EEVblog #540 - HP35670A DSA Repair - Part 4

[EEVblog]

Dave continues the investigation into repairing the HP 35670A Dynamic Signal Analyser. This time using the Flir E8 themrla camera to check for any overheating components on the suspect A5 analog board.

Part 1:
Part 2:
Part 3:

[jesuscf]

One quick test is to check the output of the op-amps.  If the output is stuck to +-15V the op-amp may be bad.

[EEVblog]

One quick test is to check the output of the op-amps.  If the output is stuck to +-15V the op-amp may be bad.

Yep, can do. I might finally be getting the schematic for this board though, that would be nice.

[synapsis]

Thanks for explaining the quiescent current relationship to heat. I have a Flir (damn you Mike!), but I've so far been lucky to find things either hot enough to cook an egg, or dead cold.

So far the best use I've found for the Flir (besides tuning engines and looking for house insulation leaks) is aiming IR lasers.

[pickle9000]

EEVblog #4769854 - HP35670A DSA Repair - Part 764.5

For those of you still following.......

[EEVblog]

EEVblog #4769854 - HP35670A DSA Repair - Part 764.5

yes, it may come to that, because I suspect this one is BER. But a lot of people do want me to persist with it.

[ludzinc]

I had a weird fault with an almost_but_not_quite working Space Invaders clone (IREM).

No schematic to hand, it was time to check everything and reverse engineer.  Only took me a decade, but I got there and the kids love it. 

[han]

simple 5 minuite DMM/Oscilloscope on pin - , + and Out will tell everything..


Out = positive if in(-) < in(+)
Out = negative if in(+) < in(-)

if not put X mark on the opamp

[Rutger]

Maybe a silly suggestion, but isn't is quicker to just replace all the opamps with your new desoldering station. I know this a kind of a brute attack to the problem, but with the low the price of these opamps and the time it takes with a desoldering gun you might have fixed the board in 1/2 hour. Onces you have put the ic sockets back you can always put the old opamps back and try to debug the problem one opamp at a time.

[c4757p]

Maybe a silly suggestion, but isn't is quicker to just replace all the opamps with your new desoldering station. I know this a kind of a brute attack to the problem, but with the low the price of these opamps and the time it takes with a desoldering gun you might have fixed the board in 1/2 hour. Onces you have put the ic sockets back you can always put the old opamps back and try to debug the problem one opamp at a time.

AD845 is $7 each! And I think they're all AD845, or most at least.

Besides, it's not necessarily the op amps (and if you replace them all, Murphy'll make sure it's not... ), and op amps are pretty easy to check individually. Make sure the output isn't pegged, and make sure the inputs are equal.

[Rutger]

AD845 is $7 each! And I think they're all AD845, or most at least.

Wow, I didn't realize that this opamp was so expensive. So that was a silly suggestion, oh well back to troubleshooting.

[echen1024]

Just check a few op amps randomly, and Bob's your uncle!

[pickle9000]

Maybe a silly suggestion, but isn't is quicker to just replace all the opamps with your new desoldering station. I know this a kind of a brute attack to the problem, but with the low the price of these opamps and the time it takes with a desoldering gun you might have fixed the board in 1/2 hour. Onces you have put the ic sockets back you can always put the old opamps back and try to debug the problem one opamp at a time.

In a business situation it's a valid method. You may have a group of 5 transistors that always cause issues part of the service procedure could be to pull and replace those. Ideally you would test them as well (on or off board) and then proceed with a your regular tests.

In this case you are looking for education so that changes the equation. There are things you can do to help but basically a schematic is best. Because of the number of op amps Dave could take an 8 pin test clip and solder on some jumpers the hook those up to a couple scopes. That would save time and help ensure proper connections. In reality though that only reduces test time and it's just so much easier with a schematic.

 

[vaualbus]

First like a said last time there is another chip like the one that you replaced so replece them as well. Then with this expansion board we've to figured out a way to connect the coax cables but this is another story.
If i were you would try see the jocking voltage reference problem you 've had last time (vref+ is vref- and vice versa).
It securly be an opamp problem. Than asked again on the same guy that send you the schematics for this board schematics.
Also i think that there also a -18v regulator on the back board.
Wish you keep on withe the series.
To be sure take also a look at the signal path from the coax through the board and see if at the adc input the signal are present. For me the transistor on near the coax connector are bad.

