Tektronix Scope TDS 524A LV PSU Repair Advice - Page 1
Forum section: electronics / repair

author: vectra_1098
author_rank: Newbie
timestamp: April 06, 2019, 12:03:17 pm
content: Hello,I am working on a faulty Tek Scope TDS 524A LV PSU, the scope will not power up!I have removed the LV power supply from the scope placed it on the bench.Powering the PSU and monitoring the low voltage outputs at J5 & J6,also as per the manual linking pins 3+5 of J5 together to allow start-upThe no-load outputs at J5&J6 are all zero ?I have checked the ac input supply and filters (and fuse ) and Bridge(CR5) output and there is a335V dc across C12 the Bulk capacitor.I suspected the Standby circuit , I removed Q9 and transient suppressors VR4 & VR5.I have obtained new components for VR4 &VR5 ok.Q9 in this power supply is a MJE 1320 Power switching transistor,But they are very difficult to get hold off and other similar power supplies use theBu508A as Q9. I don’t think there parts are interchangeable unless the stand-by circuit is modified to take the BU508A part in this power supply ?Is there an equivalent part for the MJE 1320 ??**Referring to TDS524A/TDS520B service manuals for TDS 524A ScopeAdvice would be nice to have.ThanksT

author: pbarton
author_rank: Frequent Contributor
timestamp: April 07, 2019, 05:15:45 pm
content: Can you identify the source of the manuals you are using, as it often helps if we are all referencing the same document?

author: vectra_1098
author_rank: Newbie
timestamp: April 07, 2019, 08:51:45 pm
content: Hi,In my opion Tek did not want anybody to repair these scope power suppliesand so to find the correct schematics /pcb layouts is made difficult.Ref Docs:The Manuals reference are -1.TDS 520B Mod CmDigitising OscilloscopeComponent Service Manual070-9710-032.) TDS 520A,524A,540A & 544ADigitising Oscilloscope070-8713-033.) A schematic drawing from an EE blog, Post” Help with my TDS 520B showing a standby Supply”.My Notes:Ref 1: Manual does not give component level detail although its for the TDS 524A scope.Ref 2: Does provide schematics which are similar but they are for the TDS 520B not the TDS 524ARef3:EE blog post Standby Supply,shows a bipolar transistor Q9 type MLE 1320 and transorbs VR4/VR5.My Power supply/custom rectifier LR40071Zytec part number:22904701 rev g300w,3.8A,1phaseThanks for the reply

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: April 07, 2019, 08:57:12 pm
content: Hi.There are at least two standby power supplies used in these TDS5xx and TDS7xx scope.There is some useful information in this thread:https://www.eevblog.com/forum/testgear/fixing-tek-tds684-psu/msg453682/#msg453682This is one:This is the other:Regards,Jay_Diddy_B

author: Ordinaryman1971
author_rank: Frequent Contributor
timestamp: April 07, 2019, 11:48:37 pm
content: I have identical problem with TDS744A, what was your replacement for VR4 and VR5... would they be the same in both power supplies?

author: vectra_1098
author_rank: Newbie
timestamp: April 08, 2019, 08:43:09 am
content: Hi,=======================================================Jay_Diddy_B Wrote Earlier :-There are at least two standby power supplies used in these TDS5xx and TDS7xx scope.There is some useful information in this thread:https://www.eevblog.com/forum/testgear/fixing-tek-tds684-psu/msg453682/#msg453682==========================================================I can see the Stand-up supply schematic version with the Bipolar transistor Q9 and VR4/5.I removed these components from the PSU and checked them with a Multimeterand other test equipment and noted their part numbers.My test equipment does not like Q9 and VR4/5I have obtained the Transorbs via EbayTVS Diode VR4=part number 1.5KE220CA St MicroelectronicsTVS Diode VR5=part number 1.5KE250CA St MicroelectronicsAs I mentioned in an earlier post My Transistor(Q9) is a MLE 1320 which I read is obsolete ??. What Q9 do you have in your PSU ?I am going to reverse engineer my pcb around the Standby supply area to see how close it is with the schematicand possibly do a spice simulation for he circuitThanksI can identify my bip

author: Ordinaryman1971
author_rank: Frequent Contributor
timestamp: April 08, 2019, 02:28:16 pm
content: I will look it up today and take some pictures.

