[Hammarlund] SP-600 the fun continues

[stbradford]

Russ WQ3X wrote:
> Hi Smith -
>
> I have seen this before. It is killing the AGC so you get distortion. 
> Now I just replace these caps before even turning a new SP-600 on. If 
> you use a plumber's iron you can remove the bathtub covers and put new 
> 10uF caps inside the cans. No need to replace the covers afterwards. 
> Then your radio will not look molested!
>
> 73, Russ WQ3X
>
>
>
>
>
> ----- Original Message ----- From: "stbradford" 
> <stbradford at insightbb.com>
> To: <hammarlund at mailman.qth.net>
> Sent: Wednesday, February 20, 2008 10:12 PM
> Subject: [Hammarlund] SP-600 the fun continues
>
>
>> Good evening List
>>
>> Having reworked the BFO oscillator components inside the L-44 can I 
>> have moved on. The various voltages and resistance checks on the 
>> oscillator, buffer are reasonable. So as to not beat a dead horse I 
>> have moved on to other areas to inspect.
>>
>> After reviewing the schematic I am now including the i.f. driver, 
>> V11, in the areas to be checked. And lo and behold the -51 volt bias 
>> source that feeds the g1 of V11 is way down in volts. Tomorrow I will 
>> look into the reason for this low voltage. I will start with the 
>> filter network attached to the 6AL5 rectifier tube. Those old bathtub 
>> filter caps, C-158, 158 and C-160 are prime subjects. The -51 volts 
>> is more like -20 volts, and I am sure that has screwed up the bias on 
>> the V11 driver. We will see....
>>
>> So, until tomorrow thanks for all the information and I will let the 
>> list know what I find.
>>
>> Smith
>> W9HAK
>> _______________________________________________

Thanks for the info Russ, great idea on the bathtub cap modifications.
I'll dig out the Big Bertha solder iron and remove the backs. This a
near end of production version of the JX-17 diversity receiver built for
the Air Force. All ceramic bypass caps through out the receiver, thank
goodness....

I discovered another modification they did to the agc system while
looking into the -51 volt line. The 2nd i.f. V10 G3 doesn't go to ground
as per my schematic. Instead it has an 100k resistor back to the -10
volt line line with a .001 decoupling cap.
I read where Hammarlund made many modifications to the basic radio and
many were undocumented. This is surely one of those modifications. It's
clearly a factory modification including that damn fungicide coating on
the solder connections.

Smith
W9HAK