I’m uploading a pic with the components labelled and the details I could ascertain. I’ve included a couple you didn’t ask for as there were some mysterious blobs on them. Attachment 66408Attachment 66409Attachment 66410Attachment 66411
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I’m uploading a pic with the components labelled and the details I could ascertain. I’ve included a couple you didn’t ask for as there were some mysterious blobs on them. Attachment 66408Attachment 66409Attachment 66410Attachment 66411
Short reply to another question. The box has been powered up for a few hours now. The CPU heatsink isn’t stone cold, but I can’t see that it’s any warmer than the chassis. The heatsinks on the PSU are only very slightly warm, but at least the kit below them is doing something, whereas I’m guessing the CPU has nothing to do at the moment.
Back to the serial voltage. We established my male RS232 port on the PC was delivering 11.2v to the box, but the USB is limited to 5v. Does that matter?
I do not think the blobs are anything mysterious, the 2 devices are programmable and would have been programmed before being fitted to the board. The marks I think are 2 show that they have been programmed and the blue mark on the flash device (U12) is probably to show that it has been verified - double checked to avoid having to remove it during production. The device in the socket (U17) can be removed if it has a problem the flash device would have to be unsoldered to replace it.
I am struggling to find data on the devices but will keep trying. I would really like to know what voltage is on CB67 small capacitor near the flash device U12, the one end will be 0V and the other (I think) is the supply for U12. The voltages on U3 (above the CR2032 cell) may also be interesting (middle pin is 0V). To be continued...
OK, will check tomorrow, but…
Oops! Didn’t notice the last para to Stroker’s post of 20/02.
The bit about the component to the right of the heatsink I think I answered inadvertently because I wondered what the blob was.
Now big resistors? Are they the handsome green things with stripes labelled R200 and R94 – hey what’s that doing here, R93, R95 and R96 are old friends from the PSU board. Zero ohms – both multimeter. But then the green one on the PSU boars, R93, also shows zero ohms. Is that close enough to 0.5 ohms?
Sorry I did not notice your earlier post - It would seem to me that the CPU is not running but this is no real surprise since we have forced the main power on with the link on the PSU so it has not powered up in the normal controlled manner. Having said that I would assume the CPU should also be running in standby but perhaps using a low power mode. The CPU needs a program to run (in the Flash device), memory (the 4 Hynix devices) and power supplies to these devices. The CPU also needs a clock and a power-on-reset signal to kick it off. So the 64 million dollar question is what are we lacking and why? I need to do some thinking - I notice there are some test points near the CPU marked as TP followed by a number but we don't know what each is meant to show us - life's a bitch!
Just had a thought perhaps you could replace the link on the PSU with a switch, then we could power up with just the standby supplies and then close the switch to bring on the main supplies. If you do this be aware that high voltages will be on the switch - any switch will do - perhaps a light switch. But we really need to work out how the standby switch is supposed to bring on the main supplies - perhaps the switch idea would just be a waste of time.
Concerning voltages USB uses levels of 0V and 5V, RS232 levels of +12V and -12V so although they are both serial interfaces they use different voltage levels and are not directly compatible.
R200 and R94 was expecting low values so that sounds about right - a stab in the dark really. Component numbers usually only relate to the board they are on, so you could have for instance R5 on the power supply another R5 on the motherboard and yet another on the display board and each of these could be different values and be in no way be related or connected.
I think I need a drink!
Right, perhaps you could check this out first, The memory runs on 2.5V not sure where that comes from just yet and U3 (NE57811) produces some related signals including a 1.25V reference. The 4 Hynix memory devices have the supplies on the corner pins with one side's corner pins being 0V and the other 2.5V
U3 has 5 pins if you look at it with the pins at the bottom and the body at the top the pins are numbered left to right 1 to 5 with the middle pin being 0V. Pin1 and 2 should be 1.25V and 2.5V, pin5 should also be 1.25V, pin4 is a called shutdown, not sure what to expect on this. Again be careful not short pins with the meter probes.
I think this is a good place to start and will hopefully give us a clue as to is or isn't happening.
