hp 54503a scope psu

[SoundTech-LG]

Hello,
I have two HP 54502 and one 54510. To repair the Boschert power supply you need to replace all the electrolytic capacitors. This issue has been fixed in
https://www.eevblog.com/forum/repair/hp-53310a-power-supply-troubles/msg934342/#msg934342

It will take some time to do it but you'll get a full working supply after that.
Fact is that there is a crowbar to protect its hardware. The SCR is triggered by some voltages unbalance. Regulation is done on the +5V only as the schematics shows. Someone has tried to replace it with standard MEANWELL PSUs. He finally let it down, results were not good.

Most of the time...  but NOT always. I have repaired multiples of these Boscherts, once I had one where the capacitor electrolyte was wicked up into the transformers, and you could measure leakage from primary side to secondary side. Replacing the transformers cured the problem of blowing the switching TO-3 transistor. Also, have seen where the electrolyte is wicked up into the main board mounted trimmer pot and caused it to be intermittent. I have one right now I have not swapped the caps yet, but have noticed some slight amount clear liquid near some of them, so obviously on their way!!! I think these were part of some bad manufacturing runs at the cap factory. I have seen some of the very old Boscherts not have any issues at all.

[Zappy]

I think you are right. Most of the time, leaking capacitors are the 1000uF 35V on the 5V, -5.2V and 15V. Once the electrolyte came to pollute the potentiometer setting the -12V and the voltage drifted to -13V causing the power supply cut off by activating the SCR.

[m3vuv]

this is a translation from what i found from a french guy who reversed engineered one of these:Type text to translate here..The breakdown repairer ... Here is that my favorite oscilloscope no longer works (November 2010) after a few months without use!


Symptoms: Still lights up but no longer accesses any menus or normal images, just displays an error message. Sometimes the internal buzzer works when switching on.

My first fear in noticing this defect was that the EPROMs are defective, because these components can lose their contents after 15 to 20 years. And this device has a certain age. Fortunately, after removing the hood, I hear the food making funny noises (tchik-tchik-tchik ...)!

"It should be fixable without too much trouble" I thought ... If I had known!


Click on the photos to see them larger in another window ...

open device

Once the top cover is removed, we discover complex electronics. The power supply sits in the airflow of the fan, which is a good place to be. Against the left wall is the control and power supply for the CRT tube, and at the bottom of the box the management board takes up all the space.

open device

You can better see the inside of the device once the power supply unit has been removed ...

open device

Top view: the main board is visible. On the left, all the microprocessor part and its periphery (memories, inputs / outputs, ...). On the right the input attenuators and the A / D converters. This device has 4 channels of 500MHz.

open device

A look at the microprocessor: it's a 68000. Logical choice for when this device was designed, the late 1980s. User data is stored in non-volatile RAM memory from Dallas Semiconductor: those circuit horrors including a battery molded into the housing! 20 years later, they have obviously become bad!

open device

The power supply extracted from its internal box. I thought the troubleshooting was simple, but ... This card (built by Boschert) is particularly complex! It is indeed a switching power supply but also comprising conventional regulations ... In addition, the control technique of the power transistor does not appear at first glance: we do not find here the classic circuits of ordered. Ouch!

The failure is "pumping": the power supply starts then stops, then resumes, ... Usually this is a short-circuit in one of the secondaries (diode, capacitor) or a failure of the protection circuit or the PWM oscillator. Not always easy to diagnose in these power supplies because everything is linked: if there is a problem with the secondary, this has an impact on the primary, and vice versa.

Visual discovery: This step is necessary to identify the different parts and understand how this power supply works.

It seems impossible to find the diagrams of this machine on the internet, this firm even seems to no longer exist. After a few initial searches for "classic" failures, we must look further! And given the complexity, without a diagram it will be difficult ... We will have to reverse-engineer!

open device

Primary side view. We can clearly distinguish the mains filter (mainly the two blue capacitors and the double self-inductor in the form of a small transformer), the rectifier and the two filter capacitors (560µF). The power transistor is a TIPL755, mounted on a heat sink. The other components are mounted nearby. There is a TIP41, a 2N2222 and a 2N2647 unijunction. Not easy to understand the operation at first glance ...

open device

Secondary side view. Quite a few capacitors, large cooled diodes and filter chokes: this power supply gives several voltages for the different parts of the device. But we also see, curiously, classic LM350 regulators and MOS transistors on heatsink. Only the drawing of the diagram will help to understand ...

open device

The voltage control board. An LM339 (14-pin integrated circuit) serves as a voltage comparator, an LM317 (TO220 box on the right) gives the reference voltage for the comparators. A thyristor “crowbar” circuit (left TO220 box) short-circuits one of the outputs in the event of voltage overshoot, so as not to “kill” the uses in the event of a power failure.

open device

View of the output connectors and test pads. I added a red LED diode on the 12V output to see the operation. The output voltages are numerous: + 5.15V digital; + 3.5V analog; -5.2V analog; + 12V analog; -12V analog; + 12V display; + 15.5V fan. All the masses (0 volts) are common. There are also a lot of potentiometers everywhere.

open device

First steps in troubleshooting (November 2010): static test of all power semiconductors, deactivation of the "crowbar" circuit (to see if this http://www.radiocollection.be/fr/fr.htm

[m3vuv]

this may be better? he is not the one causing the pumping), ... Some secondary and decoupling capacitors replaced, ...

