Metrix 803B Oscilloscope power supply issue

[CapLeaker]

Re: Metrix 803B Oscilloscope power supply issue

great to hear your success! Thanks for the follow up!

[itsthatidiotagain]

Re: Metrix 803B Oscilloscope power supply issue

Progress at last. I finally got an opportunity to take another look at this, and decided in light of the fact that the parts were dirt cheap to just go for the shotgun approach.

I changed all of the chief suspects except the optoisolators.
The 220K resistor was open (confirmed once I had extracted it).
The power mosfet (P4NB80FP) transistor and TDA4605 I replaced on the assumption that if the resistor had cooked, then it was likely that it had done so because of excess load cause by the failure of those other two devices, and I replaced the 22uF 50V 85C cap with a 22uF 63V 100C just bacause it looked iffy. Total cost for repair... around £5.00 Total cost of 'scope including repair £35.00

Cost new... £758.00 ( http://uk.farnell.com/metrix/ox0803b...03b/dp/3430753 )

The PCB material is a little fragile (i.e. if you look at it sideways the tracks tend to lift), so I had to add one small mod wire to cover up my over enthusiastic desoldering.

A quick clean and inspection, then on with the juice... worked first time. It is now sitting doing a burn in test before I check its callibration.

Thanks for all the advice. I am now grinning from ear to ear.

[budm]

Re: Metrix 803B Oscilloscope power supply issue

They use 1% because the value will drift a lot less with temperature than typical 5%, it is use for sensing the main B+ voltage and shutdown the circuit if the B+ is too low.
You can see the application circuit in the spec sheet on how pin 3 is used.

[itsthatidiotagain]

Re: Metrix 803B Oscilloscope power supply issue

I figured 1W sounded right too. I don't have anything suitable, and its now pretty late here, so I think this will have to be patient and wait for another day.

I ordered a pack of 6 x 220K 1W 1% resistors from ebay, for pennies so I may chance just replacing that first to see if it works, since if it pops instantly I'll have four more to play with.

I noticed that all of the resistors appear to be 1% tolerance, so I went for 1%, but I suspect 5% would work fine. All of the other TDA4605 circuits I have looked at online dont seem too fussed about this, and I suspect it isn't critical. I doubt if it would affect the accuracy of the scope in any way since all of the DC side is regulated.

[budm]

Re: Metrix 803B Oscilloscope power supply issue

R4, at worst case the voltage will be around 330VDC, so that is about 1.5mA which means the power dissipation will be about 1/2W so I would use 1W.

[itsthatidiotagain]

Re: Metrix 803B Oscilloscope power supply issue

For future reference, here are my notes from the examination of the patient to date (to assist Google and anybody else with a similar issue).

Metrix 803B PSU - X00391C95A - 60286A

12 pin power connector on green cable to main PCB.

Although the board has no obvious markings, assume pin 1 is the one with the black stripe.

Note: DC ground is tied to main earth, and that for safe operation of the scope it must be earthed.
As with most scopes, all scope input BNC connectors therefore have the Ground side tied to mains earth.

Pin 1) -12V ?? Unverified
Pin 2) -5V ?? Unverified
Pin 3) +12V ?? Unverified
Pin 4) STD-BY> Unverified
Pin 5) +5V ?? Unverified
Pin 6) GND
Pin 7) GND
Pin 8)
Pin 9)
Pin 10)
Pin 11) GND
Pin 12)

PSU Components to check.

R1 - 12K 2W 1% (5 band Brown Red Black Red Brown)
R2 - 12K 2W 1% (5 band Brown Red Black Red Brown)
R28 - 1M2 1/2W 1% (5 band Brown Red Black yellow Brown)

C5 - 100uF 50V - Checked - replaced with 100uF 100V
C15 - 22uF 50V - http://www.ebay.co.uk/itm/ECA1HM220-...item2ed0cb867d

Z1 - TDA4605 - Control IC for SMPSU

https://cdn.badcaps-static.com/pdfs/...731590de00.pdf

Z10 - 4N25 - Opto coupler - https://cdn.badcaps-static.com/pdfs/...9b513c4a42.pdf
Z11 - 4N25 - Opto coupler - https://cdn.badcaps-static.com/pdfs/...9b513c4a42.pdf