Please check for 2.5V on each of the 4 Memory devices - I think they may not all be supplied from the same source.
I'd like a pic of devices below the JTAG connector and to the left of the CPU ie from U23 to U3 include the 2 memories - thanks.
Attachment 66415
U3 voltages:
Pin 1 1.31v
Pin2 2.62v
Pin 4 2.62v
Pin 5 1.31v
Hynix memory chips: 2.62v on all four.
I did find it very difficult to measure Pins 2 and 4 – several times got nothing, and several times killed off some voltages completely – HDD stopped clicking and failing fan stopped grinding. So tried to heed your warnings but a probe obviously slipped – maybe I shorted 1 and 2 for instance. When that happened +5VS ws still present.
Hope the pic is OK.
Pic is good thanks, voltages seem reasonable, not sure where we go from here. Worried about probes shorting could do with finer probes.
Returning to think mode....
Sorry for the pause in posting I'm still searching, mainly in vain for data on some of the chips on the motherboard.
I have found data for U21 STV6412BD which is just used for switching video and audio inputs and outputs so we can probably rule this out as causing our problems.
The SM8952AC device in the socket near the battery is quite a complicated CPU/Controller/IO/Flash device which is much more interesting as regards our faults but I am still looking into this and may take some time to digest.
We have discovered 8 fuses (names starting with FB) on the motherboard next to the main power connector but unfortunately all are intact.
List of components next to connector below, courtesy of jphilweybr.
Between the caps and the PSU connector, there are lots of tiny components. Running from Pin 1 towards the back, they are:
CB144
Q101
R222
CB138
FB29
CB139
FB28
FB27
CB140
CB141
FB26
CB124
FB24
FB25
FB23
CB137
FB22
CB143
I will not be posting quite as often over next couple of weeks due to other commitments but I will still looking at this when time allows.
I am taking a break from looking at the motherboard for the time being and concentrating on trying to get the display to work instead as this could give us some badly needed clues as to what is actually happening. Some of your recent voltage measurements suggest to me that there may be display data getting to the front panel board.
I am trying to convince myself that due to the loss of the 5VS supply the two transistors driving the transformer Q3 and Q4 could have blown.
There is a simple check on transistors you can do with a meter where the transistor can be tested as being two diodes with the common pin being the base. So you should see a diode between base and collector and another between base and emitter. If you look at the Q3/Q4 with the two pins at the bottom I think the base is the left hand pin of the two, so check for diodes from this pin to each of the other two pins.
If you don't follow this and you are feeling brave you could just go ahead and change Q3 and Q4 which I think you may have replacements for. If my theory is correct and the transistors you have are equivalent to the old ones I am hoping the display will then work - fingers crossed!
Attachment 66497
Many thanks for your continuing help Stroker. And a good time for you to take a break, because I took a nice holiday and found lots of things to sort out when I got home. One involving an Axxr laptop merits a thread in its own right but probably not on this site!
Anyway, I’ve managed a little bit of time today and here’s a brief report.
I’ve had a look at Q3 and Q4. I’ll call the pins Left, Right and Top. The following results were identical for both Q3 and Q4.
1. There is continuity to GND to the Right hand pin
2. The diode test was normal for the bottom two - + 6.5v with the red probe on the left and Infinity with it right.
3. Similarly, Red probe Left, Black probe upper +6.5v, infinity the other way around.
4. With red probe on the Right and black probe upper about 0.12v and infinity the other way around
5. There is no voltage between the chassis and the bottom pins. There is 5.07 to the top pins – the same as at the transformer middle pins on the back.
6. I did buy two of the BCW65 transistors, when they were a mere £1.21 each, but at the moment cannot find them.
I don’t know about D9. There is a blob on the L31 side, and I can measure 4.6v to it. However, while the pics show something at the other end, I get no voltage there. Nevertheless I can do a diode check on it. With the red probe on the L31 end, I get 6.5v and infinity the other way. I realise that I didn’t correctly identify D9 in my post of about three weeks ago, I was referring to a larger component – probably the label Y5 refers to it.
