Re: Philips PM 3070 100MHz Scope Repair No.1 - Page 1
Forum section: electronics / repair

author: dazz1
author_rank: Frequent Contributor
timestamp: September 14, 2022, 04:07:34 am
content: HiI paid far too much for a broken Philips PM3070 100MHz analog oscilloscope. When these came out, I would have sold my children for lab experiments to get one.This particular scope came from the Royal New Zealand Navy. It is in good physical condition. Clean and no physical damage.Note that I incorrectly titled this thread for a PM3072 scope. Although the service manual is for both PM3070 and PM3072, I can't find evidence that a PM3072 was produced. If it was, it was probably a higher frequency version of the PM3070 that I have.Now I have to fix it.The power supply emitted a high pitched note, indicating distress. Early checks of the power supply output voltages were all close to zero. The service manual described a test load. I made that. This isolates the power supply from the rest of the scope. This confirmed that the fault was on the power supply board.

author: dazz1
author_rank: Frequent Contributor
timestamp: September 14, 2022, 04:21:10 am
content: I don't mind admitting that the kilo Volts present on the CRT scare me. There is no way I was going to stick in a screw driver to discharge the tube. There is too much risk of the HV discharging across the handle through me. If my grip slipped, my hand could end up too close to the screwdriver metal shaft. If the crocodile clip lost contact, the screwdriver would be floating at kV above the ground I was standing on.I used a piece of plastic electrical conduit to add greater separation and insulation between me and the kV. I cleaned the conduit to ensure no surface break over. The probe at the end is sharpened to make it easy to get under the insulator and to maximise electric field intensity. Even if I didn't touch the anode, it should arc over to discharge. This is much safer than the usual screw driver probe.

author: dazz1
author_rank: Frequent Contributor
timestamp: September 14, 2022, 04:36:27 am
content: Having determined the fault is on the power supply board, I had to isolate the fault. There are 8 separate output voltages plus other circuits on the board.None of the supply outputs were greater than 0.5V. Something is dragging down all supplies.With mains power disconnected, I applied DC voltage to each of the outputs. If the diodes inductors and capacitors are good, the current settles to near zero. This also reformed the capacitors.If a diode had failed closed circuit, the injected voltage would have excessive current.Same if a capacitor had shorted.DC voltage injection measured milliamps of current. Nothing that would drag down all of the supply outputs with mains power applied.So now I am working to the hypothesis that the fault is in the EHT converter. The voltage multiplier is a prime suspect.That is where I am now.I need to figure out a strategy for testing this circuit. I do not like working near HV.

author: dazz1
author_rank: Frequent Contributor
timestamp: September 14, 2022, 04:49:40 am
content: I have already removed the Rifa exploding capacitor. It looked factory original and OK but not something I want to be near if it exploded.I was originally going to re-cap the whole scope, but the cost of the components is much more than I paid for the scope. The current plan is to identify and replace faulty components, rather than just replace them all.

author: tautech
author_rank: Super Contributor
timestamp: September 14, 2022, 09:19:10 pm
content: Expect an identical donor from the same source coming your way soon.I've had it for a few years but never looked at it or turned it ON and it was to go to recycling but you can have it Dazz.I can't venture down your ways this time and we have to arrange the handover between yourself and wifey but we have 2 or 3 weeks to sort that out.AFAIK she's booked somewhere in the Hutt so maybe you can collect from her.Your address and contact #'s still the same ?

author: dazz1
author_rank: Frequent Contributor
timestamp: September 15, 2022, 07:58:43 am
content: HiThanks for that. A donor would be very much appreciated.There is also a combi-scope Philips PM3375 for auction, but it has taken a bash. A knob is missing and the pot shaft broken. There may be other damage. I was tempted but the sample frequency and memory depth are not enough to justify buying it. It could easily be bricked with an unobtainable part. No one has bid on it.I have given serious consideration to buying a modern Siglent DSO, but I simply don't need that level of sophistication. A good old fashioned analog scope is sufficient. I don't even need a 100MHz scope. I am either dealing with RF or slow analogue.I could easily collect from SWMBO where ever she stays. My details unchanged. I foresee the biggest risk is baggage handlers.I am currently trying to figure out a diagnostics plan. I don't have kV type test gear. I don't want to be working close to HV. It takes 3 supply voltages to make the EHT work. They are all being dragged down somewhere.