author: Ordinaryman1971
author_rank: Frequent Contributor
timestamp: April 17, 2019, 05:07:34 am
content: Took few days but... here are some pictures.Q9... I have BU508A in mine.I've just installed the suppression diodes hoping that that's the only damage, but q9 is shorted as well. Don't have those transistors so that's the next purchase.I hope it's good news about the replacement transistor. The BU508A are about $2 a piece.

author: radioguy123
author_rank: Regular Contributor
timestamp: December 09, 2019, 11:11:44 pm
content: So what was the outcome of this repair ? Was the power supply fixed ?I have a similar problem with a 620-0063-04 psu.Mine has the Q9 BU508A but it test's good.The TVS diodes were bad but that didn't fix it.All components on either side of the standby circuit test good now.But still want's to overheat the TVS devices and still no standby output ??

author: chronos42
author_rank: Regular Contributor
timestamp: December 09, 2019, 11:22:58 pm
content: Quote from: radioguy123 on December 09, 2019, 11:11:44 pmCheck C17 (47µF) again for ESR or replace it with a very high quality, low ESR Capacitor.

author: radioguy123
author_rank: Regular Contributor
timestamp: December 10, 2019, 01:12:14 am
content: Hello chronos42Have you actually seen a case when C17 was a cause for this problem ?I did check the capacitance of C17 and it looked good.I have a new capacitor on order.Also I'm getting an ESR meter.Thanks for your reply

author: madao
author_rank: Frequent Contributor
timestamp: December 10, 2019, 06:02:06 am
content: Old Standby PSU is very sensitive by bad ESR of C17 eletrolytics.If ESR is too high = more spikes, which it is absorbing by TVS-diode -> overheated ! If TVS diode is dead, BU508A play kamikaze.I have similar failure by TDS540A, but C17 has good ESR, only Transistor is dead. I have not MJE8501, it is replaced by TIPL760 (TIPL760 has much less limit voltage, not recommed!) BU508A is in europe cheap, but i have MJE8501 w TO220 case in PSU.

author: radioguy123
author_rank: Regular Contributor
timestamp: December 10, 2019, 02:03:54 pm
content: Hi madaoI think you are right. This version of psu is very sensitive.And if you leave your oscilloscope plugged in all the time, that makes matters worse.The stand by supply is running continuously.I'm just about at my wits end on this one. I've removed nearly every part and tested it out of circuit.Of course not the transformer, it has continuity but still could be bad.Probably a lot of these psu's have hit the dumpster for just this reason.

author: Ordinaryman1971
author_rank: Frequent Contributor
timestamp: December 10, 2019, 04:21:31 pm
content: I've had to put this project on the shelf for a while.The outcome of changing the suppression diodes and the transistor was none. The power supply is still dead.I was just looking at it the other day. I've ordered working power supply few weeks ago so as soon as I have some time I will have it changed and see if the scope is working ok with the "supposedly" working power supply.Then I will go back to fixing the not working one.... that's the plan.I didn't really test it outside of the scope.Let me look at the way to actually have it tested by linking the pins on the connectors.Didn't change any caps... that will be the next on the list.

author: rf+tech
author_rank: Frequent Contributor
timestamp: December 10, 2019, 05:12:07 pm
content: Hello radioguy123, Ordinaryman1971,By disconnecting Q9 base, an in-circuit static DC bias test can reveal useful information about multiple components.With Q9 base connected, what DC voltages are measured on Q9 base and collector in a non-functioning standby supply?It is expected that T3 pin 4 produces a negative-going pulse to start oscillation, when Q9 base becomes sufficiently forward biased to cause collector-emitter conduction, as the DC bulk line rises following application of AC mains/line voltage. Scope this point, with triggering derived from Line Trigger on J5 pin 1.RF+ TechEdited to resize image

author: radioguy123
author_rank: Regular Contributor
timestamp: December 10, 2019, 07:21:59 pm
content: So there should be a +0.6v positive voltage on the negative terminal of C17 ?And this is with the Q9 base still disconnected ? right ?

