Resistor Numeric Code Decifer

[MonarkeIV]

I am repairing a HV PWR supply board for a General Aire GA50A20 electro-static air cleaner.

One of the 2000pf 5KV ceramic capacitors gave up it's magic smoke.

With the exception of the regulator which appears to have over heated a bit, all of the other components check out okay.

Seems this resistor (R10) has a value above 10 mega-ohms, which my DVM cannot measure.  The resistor is connected from the collector plates ground and a HV output fast-on connector.

I've been unable to decipher the value code.

The only other component I cannot verify with the DVM are the FV5M-10 10KV diodes which seem to have a forward resistance above 10 mega-ohms as well.

I have the "service" documentation however there is no component level schematic or parts list for this PCB.  It was manufactured before the "right to repair" law in California went into effect therefore I can't request that additional info.

Any ideas?

[RoGeorge]

For R10, my guess is the marking '275' means 27*10⁵ = 2_700_000, so a 2.7M\$\Omega\$.

[donlisms]

I think you can make a voltage divider out of the resistor in series with the input impedance of the DMM to get a reasonable measurement of high resistance. Would that help?

[MonarkeIV]

I did try to decipher the 275 number as a resistor code.

The problem is if it is indeed 2.7 Mega Ohms I should have been able to measure it with my Amprobe DVM.

As you can see there is no physical evidence to suggest the resistor is open. No signs of overheating burning, cracking etc.

[MonarkeIV]

I did try to decipher the 275 number as a resistor code.

The problem is if it is indeed 2.7 Mega Ohms I should have been able to measure it with my Amprobe DVM.

As you can see there is no physical evidence to suggest the resistor is open. No signs of overheating burning, cracking etc.

I saw reference elsewhere to measuring the voltage drop through the resistor calculate it's approximate resistance.  Unfortunately I've not been able to locate that information again. 
Ah the Internet.

[MarkF]

I did try to decipher the 275 number as a resistor code.

The problem is if it is indeed 2.7 Mega Ohms I should have been able to measure it with my Amprobe DVM.

As you can see there is no physical evidence to suggest the resistor is open. No signs of overheating burning, cracking etc.

I saw reference elsewhere to measuring the voltage drop through the resistor calculate it's approximate resistance.  Unfortunately I've not been able to locate that information again. 
Ah the Internet.

Ah, the old V=I*R  equation.  A novel idea.

resistance = voltage / current

Apply voltage -> measure current -> a little math -> and wolla, resistance

[RoGeorge]

Desolder the HV resistor, then put in series the resistor, your DMM and a 9V battery, all 3 in series.  You can use a power supply set to 10V in case you don't have any 9V battery at hand.  So all 3 in series.  At plus of the battery the resistor, then the + probe of the DMM, then the minus probe of the DMM to the minus of the battery.  Set the DMM to measure DC Volts.

The idea is that the 2.7G\$\Omega\$ resistor together with the internal resistance of the DMM will make a voltage divider.  The internal resistance of a DMM on DC V is usually 10M\$\Omega\$.  If yours is different, then you should find the internal resistance of your DMM in its datasheet specifications.

If the HV resistor is OK, you should read a few mV on the DMM.  The exact value read can be used to calculate the unknown HV resistance:  Rhv (in G\$\Omega\$) = 9 / mVread * 10

[MonarkeIV]

Sorry, for the late reply.  The previous week kept me preoccupied and slowed me down over the last week, including a lack of quality sleep.

If I hadn't found a shortcut, I'd do the calculations.  I may still do the calculations just to be sure.

I recently purchased a lightly used in excellent condition Triplett MG430 MegaOhm Meter for $49.95 plus shipping from northern California to my home in Southern California.  When you consider the price of a new one from Triplett I feel extremely lucky to have found this one.

With the MG430, R10 measures 94.5 Mega Ohms.  A quick search on Mouser verified this may actually be a 100 Mega Ohm Resistor as seen here:

https://www.mouser.com/c/passive-components/resistors/?resistance=100%20MOhms&termination%20style=Axial%7C~Radial

Carefully forging forward. 



 

[Terry Bites]

Use Rmystery in a voltage divider.
Maybe with Rmystery on the top and 100k on the bottom. Put 10V into it. If its 2M7 then you'd expect to see 360mV across 100k.

[Jeff eelcr]

2.7Mohm, measure out of circuit, does not have to look bad to be OPEN or changed value.
Jeff

[MonarkeIV]

Sorry have delayed my response...

I was reviewing the parts once again and realized I did not explain R10 is connected between case GND, across D1-D6, onto R9 (10K), then finally onto the HV2 (7.2 - 7.8 KVS) faston connector. 

This leads me to believe and it was explained by someone else, this is a bleeder resistor.  It would need to be a quite high resistance in order to not appear as a short circuit.

I've included another image (not my board) but one which was previously repaired by someone else, where the traces are easily seen from the component side.

Also included is the basic wiring diagram from the Op/Maintenance manual.

I hesitate to remove R10, as if I damage it, I don't know that I can easily get a replacement not knowing it's marked value.

........

I removed R10 and measured it.  First with my regular DVM on the highest scale (10 Mega Ohm) which produced nothing.  With my Triplett Mega-Ohm meter it measures 94.5 Mega Ohms.

........

After all of this, I now realize some of the parts used aren't readily available. 

The 302M capacitors seem to be 3000pf @ 5KV.  Neither Mouser of Digikey has a small reasonably priced 3000pf capacitor with at least a 5KV voltage rating.  The 10KV FV5M-10 high voltage diodes are not readily available as well.

I probably should pop for the new replacement board and be done with it.

[smashedProton]

checks out, its adjacent to a cockroft walton.

[MonarkeIV]

checks out, its adjacent to a cockroft walton.

Ummmm, that seems to make no sense.