author: tautech
author_rank: Super Contributor
timestamp: September 15, 2022, 08:25:03 am
content: Quote from: dazz1 on September 15, 2022, 07:58:43 amIt's a 3070 c/w the front panel hard cover and probe soft case on top. Top physical condition but unknown if operational.QuoteHutt Park Rd so not far from you and sometime 1st week Oct IIRC but we can sort the rest by email or on the blower. She'll be well pleased to get some stuff outta the garage.QuoteJust checked the PC manuals folder and I have the manuals for 3070 and 3072 and there's a PM3217 here you can have too....from same source as your 3072.I guess I'd better study up on the 3072 to see if I can be of any assistance in the meanwhile.

author: wasedadoc
author_rank: Super Contributor
timestamp: September 15, 2022, 07:18:29 pm
content: Removing L6201 will ensure that the EHT generator cannot come to life. If the power rails come up check the transistor driving the EHT transformer. If the rails don't come up you can fault find the power supply safe in the knowledge that the EHT generator isn't going to unexpectedly start producing those kVs.

author: dazz1
author_rank: Frequent Contributor
timestamp: September 16, 2022, 08:08:47 am
content: Quote from: wasedadoc on September 15, 2022, 07:18:29 pmHiMy power board is slightly different to the service manual version and there is no L6201. Also, I needed the EHT converter powered up because the +48V was down to about 32VDC. I suspected a fault on the low volt driver side, or the fly back transformer.I considered removing C6209 which is the capacitor that connects the 2kV output of the transformer to the voltage multiplier but I figured it was safer to keep the transformer loaded.Just before I started writing this, I checked all the power rails driving the dummy test load I made. All power rails are good and no high pitch tone. The test load smells (and is) hot. I think driving power back into the power supply outputs (mains power disconnected) has reformed the capacitors. In hind sight, a break-out adapter that provided easy access to the power rails would have been a better option than the dummy load. For the dummy load, it would be good to add jumpers to be able to isolate each load from the respective voltage rail.I anticipate someone will say I should replace all the caps, and they would be right. If the scope works OK and is in spec, then I will leave the caps alone. I will replace as required. I need to order enough parts to get free shipping. That means I will end up replacing the caps in batches. One fails, I replace ten.The next step is to plug everything back in and switch it on. Hopefully all the smoke stays inside.

author: dazz1
author_rank: Frequent Contributor
timestamp: September 16, 2022, 08:15:02 am
content: Quote from: tautech on September 15, 2022, 08:25:03 amI am starting to be infected with a serious disease. It's called "I like fixing old test equipment that I can fix". I'll be ready to catch anything you throw my way.

author: dazz1
author_rank: Frequent Contributor
timestamp: September 16, 2022, 10:33:47 am
content: OK so one step forward.I reconnected everything and turned on the mains power.All the smoke stayed inside.Illumination worked and I got a single focused dot on the CRT.Nothing seen on the LCD and none of the front panel controls work.So fixing one fault has revealed another. Just like whacka-mole.Measuring voltage rails, even with the power supply partly inserted, is harder than it should be. I think I will be making a breakout connector set because it looks like I might have to fix more than one Philips scope.

author: tautech
author_rank: Super Contributor
timestamp: September 16, 2022, 11:18:29 am
content: Quote from: dazz1 on September 16, 2022, 08:15:02 amThat reminds me of me prior to 2013 before I was in bed with Siglent.Back then I had a hobby fixing scopes then there was no time for that hobby .....and now there is no time for any hobbies !Quote from: dazz1 on September 16, 2022, 10:33:47 amSo you can't move the focused dot around the display ?Is it in X/Y mode ?

author: dazz1
author_rank: Frequent Contributor
timestamp: September 17, 2022, 12:53:39 am
content: HiThe power supply board on my scope is slightly different to the one in the service manual.The differences are evident when comparing the photo and the service manual diagram.They include different (larger value) caps and a couple of additional connectors.I suspect my scope is a late model, given the colour of the enclosure.

