Parabee resistance check

[Doc B]

Try swapping the leads on your meter when testing the rectifiers. They should read high like you are seeing in one orientation, and with the meter leads swapped  end for end on the rectifier they should read lower, like maybe 1500 ohms. The only time to be concerned is if you see a really low reading like under 100 ohms in each orientation. That would mean that the rectifier is shorted out. When the rectifiers are measured in circuit the values can differ somewhat from what I describe here, but very low ohms measured across a rectifier is always bad.

 

[MarimbaMan]

I swapped the meter leads and remeasured:


Right amp
T1 to T5 = inf
T2 to T5 = inf

Left amp
T1 to T5 = inf
T2 to T5 = inf

 

[Paul Joppa]

This is problematical - it may be that the diodes are dead, or it may be that your meter does not use enough voltage for this test to be valid, or it may be that you are using the Cree SiC diodes which need more voltage to turn on.

 

[MarimbaMan]

I broke out my $10 analog meter and got the same readings as above across the diodes, and inf again when I swapped the meter leads.  Is there a way to determine whether or not I have Cree SiC diodes?

 

[Paul Joppa]

The Cree diodes were a change that some people made; if you did not make this change then you don't have them. (They are TO-220s, large chips with a large metal heatsink, and inconvenient to mount - made for PC boards.)

So, the diodes are toast apparently. They were 1000v UF4007s originally (the Crees are rated 1200v, for more margin - that's why some people used them.). The UF4007s should be easy to find.

 

[MarimbaMan]

The diodes came with the kit, so they are not Cree SiC.  I am the only one who touched them, so it appears I fried them when soldering.  I'm regrouping and ordering new diodes, and will post again once the amps are put back together.  Thanks all for sticking with me through this!

 

[MarimbaMan]

I got the replacement diodes in the mail from Queen Eileen, and did a quick measurement straight out of the box:

.718 Megaohms, reversed the leads, and got inf.  It appears these are the same measurements as the old diodes.

I then installed the new diodes, and did the resistance check:

T1 = .718 megaohms
T2 = .830 megaohms

This appears to be correct since the diodes pretty much read the same uninstalled as they do installed.

At this point, I'm wondering if I should approach this from a different perspective:


The manual states that the resistance check should be:


T1  155-165 ohms
T2  155-165 ohms

How is this range calculated?  Or, what components are included in these measurements from the terminal to ground?

 

[Grainger49]

Sorry, I have to ask, because I'm an Engin-nerd, what meter are you using?  The meter has a lot to do with the readings you get.  Some aren't so good.  Some are.

New Cree diodes should measure a forward diode drop in the forward direction and a high resistance (or infinity/overload) in the reverse direction.

 

[Doc B]

.718 Megaohms, reversed the leads, and got inf.  It appears these are the same measurements as the old diodes.


This appears to be correct since the diodes pretty much read the same uninstalled as they do installed.

It's a problem that revolves around using an autoranging meter.  It should read more like 500 ohms, not 718K ohms. I'm measuring about 450 ohms in the forward direction on some UF4007s out of the same bin that your replacements came from using our old reliable Radio Shack meter, and 490 ohms with an HP3466A that we keep on the bench.  Both are set on diode check, not autorange. On autorange the HP reads something crazy like 256K ohms. If you have a diode check setting on your meter, use that. If not, try using the meter on a manual range setting of 2K ohms.

 

[MarimbaMan]

I am using a Radio Shack digital mm, model 22-805, fresh battery. I'm not using Cree diodes, I'm using the stock UF4007's.

Doc, I measured the newly installed diodes with the diode check on the mm:

T1  474mV
T2  481mV

 

[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.