Re: Philips PM 3070 100MHz Scope Repair No.1

[dazz1]

So today I did some more checking to see if there were any latent defects I don't yet know about.
An obvious thing to look at was the condition of the power supply caps.  Rather than measuring cap condition ( ESR ), that would would require removing the caps (to do it properly), I looked at the power supply performance.  If there was a problem with the caps condition, I should expect to see degraded performance.  Specifically I used my trusty PM 3217 50MHz analog scope to look at the noise  on each output.  I also measured the DC voltages.

Using a scope as a noise meter is a reasonably average method, but good enough.  Peak to peak, each output displayed about 10mV noise.  Nothing alarming. 
The exception was the 48V outputs, which showed 10mV white noise plus about 40mV mains freq switching noise.    As far as I can tell, the +/-48VDC is only used on the output stage of the XYZ amplifier.  I don't see any noise on the displayed trace. 

The main difference between the trusty PM3217 and the "new" PM3070 is that by necessity, all the controls of the PM3217 analog scope are in the open on the front panel.    The digitally controlled PM3070 has multi level menus.   More features than knobs, but more to learn to find the features.  I have decided that the PM3070 is a very "cool" instrument.  Definitely a step up and a keeper.

[dazz1]

Hmmm!
When I put on the top cover, I noticed lots of holes over the really hot transistors.  The holes are partially obscured by the envelope that holds stuff like the manuals and probes.

I added a smear of heat sink paste to the contact points between the heatsink and the transistor casing.  Should make at least 0.0001% difference in temperature. 

[dazz1]

some photos showing details of the brackets that hold in the power supply PCB.

[tautech]

Note that I incorrectly titled the thread. 

Use Modify on the OP to edit the Subject/Topic line as I have in this reply.

[dazz1]

Hi
The PM3070 is back together again.  I haven't finished with it yet.  I still need to fit a replacement for the Rifa bomb and I want to do a LED conversion.  I need to wait until I have an order for parts large enough to avoid paying shipping costs.   

[dazz1]

Hi
I have purchased a 1m length of LED strip from Aliexpress to replace the hot wire lamps inside the PM3070.  It has 240LEDs/m and I can chop it up to 3xLED "lamps" that should fit within the available space without too much modding.  Each LED lamp will run on 12V. 
Cost is only about $USD7/m including shipping.  That puts the price of an LED lamp at about $USD 0.05 each.  I could not buy the parts for this cost. 

I just need to wait 3 months for them to arrive.

I expect to have about 200LEDs left over.   If anyone needs some LED lamps, I could chop up the strip and sell them separately. 
Of course there would be some costs to cover including:

• Customs clearance
• Bond store fee
• ISPM15 compliant packaging
• Carnet declaration
• Escrow service fee
• Insurance.

so the price should be around $USD68.37
For all those interested, just send a few payments to ripoff.a.holic@scam.me.com

[HighVoltage]

Looking forward to see, what that looks like with the LED mod.

[tautech]

Looking forward to see, what that looks like with the LED mod.

Dazz has another PM3070 incoming so he might get the chance to do another. 

[dazz1]

Hi
I have the LED strip .  I chose the warm LED and that is a good colour.   These things are BRIGHT.  I can't be bothered doing a quantitative  comparison but single LED looks brighter than the hot wire lamp.
The LED strip as delivered, is a strip of sixty segments connected in parallel.   There are 3x LEDs per cut-off segment.  Each segment includes a 150R current limiting smd resistor.
The strip has an adhesive backing.

I hit my first problem quite quickly.
I purchased 12V LED strip, but the lamp circuit for the graticule  provides 24VDC.  That is not a major problem because the only difference is the current limiting resistor.  This is set to limit current to 20mA.
Another option is to put two 3xLED segments in series. 

The lamp for the LCD display is 12VDC so that is OK.

I have found that these LEDs get quite warm to the touch.  Replacing the SMD resistor with a link, and mounting the resistor separately will reduce the LED temp, which can only be a good thing.
The other problem it solves is flexibility.  Trying the bend the strip with the smd resistor in-place is expected to snap the resistor.  I unsoldered the resistor, then resoldered after the bend. 

Taking the smd resistor off-strip has another advantage.  I can selectively replace LEDs with links.  Rather than being limited to 3xLED segments, I can choose how many I want.  Although I could fit 2x 3LED segments in the same space as the original lamp. I think that would be waaayyy tooooo bright and too hot.   Easy to adjust by increasing the resistor.


At 12V, the 150R resistor produces 60mW, but a 330R resistor will more than double that.  Not a lot but it adds up.

