FRAUDS IN NIGERIAN BANKS: NATURE, DEEP-SEATED CAUSES, AFTERMATHS AND PROBABLE REMEDIES
Repair of the TDS 7x04 PSU Front End · To trip the supply breaker, or blow a fuse, I would expect...
Transcript of Repair of the TDS 7x04 PSU Front End · To trip the supply breaker, or blow a fuse, I would expect...
Repair of the TDS 7x04 PSU Front End
NOTE: THE TDS 7X04 PSU EXPOSES POTENTIALLY LETHAL VOLTAGES. DO NOT ATTEMPT REPAIR UNLESS YOU HAVE
THE KNOWLEDGE, EXPERIENCE, COMFORT LEVEL, AND TOOLS TO WORK ON SUCH CIRCUITS SAFELY. NEVER WORK ON
A LIVE CIRCUIT ALONE. THIS NOTE PROBABLY CONTAINS ERRORS. ALWAYS CHECK ESSENTIAL FACTS PERSONALLY.
I recently purchased a dead TDS 7404 on eBay. The seller said that the unit was not working, and blew the mains fuses
when turned on. The unit was otherwise cosmetically in good shape. I powered up the PSU outside of the scope and
discovered that the seller was incorrect about the fuses; the PSU with no load tripped the 20A arc-fault circuit breaker
feeding the outlet into which it was plugged before the fuses had time to blow. There was also a momentary un-
localized arcing sound.
Tektronix was happy to quote $3800 for a new PSU. Neither Tektronix nor Martek (who made the custom PSU for
Tektronix) answered email inquiring about the availability of schematics. I was unable to locate schematics or further
useful details on the usual Tek scope web sites.
Preliminary Investigation:
The TDS 7404 service manual (pg. 6-66) provides a list of output voltages for this supply. Simple inspection tells us that
this is a multi-output, line voltage primary, switching power supply. The PCB has a white line delineating the high voltage
and low voltage parts of the PCB. There is a sticker on the side of C18 (one of the large, 470uf, 450V primary filter
capacitors) that provides typical current loads for each output. From those two sources, I was able to construct the
following table (note that +5V is produced and filtered separately for each connector, as opposed to +3.3V, which
appears on both connectors from the same source):
J2 V A Pins Source
+3.3 (see J1) A/B/C 1,3,5,7,9,11 T1
-15 3 A/B/C 13 T7
+15 3 A/B/C 15 T7
-5 15 A/B/C 17, 18, 20, 21,22 T7
+5 22 A/B/C 24,25,27,28,30,31 T7
J1
+12 6 A/B/C 5,6 T10
+5 18 A/B/C 8,9,11,12,14,15,17 T1
+3.3 35 A/B/C 19,21,23,25,27,29,31 T1
+9.8 (Vfan) 2.5 B/C 3 T10
Vcontrol
+5 1 N/A T10?
The label affixed to C18 also tells us that the design parameters for the PSU were 100-240VAC, 50-440Hz input voltage,
at 380 watts maximum.
The PSU actually consists of three multi-output switching supplies with a common front end. The “Source” column
above identifies which outputs derive from which supply. The PCB part number of the switching transformer of each of
the three supplies is shown in this column. “J1” and “J2” refer to the two 96-pin main connectors by which the PSU
provides power to the Acquisition Board (J2), the NLX Board (J1, via the PPC Board riser), and the PPC Board(J1).
The picture below identifies the essential elements of the PSU. The red circles identify which of the 1500uf, 35V
capacitors filter which output voltage. The three switching transformers and their PWM switching transistors are located
as shown. Each of the three subordinate power supplies is protected by an in-circuit ceramic fuse, with values as shown.