author: dazz1
author_rank: Frequent Contributor
timestamp: September 17, 2022, 01:13:46 am
content: Another step forward.After lifting one of the PCBs, I found loose connectors. This is bad sign because it indicates someone has already attempted a repair and failed. I used to work where this equipment was repaired. I know the quality of the techs there. Failure to repair would not have been due of lack of knowledge or skill. Worst case is that they discovered a faulty part with no available replacement. Best case is that the cost of the repair (time and parts) was more than the value of the scope. It would have been disposed of "Beyond Economical Repair". So right now, I am not confident I can succeed in fixing this scope where others have not.So with the connectors connected, I now have text displayed on the CRT and the LCD appears to be OK. This is a major step forward because it indicates some critical unobtainium parts like the ROM are OK.I have no control with the buttons and no trace. So a new phase starts in the fault finding process.

author: dazz1
author_rank: Frequent Contributor
timestamp: September 17, 2022, 01:59:06 am
content: Another step forward.I now have a trace A and the front panel controls all appear to work. I think the problem was sticking buttons. Some of the buttons are reluctant to return when pushed.The B trace is displayed but not displaying a signal. This should be relatively straight forward to fix unless something weird is causing the fault.The strategy is firstly to make sure the problem is not operator error. I need to read the user manual.Next I will apply the cal signal to both A and B channels and compare the signals inside the scope. I can use my trusty Philips PM 3217 scope to do this.I will apply a binary division approach.That is, I will identify the mid point of the signal path and compare A and B.If both A and B are good, I will go down stream to find the next half way point along the signal path.If B is bad, I will go up stream.Repeat.This binary approach is MUCH faster than linear signal tracing. Linear tracing is starting at one end and following the signal path.I am starting to feel more optimistic about getting this scope fully operational, but I am not there yet. I will need to go through all of the controls and features to see if there are any latent faults.

author: dazz1
author_rank: Frequent Contributor
timestamp: September 17, 2022, 02:26:26 am
content: In my experience, you can always rely on operators to make stupid mistakes with complex equipment, and this is no exception.So when testing the B input, this operator simply plugged the scope probe into the connector next the A input.There are 3x BNC connectors on the front of the PM3072 labeled A, Ext and B, in that order.So when the operator moved the scope lead from Ext to B, suddenly the B signal appeared on the CRT. Amazing!!!In my defence, I can only say my trusty PM 3217 has the external input on the back panel.So at the moment, I am feeling reasonably confident that I most likely have a fully operational scope. I need to read the user manual and work my way through every feature and function to make sure it all works. I then need to do a calibration. That will include looking for bad caps doing bad things to the performance.

author: dazz1
author_rank: Frequent Contributor
timestamp: September 17, 2022, 03:08:31 am
content: Just taking a look around the insides while things are running OK.I think I read something about really hot transistors in these scopes.I have found 4x on the XYZ board. These are too hot to keep my fingers on. I see no indication of a fault.The heat sinks appear to be too small, but there is not enough room to fit bigger ones.I am already thinking about how I could improve cooling. Maybe 3D print cooling air ducts from the chassis fan. I could even fit a tiny cooling fan. Standard sizes go down to 25mm.

author: tautech
author_rank: Super Contributor
timestamp: September 17, 2022, 04:18:42 am
content: Quote from: dazz1 on September 17, 2022, 02:26:26 amCan I see myself do that....well yes of course.Might turn out to be an easy fix Daz.From just a few second on Gurgle:BFW16A = TO-39 Final stage of the wideband vertical amplifier in high speed oscilloscopes.IME all final stage CRT plate amps are under some stress and as such are a common failure point.Those that appear to be their drivers OTOH need be thermally linked and as such will be harder to heatsink although with your skills Daz you could bend something up outta a wisp of copper sheet that could easily be better than OEM.I'll have a rat through the dead PCB's stash to see if I have any TO-39 heatsinks and if so pop them into your October Red Cross package.

author: dazz1
author_rank: Frequent Contributor
timestamp: September 17, 2022, 08:27:35 am
content: So on further consideration, I think the unconnected connectors were not the result of someone else trying to fix a problem. I think it most likely they simply vibrated out. That connector type has white plastic clips to hold it in place. These were missing on the downward connectors. I will 3D print some replacement clips to prevent the same problem happening again.Many years ago, this type of fault would have been repaired. I introduced the concept of run-to-failure or repair-by-replacement. For certain types of test gear it makes sense to use an item until it stops working, then repair by replacement. It maximises the serviceable life of expensive equipment past the point of being supported. Unsupported is does not make equipment unserviceable. I didn't invent this concept. I just introduced it.It is very likely that this scope was run to failure, then simply replaced with no attempt to repair.

