TongHui TH2821A help needed

[djrobert]

I have a Tonghui TH2821A LCR meter I acquired on ebay a couple years ago. Back then they had a US representative in CA. It has never properly charged the internal ni-cad. I was using it only on my bench so it was not much of a problem and I never bothered to get it fixed. Now I would like to use it portable and not keep opening it up to replace the 9V alkaline battery.
One of the things I have found on inspection of the PCB is a medium size 1W (I think) resistor that is burned and open. this resistor is on the battery end of the PCB and is identified on the silkscreen as R82.
The resistor is burned and discolored to the point where the color code is not readable.
I was hoping that another TH2821A owner might be able to have a look at their own instrument and tell me what the color code of the resistor is.
I was also wondering if anyone had ever seen any service information for the instrument.
I have emailed Tonghui in China but have received no response as yet.
Thanks in advance
Dale Robertson NV8U

 

[djrobert]

I should also mention that no disassembly (aside from that normally required to change the battery) is required in order to check this.

 

[john monks]

I'm taking a wild guess. I believe the resistor is the dropping resistor for charging a 9 volt nicad battery. I would plug in the unit as you normally do, measure the voltage across the burned resistor, then replace the resistor with a value selected to dissipate approximately 1/4 watts. Then I would put in a new 9 volt nicad. My experience with 9 volt Nicads is terrible. They either begin to look like an open circuit, they short out, or corrode. Plus they have a short shelf life. I never had one last very long and I quit using them. I suspect this may be the root of your problem. Your resistor value should be your voltage squared divided by the power dissipated by the resistor. I would still use a 1 watt resistor to prevent it from burning out again.

 

[velocina]

...One of the things I have found on inspection of the PCB is a medium size 1W (I think) resistor that is burned and open. this resistor is on the battery end of the PCB and is identified on the silkscreen as R82.
The resistor is burned and discolored to the point where the color code is not readable.
I was hoping that another TH2821A owner might be able to have a look at their own instrument and tell me what the color code of the resistor is.....

Hi,
if it can still help, in myTH2821A the resistor color code is Orange-Black-Black (30 Ohms, verified by an ohmmeter).
My instrument has a big problem: it doesn't power-up, charging the battery out of it is the same. Display only blinks a while when depressing power button a first time, then no effect at all on successive tries. I suspect a fault of internal soft-start circuit....

Bye.

Luca