author: rf+tech
author_rank: Frequent Contributor
timestamp: December 10, 2019, 10:09:41 pm
content: radioguy123,QuoteCircuit analysis reveals this to be true. If this tiny reverse voltage is a concern, disconnect C17.QuoteAttempting to diagnose this non-oscillating condition with Q9 base connected is a bit problematic. RT5, RT4, T29 and R25 provide forward bias to Q9 base-emitter junction. At the same time, Q9 collector-emitter junction begins to conduct, lowering the voltage at RT4/R29 junction. In the non-oscillating condition, where feedback is not present, Q9 base forward bias and Q9 collector voltage settle to a point of static equilibrium where collector voltage limits base forward bias.How is one to get around this obstacle and better evaluate components in the feedback path?The objective of this seemingly unconventional test is to check the complete series circuit of RT5, RT4, R29, R25, CR11, VR1, CR10 and T3 secondary pin 4 to 3. If Q9 base is not disconnected, the base-emitter junction acts as a DC bypass around CR11, VR1, CR10 and T3 secondary pins 4 to 3. Again, circuit analysis shows Q9 base must be disconnected, as a prerequisite for this test. The voltages measured can confirm these components to be okay, or provide valuable clues that help pinpoint a fault.Now for another less than obvious potential cause of failure to oscillate. A shorted rectifier or bypass capacitor on one of the T3 secondaries could reduce feedback amplitude to the point where the circuit cannot oscillate.For the record, it would be advantageous to know what resistances RT5 and RT4 exhibit at room temperature.Hope this insight and detail helps understanding,RF+ Tech

author: chronos42
author_rank: Regular Contributor
timestamp: December 10, 2019, 10:24:43 pm
content: Quote from: radioguy123 on December 10, 2019, 01:12:14 amHi,yes, i had one broken PSU with a C17 with too high ESR. The complete circuit fails in this case, because the bias voltage is not clean (spikes) and can trigger Q9 in a wrong way. Also the bias can be too low, therefore the circuit is not working as intendet. In this case the TVS diodes were overloaded, too. From my experience C17 is a very critical part in this circuit.I would also follow the recommendations from rf + tech to exclude other parts as failure source.

author: radioguy123
author_rank: Regular Contributor
timestamp: December 10, 2019, 11:34:47 pm
content: Wow a lot of useful information here. Thanks guys. Now were really getting somewhere.

author: radioguy123
author_rank: Regular Contributor
timestamp: December 13, 2019, 09:30:17 pm
content: I replaced C17 and there is no change in the operation. A few seconds and the TVS diodes are about to melt their own solder.Ok so I got a chance to measure the static voltage points that rf+tech has shown.This is with the base of Q9 disconnected and board powered up.The voltage at the disconnected base of Q9 connection is: +6.55 voltsI measure +5.96 volts between the Zener VR1 and diode CR11And at the negative terminal of C17\anode of VR1 I see +0.43 volts.rf+tech do you make anything of my measurements ? I think they are pretty close to yours. Still isn't working.

author: rf+tech
author_rank: Frequent Contributor
timestamp: December 13, 2019, 11:54:53 pm
content: radioguy123,Yes, these voltage measurements are good. Did the TVS heat up with Q9 base disconnected?The total withstand voltage of VR4 and VR5 should be a minimum of 470 Volts. If the peak to peak voltage across VR4 and VR5 exceeds some 500 Volts, then they will begin to conduct and heat. Are these original or have they been replaced? If replaced, what part number was chosen?I’ve been under the impression that this circuit is not oscillating.So this raises the question, have you actually scoped the signals at Q9 base and collector?What DC voltage is measured at VR1 anode? What is the DC Bulk voltage?RF+ Tech

author: radioguy123
author_rank: Regular Contributor
timestamp: December 14, 2019, 01:24:51 am
content: No with the base of Q9 disconnected the TVS did not get warm at all.The TVS devices I have in the circuit right now are 1.5KE250CAWell I guess it's oscillating. But in a very wrong way. There must be tremendous voltage spikes to heat the TVS that much.No I have not yet scoped the points you point out. I'll have to get back to you on that.I show the voltage at VR1 anode above. It is +0.43 volts.I'm running on a mains voltage of 123vac. The bulk voltage I measured is about 169vdc.I'm wondering if the BU508A could be bad even if it tests good ? This could make it operate strangely. I think I will change it just because.

