Parabee resistance check

[Grainger49]

Readings from different meters will vary.  That is, one might show 200k ohms, one infinity, one 500k ohms and some read to over a mega ohm.  The RS meter might not show diodes the same as a more expensive meter.

Yes, I'm a big fan of Fluke meters.  You see one in one of the pictures of Doc's workbench somewhere on the site.  They give very reliable readings and usually have a diode checker position on them.  You can get a resistance in one direction and swapping the leads with my meter you get the forward voltage drop, most are under 1V, some just over, in the other direction.

[Doc B.]

The values look fine but I think we have a bit of an issue with the unit of measure. mV is the abbreviation for millivolts. We would be looking for 471 ohms which is equal to .471 Kohms, and the scale on the diode checker setting on the meter should indicate one of those.

[MarimbaMan]

I reread the diodes with the diode setting on the RS meter, it is indeed measuring in mV.  Since every resistance reading was spot on except T1 and T2, I'm now wondering if the meter is defective measuring across diodes, or maybe wasn't built for this application. 

In the interest of completing the project, I wouldn't mind spending the money on a good Fluke, although I don't want a $500 meter.  Doc/Grainger, any recommendations on a good Fluke model?

[Doc B.]

OK, so perhaps you meter is attempting to measure the forward voltage drop across the diode. I think the UF4007 is spec'd at something like 1.7V rather than .48V so something still doesn't seem quite right with the meter reading. At any rate the diodes seem to be functional and I think you can move on to whatever else might still not be working right. Rather than buying a meter can you possibly borrow one from someone? That way you can compare its readings with the RS meter before deciding if you need to replace it.

[Paul Joppa]

1.7v is at high current, 1-2 amps. Meter test current is likely around 1mA and should drop around 0.6v according to the data sheet. 470mV seems close enough to me.

Here's a link to a data sheet:

http://www.datasheetcatalog.org/datasheet/GeneralSemiconductor/mXyzrxzq.pdf

[MarimbaMan]

I'll see if I can scare up another mm at work tomorrow.

[Grainger49]

MarimbaMan,

PM me and I will send you some links to reasonably priced meters.

[MarimbaMan]

I borrowed a Fluke 70 III from work and remeasured:

diode check:

.448  reversed leads = inf
.451  reversed leads = inf

the value above was not specified, I'm assuming it was ohms

resistance

T1 = 1.3 Megaohms
T2 = 1.4 Megaohms

I'm still miles away from the spec of 155-165 ohms

[Doc B.]

Are you clipping the black DMM lead to a solder terminal on the ground buss when you do this test? If you are clipping to the copper ground buss wire rather than a solder joint that it attaches to, you may not be getting continuity because of the varnish coating on the wire.

[MarimbaMan]

The kit shipped with naked ground buss wire, not magnet wire.  I attached the negative lead to the buss near the rca tab to do all my measurements, to be on the safe side, I clipped the neg lead to the solder joint on the rca tab, and got the same readings.

[Doc B.]

OK that's good. What's the resistance reading from the ground buss to 1T on the power transformer?

[MarimbaMan]

ground buss to 1T = 0 ohms

[Doc B.]

OK, now try measuring resistance from 1S on the transformer to that same point on the ground buss, and then from 1F on the transformer to the ground buss. Use either autorange or manually set the meter to the 20K ohms range.

[MarimbaMan]

set meter to autorange:

ground buss to 1S = .820 Megaohms
ground buss to 1F = .923 Megaohms

[Grainger49]

That sounds like the ground that the transformer gets might be wrong.  I don't have a schematic so I have no idea where the transformer/power supply ties back to the ground bus.  Try heating up the grounding connections and the ones that should give you the resistance (to ground??) Doc called for.  I tried looking but couldn't find the number for 1S and 1F.