author: dazz1
author_rank: Frequent Contributor
timestamp: September 17, 2022, 08:42:18 am
content: Quote from: tautech on September 17, 2022, 04:18:42 amA Red Cross package for someone needing help. Yes please.The power supply board has an unused 5V connector, which I could use to power a small 25mm fan to supply cooling air directly to the hot transistors.This connector is not shown in the service manual so this fix may not be available to similar scopes.There is a gap where I can 3D print a bracket that will just slide into place. No holes, no cutting.

author: dazz1
author_rank: Frequent Contributor
timestamp: September 17, 2022, 09:59:27 am
content: The sticky buttons on the front panel were very quickly becoming a serious issue. I figured they had probably built up a sticky film of gunk over the years.I took the panel apart to clean the buttons with alcohol.All much better now.

author: dazz1
author_rank: Frequent Contributor
timestamp: September 17, 2022, 10:24:29 am
content: I am noticing that the LCD back light is feeble. Looking at service manual, I could not find a part number for the lamp. The manual says the LCD panel is not serviceable. When I took apart the front panel, I did not see an obvious way to change the lamp. Ideally I'd like to upgrade to LED back lighting.Any suggestions?

author: tautech
author_rank: Super Contributor
timestamp: September 17, 2022, 10:49:50 am
content: Quote from: dazz1 on September 17, 2022, 10:24:29 amYes IIRC it's been done and written up a few years back in the forum. We need to hunt it out and again IIRC it was in a 3070.

author: tautech
author_rank: Super Contributor
timestamp: September 17, 2022, 10:56:42 am
content: Gunna drop a few links here with worthy/valuable comments about these scopes:https://www.eevblog.com/forum/buysellwanted/philips-scopes-opinions/msg119557/#msg119557https://www.eevblog.com/forum/repair/philips-pm3070-scope-runs-very-hot!/https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/msg1459884/#msg1459884And jackpot !LCD backlight repair:https://www.eevblog.com/forum/repair/philips-pm3065-scope-repair/

author: dazz1
author_rank: Frequent Contributor
timestamp: September 17, 2022, 11:14:14 am
content: I found the LCD lamp replacement instructions here:http://www.jetecnet.com/Scopes/backlight.htmlThat hot wire lamp looks like unobtainium. I strip of LEDs would be an improvement.Like the one here:https://www.eevblog.com/forum/repair/philips-pm3055-scope-repair/

author: dazz1
author_rank: Frequent Contributor
timestamp: September 18, 2022, 08:12:16 am
content: 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.

author: dazz1
author_rank: Frequent Contributor
timestamp: September 19, 2022, 03:25:05 am
content: 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.

author: dazz1
author_rank: Frequent Contributor
timestamp: September 19, 2022, 03:28:23 am
content: some photos showing details of the brackets that hold in the power supply PCB.

author: tautech
author_rank: Super Contributor
timestamp: September 19, 2022, 04:17:02 am
content: Quote from: dazz1 on September 14, 2022, 04:07:34 amUse Modify on the OP to edit the Subject/Topic line as I have in this reply.

author: dazz1
author_rank: Frequent Contributor
timestamp: September 19, 2022, 07:08:31 am
content: HiThe 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.

author: dazz1
author_rank: Frequent Contributor
timestamp: October 04, 2022, 07:14:48 am
content: HiI 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 clearanceBond store feeISPM15 compliant packagingCarnet declarationEscrow service feeInsurance.so the price should be around $USD68.37For all those interested, just send a few payments to ripoff.a.holic@scam.me.com

author: HighVoltage
author_rank: Super Contributor
timestamp: October 04, 2022, 07:38:03 am
content: Looking forward to see, what that looks like with the LED mod.

author: tautech
author_rank: Super Contributor
timestamp: October 04, 2022, 08:39:07 am
content: Quote from: HighVoltage on October 04, 2022, 07:38:03 amDazz has another PM3070 incoming so he might get the chance to do another.

author: dazz1
author_rank: Frequent Contributor
timestamp: October 17, 2022, 08:36:00 am
content: HiI 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.

author: dazz1
author_rank: Frequent Contributor
timestamp: October 18, 2022, 01:01:06 am
content: HiI 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.

author: dazz1
author_rank: Frequent Contributor
timestamp: October 18, 2022, 11:32:09 pm
content: HiHaving 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.

