Forums :: NEW! Web Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives
Support This Site: Contributors :: Advertise


It is currently Jan Sat 16, 2021 12:40 am


All times are UTC [ DST ]





Post New Topic Post Reply  [ 24 posts ]  Go to page Previous  1, 2
Author Message
 Post subject: Re: Heath SB-300 Tuning
PostPosted: Nov Tue 17, 2020 9:30 pm 
Member
User avatar

Joined: Jan Fri 06, 2012 8:47 pm
Posts: 6941
radiowizard wrote:
The S-meter is not functioning correctly. It goes "negative" after the radio is powered up and warmed up. The meter zero control does nothing. I've used DeOxit Fader Lube on it. I just replaced the 6AS11 audio, 6HF8 and 6BZ6. Same old. Not sure what to check in that area. I do have a schematic, but it's in four parts...probably should print them off and then cut and tape it together so it makes more sense.
RW
If the S-meter seems OK after the radio is first turned ON then as it warms up (like over 30 seconds) it starts creeping negative and then eventually stays stuck down below zero you probably have a Grid Leakage problem with V6 (6BA6 1st IF) and/or V7 (6BA6 2nd IF). Grid leakage typically goes up as the tubes warm up and cause the AGC line which is connected to the grid to be drawn in a positive voltage direction which makes the S-meter pin below zero. Tube V1 can also do this but I see that you have already replaced it without improvement (6BZ6 RF Amp).

The remedy is replacement of one or both 6BA6 tubes.

I had the trouble with my SB-101 which used 6AU6 tubes and experienced the same issue. Sometimes I would have to go through several tubes to find some that would stay OK. I think the circuit stresses the tubes a little beyond their normal operating range and tends to promote this problem.

Other components that could be checked to see if they are having an effect on this are R163 (33K), R164 (100 ohms) and R167 (68 ohms). If they have shifted in value they can make it hard to zero the s-meter. Also if CR1 and CR2 become leaky in the reverse direction they will have a similar affect on the AGC line.

Curtis Eickerman

_________________
http://curtiseickerman.weebly.com


Top
 Profile  
 
 Post subject: Re: Heath SB-300 Tuning
PostPosted: Nov Tue 17, 2020 10:36 pm 
Member
User avatar

Joined: Jul Sat 15, 2006 3:54 am
Posts: 3212
Location: Zeeland MI
Thanks..will check that all out. I at least know a bit about tubes screwing up voltages and making things go negative.
RW

_________________
"And now, for something completely different.."


Top
 Profile  
 
 Post subject: Re: Heath SB-300 Tuning
PostPosted: Nov Wed 18, 2020 1:34 am 
Member
User avatar

Joined: Jul Sat 15, 2006 3:54 am
Posts: 3212
Location: Zeeland MI
While waiting for soldering iro to heat up, I found a terminal near piwer supply that was not soldered.

_________________
"And now, for something completely different.."


Top
 Profile  
 
 Post subject: Re: Heath SB-300 Tuning
PostPosted: Nov Wed 18, 2020 2:06 am 
Member
User avatar

Joined: Jul Sat 15, 2006 3:54 am
Posts: 3212
Location: Zeeland MI
The 68 ohm was high at 91 ohms. Didn't have a 68, but made a 74 ohm, and got it to zero. Oh, got the speaker working as well. More DeOxit and a little more work with a relay contact burnishing tool. Just a few more things, and we will be good. Well, have to learn how to read the dial...but at least I have that part of the manual.
Thanks for suggestions, Curtis.
RW

_________________
"And now, for something completely different.."


Top
 Profile  
 
Post New Topic Post Reply  [ 24 posts ]  Moderator: Sandy Go to page Previous  1, 2

All times are UTC [ DST ]


Who is online

Users browsing this forum: Peter Bertini, rsingl, Spidermonkey, WA1KPD and 18 guests



Search for:
Jump to:  


































Privacy Policy :: Powered by phpBB