HP 1660AS XL130-3604E PSU problem

[modrobert]

I have an HP 1660AS logic analyzer with power supply 'Computer Products XL130-3604E' where the +5V reference (test point in service manual) is ~+2.50V without load, with acquisition board as only load the +5V test point voltage drops to ~+1.14V. Adjusting the pot (+5.00V ADJ) next to the 5V test point have no effect.

The voltages on the PSU 20-pin output connector without load are as follows.

Code: [Select]

pin 1,2,3,4 +2.50V (expected +5.00V)
pin 9       +2.49V (expected +3.50V)
pin 11      -0.01V (expected -5.20V)
pin 13      +8.24V (expected +12V)
pin 15      -5.99V (expected -12V)
pin 17      +5.37V (expected +12V)
pin 18      -0.01V (expected -5.20V)
pin 19      +6.36V (expected +15V)
When the PSU started failing it was intermittent, it could work for like 20 seconds, all LA self tests passing, then fail again. Now it's "constantly failing" with output voltages as mentioned previously.

I have since then replaced all electrolytic capacitors (31pcs total), roughly 20% of the old ones were leaking, but the problem still remains. Also checked power transistors in-circuit which seem OK, at least no shorts between collector and emitter.

When measuring the diode bridge from the main transformer I get +2.50V, same as pin 1,2,3,4 on the PSU 20-pin output connector which is supposed to be +5.00V, so I'm assuming something must be wrong before the regulation, perhaps the diodes in the bridge or the transformer?

Here is a photo of the PSU linked...
http://modrobert.ddns.net/pics/HP_1660AS_PSU_XL130-3604E_top.jpg

...and the small board soldered vertically in PSU:
http://modrobert.ddns.net/pics/HP_1660AS_PSU_XL130-3604E_side.jpg

If anyone have any ideas or what to look for it would be appreciated, power supplies are not my strong suit. I can't find any schematics for the PSU (XL130-3604E) when searching online, and the HP 1660AS service manual does not cover it.

[Toasty]

What/where is the 5V adjustment pot connected to?  I would check in that area first.

Note: Transistor/device A8 is not secured to heatsink...

T

[modrobert]

What/where is the 5V adjustment pot connected to?  I would check in that area first.

Note: Transistor/device A8 is not secured to heatsink...

T

OK, I will check where the pot leads.

Yes, I removed the A8 LM350T voltage regulator from the heatsink to make sure it was not shorting out against it (Vout, pin 2, also leads to heatsink according to datasheet). There is a plastic film on the heatsink protecting the metal part of the TO-220 package from making a connection, was thinking it might have gone bad over time.

A8:
----
Vin = +1.38V
Vout = 0V
R1 = 0 Ohm
R2 = 233 Ohm

At first, after measuring voltages and resistance, I thought the problem was with the A8 LM350T, because the output was shorted straight to ground, but after checking the board visually the trace from Vout actually lead straight to ground by design.

[Toasty]

Hmmm...?

I would check that again. There's no sense in a voltage regulator output being connected to ground... 

Remove the device and test it using a DMM on Diode Check.  While it's out, check what Vin is on the board.

T

[modrobert]

I checked it several times, the trace from Vout (pin 2) goes to ground.



The "+5.00V ADJ" pot goes to the small vertical board (second photo linked in first post), can't trace the leads there without desoldering the small board.

[Toasty]

I see it, but I don't grasp the concept, yet.

Measure from Vout to Vin please.  Still get 1.38V ?

Where does the trace I highlighted in the attached image go to?  It's the input leg of A8.

T

[modrobert]

I can't measure A8 live at the moment, have the S1 component labelled "MTS90A" on the first photo desoldered along with some other components. Following suggestions from a nice fellow called George over a the "HP/Agilent Test Equipment" group on Yahoo.

Having now taken an even more careful look at your photographs the reason for your problem is most likely the device above and to the left of Q2 (main switcher?), it is marked "MTS90A". This is a Currie effect over temperature switch which is used to reduce the drive to Q2 when Q2 gets too hot.

As a general rule SMPSU's either work or they do not, this is a very odd one that has a protected mode due to the MTS90A reducing the drive to Q2. From your photograph the MTS90A appears to be "burnt" at one end due to it being overheated.

Normally if a SMPSU is unhappy about the total load on it it starts to tick, also a lot of SMPSU's require a minimum load to operate.

However, I did follow the trace from the highlighted pin on A8 as you suggested, it ends up at the orange wire of the main (biggest) transformer. Following from the transformer side it first passes through a diode with heatsink marked CR25 (close to the pot we discussed earlier), then through a capacitor (100uF 50V) marked C35, and finally to Vin pin on the A8 LM350T voltage regulator.

[Toasty]

Could you tell me where C35 and the diode are?

I understand the MTS90A action, but is it defective?  If it's operating as it should, then surely when doing a cold startup it should not be active.

I just noticed a tantalum cap glued to the back of Q2.  Tantalums are known troublemakers and I am suspicious of it.  The overall issue seems to be a lack of "drive" for the switching circuit(s). If that cap is causing a problem with Q2, it could be the root of the problem.

