[Heathkit] SB-200 issues.

[Rodger]

Hi Ken,

It sounds like either the original choke was damaged and replaced or 
someone was thinking about trying to add 160 to the SB-200.  If the 
replacement choke was working properly it shouldn't affect the loading 
on 80; I would suspect it either is close to resonance on 80 which 
should make it go BOOM or more likely it is damaged (internal short) and 
no longer is working as intended.  I would replace it with a known good 
unit since it is designed to short the HV if the plate coupling 
capacitor fails.  My homebrew triple 4CX800 amp suffered a choke failure 
which caused similar loading issues on 160.  I was operating during a 
minor ice storm when the wire part of my Hy Tower/160 inverted L broke 
and it apparently caused an arc across the output choke before the 
protection circuit bypassed the amp; after that it had some 
"interesting" loading issues until I replaced the choke.

On the tube that is not as bright, is it the filament or the plate that 
is showing less color?  If the filament is the issue, check the socket 
contacts and the connections for the filament choke as any slight 
resistance will create problems here.  The SB-220 big brother is fairly 
famous for poor socket contacts causing the solder connection to fail in 
the 3-500Z tubes and your SB-200 could have a similar problem.  It has 
been awhile since I have been inside my SB-200 but if you could measure 
the AC filament potential at the actual tube pins this would help you to 
narrow down the problem.  I cannot remember if the 200 has a HV 
interlock like the SB-220 but if you need to defeat this to make the 
measurement be extremely careful!!!  In any case, there is no reason to 
get near the HV while making filament voltage measurements so stay away 
from the chassis top while measuring.  If one tube is not at proper 
filament/cathode voltage then the gain for that tube will be very low 
and the other tube will be drawing too much current.  The tube that had 
low filament voltage may be damaged, there is a procedure for trying to 
restore emission in this case and it is covered in the older handbooks.  
Let me know if you need more info on this.

In general, it sounds like you need to examine every aspect of your 
SB-200 construction to end up with a reliable amp that you can trust.  
Fortunately linear amps are fairly simple circuits so it shouldn't take 
too long to make a detailed examination of what was done/needs doing to 
yours.

I saw a very strange SB-200 at the Princeton hamfest this weekend.  It 
definitely started life as an SB-200 but the panel had been redone 
(looked factory) and it was labeled "Heathkit single bander" and the 
bandswitch was now a coarse loading control and the plate tune was 
labeled 1-10.  It was also now mounted in what looked like a Collin's 
case; I presume it had been put on 160 given the separate coarse loading 
control.  The panel looked like it could have been done by Heath 
although I presume someone had it made up this way.  Strange that it 
wasn't in its original cabinet but the replacement cabinet fit it perfectly.

73, Rodger WQ9E

Kenneth G. Gordon wrote:
> On 3 Jun 2007 at 21:10, Ian wrote:
>
>   
>> Ken,
>>
>> It sounds like it's time to get out the ohmmeter and start tracing the
>> entire circuit.  ???????
>>     
>
> Well, I found one problem that is fairly interesting: RFC-3, the RFC 
> which is across the antenna connection for safety, appears to be MUCH 
> larger than 1.1 mH. It looks a lot like a 2.5 mH or larger choke. When I 
> disconnected that so that I could check to see if the bandswitch was 
> adding the extra loading cap on 80, now the rig loads properly. At the 
> bottom end of 80, the loading cap is still open a bit.
>
> I wonder what else I will find?
>
>   
>> Otherwise it would seem that you are in for a long series of finding
>> the problem only to have another pop up down the stream.
>>     
>
> Thanks, Ian. However I have gone pretty much completely through the 
> circuit. It IS pretty simple, after all.
>
> It appears that one tube, the one that wasn't lighting, is not drawing the 
> proper current. All grid resistors seem to be OK. I am wondering if one 
> of those little 200 pfd mica grid bypass caps is open.
>
> And the difference in filament brightness still bothers me...one tube is 
> considerably brighter than the other.
>
> Ken Gordon W7EKB
> _______________________________________________
> List Administrator: Duane Fischer, W8DBF        
> ** For Assistance: dfischer at usol.com **         
> $$ See the vintage area on the HCI web site - http://www.w9wze.org $$ 
> Heathkit mailing list
> Heathkit at mailman.qth.net
> http://mailman.qth.net/mailman/listinfo/heathkit
>
>
>   

-----------------------------------------------------------
Products bought, sold or traded here is the responsibility of the
parties involved.  This list and the City of Tempe are not responsible
for losses or misrepresentations of any kind.  Buyer beware!

-----------------------------------------------------------
This list is a public service of the City of Tempe, Arizona
-----------------------------------------------------------

Subscription control - http://www.tempe.gov/lists/control.aspx?list=HEATH
To post - HEATH at LISTSERV.TEMPE.GOV
Archives - http://listserv.tempe.gov/archives/HEATH.html