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

Lou Amadio lou.amadio11 at gmail.com
Tue Jun 7 23:09:35 EDT 2016


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

On Wed, Jun 8, 2016 at 1:29 AM, Dallas Smith <dosmith at outlook.com> wrote:

> Hi  Lou,
>
> What voltage are on the 15 volt rails? They are not regulated ±1 volt
> seems reasonable. Check/Replace C117, C118, Q111, Q112; these are the only
> components that can significantly draw the supplies down. If your unit is
> like mine the carbon resistors have changed 30 % by now in the positive
> direction. Also check for the regulated +10 volt supply at CR105. What is
> the reading on your meter semi range of your digital ohm meter across the
> emitter/collector? When using digital ohm meter to check semi's, the
> reading is actually the voltage across the junctions. What is the six
> junction readings you get on the EBC on the transistors? (forward and
> reverse readings).
>
> Dallas
>
>
> On 6/7/2016 2:05 AM, Lou Amadio wrote:
>
>> Hi Dallas, no, I do not have a curve tracer but I have an oscilloscope and
>> function generator. What do you suggest?
>>
>> The +/-15v is really low so I need to do something.
>>
>> If I change to Si, are R207_8 likely to be lower or higher?
>>
>> Regards
>>
>> Lou
>>
>>
>> ..............................................................................................................
>> On 7 Jun 2016 2:26 am, "Dallas Smith" <dosmith at outlook.com> wrote:
>>
>> Hi Lou,
>>>
>>> It's normal for some leakage in germanium transistors. Do you have a
>>> curve
>>> tracer?
>>> If you change them to Si you will have to re-bias them (i.e. R207_8).
>>> The 84 Hz may change also, might have to adjust C203.
>>> But should be possible.
>>>
>>> Dallas
>>>
>>>
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