Diagram survey. It is indeed a switching power supply, but certain outputs (+12, -12 in particular) are stabilized by conventional regulators. The capacitors are very broadly sized.

It is by testing the power supply via a variac and slowly increasing the AC supply voltage that I see this: it starts up and is quite stable around 150V AC, up to approximately 180V. Between 180 and 230V, it starts to oscillate again! My research then focused on the primary side, and I discovered a small defective 100nF ceramic capacitor! The 470µF on this side was bad too.

open device

For safety, I decide to replace all the electrolytic capacitors, some having leaked a little.

Here is the set of capacitors replaced, on my scrap sheet ... You can now download my survey notes for the schematic of this power supply is available: see the end of this page.

open device

But despite troubleshooting the power supply, the device does not start more than before, and even worse: now there is no error message at all! And yet, all the supply voltages are present, and very stable ...

The fight was not over yet! I then received some interesting information from a former engineer at HP. If the starting of the power supply is not correct (shift in appearance of the different voltages), the motherboard may not work at all. This gentleman sent me, for testing, a complete and tested diet! You can find visiting his eBay store: "watronics92".

Restart with the tested power supply ... And still the same problem, no start! That's when I decide to check the signals at the 68000 pins. All data and address lines are frozen, while the power supplies, clock, and other signals are OK. Weird! By checking the power supplies of the 4 EPROM circuits, "something" is happening. A reboot, and ... It works again! I probably moved an EPROM in its socket while working in the scope.

I also know now that my power supply repair is OK and that the failure to restart is not due to my troubleshooting. Phew!

motherboard

Now the total. The device can operate again, so it's time to do some good maintenance on it. And to start, replace the NVRAM memory (built-in battery). In fact, it no longer stores anything, and the calibration is lost after each cut. To do this, you have to extract the large motherboard ... In this photo, the NVRAM is already de-soldered and replaced by a 28-pin support. (sorry for the blurry photo ...)

NVRAM

The Dallas Semiconductor type DS1235YW NVRAM circuit, widely used by HP in this series of oscilloscopes. The date code is 8912 (12th week of 1989): the internal battery will still have held up well! This circuit is no longer available and is replaced by the reference DS1230Y.

anti-static

To work on such a circuit, anti-static precautions are required: earthing of the plate and the soldering iron, bracelet, etc.

Home NVRAM

Not having a new Dallas NVRAM on hand, I built one with components from my stock, to test. The 51256L RAM is pin-to-pin compatible with the DS1235Y. All that remains is to assemble a few components, and a 3.6V battery. Diagram below ...

NVRAM diagram

The diagram of my assembly. The diodes provide a logical “or” between the supply or battery voltages. The 47k resistor keeps the CE pin high when the power is off, so the RAM consumes almost nothing on the backup battery. But here in this oscillo, "something" is keeping this pin low when the power is off. Too bad, I will put an NVRAM back! But this allowed me to test the memorization of the calibration, and it works!

March 31, 2011: Continuation and end! Here it is fixed! After repairing the power supply and replacing the Dallas memory, saving the EPROMs then recalibration. It took a while, but I'm glad I saved it. It saves money (avoiding buying another one), and it's ultimately a good experience!

Calibration

At the end of the calibration, the messages are reassuring: everything is ok! Here is this device good for the service again.

Documents:

My statement of the "Boschert" power supply diagram (also valid for HP54051A, HP54502A, HP54503A, Hp54504A) is available HERE (PDF file 480kb).

NOTE (not present in the drawings): the voltage Vref must be adjusted with R41 to 2.75V (output of A1, the LM317).

The copy of the 4 27C101 EPROMs can be downloaded HERE (280kb ZIP file containing the 4 binary files).

© 2010-2018 Radiocollection.be

Back to repairs page

Back to home page

[m3vuv]

well i spent an hour digging off all the silicon goop covering this thing and have so far  renewed 10 of the 1000uf 35v caps,ive used rubicon 105deg types,they are only about 2/3 the size of the originals,was a struggle to remove them,ive a 50w iron and still had to use a smd hot air gun to preheat the board!,it sucks heat like crazy!,im now waiting for all the other caps to arrive,so far all of the 1000uf caps were leaking,ive decided to swap every electrolytic on the psu,just hope this cures to ad+da converter and analog trigger fails,i found the 5v rail when using the atx psu sank to just over 4v when connected to the main board,as i see it ,its either the atx psu not meaty enough on the 5v rails or sommat shorted on the main board,just hope this sorts it as otherwise the scope is mint condition,i did think of feeding the 5.2v rails from a 30amp lab psu but decided to hold off untill the original psu is working as i think feeding one rail on its own could be a bad idea,i recal years ago reading some datasheet on a certain ic that if one pin went above another pin it killed it.

[m3vuv]

hope this helps

[m3vuv]

Well i fixed this,had to replace every cap on the secondary side,connected the psu to the mains and switched it on,could see a fireworks display from under one of the caps!+ the psu was pumping/cycling,i removed the cap with the light display under it and found a burn mark on the pcb,it wad already been cleaned,i got o bur on a dremmel and dug out the black part between two ajacent tracks down to about 1.5mm deep and cleaned it againe,then put my fingers in my ears and connected the scope,was a bit concerned as the 5.2v rails were not set due to not having a 2 ohm load resistor!,anyway long story short it works and passes all cals and self tests,i fitted a socketed nv ram as well,happy days,just got to learn how to operate it now,seems a lot more complex compaired to a normal cro.