author: rf+tech
author_rank: Frequent Contributor
timestamp: December 14, 2019, 03:12:30 am
content: 169 Vdc is expected, before the Boost Converter takes over. The boost converter needs +12 V Bias to operate. That comes from T3 secondary.I understood +0.43 Volts at CR1 anode to be with Q9 base disconnected. What is VR1 anode voltage with Q9 base connected? Is it anywhere close to -6.08 Vdc? If so, then yes, the circuit is oscillating.CR7, R21 and C15 form a snubber that dampens the flyback pulse on T3 primary. In normal operation, VR4 and VR5 would never conduct enough to get hot. A failure in the snubber network could cause the waveform amplitude to exceed the breakdown voltage of VR4 and VR5 and explain their overheating.The symptoms suggest Q9 could be struggling to overcome excessive load on T3 secondary. This is why it is important to make lots of additional measurements. Effective troubleshooting skills are gained by measurement and analysis, not replacing parts just because.What voltages are measured at J4-7, J4-5 and J2-7? If any of these are significantly out of range, try disconnecting one end of the associated rectifier (CR17, CR15 or CR8) to isolate that particular load.RF+ Tech

author: chronos42
author_rank: Regular Contributor
timestamp: December 15, 2019, 09:37:57 am
content: Hi Radioguy,maybe a dumb question, because I assumed that you did check this first, but: Are you really sure that there is no short or overload in the +12V and +15V output rails?

author: radioguy123
author_rank: Regular Contributor
timestamp: December 17, 2019, 08:13:07 pm
content: Hi chronos42Yes I did check if those were shorted. I went one step further and totally disconnected them. Still no go.But I think I have found the problem. I tried another BU508A transistor and got no oscillations at all.Since I wasn't getting any oscillations I measured the voltage on the BU508A collector. 8 volts ?!!?!?It seems that one of those (unobtainium) thermistors was just going open when under load.It measures fine when no current is being drawn.They both measure around 30 ohms. I just don't have any information about these devices.There is a single marking on them " SL "? Does anyone have a part number or any more info on what these are ??thanks

author: nctnico
author_rank: Super Contributor
timestamp: December 17, 2019, 08:22:36 pm
content: Make sure to load the 24V output with at least the fan. Without load on the 24V output these power supplies won't start properly.

author: radioguy123
author_rank: Regular Contributor
timestamp: December 17, 2019, 11:30:52 pm
content: nctnicoYes but you can't even get to that point without the standby supply working.When the standby is working I'll worry about output load.

author: rf+tech
author_rank: Frequent Contributor
timestamp: December 18, 2019, 02:24:52 am
content: radioguy123,An after-thought about the DC voltage test results previously suggested (Q9 base disconnected), +0.43 Vdc at the junction of CR10 and VR1, is too low. The forward drop really should be higher, 0.56 to 0.61 Vdc. Pull one end of CR10 loose and measure the forward drop using the diode-test function on your DMM. Then check the reverse resistance using the regular Ohms setting. For silicon, this should be infinite.While you're at it, check CR9, CR11 and CR12 for forward drop and reverse leakage.RF+ Tech

author: radioguy123
author_rank: Regular Contributor
timestamp: December 18, 2019, 02:10:33 pm
content: rf+techDo you think it's possible that the readings on your static test were low because of the thermistor I found bad ?Sure wish I had a part number for that thermistor.