I think in this application, a simple resistor is entirely adequate.
The single 3x LED segment bent in the photos is not great.
Note that the 3x LEDs would be fitted standing up, not lying down as shown in the photo.

So current thought is to use 2x segments configured with a total of 4x LEDs. 

[dazz1]

Hi
I came up with a final configuration.  I wrapped 2x segments (= 6LEDs) around a piece of sheet metal that holds the LEDs within the existing lamp holder.  The aluminium doubles as an unnecessary heat sink.  This LED assembly is a direct replacement for the hot wire lamp.  No other modifications required to the scope.

The strip is flexible, but not flexible enough to wrap around the tight bend of the aluminium.  The Cu split on the front edge.  Fixed with a splash of solder.    I removed the 150R resistors before bending.
The lower front edge has a loop of wire soldered on.  The purpose of this is to prevent the LED assembly moving.  It hooks under the light pipe.

The 6x LEDs are much brighter than the hot wire lamp, and one of the reasons for that is probably about 80%-90% of emitted light actually ends up in the light pipe.  In comparison, probably at least 70%-90% of the hot wire light ends up somewhere inside the scope.

I was expecting to add in a reasonable sized series load resistor to reduce the current from 20mA/segment (40mA for 2x segments) off 24VDC.  It turns out that too much light is just enough.  The graticule is easy to see in full daylight.

The light circuit provides about 8V to the lamp, even when turned fully down.  So the hot wire lamp was always illuminated.  It just so happens the LED assembly requires 8V to just turn on.  A happy coincidence. 
To convert the LED strip from 12V to 24V, I just added a 300R resistor.  This is in addition to the 150R resistors already fitted to the LED strips.

So all of this has been a dress rehearsal for doing an LED conversion to the LCD back light.  At present, this is anemic and makes the scope more difficult to use than it should be. 
Unlike the graticule, there is no front panel brightness adjustment for the LCD back light.  The lesson from the graticule conversion is to add at least twice as many LEDs as reasonable, then double that number. 

[dazz1]

Hi
Having completed the relatively simple graticule LED conversion, I am now focusing on the LCD back light LED conversion.
This will require an entirely different approach.

First, some self imposed constraints.

• I don't want to take the LCD unit apart.  It is held together with bend-over metal tabs.
• This means I can't make major modifications to the light pipe.  Specifically, there is a slot where the existing hot wire lamp is fitted.  That is where the LEDs need to go. The slot could be slightly modified, but significant mechanical material removal is off the cards.  A hot knife might work for rough cuts if the light pipe is thermoplastic.   
• Polishing anything is not an option because of the risk of static electricity frying the electronics and also spraying polish and dust around the place.

The slot is 10.1mm W, 2.45mmH, 3.00mmD.  That is too small to fit a segment of LED strip.  I need something custom.
The 2835 LEDs are 3mm L, 2.47mm W, 0.94mm H.

If I made the slot slightly longer (11mm), then I could possibly fit 4x LEDs side by side.

[dazz1]

Jamming in that many LEDs into such a small slot, which sits near the top surface of the enclosure, would likely create an overheating issue.

I tried to simulate this by squeezing a 3x LED segment between my fingers.  My fingers instantly turned the colour of yellow hot steel.
That indicates how bright these little LEDs are.

[dazz1]

But the LEDs stayed surprisingly cool.

I then started looking at how I could make the back lighting sun-bright within a tiny space.

I started to look at datasheets.  2835 is a size format, not a spec.
The specs are reasonably consistent between different brands.

Attached is a curve that translates current to flux.  The LED strip drives 20mA through each LED.  The curve tells me if I increase the current to 60mA, the light output increases ~2.5x.    That is useful to know.  I can increase light output by either increasing current, or increasing LED count, or preferably both.

I could try and squeeze 4x LEDs @ 20mA, or I could comfortably fit 3x LEDs @60mA to end up with double the light output.

So that raises the question, why doesn't the LED strip run 60mA through each LED.  I think the reason is stability.  LEDs are a negative resistance.  The greater the voltage, the less the resistance (as the current increases).  The forward voltage across a 2835 LED is about 3.0VDC.  For 3 in series, that is about 9VDC.  If the supply is 12V, then there is only 3VDC for the 150R resistor to stabilize the current.  A higher current would probably not have enough resistance to make the system stable.

So I need a 60mA current source that will supply each LED from -12V. 

[dazz1]

Hi
I may have found out why the LCD back light was so feeble.  Checks on the lamp showed open circuit. 
There is absolutely no danger of finding a replacement lamp locally.
Obtaining one from over seas will be more difficult and expensive than an LED conversion.

