[time-nuts] Re: TEMEX LPFRS-01 LPRO repair

[VE7HR]

Changing the 4 47 uF 63V caps did not change anything.
I tested the old ones and my Sencore LC 102 said they were good so I was not surprised. 
I will log data from a cold start tomorrow to see if that tells me something. It does lock for a bit from a cold start and when locked the numbers look believable. But I certainly am not seeing the sweeping on the Control voltage but it does change when it locks. 

This unit has a sticker on it saying Pin 7 is not connected. That’s the analog adjustment input. 

When I had it apart I was unable to see any trimmer caps on the board. Where should I be looking? There were a few trimmer pots and a couple of tuning cans. 

It’s not clear to me but what frequency am I supposed to be adjusting?  The 10 MHz output frequency that is on some units adjusted by the external trimmer on pin 7 or something else. ?

I will have to check calibration but I think the unit outputs a signal that is 249 mHz low in frequency.  I did not have a chance to see if that changes when it briefly locks. 

Thanks
Dave

Sent from my iPad

> On Apr 25, 2024, at 10:37 PM, Hans Georg Lehnard via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> Hi Dave,
> 
> I suggest recording the data from the serial interface and displaying it in a diagram.  I did this with my Temex MCFRS-01 at 1s intervals and it looks like the attached picture.
> 
> The manual says:
> 
> 1. Rb Signal level.
> During warm-up time this signal is approximately 0V and after it stabilises to a nominal value of 1 to 3.3V.As
> long as this signal is too low (<0.6V) the internal LPFRS control unit sweeps the Xtal frequency in order to
> find the Rb absorption dip.
> 
> 2. DC-Voltage of the photocell
> this signal corresponds to the transmitted Rb light level.
> This is the light of the Rb lamp which is partly absorbed by the Rb cell.
> The nominal photocell voltage is in the range 2.0 to 3.5 V but must stay stable after the warm-up time.
> The photo cell voltage is related to the internal reference 5 V voltage.
> The full scale corresponds to the coded value ‘00’ and the zero (no light) corresponds to the coded value ‘FF’
> 
> hgl
> 
> 
>> Am 24.04.2024 um 16:32 schrieb VE7HR via time-nuts:
>> I have been having lots of enjoyment out of my TinyPFA. It’s so small, easy to carry and quiet compared to my HP 5370B.
>> 
>> I am testing it out on my existing pile of frequency sources. One of them is a LPFRS-01. It no longer locks. It locked for a few minutes when I first powered it up.
>> 
>> Looking in the archives there has been a long standing thread on repairing these devices with several people in the same spot I am in on a used to work LPFRS-01.
>> It no longer locks.
>> 
>> Output from Serial port on mine is
>> 8A 1C C3 8A 7C 86 A7 DC
>> The 4th column is the VCO control voltage that should be sweeping when unlocked. So that is what the other threads seem to indicate.
>> 
>> There are some nice teardown pictures of a unit at https://egidy.de/temex_lpfrs_lpro/
>> With similar data output.
>> 
>> I don’t think I have found any service information yet. If I have missed it please point me in the right direction.
>> 
>> I have not pulled my unit apart bit I am not seeing the trimmer cap that has been speculated to be the fix in the pictures.
>> 
>> Can anyone shed any light on what to twiddle?
>> If I get it fixed I promise to post my results. It’s somewhat puzzling that there is a common question but no answer. Does this mean not one has fixed it or just not one has told us they have.
>> 
>> Dave
>> VE7HR
>> Sent from my iPad
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> <Rb_1265_OK.JPG>
> <Temex_LPFRS_Manual.pdf>
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