I think this is the best pic.
From your results I would say that Q3 and Q4 on the front panel are OK.
Your results for D9 on the motherboard are much more interesting. From your pics it looks pretty certain that the bottom end of D9 is connected to pin 10 on the Syncmos chip which according to the datasheet I downloaded is the reset pin. If I understand you correctly the bottom end of D9 is at 0V and is holding the chip in a reset state which would prevent it from doing anything!
I would expect the reset pin on the chip to be connected to a resistor and capacitor. D9 is there to discharge the capacitor quickly when power is turned off. So the resistor could be open circuit, the capacitor could be short circuit or there could be an internal short in the chip. There is also a possibility that there is something else connected to this circuit which is causing a short.
First thing to do is check for a short from bottom end of D9 to GND, if there is I would first suspect the capacitor next to D9 unfortunately to check this the motherboard will need to come out (as I am sure you already guessed). Check for a short across the cap to confirm we have the right cap, you will then have to remove the cap to check it is shorted and also check the short has gone from the board itself. If the cap is not shorted and the short is still present on the board take a pic of the underside of the board in this area so we can try to see if anything else is connected. If the cap is shorted replace it with the nearest value you have as long as the voltage rating is greater than 5V.
If there is not a short we need to check the resistor is intact and soldered properly. One end of the resistor would be connected to 5VS and the other to the capacitor and the bottom end of D9. I think the resistor is R228 on the other side of the capacitor - first try to read the details from the resistor, expect 3 digits - the first 2 digits are just digits the third is the number of noughts to stick on the end, for example 103 is 10K (1,0 and 3 more noughts). See if you can measure the resistor and check continuity, one end should be connected to the top end of D9 the other to the bottom end - you should be able to measure the resistor with your probes across D9 using a higher range (you should measure the resistor with the probes one way, and see D9 with them reversed).
I think the Syncmos chip, which includes a CPU, does all the low level and standby tasks, while the main CPU only operates when out of standby. The Syncmos chip presumable drives the front panel display and takes in signals from amongst other things the front panel push buttons. If I am correct on this the box will not do anything until we get the chip out of it's reset state. This could well be a huge breakthrough I just hope the cause is something easily fixed.
Attachment 66515
Thanks.
If Q3 and Q4 are OK, could there be something wrong with the transformer? Is there a way of checking that?
Resistance from chassis to the bottom end of D9 is 7.6kΩ. To the other end, I get OL.
Just to double-check, the next checks I have to do need access to the underside of the motherboard? I think R228 is 822 – see pic, so 8.2 kΩ?
I tried to post an example diagram of a POR circuit but was unable to upload it as usual!
I have sent it to you by email - perhaps you can post it on the thread for me - thanks.
Do not take the motherboard out just yet - you may not need to.
Since you measure 7k6 from bottom end of D9 to GND we do not have a short.
Try to measure R228 it should be 8k2 as you say, also measure voltage at each end - the cap should charge up to the same level as 5VS so you should have about 5V at both ends of R228. Because you measure 7k6 to GND the cap may not charge up enough.
I need you to confirm these measurements (R228 and voltage each end) before I ask you to do anything else. The cap may well be faulty but I need to be sure of the details - can you see what value the cap is? do you have something similar? I think the cap is between R228 and D9 - looks like CE47.
I would expect the transformer on the front panel to be OK since you have 5V on the top pins of Q3 and Q4. The 5V would be fed through the transformer primary windings.
R228 is tiny and not easy to get probes to make contact, but I finally managed to get a resistance reading of 7.7 kΩ. I didn’t managed to get a voltage though. I think I was making contact at least at one end, because the reading settled at 12.6mV, whereas without any contacts the meter usually runs back to zero.
CE47 is 50V 10µF. The one next to it is 16V 100µF. I have some 50V 22µF and some 10V 470µF.
Sorry, I can't upload that diagram either.
Yep those little resistors are a real pain.
Can you measure the voltage on the bottom and of D9? We need this to be over 3.5V
I think we need to bite the bullet and get the motherboard out, remove CE47 and test it, which I hope is faulty.
