 2 of the regulator with my meter, I got the buzzing to stop. I tried re-flowing the solder, but it didn't help. I know this regulator needs some load to work, so I figure my meter changed something enough for it to stabilize for a moment?-5 volts is measuring -7.23, with a spec of -7.1V from low voltage board. It is also stable through the described malfunction.-19.2 is dead on and stable.C1331 is 0.1 uf 20% 50V ceramicCR1330/32 are 400V 1A Rect. DiodesI'm leaning towards the regulator or a diode. Your best guess?Bad regulator(LM7912CT), supporting components, leaky diode, or U1207(LM324N) OpAmp?Thanks,Lawrence

author: tautech
author_rank: Super Contributor
timestamp: September 11, 2014, 08:49:10 pm
content: QuotePin 2 is the -19.2 V supply, that doesn't make sense?Have you checked the supporting resistors 1331-34 out of circuit? Lift one leg.

author: okney1lz
author_rank: Contributor
timestamp: September 11, 2014, 10:31:04 pm
content: I know it doesn't make sense, but as they say on the internet, true story. lolResistors checked fine. Added in blue:Suggestions for next area/component to test?Thanks again for all the help!Lawrence

author: tautech
author_rank: Super Contributor
timestamp: September 11, 2014, 11:34:04 pm
content: The 7912 is configured as adjustable with the 324 controlling it.Does the 7912 get warm/hot? this may indicate it is overloaded and the reason the -15 rail is low.I suspect all is well here and the problem is elsewhere. But I have been wrong before. If you have the parts available, replace them to eliminate this part of the circuitry as a problem.Study the schematic for any tantalums on the -15 rail.QuoteDon't be too sure of that. HV stresses componentry much more and has more frequent failures.Can you post the HV and CRT schematic so we don't need to download it.

author: okney1lz
author_rank: Contributor
timestamp: September 12, 2014, 01:00:52 am
content: So, I decided to test the diodes too. I got a iffy reading on cr1330, so I replaced it with a 1N4004 I had.This made the scope stable for about 25 minutes. I ran all of the tests and exercises. Thought I was home free, however, I noticed something new, the trace seems to bend down left of the center graticule marker. It definitely wasn't doing that before. So either a new problem that I caused or something that was masked.Then it started again with the buzzing and carrying on. So, I'm thinking there's still a problem somewhere. Argh!!! lolI'll have to disassemble again to check the voltages and if heat is an issue. This will probably have to wait until next week though, I have a ton going on right now!Here's a try at posting the schematics:And a link to, hopefully, a larger/clearer pic:http://thefinezzagroup.com/2445A%20Info/CRT_HV_Schematic_002.JPGThanks for the input so far everyone!Lawrence

author: tautech
author_rank: Super Contributor
timestamp: September 12, 2014, 01:14:53 am
content: C1951 looks suspiciously like a Tantalum, please confirm? At location B7 on the CRT HV schematic.Also check & test C1971 100 uF. After the inductor on the -15 V rail.Confirm voltages listed around T1970.EDITQuoteIs the trace a straight line? If so, Trace rotation adjustment required.