[volt-nuts] Replace neons with LEDs in Fluke 845AB

[Lou Amadio]

Hi Dallas
So sorry. I understand how confusing it can be coming in part way through a
repair. My apology.
I have managed to implement your LEDs mod and I can see the LEDs light up
but the voltage driving them, and the rest of the meter, is way too low.

The history of my 845 can be found here:
http://www.eevblog.com/forum/metrology/teardown-fluke-845aabar-tweaks-and-mods-%28and-repairs%29/.You
will need to scroll down somewhat.

Briefly, When I first received the 845 it was not working and had nicad
corrosion on the PS PCB. I have not really gotten it going properly yet.

I then started on your LED mod hoping that would help, spurred on when I
discovered the neons had turned black.

So, I am now back to square one and working on getting a stable +-15v.I
have managed to separate the PS and Main PCBs and will conduct experiments
on the PS by emulating CR104/105, C117/118 off board.

I live on the east coast of Australia, near Sydney.
Regards, Lou

On Thu, Jun 9, 2016 at 3:19 AM, Dallas Smith <dosmith at outlook.com> wrote:

> Hi Lou,
>
> I think we should talk to each other. Send me your phone number to
> dosmith at outlook.com.
>
> I'm confused to the state your meter is in, you already made the mod and
> now you are having problems? Or it's not working before the mod? When I
> made my mod it pulled the rails to ±14 volts. It was a compromise between
> brightness and performance. I tried many LED's and the ones I selected were
> just right. The right brightness to current draw and the correct wavelength
> for the photo cells. My instructions are not as clear as it should be for
> the T202 move wire from pin 9 to 7. The high voltage winding are abandoned
> and the led's are powered by the winding that supply the ±15 volts. I may
> have to look at my meter to see better what was done.
>
> I work second shift, whats a good time to talk? I go to work at 3pm and
> return at midnight.
>
> Dallas
>
>
> On 6/8/2016 1:24 AM, Dallas Smith wrote:
>
>> Hi Lou,
>>
>> I believe that the transistors are good. If they do not run to hot to
>> hold on to.
>> Yes CR105 feeds the negative supply. Did you check CR104 and CR105?
>> Q203 and Q204 should also be check since they could shunt the transformer.
>> Is transformer T202 and T203 hot. The high voltage windings on T202 could
>> be leaky.
>> Try taking out C119, did rails go to approximately 15 volts?
>> Time to start isolating the supplies to various circuits to find the
>> culprit by removing one leg of various resistors.
>> You can deal with the carbon resistor accuracies later.
>> I had all many issues with my meter, after all their very old.
>>
>> Dallas
>>
>> On 6/7/2016 11:09 PM, Lou Amadio wrote:
>>
>>> Hi Dallas
>>> I set my DMM on Diode Check mode:
>>> Q201, PNP, no part number: EB/BE=0.23/Hi, CB/BC=0.21/Hi, EC/CE=Hi/Hi
>>> Q202, PNP, no part number: EB/BE=0.22/Hi, CB/BC=0.20/Hi, EC/CE=Hi/Hi
>>> Q111, NPN, 2N1304: EB/BE=Hi/0.28, CB/BC=Hi/0.26, EC/CE=Hi/Hi
>>> Q112, PNP, 2N1305: EB/BE=0.26/Hi, CB/BC=0.25/Hi, EC/CE=Hi/Hi
>>>
>>> The two 15v rails are down to about 5v each. The oscillator is working
>>> but
>>> fluctuating badly on frequency.
>>> Replaced all the electro caps. Have not replaced the resistors or diodes.
>>> Will check these next.
>>>
>>> CR105 feeds the neg 15v supply cap??
>>> Regards
>>> Lou
>>>
>>>
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