Also measure the resistance from bottom end of D9 to GND which I hope will be much higher than 7k6.
I would fit 16V 100uF test it first so we know it's good then put it all back together and see what happens.
I have high hopes for this - good luck!
Having internet probs.
If CE47 is not faulty check the cap next to it - I may have picked the wrong one.
Well with the motherboard still on, the resistance to the bottom end of D9 is 7.7 kΩ, 15Ω to the L31end (not OL as reported yesterday). There is just 12mV to the bottom end, 4.7v to the L31 end.
So I’ll get the motherboard off. It promises to be fiddly to get all the ports at the back loosened and removed. To double check, the cap I’m removing is 50V 10µF, so would my nearest replacement be some 50V 22µF?
I’ve managed to make a picture, albeit third rate, of your diagram, hopefully now uploaded. And if it’s the other one that’s faulty, would my 10V 470µF be OK?
Attachment 66530
Hang on a minute
You've only got 12mV on the bottom end of D9 - it's looking like R228 is either faulty or not soldered properly. Check this first before taking the motherboard out. Beware the resistor may come loose when you heat it and come off the board.
Perhaps a better thing to try for now - short D9 top to bottom with a pair of tweezers or pliers and see if anything happens. That will raise the reset pin to 5V and get it out of it's reset state. It will return to reset when you remove the short.
And yes sorry 22uF would be best - I miss read your post - still wrestling with my router.
And 470uF for the other but it is more likely to be CE47.
I think the top end of R228 may not soldered properly - you saw 12mV on the bottom end nothing on the top (should be about 5V) - be very careful soldering as these resistors come lose very easily, if it does it may be on the end of your soldering iron bit.
Check for continuity from top of R228 and top of D9.
Attachment 66533
Too late! I’d already removed the motherboard when you posted.
R228 looks clean through a magnifying glass. I can’t get a better picture though than the one I posted yesterday. If I’m going to get closer, I think I will have to remove both the capacitators next to it.
Shorting D9 didn’t seem to have any effect. I removed the R98/R95 link so SATA stopped working. Shorting D9 didn’t bring it back on produce any light on the front panel.
Resistance between the top end of D9 and the top end of R228 starts off at zero and creeps up to 15Ω. Between the two bottom pins there is continuity. Diagonally, there is 7.7 kΩ. Voltage is 12mV to both bottom pins, 5.7v to the top of D9 and say 2mV to the top of R228.
Attached is a pic of the underside of this area. Neither D9 nor R228 are connected to the underside. In the centre right, there are signs of some damage. I assume that happened during assembly and has been there ever since.
I think I need to look at this again the reset circuit is more complicated than I thought. Looking closer at your earlier pics the top end of R228 is connected to ground and not 5VS which is why you measured 7.6K to GND from the bottom end of D9.
I am convinced we are onto something here but it is not as simple as I thought. I think we can leave the link off R98/R95 now.
It looks like U4 (above the 2 caps) has something to do with the reset circuit. Please check continuity from bottom end of D9 to each of the pins of U4 - pin1 is nearest R41 then count anti-clockwise around the device. Check CE22 is not shorted. Is R42 missing (may not be needed)?
This circuit is really strange and if I understand it correctly could be a very dodgy design - I'll be back!
Thanks for the pic - leave motherboard out for now.
Thanks Stroker. Happy cogitation!
On U4, Pin 7 on the corner near the Syncmos chip is connected to ground. To that pin from D9, resistance starts at zero then climbs to 7.7kΩ. All other measurements are in the MΩ range or OL. Measured on the underside, CE22 reads 270Ω
OK I take back what I said U4 appears to have nothing to do with it. I need some more facts so I need you to check a few things.
1. Remove CE47 and test it just so we know where we are, leave it out for now but keep it safe.
2. Between the top edge of the Syncmos chip and the big resistor R194 there are 2 small resistors, check for continuity between R229 and R228/D9 (bottom ends?)