Since none of these fuses were blown, I concluded that my problem had to be in the front end. Unfortunately, in-circuit
testing (powered down) of the front end did not identify any shorted or otherwise failed components. In particular, the
bridge rectifier, which was a likely suspect since it sees both high inrush current and high-voltage inductive spikes,
appeared fine with an ohmmeter diode tester. Ditto for the front-end caps and semiconductors. I considered bringing up
the PSU on a variac until something bad happened, but I was concerned that internal arcing could create damaging
transients elsewhere in the front end. At that point, I needed to figure out how this thing actually works.
Theory of Operation:
Note: This section describes how I think things work. Feedback and corrections are welcome.
Input Filter - The PSU input supply voltage can range 100 to 240VAC, 50-440Hz. This means it can operate essentially
anywhere in the world without adjustment (other than fuse type). Using the simplified schematic below (created by me
from inspection; the inductance values are as measured), the line input passes through a simple canned RFI filter
module, then through fuses on both sides of the line (required if the line input is 220V), though the power switch (again,
both sides are switched), before reaching the PSU PCB through a two-pin AC-rated connector. The line voltage then
passes through a common-mode LC RFI filter network consisting of L1, L2, and L5; and C1, C2 and C69. Each of the
inductors consists of two separate but equal (same number of turns) windings sharing a high permeability toroidal
ferrite core. This makes it easier to get a lot of inductance with a small number of turns (important, since the wire size is
large for current-handling reasons). The anti-phase wiring arrangement ensures that the magnetic field resulting from
series-mode AC line current cancels to zero in the core (except for small leakage inductance). On the other hand, for
common-mode noise (high frequency currents or voltages that appear on both supply lines at the same time with
respect to ground); the two windings are in parallel and in phase. This presents a very high inductance between any
power supply noise source and the line input, so common-mode noise currents from the switching circuits in the power
supply will be bypassed to ground by C1 and C2 before they are passed to the input. C69 is a series-mode decoupling
capacitor; R1 ensures that any residual charge is dissipated when power is off, and perhaps helps a little to mitigate
large turn-on transients. I see no reason why R1 needs to be a 1% resistor.
Bridge Rectifier – The PSU uses a GBPC2506W (600V, 25A) full wave bridge rectifier, whose output voltage is 120 x 1.414
= 170V (for 120V) or 240 x 1.414 = 340V (for 240V). If replacement is indicated, a GBPC3508W (800V, 35A) provides
better protection from transients (IFSM is 400A vs. 300A). Capacitor C6 acts as a small reservoir in front of the boost PFC
circuit, preventing its input voltage from dropping to near zero when the input AC changes polarity. This limits any duty
ratio / switching frequency discontinuity. The design rule of thumb for this kind of capacitor is 3uF per KW, so with a
380-watt supply, 1uF is right on target. NTC Thermistor R7 has an initial resistance of 7 ohms, which helps to limit inrush
current on startup (when C18 and C19 are discharged), and then decreases in resistance to near zero as the thermistor
warms up.
Boost Power Factor Correction – R73, L3, CR15, Q3, Q6, C16, and CR9 (plus all of the circuitry in the green box) comprise
a boost-mode power factor correction circuit. This circuit is controlled by a TI (Unitrode) UC3854 power factor
preregulator, which uses average current-mode control to maintain sinusoidal line current, keeping the input power
constant with varying line voltage. Average current-mode control is a subset of Fixed Frequency Continuous Conduction
Mode (CCM) that uses current and voltage difference amplifiers, an analog multiplier and divider, and a fixed frequency
PWM to monitor the current in boost inductor L3 (kept continuous over the switching cycle), and make it track a
sinusoidal reference by selectively (and simultaneously) turning Q3 and Q6 on and off.
