TDS 400 Series TDS460A Soft Start Circuit stopped working [SOLVED] - Page 1
Forum section: electronics / testgear

author: icezebra
author_rank: Contributor
timestamp: April 06, 2025, 07:24:38 am
content: Thankfully someone from Oregon recently posted schematics for the TDS460AThis repair would not have been possible (in a reasonable time) without themSee attachments for details<EDIT> Row 34 in the spreadsheet is too short and is hiding some text. Stretch it to 3 rows high to see all the text.Problem: On/STDBY Power Switch on front panel of my TDS460A would not turn the scope on.Solution: Repair resistors under the conformal coating near the calendar circuit.

author: icezebra
author_rank: Contributor
timestamp: April 06, 2025, 07:26:48 am
content: File size limit requires multiple posts

author: icezebra
author_rank: Contributor
timestamp: April 06, 2025, 07:28:29 am
content: File size limit requires multiple posts<EDIT>Correct Output Photo to show that ON/OFF signal is at D4

author: icezebra
author_rank: Contributor
timestamp: April 06, 2025, 03:18:47 pm
content: Huge shoutout toFerrous Steinkafor making this repair possible!https://groups.io/g/TekScopes/topic/tds420_460_schematics/110671692Onhttps://w140.com/tekwiki/images/6/60/Tek_420_460_schematics.pdf(PDF page 86) there is a note that implies that (when it is grounded) the white wire from the A07 Aux Power to 'E5' on A25 Low Voltage Power Supply will put the A25 LVPS into STANDBY. My observation is that A25 Low Voltage Power Supply is in normal operation when this wire is grounded and in STANDBY if this wire is open/disconnected. This may be a LeCroy vs. Tek supply config thing. This white wire at E5 is electrically connected to ON/OFF signal on D4 on A03 CPU Card.Also, if you wish to probe the ON-OFF J-K FLIP-FLOP power control soft start circuit (from underneath) without disturbing the conformal coating, see the signal diagram posted above.I used the attached grid drawing to figure out what each via is connected to. I was able to determine the state of R28, R29, CR3, U44 pin 8 and U44 pin 9 without disturbing the conformal coating. Also, if you connect the Calendar Back-up battery to J5 then you can test the operation of the soft-start circuit with the A03 CPU CARD on the lab bench, disconnected from the A01 backplane and disconnected from J4/P38.<EDIT>It may also be possible to probe U44 pin 12 from underneath (at the via just to the left of C38 on the topside) but I have not verified that. This via is located just above the '5' on the barcode label bottom side in the attached grid drawing. If you confirm this via is connected to U44 Pin 12, kindly post your finding in this thread!Note that the bottom side photos in the grid drawing are flipped left/right (to enable mapping the through-hole vias, using the grid.)

author: icezebra
author_rank: Contributor
timestamp: July 19, 2025, 04:31:55 pm
content: I previously attached a detailed repair summary to the article above in an Excel spreadsheet with photos and a schematic walkthrough, but is seems to have been deleted. Here is an abbreviated version:Repair summary:R29 the 56.2K was failed open and replacedR30 the 100K was bad solder joint and reflowed.R30 Measured 150K after repair, so it may need to be replaced in futureDetail:Soft standby circuit is described in Tek_420_460_schematics.pdf located here:https://w140.com/tekwiki/wiki/TDS460https://w140.com/tekwiki/images/6/60/Tek_420_460_schematics.pdfExplanation is on PDF page 88 and detailed schematics on PDF pages 20, 21, 22Also refer to "TDS460A Service Manual 070970304.pdf". It is not for serial numbers below 5000, but it is text searchable and it is close enough even if your serial number is below 5000https://download.tek.com/manual/070970304.pdfSignal flow:The ON/STANDBY signal is referred to as ON/OFF in the schematicThe ON/OFF signal begins on the Elastomer Keypad (assembly A17) at the MENU FLEX circuit as shown in Figure 6-2 on PDF page 172 (pp6-16) of the Service ManualBezel Buttons "A17" are also shown schematically on PDF Page 265 (pp9-2) Figure 9-1 of the service manual. They can also be seen 7:29 - 9:00 in this videoThe MENU FLEX circuit connects into J103 of front panel assembly A06 as shown in Figure 6-11 PDF page 191 (pp 6-35) of the Service ManualAs shown on PDF page 70 of the schematic (A06 quadrant 1C) the ON/OFF signal goes from the MENU FLEX circuit (A06 J103 pin 2) to the CPU-to-front panel FLEX CABLE on A06 J101 Pin 10As shown on PDF page 265/266 (pp9-2/9-3) figure 9-1 of the Service Manual, the ON/OFF signal on the CPU-to-front panel FLEX CABLE goes from A06 J101 to CPU Card A03 J4 (pin 10 on both)(Also shown on PDF page 2 of the schematic)Regarding off-page connector notation on the schematic:Example on PDF page 73 of schematic quadrant 6B (J103): TO A17  <1> (Where <1> means a 1 inscribed inside a diamond)This means the signal goes to the drawing for assembly A17 (The Menu Elastomer Keypad Assembly) quadrant 1A, drawing sheet 1 (that sheet will have a 1 inside a diamond near the title block)Unfortunately this drawing appears to be missing from the schematic, however we can infer the key matrix from J103 on PDF page 73 quadrant 6B and J103 on PDF page 70 quadrant 1CSpecifically, the ON/STDBY button on the elastomer keypad shorts J103 pin 1 (GND) to J103 Pin 2 (ON/OFF) as described on PDF page 89 of the schematicOn PDF page 21 quadrant 1A we see that J4 Pin 10 is connected to signal ON-OFF on the CPU card.On PDF page 22 quadrant 1C we see that ON-OFF is connected to input of the standby control circuit which is powered by the auxiliary batteryThis circuit is a simple J-K toggle FLIP-FLOP embodied on the B side of U44 a 74HC74 CMOS Device powered from the battery. The output of this circuit (signal ON/OFF) goes to PDF page 20 quadrant 5APDF Page 20 shows signal ON/OFF goes to the back plane A01 J3 Pin 35Not shown on the schematic A01 Backplane J3 Pin 35 is connected to A01 J6 Pin 9 signal REM-PWRSchematic PDF Page 2 shows that A01 Backplane J6 is connected to A07 Auxiliary Power Supply J7Schematic PDF Page 75 shows that A07 Auxiliary Power Supply J7 Pin 9 signal REM-PWR is routed toA07 W1 then to the A25 Low Voltage Power Supply wire E5Schematic PDF 86 shows that signal REM-PWR is connected to E505 (which is actually E5). This is a LeCroy Power supply schematic so it is a little different as U701 is shown as a 16PIN LM339 in the LeCroy SchematicOn the real Tek Power supply U701 is an LM393 8 pin DIP device. See photo below which shows E5 is connected to U701 pin 1

author: icezebra
author_rank: Contributor
timestamp: July 19, 2025, 04:36:41 pm
content: Other TDS400 series power supply and scope repair info:https://www.eevblog.com/forum/projects/tektronix-tds-460a-repair(power-supply