3. Take another pic of the area above your previous pic, I need to follow the 4 thin tracks in the middle going upwards.
4. Take a pic of the top side of the board showing the area to the right of the Syncmos chip to the far right . It might be easier to take a full width pic of the board below the main CPU as long as the detail is good.
5. From your previous pic the tracks going to the big resistor R194 appear discoloured or possibly burnt, perhaps you could take a closer pic.
Don't expect a quick reply - other things to do today.
Attachment 66538I’m going to have to leave CE47 until tomorrow.
Meanwhile, here are some other pics I took a couple of days ago.
Again difficult to get readings from R229 and R230. Assuming the covering of the Syncmos chip is non-conductive so I am actually measuring the resistors, they both come in at 100Ω. From their bottom ends to the bottom of D9/R228 resistance is 4.2MΩ.
Yes, I had noticed that some areas of the motherboard (green part) look a darker shade, but looking at them now, they all seem clean. Let me know if there is another pic you want me to take.
Attachment 66539
Another pic. Sorrry, the system has started outwitting me again.
This is a pretty good pic, I would like a similar pic or the top side of the board please.
From this pic it looks like pin3 of the power connector (third from the right in pic) is not soldered properly but it's right on the edge of your pic. The other pic you posted today shows this problem more clearly. By the way it looks like pin1 of each connector is soldered to a square pad on the board while all other pins have round pads.
I am trying to work out what signal/voltage is on pin3, if it is the same as pin3 on the power supply it is 3V3 which is a very important supply indeed - could this be the cause of our problems?
I could do with a close up pic of this end of the power connector to try to make sure that it is just a fractured solder joint and not more serious damage.
I should point out that this will not be the only problem since the 3V3 (and main 5V) supply only comes on when the box is out of standby. However if we can get the box powered up with the link on R95/R98 on the PSU we may be in a position to flash the firmware. But this is no time to get carried away - one step at a time!
Anyway back to IRLvWAL - come on Wales!
Attachment 66545Attachment 66546Attachment 66547Attachment 66548Attachment 66549
I assume you’re referring to the 11-pin connector to the front panel, so I’ve taken a couple of pics showing these from the side. To me they all seem to have the pin protruding from the solder.
The square pads on the PSU connector and on the front panel 11-pin connector are both GND.
For the PSU connector, Pin 1, defined as the one with the black wire, is on the left of the picture and carries +FL. So the third from the right, call it Pin 13, carries 3VS, and Pin 12 5V.
For the 11-pin connector, Pin 1 with the red wire is on the right of the pic and is connected to GND. Pin 2 carries +5VS.
Now here’s an odd story. You may even regard it as good news, but more likely bad news.
Prompted by your query about the solder, I set about checking the voltages on the front panel.
At this point things are normal. The failing SATA disk is clicking away. To remind you, I’ve always had all the voltages on the PSU output connector, except +5V and +3Vs. Now with the R98/R95 link in, I have those as well.
So checking the front panel I measure in the following order:
Pin 2 5.1
Pin 11 –
Pin 10 17.3
Pin 9 24.0
Pin 8 26.5 (could be minus – didn’t check)
Pin 7 3.5
This seemed significant, so I stopped to write this down and went to re-check in case I’d missed a pin. Now, I couldn’t find a voltage at all in any of them.
So I check the PSU output at the motherboard pins. Apart from two, no voltages there either – no 8v for instance and no +12V though the SATA HDD is clicking away. The voltages I do have are 3.4v at+3VS and 4.4v at +5V. I also have 336v between the rear heatsink and TH81.
It may be that just powering down and back on again may restore things. But I’ll leave it on in case there’s something else you want me to check.
Attachment 66550Attachment 66551Attachment 66552
A few more pics
Sorry for the delay in answering I took advantage of the good weather to catch up on a job I needed to do on the car.
Thanks again for the good quality pics.