author: dazz1
author_rank: Frequent Contributor
timestamp: October 18, 2022, 11:35:53 pm
content: 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.

author: dazz1
author_rank: Frequent Contributor
timestamp: October 18, 2022, 11:57:07 pm
content: 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.

author: dazz1
author_rank: Frequent Contributor
timestamp: October 19, 2022, 06:51:46 am
content: HiI 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.

author: dazz1
author_rank: Frequent Contributor
timestamp: October 19, 2022, 07:13:16 am
content: HiI 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.

author: dazz1
author_rank: Frequent Contributor
timestamp: October 20, 2022, 01:21:27 am
content: HiHere 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.

author: dazz1
author_rank: Frequent Contributor
timestamp: October 20, 2022, 08:44:58 am
content: HiI 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.

author: dazz1
author_rank: Frequent Contributor
timestamp: October 20, 2022, 08:53:53 am
content: HiWith 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.

author: dazz1
author_rank: Frequent Contributor
timestamp: October 20, 2022, 10:27:01 pm
content: HiHere 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.

author: HighVoltage
author_rank: Super Contributor
timestamp: October 21, 2022, 08:19:54 am
content: Nice job, looks good.

author: dazz1
author_rank: Frequent Contributor
timestamp: October 22, 2022, 04:58:35 am
content: HiHere 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.

author: tautech
author_rank: Super Contributor
timestamp: October 22, 2022, 09:22:50 am
content: 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 ?

author: dazz1
author_rank: Frequent Contributor
timestamp: October 22, 2022, 09:45:19 am
content: HiI 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.

author: HighVoltage
author_rank: Super Contributor
timestamp: October 22, 2022, 11:46:17 am
content: Quote from: dazz1 on October 22, 2022, 09:45:19 amYes, they like to multiply...I have plenty of the PM3394B and they also could benefit from a LED conversion.Thanks for sharing your project.

author: dazz1
author_rank: Frequent Contributor
timestamp: October 23, 2022, 07:01:25 am
content: Quote from: HighVoltage on October 22, 2022, 11:46:17 amHiI would sell my neighbour's wife to white slave traders to get a PM3394B.

author: dazz1
author_rank: Frequent Contributor
timestamp: October 25, 2022, 01:53:18 am
content: HiThe Philips PM3375 Combi-scope (photo attached) has just sold at on-line auction. It is a good example of the limited supply of used test equipment available locally in my country. It is faulty and damaged. There is no trace and the front corner has taken a blow. The chassis is cracked and a pot shaft with knob has snapped off. Both are unobtainium.This broken scope sold for about $USD155, including local shipping cost. That is a lot for a faulty and damaged scope.If this scope was a working PM3394B, it would probably sell for at least twice as much. That price would be very close to the cost of a more capable, lower end, new Siglent scope.I have looked at importing used test equipment from Europe and USA, but shipping cost for a scope size item is about $USD200. If the item was worth $20,000, then the shipping cost would be insignificant, but for something like a used scope, the cost of shipping is significant. It is also dead money. Paying for shipping does not improve the quality of the item.So the outcome is that the limited local supply of used test equipment pushes up the prices. The alternative of importing used test equipment is generally not economically viable, which ensures local supply remains constrained, which supports high prices.The rational choice is to buy new.

author: kiwical
author_rank: Newbie
timestamp: March 26, 2025, 08:35:25 am
content: I can get hold of more 3070s. Same supplier (NZ navy) Just how many of these did they buy? Anyway, maybe let me know if interest. BTW I am the Trade Me seller you referenced on the final page of this thread.

author: dazz1
author_rank: Frequent Contributor
timestamp: March 26, 2025, 10:11:37 am
content: Quote from: kiwical on March 26, 2025, 08:35:25 amNZ Navy had a big clean out of old broken, obsolete test equipment a few years ago. My PM3070 was one of them. I don't know how many they purchased but it would have been a few. The equipment that went to sea had a hard life.I repaired and up-modded my PM3070 so not looking for another.Then I got a HP 54645D Scope with Logic probes and GPIB/Maths interface. Much better than a PM3375/PM3070. I sold 3x scopes but kept the PM3070.I am still restoring my GEC Miniscope with an advertised bandwidth of 50kcs. Can never have too many scopes but I don't have enough bench space.