Krolroger wrote:

Can I use a DMM (cheap Chinese) to measure voltage outputs from transistors?  (The service manual sometimes calls for use of a vacuum tube voltmeter).

The "vacuum tube voltmeter" is an old times solution for a high input impendance meter which doesn't load the circuit under test (as opposed to an analogue mechanical voltmeter). A modern DMM is just that, so you can use it for all measurements.

Krolroger wrote:

If I am measuring voltage relative to ground/chassis, where do I ground the negative test lead?

Any place that is directly connected to the ground in the circuit diagram. Ideally close to the point being measured, but that isn't too critical in a 4002. Use a spring clip if you have one, we have read too many "my probe slipped and I fried this and that" stories lately...

Krolroger wrote:

Can I measure resistor values in situ?

In most cases, no. You'll need to disconnect one end of the resistor if you need to measure it.

Thanks for this detailed response.

Simon

tournedos wrote:

Krolroger wrote:

Can I measure resistor values in situ?

In most cases, no. You'll need to disconnect one end of the resistor if you need to measure it.

tournedos is correct - you won't get an accurate measurement in circuit, but, since resistors usually (although not always) fail open circuit, and because any parallel paths with always reduce the measured value, it can sometimes still be useful to take measurements across resistors in circuit as a first step.

The measured value in circuit should always be that of the resistor itself (taking into account normal tolerance), or lower (because of the parallel paths within the circuit). If the measured value is higher then it indicates the resistor has likely failed open.

In summary, you can't prove a resistor is okay when testing in circuit, but you *may* be able to identify a faulty resistor before you resort to desoldering legs.

That said, I would always be looking at any electrolytic capacitors first as these are far more prone to age related failure.

Kind Regards,

Martin.

Thanks, Martin.

Is there a rough and ready way to identify (aside from visual inspection) if an electrolytic cap might have failed without removing it?

Simon

Krolroger wrote:

Thanks, Martin.

Is there a rough and ready way to identify (aside from visual inspection) if an electrolytic cap might have failed without removing it?

Simon

Only if it shorts, really. Electrolytic caps can fail in a lot of other ways.
Replace the lot. There isn't that many of them in there.
(Don't touch the decoder board - if fitted).

Martin

Original question was Trying to diagnose a non dropping tone arm on a BG4002.  I would suspect hardened grease, clean and lubricate,  Hopefully no meter required.

    John.