Sorry that some of this is getting confusing. Perhaps I should have mentioned that it was the 15 pin connector with the badly soldered pin. This is best shown in your pic img_4960.img if you look at the third pin from the right you can clearly see the (empty?) hole that the pin goes into below the solder. It is not the solder further up the pin that is important but the (lack of) solder joining the pin to the pad on the board. The solder needs to connect the pin to the pad. The hole in the board is probably what they call "through hole plated" and the pin may be touching the plating but for good connectivity the pin has to soldered to the plating in the hole. The quality of the connection is even more important with low voltages because the currents involved are much higher.
Please re-solder the pin I have high-lighted leaving the soldering iron on the joint for at least a few seconds to get the solder to flow freely through the hole. The pin on the far right could also do with similar treatment and also the 3 pins on the far left - these are not as bad as the one mentioned above but are not the best.
I am not sure what to deduce from your voltage measurements but please continue to mention such things as I am still looking for clues. If the hard drive you are using is a 2.5" one it will not use 12V only 5V.
I am still concerned with the D9/CE47/R228 situation - all I have seen so far are things that discharge CE47 but there must be something that charges it up. It could be helpful if you were to remove CE47 and take a (top side) pic of the area so I can see tracks going to it if any.
Thanks – amazing how you can spot these things from pics, but I can’t see them in real life.
Tomorrow will be soldering/desoldering day.
So the pin you noted, Im going to call it 13, is definitely +3VS. Fingers crossed.
And SATA does not use 12V? I learn something new again – yes some websites show it, but not obvious in Wikipedia!
I think 12V is present on the SATA power connector. The point I was trying to make was that 3.5" drives use 12V and 5V but 2.5" (laptop) drives only use 5V. This is also true for IDE drives. If you look at the label on some drives they state the current consumption on 5V and 12V I don't think I have ever seen a 2.5" drive mention 12V.
Anyway an hours less sleep tonight usually takes me week to adjust! Good luck!
Attachment 66557Attachment 66558
Sorry, I meant SATA 2.5”, which is what I’m using here.
Anyway, a couple of steps forward I think.
I could actually see with the ***** eye what Stroker meant. Pin 13 protrudse further from the board than the others and its cloak of solder sits a little high, whereas all the others sit nicely on the board. I’ve filled round with a bit more solder. I can’t promise that I was actually getting it down into the hole. In fact it kept moving around, forming joints with the adjoining pins, but finally seems to have sealed where the gap was. Pics attached. Actually I see there is a blob of solder worryingly near Pin 14 so I’ll have to fix that.
Secondly, have taken CE47 off and it displays resistance climbing from modest to OL. So I assume OK.
BUT another big step backwards. Before powering down with the mysterious loss of voltages in place, I thought I should check voltages at R98 and R95, which were in fact as before. However in doing so, I must have disturbed the link and trying to restore it, I did something which produced a bang, a burning smell and a soot- covered R93 at the other side of the board. The main fuse had also gone. I have replaced the fuse, and spent an hour (I timed it) trying to release R93, and am not yet there by some margin. Mercifully, we do have a pic of it in its younger days. It’s blue and from the hot side its stripes are brown, white, white, bigger gap and grey. Obviously I can’t power up until it’s replaced, and I suppose I should take a little time out to see if anything else is damaged.
Sorry!
OK - Robot Wars and Top Gear finished - back to business. I wouldn't worry too much about R93 just yet - check for a short across the biggest capacitor C85, if there is a short I would think you have blown IC91. R93 may have burnt out as well, it is a low value so should measure about 0.4 ohms - your problem removing it is probably because the legs are bent over on the bottom of the board - you need to melt the solder and straighten the legs using a small screwdriver of the soldering iron bit, then melt the solder while pulling the resistor out. If you need to replace IC91 check the short is gone before fitting the new one. In fact I would suggest leaving IC91 out until you check the standby voltages are present first.
I was going to mention before that your linking method was a bit dangerous - from the pics I got the impression you had just twisted a wire around the resistor legs, I'm sorry if I have got this wrong. I really wanted you to put a blob of solder on each resistor leg and solder the wire to these without any loose strands sticking out - however I assumed you would be very careful using your method so I decided not to mention it.
