Parabee resistance check

[MarimbaMan]

Voltage measurement:

1S to 1T  334 VAC
1S to 1F  679 VAC

[MarimbaMan]

I forgot to mention, I am using autoranging on the mm

[Paul Joppa]

Other than being a bit lower than expected, those look good. Perhaps you line voltage is a bit below 120v? Anyhow they are symmetric, so the transformer is good (all the winding turns are there). Your previous resistance measurement problems must have been due to poor connections. Time to solder the 10 ohm resistors to 1S and 1F, and check resistances from each end of each resistor to 1T - i.e. confirming the solder joint. Then proceed downstream, one joint at a time.

[MarimbaMan]

The 10 ohm resistors I mangled are being shipped by Mouser.  In the meantime, here are the resistance checks from the other amp (wired identically):

1F to T2 = 10.1 ohms
1S to T1 = 10.9 ohms
T1 to T5 = .740 Megaohms
T2 to T5 = .705 Megaohms
T5 to 1T = 7 Megaohms and dropping
T4 to 1T = 0

Hope these are the measurements you are looking for, otherwise, let me know.

[MarimbaMan]

Ok, the RRSF has been rebuilt with new wirewound resistors, and another fresh battery in the MM.

Left amp measurements are in the previous post, here are the new right amp measurements:

1F to T2 = 10 ohms
1S to T1 = 10.1 ohms
T1 to T5 = .668 Megaohms
T2 to T5 = .713 Megaohms
T5 to 1T = 7 Megaohms and dropping
T4 to 1T = 0

T1 to T5 and T2 to T5 are across the diodes, do these look correct?

[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