author: radioguy123
author_rank: Regular Contributor
timestamp: December 18, 2019, 05:16:19 pm
content: Ok here you go RF+ TechCR9 forward voltage 0.741 reverse resistance greater than 20 meg ohmCR10 0.542 20CR11 0.742 20CR12 0.732 20

author: rf+tech
author_rank: Frequent Contributor
timestamp: December 18, 2019, 09:11:16 pm
content: radioguy123QuoteLook at the schematic and apply some theory - this is a series circuit. Once a thermistor goes open circuit, or extremely high in resistance (100s of kOhms), current flow ceases or is greatly reduced and all voltage drops equally affected.There are two 200 kOhm resistors in series with the thermistors and the diodes. You have confirmed current flow through both thermistors with your DC bias measurements. No, the suspect thermistor that measures about 30 Ohms at room temperature and fails with excessive current cannot have any significant effect on this one measurement (CR10) and at the same time, the other measurements are as expected.Your test results also show these thermistors to be NTC, not PTC. Thus, we now have four data points in favor of NTC. Until a BOM for this power supply is located, all one can do is spec a replacement part that is parametrically close in value to known good parts. There is really nothing magic about the thermistors Tektronix used. Even if one had the original part number, it is likely to be a discontinued part. And what does one do if a datasheet cannot be found?Resistance at 25°C is easily measured. Beta (delta_R/Temp) takes a little more effort. Between these two measured parameters from a known good part, one can be confident in the selection of a replacement. The Epcos part would be as good a starting point as any.QuoteWithout the benefit of part numbers on each of these diodes, and on the assumption all four are the same part number, CR10 looks defective. Are each of these 1N4148, by chance?You mentioned a few posts back that T3 secondary loads were disconnected. So there is no uncertainty, the feedback winding must remain connected, or else Q9 can sustain damage. Without feedback, Q9 conducts continuously and the thermistors are subjected to excessive current. In this failure mode, the TVS remain cold.RF+ Tech

author: radioguy123
author_rank: Regular Contributor
timestamp: December 18, 2019, 09:42:33 pm
content: CR9. CR11, CR12 are 1n4148CR10 is a completely different diode. Couldn't read the number.You suggested I lift the diodes to disconnect the loads.I tried with the three loads disconnected. Still didn't work.I didn't disconnect T3 pin four.What feedback line are you referring to ?

author: rf+tech
author_rank: Frequent Contributor
timestamp: December 18, 2019, 10:19:29 pm
content: Ah, CR10 is a fast or ultra-fast recovery rectifier. These do have lower forward drop than 1N4148. I've just checked a few 1N4148 with a Fluke 12, the forward drops were all 0.594 to 0.596 Volts. The diode-test current supplied by the Fluke 12 is 635 microamps. Higher test current and lower temperature both increase forward drop.By contrast, some fast and ultra-fast recovery diode forward drops were 0.474 to 0.484 Volts. So CR10 should be good.Feedback is from T3 pin 4. There are two paths, one DC and one AC. Do you understand how this part of the circuit functions?

author: radioguy123
author_rank: Regular Contributor
timestamp: December 18, 2019, 10:51:22 pm
content: At this point I'm not sure I understand any part of this circuit

author: rf+tech
author_rank: Frequent Contributor
timestamp: December 18, 2019, 11:54:39 pm
content: R29 and R25 forward bias Q9 base-emitter. Q9 collector-emitter conducts current through T3 1-2. Transformer action induces current flow in T3 3-4, creating a negative-going voltage at pin 4. CR10 begins charging C17. VR1, CR11 and CR9 clamp the maximum negative voltage swing on CR10 cathode. This is the DC bias path.CR12 and C18 provide an AC path to quickly yank Q9 base in the negative direction, to turn off Q9. Notice CR9 is connected anti-parallel to Q9 base-emitter junction. The voltage on Q9 base can only swing + or – 0.6 Volts.Q9 is now cut off, current flow in T3 1-2 is interrupted and T3 does what any inductor does. The voltage at Q9 collector rises above the DC level on T3 pin 2. Now CR7, C15 and R21 come into play. This snubber network tames the rapid rate of rise of the flyback pulse. VR4 and VR5 provide a backup defense, in the event of snubber failure.When current flow in T3 1-2 is interrupted and the field reverses, T3 pin 4 now swings positive, R23 discharges C18. Q9 base becomes forward biased by R29 and R24. Wash, rinse and repeat some 20-50,000 times a second.To really see what is not happening requires an oscilloscope. Everything working correctly depends on that signal from T3 pin 4. Is is badly distorted? Is it very low amplitude? Is it non-existent? What does this signal look like at Q9 base? Is it too low to turn Q9 off?Inquiring minds want to know.