Your latest pics of the 15 pin connector show more problems - the first 5 pins at the dodgy end are not good and the first 3 pins the other end I would not be happy with. I think you have a flux pen - put lots of flux on the pins and re-melt every pin on the connector to be sure. Flux is used to help the solder flow freely, don't be afraid of using too much, it should also prevent solder bridging to adjacent pins. The green coating on the board is called solder-resist and is also used to prevent/minimise solder bridging.
I am a little worried that the third pin we were looking at is protruding further than the others - is it still properly attached to wire (leading to the PSU)? Try to check continuity between the PSU and motherboard while wiggling the wire (with your third hand).
I hope I have covered the important points but if in doubt about anything ask questions first.
Attachment 66565Attachment 66566
First, picture of sad event and one taken of that area in the early days of this adventure. Yes, I decided not to solder that link, because we had cause to take it on and off. And yes, I was very careful – I always powered down before fiddling with it. Except two days ago when I was missing those voltages, so I wanted to check what was on there. So I was very careful, but ….
I haven’t had much time today, but I tried to check what else might be damaged. I’ve confined myself to the hot side of the PSU board. Tomorrow, I’ll try and upload a CSV file or pic thereof, of my measurements. Meanwhile I have identified the following problems/queries:
1 Sooty R93 – should be 0.39Ω; my little multimeter shows 0.00 Ω, the more sophisticated one shows 0.6 Ω. I’d be best replacing do you think?
2 R84 looks shorted.
3 R80 and R91 (the latter next door to the sooty R93) are not giving me stable readings, albeit they home in and flicker around a given figure. In fact on my more sophisticated multimeter, R91 shows 2.78 with the setting at 20k and 17.8 when set at 200k. All the other resistors I’ve checked (81,82,83,85,94,95,96,97) give rational readings but I have not checked them against the expected nominal values.
4 No bulging capacitors but sooty C92 definitely shorted. I have a replacement. C85, C86, C87 have been tested in situ and all pass the increasing resistance test.
5 D91 shorted.
6 D83, D84 and D87 show 6.7v with the red probe on the arrow, and 5.7v or thereabouts the other way. D85 and D86 look OK (4.6v and I) as does D93 (2.2v and I).
7 I reported on IC91 back last year (did we really have nothing better to do on Dec 27th?), and found continuity between three pairs. I now only have one – 2 to 3 in that post, but neither of the others. I have a replacement.
Perhaps erring on the safe side, I should replace C92 and IC91, which I have and R80, R84, R91 , R93 and D91. If you agree, I’ll probably need help to source these.
Many thanks.
Wow that's a bit of a mess but probably looks worse due to the soot. If you zoom in on the pic you can see IC91 has a crack in it on it's top edge near the screw and the face of the device has blown off so that's fit for scrap for sure.
Your best bet is to remove IC91 to get it out of the way as some of the shorts you are seeing may be due to internal shorts in IC91. Don't fit the new one yet and be careful not to damage the tracks on the board when removing it - I doubt it if we could ever get another board. It might be better to leave it screwed to the heatsink and clear the solder first to avoid stressing the tracks on the board. Although sometimes it's easier to cut the legs off the IC before clearing the solder if you can get cutters in there to do it - then melt the solder on each leg and pull them like teeth. You will have to decide which is best for you and the board.
After removing IC91 check diodes D81,82,83,84 and check for a short across the big cap.
I need to digest your info from today and possibly tomorrow as well before deciding what we need to order.
OK, but just to warn you my experience of electronics as you know is virtually non-existent. My experience of pulling teeth is totally so.
I’ve snipped off the legs of IC91. Re-checking I find:
D91, C92 still shorted; D87 still showing voltages both ways as now is D93 – I got misled because it occasionally shows infinity, but next time it doesn’t. D83 and D84 are now OK, as are D81 and D82 which I overlooked yesterday. Big cap, C85, still OK.
R93 body is 10cm long, R91 11mm, both about 3mm diameter. The R91 resistance starts off at about 100kΩ and then just keeps climbing slowly. Its stripes are red, red, yellow, so should be 110kΩ.