[dazz1]

Hi
I have carved out the slot so it is 2mm longer than original.
This will allow me to comfortably fit 4x LEDs in the space.
4x LEDs will be simpler to mount than 3x LEDs because of the symmetry. 

[dazz1]

Hi
Here is the circuit I am about to implement. 
Two LEDs in series with a 100R resistor will allow 60mA to flow.  This is the maximum rated current.

The third resistor allows fine tuning of the current to both pairs of diodes.

There isn't much space for a PCB to mount the LEDs.
I used a CAD program to figure out how things will fit.

[dazz1]

Hi
I have finished the PCB.  The shape is the way it is to get around numerous obstacles.  The tracks were scratched in with a sharpened jewelers screw driver.

The images of the PCB are low res but I did them with a scanner.  It should be possible to print them off to scale 1:1.

If I made it again, I would make the slot slightly shorter.

[dazz1]

Hi
With the current set to only 20mA per LED (40mA total), the back light was nice and bright.  It could almost be too bright (if that is ever possible).
  If the backlight is too bright, the LCD contrast will be too low.   I won't know until I get it all back together again and power up the scope.

The colour is warm white, the colour of the LEDs.  There is no hint of amber as in the original hot wire lamp.  If you follow this guide and want an amber back light, you will need amber LEDs. 

So time to wire it to the scope and put it all back together again.  I just hope I haven't static zapped the LCD driver ICs.  I have read they are quite delicate.
If they are zapped, the LCD driver chips are still available, so the LCD board is repairable.

[dazz1]

Hi
Here is a photo of the final result of the LED conversion.

The conclusions are that:
6x LEDs run at 20mA are sufficient for the graticule.  Traditionally I don't usually have the graticule illuminated, but the High brightness of the LEDs makes it easier to see.
4x LEDs run at 20mA are almost too bright for the LCD backlight.   I have the option of reducing the current by changing one resistor, but I won't do that.  The LCD is clear and easy to read, in the dark or in full day light. 

If I was doing this again I would:

use 3x LEDs run at 20mA for the LCD back light.   Three LEDs will fit in the lamp slot without modification.   If not bright enough, increasing the current up to 60mA would provide adjustment.
get a proper PCB made.  I scratched one out, but it looks terrible.  Fortunately it is well hidden.


Tips:
I swapped an inductor to the other side of the PCB to make room for the LED PCB (no photos). 
I cut off the wires to the hot wire lamp and reused these to connect to the LED PCB.  I did not want to risk solder for the lamp wire melting a hole through the matt back light diffuser.   That would have been a disaster.
I could have made the LED PCB smaller, but I used it as a heat sink.    The LEDs do not get hot, but they are also located in a heat trap and heat is the enemy of electronics.

Was it worth it?
Rationally, it would have been better to skip-bin the PM3070 and buy a new modern scope, but that would have required cash and been no challenge.
The LED conversion has significantly improved the usability of what is a nice scope, more than adequate for my needs. 

[HighVoltage]

Nice job, looks good.

[dazz1]

Hi
Here is a side by side comparison of a standard PM3070 scope with weak and feeble back lighting compared to the brilliant (pun intended) LED conversion.

[tautech]

Good work Dazz. 
I can see you going into business making LED lighting conversion PCB's for these and selling them worldwide. 

The same model but wow how different they look.  Do you think you'll keep both ?

[dazz1]

Hi
I am not sure if I will keep them all yet.  I now have 4 scopes when just a couple of weeks ago, I had one.   

I think if I keep them all, they will just sit on the shelf.  It will probably be better to sell 3 of them to some young budding electronics hobbiest that can't afford a new Siglent - yet.  I would regard that as a charitable thing to do.  There is no way I can sell these things for anything close to the value of my time fixing them.    I am finding that fixing them is quite therapeutic.   If these old scopes can gain a useful life extension as a result, that would be a good thing.  I know these scopes are really old school, but they are still useful instruments.

I'd be willing to make something but I don't see much future in making LED conversion kits for these old scopes.  Every model of scope would need a custom design.  There is no doubt that the LED conversion makes the scope much easier to use.    It was definitely worth doing, as long as no one counts the value of my time.

[HighVoltage]

  I now have 4 scopes when just a couple of weeks ago, I had one.   

Yes, they like to multiply...

I have plenty of the PM3394B and they also could benefit from a LED conversion.
Thanks for sharing your project.

[dazz1]

  I now have 4 scopes when just a couple of weeks ago, I had one.   

Yes, they like to multiply...

I have plenty of the PM3394B and they also could benefit from a LED conversion.
Thanks for sharing your project.

Hi
I would sell my neighbour's wife to white slave traders to get a PM3394B.