[R-1051] Frequency standard adventures ...

Sun Sep 24 22:18:22 EDT 2006

Jan, Steve, All

My R-1051 (plain) came with the style I think you are talking about.  A 
metal box for the oven covered with foam.  Part Number 6625-078-4718. 
Assy 4013399-0701.  Only one out of the four of my standards of this 
type has the voltage regulator circuit board mounted on the side of the 
oven.  The others run at 28 volts.  A 10 volt zener (1N758) on the 
oscillator board runs the oscillator and oven control from 10 volts. 
The resistor in series with the zener (330 Ohm 1/2 W,or 820 ohm) to make 
the regulator is often cooked.  The 28 volts in the radio often runs 
high into the 30 to 34 volt range.  I wonder if the 18-19 volt regulator 
was added to correct the resistor from cooking and to reduce the power 
in the transistor in the bottom of the oven.  The 2N697 and the 1/2W 820 
ohm resistor on the regulator circuit board have discolored the board. 
I have only one of type working.

The three newer style standards that I have are 6625-01-163-1323 Assy 
01A228203-01.  They have a square opening marked Index on the top with a 
  number showing the fine frequency adjustment.  Next to the adjustment 
screw is a white plastic cover marked coarse.  Under the cover is a 
coarse trimmer cap.
One says "Modified to TA963"
It had a regulator added.  One 2N3441 mounted to a bracket mounted to 
the bottom plate, a 1N4965 20V zener, a tantalum cap, and a 402 ohm 
resistor.
The other two marked TA973 Rev A the regulator was standard.
The new board mounted to the side if the oven has a 78M05 5 volt IC 
regulator, some TTL logic, etc. to switch from an external 5MHz source 
to internal if the external fails.
Some have a 78M15.  The power resistors in series with the voltage 
regulators are not all the same.
I guess they were still having problems getting rid of heat.

Dennis McLaughlin

Jan Skirrow wrote:
> Interesting. And true! It seems pretty obvious that you'd not try to 
> drop 28V (the input to the regulator) to the 27V shown on the schematic. 
> Anyway, the zener ref on the reg module is a 1N3026 - 18V at 1W.  I 
> first ran the oven without the regulator module - just put 27V directly 
> on the oven module. It ran very hot, and the frequency overshot in the 
> opposite direction from 5MHz from its cold freq. So I figured something 
> was wrong and checked the reg module. I ran the directly applied voltage 
> down to 18, and the osc was _much_ happier.
> 
> At least one of the freq std modules I have includes a very different 
> regulator module - and I think the DC amp on it is mounted outside the 
> oven box - but I've not really looked at it closely enuff to state more 
> than my suspicions.
> 
> Jan
> 
> Steve Hobensack wrote:
> 
>> I have worked on those also. One thing to look out for is the voltage 
>> regulator feeding the oven. My schematic labels the regulator output 
>> as 27 volts. I think it is a misprint and should read 17 volts. It 
>> actually works at 27 volts but if something goes wrong in the 
>> temperature bridge circuit, things really cook inside the oven. A 
>> three terminal regulator type 7815 works good here. I set the 
>> temperature using a mercury lab thermometer. Fair Radio used to sell 
>> the frequency standard modules at a reasonable price. I don't know if 
>> they still do.
>>
>> I have a freq standard module that is so old , it is not adjusted with 
>> trimmers. It is adjusted with slugs in the rf transformers. I wonder 
>> if anyone out there has a skiz diagram of that type of freq standard 
>> module? Thanks
>> ....Steve...N8YE
>>
>>
>>> From: Jan Skirrow <jan at skirrow.org>
>>> Reply-To: R-1051 Discussion Group <r-1051 at mailman.qth.net>
>>> To: R-1051 at mailman.qth.net
>>> Subject: [R-1051] Frequency standard adventures ...
>>> Date: Sun, 24 Sep 2006 10:52:18 -0700
>>>
>>> I have several 1051s, and several different versions of the internal 
>>> frequency standard module. One extra module was in my junkbox, as the 
>>> oven was kaput altho the oscillator itself worked but way off freq. I 
>>> have a need for an oven for another project, and decided to see if I 
>>> could fix this one - I know the ovens have been a source of problems 
>>> in the 1051s, and if the oven isn't working, you can be pretty sure 
>>> the oscillator will be well off frequency.
>>>
>>> As everyone likely knows, the oven element is in the collector 
>>> circuit of a DC amplifier. The DC amplifier is driven by a circuit 
>>> that contains a resistor bridge to set current to the heater coil. 
>>> One of the bridge resistors is a sensistor bonded to the metal oven 
>>> shell and responds to temperature. Another arm of the bridge has a 
>>> trimmer to set the operating temperature of the oven. Quite a nice 
>>> little circuit. It will take awhile to reach its intended temp, but 
>>> holds well once it does and the std is remarkably stable.
>>>
>>> I think Cecil Acuff once told me that a defective sensistor was often 
>>> the problem with the oven. But mine tested fine. The DC amplifier 
>>> transistor (in this version anyway) is in a TO-5 metal case, and is 
>>> press fitted to the oven shell so that it is heat-sinked. The 
>>> transistor tested open. So I decided to remove it (no small task as 
>>> it is cemented in place) and replace it with a similar transistor I 
>>> had. But when I clipped the collector lead off the heater coil 
>>> terminal, the wire fell apart (it was in a piece of clear plastic 
>>> tubing.) It had corroded thru despite being gold plated but the break 
>>> wasn't visually obvious. When I connected the remaining collector 
>>> stub back to the heater, the assy worked perfectly and the oscillator 
>>> was on-frequency once the oven was at temp.
>>>
>>> So, the lesson in all this is to not assume that a freq std oven 
>>> problem is the sensistor. The plastic sleeving on the collector lead 
>>> was heat-discolored (this baby runs hot!) and the break wasn't 
>>> noticeable until I had freed up one end.
>>>
>>> Jan
>>> _______________________________________________
>>> R-1051 mailing list
>>> R-1051 at mailman.qth.net
>>> http://mailman.qth.net/mailman/listinfo/r-1051
>>
>>
>>
>> _______________________________________________
>> R-1051 mailing list
>> R-1051 at mailman.qth.net
>> http://mailman.qth.net/mailman/listinfo/r-1051
>>
>>
> 
> 
> _______________________________________________
> R-1051 mailing list
> R-1051 at mailman.qth.net
> http://mailman.qth.net/mailman/listinfo/r-1051
> 