Basic In-Circuit troubleshooting techniques?

[jasonbrent]

Folk,

I'm working on a repair of a Valhalla 2701C. It is my first real repair and my learning curve is steep. I have more equipment than knowledge and want to close that gap.

So... I'm wondering if there are basic philosophies on how to troubleshoot comoments in circuit? Pulling components is an option, but I would like to avoid that for the moment on this repair unless necessary.

* always start with input power rails, verify the voltages are correct

* resistors - straight forward unless it's part of an LC, in which case some basic math has to be done to understand what the reading should be.

* relays- can be tested in circuit by powering the poles to get them to flip. If they are latching, you can then check for continuity on the latched pins.

* mosfets - unsure here... potentially treat them similar to a relay by monitoring source and drain while applying an appropriate voltage to the gate? In the 2701C there are a set of mosfets in a chain that act as voltage dividers, i believe. Unsure if they can be tested separately like this...

* NPN and PNP transistors - haven't figured this one out yet?

* diodes - I believe a diode tester attached directly to each would work...

* capacitors - I am not sure how to test these in circuit.

Any tricks or modifications to my assumptions on these component types so far?

Thank you on advance for any tips.

-j

[JoeO]

First - Determine what is wrong.  Attempt to locate a user's manual.  This alone can give you a good indication where to start looking.

Second - Try to locate a schematic for the item.  If not the exact item, you could try looking for a similar item with a different model number.

Third - Search the internet for a description of a similar problem and what the repair turned out to be.

Now start with your list.

[jasonbrent]

First - Determine what is wrong.  Attempt to locate a user's manual.  This alone can give you a good indication where to start looking.

Second - Try to locate a schematic for the item.  If not the exact item, you could try looking for a similar item with a different model number.

Third - Search the internet for a description of a similar problem and what the repair turned out to be.

Now start with your list.

Joe - Thank you for the response.

I've done these three things so far. Someone has repaired this unit with a variant of options at https://www.eevblog.com/forum/metrology/valhalla-2701c-schematics-100832/ and posted schematics. I'm currently going through the troubleshooting steps in the manual, and have printed the schematics out in large form for annotation purposes as I try to troubleshoot.

Otherwise, this particular instrument has very little information pre-existing on the internet from a repair perspective... which leads me back to my question of "how to troubleshoot these components in circuit?" It sounds like my list is somewhat correct at least.

Thank you again.

-j

[Neomys Sapiens]

A quick test of components in-circuit can be done by using a component tester similar to the Huntron or Vu-Data units. A simpler version of it was build into some scopes from Hameg. Also, there are diy descriptions available. Look for 'octopus' component tester.

[jasonbrent]

A quick test of components in-circuit can be done by using a component tester similar to the Huntron or Vu-Data units. A simpler version of it was build into some scopes from Hameg. Also, there are diy descriptions available. Look for 'octopus' component tester.

Ah, interesting... Huntron is... pricy.. but now I grok what a curve tracer is for. I found one description of the octopus at http://www.jammarcade.net/simple-component-tester-a-k-a-octopus-curve-tracer/ which pointed to http://www.qsl.net/kd7rem/pdf/octopus.pdf .

A basic question... if I generate a 60hz sine wave with my function generator with a Vpp of 6.3volts, is that functionally equivalent to a stepped down 120VAC to 6.3VAC?

[Neomys Sapiens]

Basically it depends on the drive capability of your FG. For directly connecting to suspect components, it's output should be well protected against short circuits and possible ringing. Old little bell transformer with a weak charakteristic is probably safer. Or put at least some R and/or a PTC in series. Or look at the manual of one of the component testers, there is a triangle wave generator in this position.

[Samogon]

Huntron is very useful if you have good mental picture of how signals should look like. But best application is to compare with good known PCB. And huntron price is high for later models with lcd displays like 2700 later. Latst CRT 2500 is very nice. I had one but traded it for 2700. 2500 still can be expensive. Look for 2000 model. It worth investing to it if you plan to do repair seriosly.

 My advise same as name of my favorite youtube channel: look for the “Signal path” in both ways.

[jasonbrent]

I just acquired^Wsniped a Huntron 1005B-1ES and a fluke 8800A combo pack for ~2 bills on rebay with a few seconds left on the auction... that'll work to start with. The 2000s are much pricier, but I'll keep a watch on them.

[Neomys Sapiens]

Definitely a good start. It is not a universal cure for all problems, but even without a complete PCB for comparisor purposes there are possibly repeating or similar structure on the very same PCB, that can serve the same purpose.

[Samogon]

Sure it is good start. But prior 2500 they have minimum excitation voltage 5 volts if i remember correctly, and it is too high to measure isolated passives without chance of enabling nearby active components and ICs. 2500 has minimum voltage of 200mV for that particular purpose.
And good luck with repairs. We will wait for success stories

[daedalux]

The first and more important step is visual inspection, anything broken, inflated, or more or less subtly burnt must be replaced. When you find something check the nearby circuitry good. Look for broken solder joints and resolder anything you may remotely think could fail, once on the job is very quick per additional joint.
You learn a lot observing the device working normally and the failure mode. Be prepared to quick shut down. In anything tube driven don't do it and go straightforward inside without powering on. If the device is freezed for good at least the power supply is bad.

After that check voltage regulator output good, BJT base voltage must usually be 0.7, and not much higher, mosfet gates must not short to ground.

Diode check function on multimeter inserts about 1,5V to be able to turn on small transistors and diodes.
Resistance check function on multimeter inserts about 0,3V to try to not turn on semiconductors for in circuit checking.
Capacitors are difficult ones, as you can't know they are good just for the capacitance and in circuit measurements are distorted. Usually replacing anything you suspect of with new ones is not that expensive.

[Ray Gianelli]

NPN/PNP transistors can be tested with the diode function of your digital multimeter.  See attached pic for details.
(Pic courtesy of EchoWars of Audiokarma).

My first question when someone brings me a repair is "what is it doing or not doing?".  Is it doing something it shouldn't, or is it not working.  Do some functions work and others don't?  Let these questions guide you.

The Valhalla may be a bit ambitious for a first repair.  I learned on discarded analog TV sets picked up on the side of the road.  Which was a good thing, as my mortality rate was not good at first!

Good luck, and keep us updated with your progress.