author: radioguy123
author_rank: Regular Contributor
timestamp: December 19, 2019, 01:07:17 am
content: That is a very good description. Now all I need is some good thermistors. The damaged ones I have aren't going to do anything.Mine read 30 ohms. But are probably damaged.Were those of yours that measured 40 ohms from a working psu ?If you think those from digi-key will work I'll order some.

author: rf+tech
author_rank: Frequent Contributor
timestamp: December 19, 2019, 04:25:21 am
content: Yes, the thermistors measured are in a working TDS640.If I were you, I would put the original Q9 and both thermistors back in the board. At least there was oscillation, evidenced by the TVS getting hot. The new Q9 appears to turn on, drawing excessive current and causing one of the thermistors to temporarily open. This will be much more difficult to troubleshoot.Wit the original Q9, something can be observed with a scope. With the new Q9 that does not even oscillate, there's nothing to see. For that matter, the new Q9 may be damaged.As you are discovering, randomly changing parts can compound problems. A soldering iron is not a measurement tool. It's a programming tool, according to the late Bob Pease.

author: Tantratron
author_rank: Frequent Contributor
timestamp: December 25, 2019, 09:23:39 am
content: Quote from: madao on December 10, 2019, 06:02:06 amQuote from: radioguy123 on December 10, 2019, 02:03:54 pmI have two TDS540C where I notice if they stay in standby mode (connected to grid - scope no display)... there is always some kind of humming faint sound near the PSU. Wether cabinet is removed or not removed, the sound is heard if putting ears close to the Oscope.Is this effect normal, known and acceptable or indicate a future to come failure of the PSU ?One remark I have for radioguy, it could be logic to always have standby mode so both NVRAMs will not deplete their internal battery energy unless both DS1486 and DS1650 do not use any internal power when TDS are completely shutdown ?Any idea how much power is consumed during standby mode plus confirmation or not this standby leads to PSU failure ?Happy New year

author: radioguy123
author_rank: Regular Contributor
timestamp: January 05, 2020, 05:03:15 pm
content: Hello TantratronI'm not sure how much power the standby circuit uses. I don't think it's very much.Someone should put a wattmeter on the scope when turned off.I got my thermistors and installed them. No change what so ever.rf-tech I did put the original BU508 back in and it's doing exactly the same thing.The TVS devices get real hot real quick.So what ever measurements I need to do will have to be done really fast.I did measure the voltage at the minus pin of C17. I see -5.55vdc there. Seems a little low.rf-tech where do you suggest I probe with my oscilloscope ? And what should I expect to see ?Whatever it is I hope it's not much above the 500v that is cooking the TVS devices.I'll have to be careful, I don't want to blow the only scope I have working.

author: rf+tech
author_rank: Frequent Contributor
timestamp: January 05, 2020, 09:26:18 pm
content: The waveforms we want to see are Q9 base (#2) and Q9 collector (#3).Refer to the waveforms file for scope settings, attached below.The troubleshooting guide details the test conditions.RF+ Tech

author: madao
author_rank: Frequent Contributor
timestamp: February 08, 2020, 06:36:11 am
content: I got few week ago a TDS744 , spare part.Their PSU is dead (possible, it wasn't dead, it is part-deliever) Missing two thermistor (RT4 &5) and Q9.I have place fuse-resistor with 240 ohm value (5W type) and BU508A and it runs. 240 ohm resistor becomes not warm, good.Now, you know: Thermistor can replaced by resistor with fuse-attribute .Greetingsmatt

author: radioguy123
author_rank: Regular Contributor
timestamp: December 01, 2021, 10:52:59 pm
content: I know, I know it's been a very long time.This power supply has been sitting on the shelf for nearly 2 years haunting me because I couldn't fix it.No matter what I did the standby power supply would not run. No oscillations what so ever.Just the vr4 and vr5 tvs devices would cook and fail.So today I thought I would try one last ditch idea.I replaced the 250v and 220v tvs devices with three 220v devices. And low and behold now it runs.And the tvs devices are not overheating.I checked all the voltages coming out of the stand by supply and they are there.I have not loaded the outputs of the main power supply, and tested them yet. But I did key the on/stby line and a relay clicked.I'm not quite ready to test this on a good scope but I will put loads on the outputs and measure them. Yes, including a 24vdc fan.If someone know why this would make this ps work please leave comments. If anyone has any reason not to do this please comment.thanksWhew !!!!!