Details of operation for the UC3854 device are readily available online. See, for example,
http://www.ti.com/lit/ds/symlink/uc3854.pdf. In short, this circuit is a boost converter, whose output voltage must be
higher than the highest expected AC line voltage. Boost inductor L3 operates in “continuous mode,” meaning that the
duty cycle (when Q3 and Q6 are on) is dependent upon the ratio between input and output voltages. This duty cycle is
governed by four inputs to the UC3854: Vsense (the DC output voltage), IAC (the line voltage current waveform), Isense
(the line current, as determined by the voltage drop across R73, the current-sense resistor), and VRMS (the RMS value of
the line input). R73 is a 50 mOhm high wattage (its size suggests 5-10watts, but there are no wattage markings on the
part) current sense resistor on the boost inductor ground return path. Two 24N50 N-Channel MOSFETs (Q3 and Q6) are
paralleled to handle the current demand. If replacement is indicated, the 24A, 500V 24N50’s can be replaced with two
faster, pin-compatible 30A, 600V IXFQ30N60X’s, or conceivably with a single 60A, 500V IXFQ60N50P3. NOTE: These
parts (both the 24N50 and the IXFQ30N60X) electrically connect the drain and device heatsink tab. This means that
the PCB heatsink will be at HV DC ground potential unless measures are taken to electrically isolate Q3 and Q6. The
fact that C21 bypasses the PCB heatsink to HV DC ground suggests that the designer intended that Q3 and Q6 be
electrically isolated from the heatsink, although this was not done on my PSU. Q3 and Q6 share a heatsink with CR15,
the boost diode. CR15 (STTA12-060) is an obsolete 600V, 12A “ultrafast” recovery (55ns reverse recovery) diode. If
replacement is indicated, a Vishay FES16JT-E3/45 (600V, 16A, 50ns reverse recovery) looks like a good substitute. CR15
must be electrically isolated from the heatsink.
Bypass Diode CR9 (MR756; 600V; 6A) absorbs a significant portion of the inrush current during startup, helping to
reduce the transient load on boost diode CR15. The MR756 can be replaced with a Diodes Incorporated 10A07-T (1000V;
10A) if replacement is warranted. Safety capacitors C20 and C21 (4700pf; 250VAC) provide a path to ground for any
high-frequency noise on the high voltage DC ground. Bulk filter capacitors C18 and C19 (470uf, 450V) are paralleled to
provide a total of 940uF. These United (Nippon) Chem-Con KMH series capacitors are still available, but the newer LHS
series part (ELHS501VSN471MA50S; 470uf, 500V), offering both longer life and higher maximum voltage, are a better
alternative if replacement is warranted.
European high line voltage (255V * 1.414 = 361V) drives the design boost DC output voltage, which is likely to be near
400 VDC.
Switching PWM and Downstream Circuits - The 400 VDC output of the boost stage is passed through three in-circuit
fuses to the switching (chopper) transistors (not shown on the schematic above), which provide high frequency pulsed
DC (assumed to be around 100KHz) to the three switching transformers. Downstream of the three switching
transformers are voltage and current regulators for each voltage required (as well as thermal shutdown protection).
Since my PSU is not likely to have a problem in this area, explanation of these circuits is left to the intrepid reader. The
picture above identifies the output filter capacitor location for each output voltage.
Analysis: What’s Wrong with My PSU?
To trip the supply breaker, or blow a fuse, I would expect to see one of the following causes (probable causes of such a
short are shown in parentheses):
1) a line short to chassis ground (C2, C3, or C20),
2) a short across the AC line input voltage (R1, C69, or BR1, and (less likely), L5, L1, or L2), or
3) a short across the bridge rectifier output (C6, Q3, Q6, C16, C18, or C19.
There are other possibilities (e.g., a faulty UC3854 might be forcing Q3 and Q6 to stay on), but it seemed wise to start
with more likely and then move to less likely. Testing ensued. All of the components highlighted in green tested fine
when removed from the circuit and tested (at the relatively low voltages hand-held testers use). C18 and C19 both read
more than 20% low, so they moved to my to-be-replaced-regardless list. C16 came apart when being removed from the
board, so it was clearly a candidate for replacement (and might even be the source of the problem, although I saw no
signs of arcing). I was able to test L5, L1, and L2 simply plugging in the PSU with the other components removed.
