[Boatanchors] SB-1000, no output on 10 meters

[Dave]

Howard;

I have pulled up pix of the R-175 and R-175A. Nowhere near the same in 
appearance.

All coil taps and capacitance value are per the construction manual and 
schematic. Input filters are passing a 28.700 MHz 20 watt CW signal at 
near input levels as observed with a 100 MHz scope.

As another person who replied noted it would be impossible, since there 
was no screen voltage at the socket, to have any (other than some 
leakage) from the tube I found in the 3-500 socket.

The only irregularity I have found, thus far, is the plate choke.

There is a slight dip in plate current at what should be resonance; 
increasing the drive to 50 watts does not increase the output. In fact, 
I end up with less power than I put in and the plate of the 3-500 starts 
going "cherry red".

No shorted or damaged caps. As mentioned, all other bands are fine.

I have not found this type of plate choke appearing in any internal 
photos of the SB-1000.

Dave, W5WP


On 7/4/2016 2:21 PM, Howard Weeks wrote:
> Dave,
>
> Choke sounds like a National R-175. Used many of them on 80-10 meters. 
> The reason it is there is probably because the original burned out or 
> quit working. If it is the R-175, it should be fine in that amp. A 
> tube failure or bad parasitic oscillation on 10 meters could have 
> taken out the tube,the 10 meter input circuitry and the original RF 
> choke. The 4D32 sure was a poor substitute for the 3-500.
>
> Do you have an O scope with vertical sensitivity in the 10 meter range 
> of frequencies? If so, scope the drive circuitry at the tube to see if 
> you are getting drive on 10 meters. If not, you need to find out why. 
> The drawing shows a set of switch contacts and pi type tuning circuits 
> for each band on the input. The switch contacts could easily be bad or 
> the 10 meter matching circuit could be bad. If you have a signal 
> generator that will put out about a half volt or so of RF on 10, you 
> can use it as a signal source or driver into the input with the amp 
> unplugged. Physically close the antenna relay on the amp and trace the 
> 10 meter signal from the RF generator with the scope through the input 
> circuit. If the problem is there, you will find it immediately. Doing 
> this with no power on the amp is much safer!
>
> If you have a RF voltmeter, you can do the tracing with it. But it 
> must have bandwidth that covers 10 meters. HP 410 is a common example. 
> I do not know of a garden variety AC voltmeter that will do the job 
> directly. Sometimes, a good detector probe on an older scope or DC 
> voltmeter can be made to work for this.
>
> You can also use the same technique to troubleshoot the final tank 
> circuit. Stick the RF generator on the ant out of the amp, jam the 
> antenna relay closed, and trace the rf with the scope up to the tube 
> plate. You can also check the tuned circuits (both input and output) 
> by sweeping the generator frequency across the band of interest. If 
> the tuned circuit is okay, you should see a nice peak in the signal 
> level across the band of interest.
>
> Howard K5JCP in GA
>
>
>
> On 7/4/2016 2:09 PM, Dave wrote:
>> Greetings;
>>
>> Working on an SB-1000 that will not produce output on 10 meters.
>>
>> I found it with a 4D32 in the 3-500 socket. Why????
>>
>> The tank circuits check out against the construction diagrams. Taps are
>> in the correct locations.The band switch is OK, no visible arc damage.
>>
>> The puzzler is the plate choke, RFC-3. It is half again as tall as all
>> the ones I have seen BUT the upper half is half the diameter of the
>> lower half!
>>
>> Is this an undocumented  conversion / modification?
>>
>> Some production variation that I cannot find in my documentation?
>>
>> All other bands are operating properly with regard to tuning and 
>> loading.
>>
>> Comments appreciated.
>>
>>  Happy 4th to all!
>>
>> GOD BLESS AMERICA!
>>
>> Dave, W5WP
>>
>> USAF - Korean conflict!
>>
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>