author: radioguy123
author_rank: Regular Contributor
timestamp: December 07, 2021, 04:45:51 pm
content: Well the power supply is still working with my mod. So I guess the extra TVS device isn't raising the voltage too much. I guess time will tell.

author: whatboy
author_rank: Regular Contributor
timestamp: January 04, 2022, 01:08:25 am
content: I have a TDS684A dead, the PSU doesn't seem to be the original one, because the pins on the conector are bended, like it is too tight but that's what I got when I bought the scope, it was working, but now, doesn't want to start, I took the PSU out and has the 12v (15v) stdby on the hot side, and the 15v (15v) stdby, I hook the fan on Pins 1,2(+) and 3,4(-) on J6 and when I use some thing to join pins 3 and 5 on J5, it won't start, sometime I can hear the relay, but still nothing happens, what else can I check???

author: whatboy
author_rank: Regular Contributor
timestamp: January 04, 2022, 01:44:05 am
content: I think I see a problem, I do not have the +5V REF, I do have 5v on pin2 of U5 (UC3525A) and 15v on pins 13 and 15, .187v on pin1, .9v on pin10... does this chip generates the 5V REF???

author: radioguy123
author_rank: Regular Contributor
timestamp: January 08, 2022, 10:29:13 pm
content: WhatboyThe problem with my power supply was I could not get the standby power supply to even start.If you are hearing a relay your standby is running.You have no +5 from that chip at all ?None of you power rails are working at all ?

author: whatboy
author_rank: Regular Contributor
timestamp: January 09, 2022, 03:22:05 am
content: No 5v from sg3525, i ordered still waiting... pretty sure it is the culprid, the datasheet says it is an 5v output... will update when I get the ic and replace it.

author: radioguy123
author_rank: Regular Contributor
timestamp: January 09, 2022, 07:01:42 pm
content: Yeah that sg3525 chip is the pulse width modulator that runs the power supply switching.If that chip isn't working then no output.So is the +15vdc bias from the standby circuit even getting to the chip ? Should be pins 13 and 15.

author: whatboy
author_rank: Regular Contributor
timestamp: January 10, 2022, 01:42:15 am
content: Yes it does! 15v stdby gets to pin 15 and 13

author: whatboy
author_rank: Regular Contributor
timestamp: January 13, 2022, 01:06:35 am
content: Got the PSU running, but still has some other issues, to channels do not work, and display doesn't work either...

author: radioguy123
author_rank: Regular Contributor
timestamp: January 14, 2022, 04:59:14 pm
content: Make sure the display controller is getting 25v. Obviously if the fan is running there is 25v present there.Have you run SPC ? If you have what does it show ?

author: whatboy
author_rank: Regular Contributor
timestamp: January 14, 2022, 05:12:14 pm
content: If I connect the display, unit will power and shut down like 1-2 seconds, and restarts like that, disconect the display unit powers on, so, the display is bad, have check the fly back and has some open windings... so fly back is bad, no other components seem to be bad!. one channel has the hybrid module broken and another one is burn, only two channels work!

author: radioguy123
author_rank: Regular Contributor
timestamp: January 15, 2022, 03:55:05 am
content: Ok so you cannot run SPC without all good hybrids. Doesn't mean you can't use the scope though.Too bad about the display. The crt high voltage winding on the flyback will be a very high resistance. So it may not be open.Let's see this is a 724. This is color right ? I've seen boards on ebay.I have a tds684a that the display is so dim it's hardly useable. When the nucolor crt display is working it's hard to beat. Even with a lcd.You may have to buy another unit for parts, to get one working scope.