T

[modrobert]

The CR25 diode is close to the pot R40 we discussed earlier (right side of the board connector output on photo, big black diode with heatsink), and the C35 is very close to the A8 Vin pin, it's hiding right behind the formation of 2x3 1000uF 35V caps there, can barely see the blue top of it.

BTW: After desoldering, how can I test if the tantalum cap is OK? I have an MCU based ESR tester, will that work for this type?

Thanks for the help so far.

EDIT:

I don't know anything about MTS90A, but found a patent which seems to match it.

Thermally actuated switch
https://www.google.com/patents/US4509029

[Toasty]

>>...I have an MCU based ESR tester, will that work for this type?<<

Absolutely.

If you ohm that switch, it should read open at room temperature.

Thanks for tracking that line down.  Pretty much convinces me that the switcher is not working.  As the other gent said, either they work or they don't.

Are there any components on the underside of the board?

T

[modrobert]

I will try to draw a simple schematic of the area of Q2 and Q3 (TIPL755A NPN transistor in TO-3 package) where the MTS90A traces leads to, assuming the board is only two layers it should be correct.

[modrobert]

The C46 tantalum cap you asked about is 37.65uF, ESR=0.35 Ohm, Vloss = 0.4%.

Still drawing the partial schematic, it will be drawn by hand and look like crap, that much is certain.

[modrobert]

OK, I received reverse engineered schematics to a similar SMPS found by George.

http://modrobert.ddns.net/pics/supplyHP54503.pdf

There are some differences between the SMPS in my HP 1660AS and the one RE:d, have found these so far:

1) I have no pots on what he refers to as the "CTRL PCB" on page 3, he has five of them.
2) There are only six 1000uF 35V caps between my diode bridge heatsinks, on page 6 he draws 8 caps.
3) Q2 is a TIP41C, not TIP42C as on schematics.
4) There is a botch/afterthought 240k ohm resistor across the legs of C1 on the bottom side of my board.

However, most of it agrees with my board, so no need to draw new ones.

[Toasty]

The cap is okay.

Is the MTS90A okay?  Does it show as "open"?

The schematic confirms 2 things.  That A8 is used to create a negative power source, -12V, and that A2 is a crowbar protection device.

Is the +5V the only line with a problem?

T

[modrobert]

OK, thanks for the info about A8 and A2, questioned my sanity first time I followed that A8 voltage regulator Vout trace to ground, especially since it looked so promising as the fault reason.

Yes, MTS90A is open when I checked resistance after desoldering, also tried to power up the scope without it, same fault symptoms.

In my first post I have a table of the output voltages, most them are roughly half of what they should be, except -5.20V which is close to zero.

[Toasty]

It sounds like you have a 220V unit plugged into 110V.  Isn't your power around 220V?

What's the output from the bridge rectifier?  You should have around 312V across the mains caps.

T

[modrobert]

I've measured the mains to 230V AC on the mains input connector, so that seems to be OK.

My voltage across R1 is 158V (between the C22 and C23 caps on the board photo).

[Toasty]

Ah-ha!  There's a switch built-in to the socket for the cord.  It wasn't in the schematic but I found it by Googling photos of the unit.  You pull it out and flip it for your local voltage.

Check what you read at the AC pins of the bridge rectifier compared to the DC output should be ~1.414 times the AC volts.

230/2 = 115 (plug is in 220V position)
115 * 1.414 = 162V - Given meter tolerances, Close enough!

T

[Toasty]

Wait!

I just re-read your previous post - "I've measured the mains to 230V AC on the mains input connector, so that seems to be OK. "

Where is that connector?  On the board?

CAREFULLY read the AC at the 2 center pins of the bridge rectifier please.

The do the same to the 2 outside legs (DC).

T

[modrobert]

I've been using the scope for years here before this, and the scope socket/switch is in the 220V - 240V position, also verified the fuse when the problem started, as the scope was dead sporadically when the fault was intermittent (as mentioned in the first post).

[modrobert]

Where are the "bridge rectifier" pins exactly? Can you give me some component identifiers on the schematic or describe where on the photo?

The mains connector is on the board, where it measures 230V AC.

[Toasty]

In your 1st picture, to the far right, between the mains caps and the rectangular light blue cap.

Thin black device with a + sign next to it's yellow box on board.

Center 2 pins are AC in, outside are DC, positive is as marked.

Where is the cord plug connection made?

T

[modrobert]

Aha, I think you mean CR1 on the schematic (page 5), it's black thin component with 4 pins. OK, I will measure as you described.

The mains connector is on the top right side on the photo, brown connector, you can see some of the pins there where I measured 230V AC.

[modrobert]

I've measured the CR1 bridge rectifier (GBL06), as follows:

234.3V AC
322.2V DC

[Toasty]

Good!

Was C5 (page 5) replaced?  What brand & series please.  It is critical to the oscillator circuit.

Also C45.   Both circled in PDF.

Do you have a scope?

T