Having found no smoking gun, and given their relatively low cost and the potential for hidden damage from arcing, I
decided to replace all of the above highlighted parts, except the inductors (which I knew were good at operating
voltage). I also replaced CR15 with a higher current faster part, as well as a few other diodes and capacitors exposed to
AC line voltage (and all electrolytic capacitors in the HV section), for the same reasons. The table below enumerates the
replaced parts and how to obtain them.
Qty PCB Part No. Orig. Part No. Orig. Value Replacement
Value Replacement DigiKey
Part No.
1 BR1 600V; 25A 800V; 35A GBPC3508W-ND
2 C2,C3 2200pf; 250V; safety cap same (300V) 490-9557-3-ND
1 C6 1uf; 450V; 10% 1uf; 630V; 10% 338-4132-ND
1 C16 0.033uf; 1KV; X7R same 490-8951-ND
2 C18/C19 470uf; 450V 470uf; 500V 565-5114-ND
2 C20,C21 4700pf; 250V; safety cap same (300V) 490-9563-1-ND
1 C69 .33uf; 250VAC; 10% .33; 275VAC; 10% 399-5971-ND
1 C93 6.8uf; 50V; 20% 6.8uf; 100V; 20% 493-11616-1-ND
4 C95,97,500,807 68uf; 35V; 20% 68uf; 50V; 20% 732-9596-1-ND
1 C808 .047uf; 630V; 10%; X2 same BC5120-ND
1 C900 330uf; 50V; 20% same (longer life) 732-9607-1-ND
1 CR9 MR756 6A; 600V 10A; 1KV 10A07-TDICT-ND
1 CR15 STTA12-060 600V; 12A; 55ns 600V, 16A, 50ns FES16JT-E3/45GI-ND
2 CR91,CR92 1N4006G-T 800V; 1A same 1N4006GDICT-ND
1 CR96 1N4936-E3/54 400V; 1A; 200ns same 1N4936-E3/54GICT-ND
1 CR97 MUR1100E 1A; 1KV; 100ns same MUR1100EGOS-ND
1 R1 1M; 1W; 1% 1M; 3W; 1% 749-2118-1-ND
1 R7 SL22 7R010 7 OHM 20% 10A 22MM same 570-1055-ND
2 R87,R90 ROX5SSJ27K 27K; 5%; 5W same A142834CT-ND
2 Q3,Q6 24N50 24A; 500V 30A; 600V IXFQ30N60X-ND
Notes:
1) The Martek designers get an ‘F’ in design for maintainability. Q3, Q6, and CR15 are mounted in the least
accessible way possible. Removing them is a pain. Getting them reinstalled with appropriate heat sink thermal
bonding and electrical isolation requires care and patience. Make sure you use non-conductive thermal paste.
Do not install C18 until Q3, Q6 and CR15 are installed and checked.
2) The PSU PCB is a .125” board with heavy copper plating. This means that anything connected to a ground plane
is a pain to desolder and solder. Solder wick won’t cut it; you will need a clean Hakko FR-300 (a 1 mm nozzle
worked best for me) or similar tool to remove components, and a high thermal mass temperature-controlled
soldering iron to install them. Especially for components that have trace connections on the top layer, make sure
that the solder fully bonds pad and pin.
3) Some of the components in the table above are larger than the components that they replace. They all fit, but a
bit of fussing (and in some cases, a bit of spacing above the board) is required.
4) If you use the 800V, 35A bridge, you will need a longer screw to reattach its heat sink (it’s a little thicker than the
lower amperage variants). A 6/32 x 1” stainless screw works well.
Final Testing:
The board came up without issue. All voltages, as measured on J1 and J2, were nominal (after reinstallation; the PSU
requires load). Here is a picture of the repaired, slightly better than before, PSU.