Q1? - P4NB80FP - N-CHANNEL 800V - 3Ω - 4A TO-220/TO-220FP PowerMESH MOSFET (Equiv = BUZ80FI?) -

R3 - 470K 1/2W 1% (5 band Yellow Violet Black Orange Brown)
R4 - 220K 1W? 1% (5 band Red Red Black Orange Brown)
R10 - 100R 1/2W 1% (5 band Brown Black Black Black Brown)

On silkscreen side of board. SOIC8 near voltage regulator - Marked 082C - TL082C smd

or

https://cdn.badcaps-static.com/pdfs/...967b60572a.pdf

Possible source http://www.ebay.com/itm/TL082C-SMD-D...-/320734331798

Similar SOIC8 at other end of voltage regulators - Appears to be TL071C -

Most likely source http://www.ebay.co.uk/itm/TL071CD-ma...item4398feb620

[budm]

Re: Metrix 803B Oscilloscope power supply issue

Can you see how that resistor is connected to the other end in the circuit? It may give you the idea why the circuit is not working.
Pin 3 is the Voltage monitoring pin, if it is too low then the SMPS will shut off, so that makes sense. You can temporary try another resistor in that resistance range to see if it will work.

Page 3 of the IC spec sheet.
Input for Primary Voltage Monitoring: In the normal operation V3 is moving
between the thresholds V3H and V3L (V3H > V3 > V3L).
V3 < V3L: SMPS is switched OFF (line voltage too low).
V3 > V3H : Compensation of the overload point regulation (controlled by pin 2)
starts at V3H : V3L = 1.7.

[itsthatidiotagain]

Re: Metrix 803B Oscilloscope power supply issue

Parts haven't arrived yet, but I supect I might have spotted at least part of the problem.

I checked most of the resistors the other day, but I thought I'd go over the circuit again and compare it with this similar one. and blow me if I didn't find a problem.

R4 which I could have sworn I checked before (220K 1W 1%) looks to be open circuit. This appears to be connected to pin 3 on the TDA4605, (suggesting perhaps a short in the TDA4605, or a thermal stress crack in R4).

I've nothing identical in my parts bins, so 5 of those are also now on order.

[itsthatidiotagain]

Re: Metrix 803B Oscilloscope power supply issue

A quick update for those following...

I have purchased the Mosfet, TDA4605 and the 4N25 optoisolators. I'll post the result of changing that lot.

My gut feeling is that I should be able to run the PSU board with the main board and HT connectors disconnected without having to add a dummy load, but that I may need some feedback to one of the optoisolators to make it stay up. One of them looks to be a voltage feedback, to control the SMPU the other looks to be some sort of standby circuit.

What is anybody elses take on this? Also what purpose do you think the two SOIC8 chips on the silkscreen side of the board serve, they look to be a TL082C and a TL071C - my guess was that they provide some sort of precision voltage references.

[stj]

Re: Metrix 803B Oscilloscope power supply issue

hard to say with old 80's designs.

[CapLeaker]

Re: Metrix 803B Oscilloscope power supply issue

I would be surprised if the circuit would be that sensitive, that the 10uf 100V cap wouldn't work.

[itsthatidiotagain]

Re: Metrix 803B Oscilloscope power supply issue

Well, I finally got 5 minutes to swap those two capacitors, however I really should pay more attention when dealing with shop assistants. The 22uF 100V capacitor from Maplin turned out to be a 10uF 100V capacitor, so back to the store for a replacement in the next few days.

I did swap the 100uF 50V, for a nice shiney new 100uF 100V but it made no difference, and since I literally had 5 minutes, I didn't even get a chance to poke around to see if I could spot anything else that might be amiss. Well that isn't strictly true, I did carefully buzz through from the the live and neutral on the plug to the bridge rectifier, and did a quick diode test on the rectifier diodes and that all looks fine.

[itsthatidiotagain]

Re: Metrix 803B Oscilloscope power supply issue

Hi stj, yes I checked the fuse, and I even remembered to re-fit it afterwards but thanks for the reminder. Good advice... I even buzzed the fuse out in circuit, as it would not be the first time that grubby contacts on one of those cheap blade style fuse holders had caused things to stop working.

Looking at the application circuit, and comparing with the PSU, I would suggest that C5 is not likely to be 100uF 250V (as I previously suggested) since this cap is the chip supply cap (i.e. connected between pin 6 (V6) and ground and is shown as 100uF in the Seimens reference design).

Since the supply (V6) voltage to the TDA4605 is stated as having an absolute maximum rating of +20V, I would think that a 100uF 100V cap would do the trick, and since I have some of those "in stock" as it were, I will swap that.