[daddario]

Well, it could be nothing but why is the -15V rail reg dissipating so much more power? On the thermal camera it looks to be 20C warmer.
The heat sink looks roughly in the 20K/W ballpark to me, thus dissipating around 1 watt more.
I don't really know what's the input voltage, but I'd assume it's dropping ca. 5V.
I=P/U ;I=1/5; I=200mA
200mA is quite a substantial current.
Of course it might just have some extra circuitry on that rail or it is designed to have some op amps driving a load with its output against the neg rail, while not in use etc.

[EEVblog]

What's the story behind getting a schematic for the CPU board but not the Opamp board?

Someone said they'd send me the schematics and that's all I got. Can hopefully get the rest as well.

[eV1Te]

Maybe a silly suggestion, but isn't is quicker to just replace all the opamps with your new desoldering station. I know this a kind of a brute attack to the problem, but with the low the price of these opamps and the time it takes with a desoldering gun you might have fixed the board in 1/2 hour. Onces you have put the ic sockets back you can always put the old opamps back and try to debug the problem one opamp at a time.

AD845 is $7 each! And I think they're all AD845, or most at least.

Besides, it's not necessarily the op amps (and if you replace them all, Murphy'll make sure it's not... ), and op amps are pretty easy to check individually. Make sure the output isn't pegged, and make sure the inputs are equal.

A quick glance on the datasheet and I'm puzzled why they are so expensive? 25 nV/sqrt Hz, 250 uV offset and 16 MHz bandwidth (1.75 MHz under load)

There must be both cheaper and better op amps today?

[digital]

Well done on having another attempt even though it is frustrating, I hope you have another attempt especially if you can get the schematic

[G7PSK]

I noticed that one of the voltage control chips was far hotter than the other 2 is that just down to the extra current on the -15 volt rail or is it faulty itself.

[DL8RI]

Hi,

I'd check TP4 on A5 (see Page 323 in the PDF).
With this TP you can check the whole Analog stuff of the source. If I remember correctly the source crashed against it high border of the DC-Offset.
Maybe the Ref-Voltage from the Signal-DAC to the Offeset-DAC is faulty.

If that's not helping maybe you should feed in a signal (1k Sine) into TP6 and look on the outside.

Same thing with TP402 for the ADC. If a signal is fed in, and the controlling works. You should see you analog signal there.

[jancumps]

Maybe a silly suggestion, but isn't is quicker to just replace all the opamps with your new desoldering station. I know this a kind of a brute attack to the problem, but with the low the price of these opamps and the time it takes with a desoldering gun you might have fixed the board in 1/2 hour. Onces you have put the ic sockets back you can always put the old opamps back and try to debug the problem one opamp at a time.

...

In this case you are looking for education so that changes the equation. ...

And let's not forget for entertainment, and the 'play-along-at-home' factor.

[BMac]

I think I would replace one of the OP amps and see if the thermal signature changes.
Dave I really like this continuing saga, I love a good fight!

BMac

[RupertGo]

Power supply overvolt failures are so often BER - even if you fix all the faults, you don't know whether the surviving components are still within spec or have been pushed close to an early death. Sometimes it's worth just not bothering to fault-find to any precision, just renewing all the affected components - which is SOP for vintage restoration, where you often re-cap (and replace known problematic resistors) before even applying power for the first time.

Dave's done this thermal imaging trick for other problematic boards in the past (was it a Lecroy scope?), but I don't remember it ever working! It's a Macbeth move: when theatre companies were in trouble, they put on Macbeth as a last-gasp, can't fail trick to get bums on seats, but of course by the time you go there it's usually too late. Which is why Macbeth is seen as a cursed play in thespian circles and cannot be mentioned by name...

[squeezebox86]

Thanks for continuing on with this one! I was just yesterday thinking yet again that I would love to see more work on this one! The three regs that you show early on with the Flir, the one appears to be much hotter than the other two. Is this the expected thermal signature for that part?

[c4757p]

AD845 is $7 each! And I think they're all AD845, or most at least.

A quick glance on the datasheet and I'm puzzled why they are so expensive? 25 nV/sqrt Hz, 250 uV offset and 16 MHz bandwidth (1.75 MHz under load)

The "AD" logo is made of pure platinum.