For C15 I have just picked up a 22uF 100V from Maplin Electronics for the heady sum of 0.49p

Both caps are physically very similar in size to the originals so I think tomorrow might be time to break out the solder sucker and see if we are in luck. Before I do that though I think I might have a quick poke around on the low voltage side to see if I can spot any shorts on the regulators or anything else that might cause the TDA4650 to sulk.

Some quick notes from inspecting the board, in case anybody else is following along behind...

Metrix 803B PSU

12 pin power connector

Although the board has no obvious markings, assume pin 1 is the one with the black stripe.

Note: DC ground is tied to main earth, and that for safe operation of the scope it must be earthed.
As with most scopes, all scope input BNC connectors therefore have the Ground side tied to mains earth.

The following pins have been tentatively identified by visual inspection only.

Pin 1) -12V ?? Unverified
Pin 2) -5V ?? Unverified
Pin 3) +12V ?? Unverified
Pin 4)
Pin 5) +5V ?? Unverified
Pin 6) GND
Pin 7) GND
Pin 8)
Pin 9)
Pin 10)
Pin 11) GND
Pin 12)

NOTE: Mosfet may be Pin-to-Pin Replaceable with IRF IRFIBE30G , but the ST P4NB80FP looks to be available from several sources.

[stj]

Re: Metrix 803B Oscilloscope power supply issue

this one has an example circuit.
i would start by replacing those caps.

btw, did you check the fuse?

[itsthatidiotagain]

Re: Metrix 803B Oscilloscope power supply issue

I popped the PSU board out from the 'scope and took a whole lot more pictures

available here..

The "mystery chip" is a TDA4605 and the two caps C5 = 100uF (250V?) exact voltage to be determined when I find my small dental mirror and poke it behind the cap to get a better look. C15 = 22uF 50V and the two other chips are as we suspected optocouplers - in this case 4N25 . Finally the switching transistor is an ST P4NB80FP

The TDA460 PDF is watermarked as "Obsolete Product", suggesting there will be a better alternative, does anybody here know what that might be?

[stj]

Re: Metrix 803B Oscilloscope power supply issue

well fwiw i replace startup caps on sight usually, because so long as the kit isnt visibly burned it often fixes it without having to get any test gear out.

when i fixed sattelite boxes made by pace, i was buying startup caps in bags of 100.

[itsthatidiotagain]

Re: Metrix 803B Oscilloscope power supply issue

Warning: Off topic post ahead.. and old f*rt alert.

I must confess that it has been quite a while since I last did anything serious with switched mode PSUs.

Many years ago I ran a small electonics repair workshop as a sideline to the software business I was with, so I cut my teeth on Wyse 50 terminals and large colour IBM CRT monitors. These days everything seems to be headed straight to the skip the moment something goes pop, which bothers me.

Just for fun I googled Wyse 50 schematic, and now I feel really old... (c) 1986.... surely not... however on the plus side I still know one end of a capcitor from the other. If anybody wants to see a "classic" CRT/SMPSU circuit, take a look at the diagrams around page 158 of that manual.

I don't have an ESR meter or even another O'scope, never mind one with an octopus component checker (these two devices probably kept me in bread and butter for quite a number of years), so this may take a little longer than would otherwise be the case.

I was pleasantly surprised when I cracked open this particular scope and discovered a through hole board with a familiar layout. All of these ramblings are a prelude to a confession that I might not get a chance to take more pictures this evening, but rest assured, tomorrow at the latest...

[sam_sam_sam]

Re: Metrix 803B Oscilloscope power supply issue

However first on the list of things to do is identify the 8 pin chip beside the 400V cap and the two other chips that look suspicious.

That would be the first thing I would do
I just work on a power board that was very dead it was a 7 pin ic switcher

[stj]

Re: Metrix 803B Oscilloscope power supply issue

probably c5, but change c15 too.
and use high temp, low esr stuff like panasonic FR
i say probably because i dont know the chip number.

psu chips need a startup current, most do it by charging a small cap.
older ones used a high-voltage bleed using a couple of high value resistors in series to the mains smoothing cap.
those often changed value but looked o.k. to the eye!!!

[itsthatidiotagain]

Re: Metrix 803B Oscilloscope power supply issue

i suspect it's the startup capacitor,
and it's only about 10p to find out.

I hope you are correct, here is a slightly better crop from the previous images (I'm at work at the moment, so I cant bunk off and take some better pictures just yet). I presume you mean C5