Hammuarlund SP 200 distorted audio with AVC on

[N1KK]

Perfect audio with AVC/manual switch in manual mode and sensitivity adjusted.

Originally the S-meter didn't work.  I found an open resistor in the plate
circuit for the AVC amplifier.  Fixed S-meter but still have distorted audio.

I found a 50K Screen resistor for AVC amplifier measuring 126K.  Changed
that and now I have a higher voltage.  Did not fix distortion.

Found another 50K bad resistor feeding Plate of 1st audio amp.
Replaced but did not fix audio.

I measured the resistors connected to the AVC RECT tube and are OK.
Did not check the cap.   This is inside the can T6 in the back right corner.

If in manual I can adjust the sensitivity for good audio, can I assume
the AVC is not producing enough voltage to cut back the gain in the RF
and I.F circuits?

Ken

[w3jn]

Is L34/T6 adjusted for max AVC voltage (IE min S-meter reading)?  Many times someone will misadjust this due to not understanding what's going on there.

Any of the .01 capacitors inside T6, if leaking to ground, could mess up your AVC.

[N1KK]

Can you confirm that for me.  When I read the manual I got the impression
you wanted to dip the meter connected to the speaker and as stated
in the manual, the s-meter will peak at this point.

I have got into this receiver a bit and have noticed many modifications.
RF,mix and OSC tubes have been changed.  The receiver has an
on board power supply.

I was told CQ magazine published an article back in 1957
Souping the Super-Pro; L.E. Geisler; CQ, December 1957

Would anyone have this?

Thanks
Ken

Is L34/T6 adjusted for max AVC voltage (IE min S-meter reading)?  Many times someone will misadjust this due to not understanding what's going on there.

Any of the .01 capacitors inside T6, if leaking to ground, could mess up your AVC.

[N1KK]

Should the required AVC voltage be close to what the manual voltage
is (after adjusting sensitivity control for good audio) be the same or
very close to each other?

I find on strong stations, measuring the AVC voltage at the output
of the switch is always much higher in the manual mode to get a good
clean audio.   In AVC position I get much less. 
Manual position could be in the area of -11 to -15 for a good non distorted
signal.  When in the AVC position I get about -8v.
If I adjust the sensitivity control in the manual position to the same
-8v I get distorted audio.

This makes me think both voltages have to be about the same
for no distortion.

What I need to know is what is the AVC voltage on a working
SP-200 (BC-794) suppose to be.

Ken

[KA2DZT]

Have you checked the negative bias voltage reaching the receiver terminal from the power supply??  It should be -50volts.

You should be able to turn the sensitivity control all the way up and not have distorted audio.  This is with the avc turned on. 

Have you checked the AVC amp. (VT-117) and the AVC rectifier (VT-90)??

Does the S-meter seem to work correctly?? If you can tell.

Fred

[KM1H]

Its all a waste of time replying as he hasnt even replaced any caps or resistors yet.
He already ran the subject into the ground on another forum.

The receiver has also been highly modified with miniature tubes in the front end and different octals in the IF's.

He doesnt bother mentioning that here....

[KA2DZT]

Carl,  thanks for letting us know that it's not really a stock SP-200.  Doesn't surprise me that the modified receiver has these problems.

Fred

[N1KK]

Update,

after coming to the conclusion that both AVC and manual voltage should be about the same
and I do get good audio in manual mode and they both appear to feed the same locations
I went back to T6. I have plenty of amplified I.F. voltage going to the plate of the AVC detector and I could
peak this voltage with C51.  

 I changed C56 .05uF tubular cap.   I also measured R52 and noticed
it was 30% high and changed it. This is part of the Time Constant circuit.
 This made a dramatic difference.  I plan to change C52 (.005uf)
the smoothing cap (on order).  Since it's a bit of a pain getting inside T6 I decided
to change C54 and C55. Both where not tubular and most likely where fine.

The other resistors inside T6 measured close enough except for R51 (2K) which earlier
in my repair was open and the reason my S- Meter did not work.  Wondering what
would cause R51 to open I decided to change C53 (.01) that hangs off of R51 to ground.
If this cap was leaky it might cause R51 to burn out.

My fixed bias is suppose to be -3 v I measured -2.7v.

The above work appears to be the main source of the problem.  Listening last night
On a few strong shortwave stations I could not hear any distortion.
I do notice a reduction in the audio volume on very strong stations but
this receiver has plenty of audio.

This receiver as mentioned on the other forum, has been modifeid.
 6BZ6 for 1st RF, 6BA6 for 2nd.   Mixer and Osc tube
changed also.  I did a little reading on those mods going back to the 50's and early 60s.
Makes me wonder with a hotter receiver if the AVC circuit should have been modified to offer
more voltage if needed.  One mod I read about was a new sensitivity control for the 2nd I.F.   My SP 200 did not have this.

I enjoyed the trouble shooting of this receiver and I am sure there will be more to come.

Ken
N1KK

[Todd, KA1KAQ]

Glad to hear you figured it out, Ken. These are excellent receivers once they are back in top form.

Though it's been posted many times, it's worth re-posting. One of the best sites on the 'net for Super Pros is Henry Rogers' site, the Western Historic Radio Museum:

http://www.radioblvd.com/hammarlund_super_pro.htm

He does an excellent job of explaining and highlighting the "issues" with these receivers, the vast majority caused by misuse, improper aerials, or mods to address problems that didn't exist. Much of it was spelled out in the manuals but ignored. The post-war mods were as much an attempt to update the receiver to 1950s standards than anything else, incorporating newer tube technology not available in the late 1930s. Boatanchors didn't really exist then as we know them, so being able to pick up something like a Super Pro or souping up the one you had was a viable (cheaper) option over buying a new 75A-4 or HRO-60.

As with any old set, it's best to go through and address aged components to establish a baseline from which to move forward. I've got a couple of pre-war 200SX models that have had the front ends butchered and are awaiting reconstruction at some point. So it's always good to hear of another one coming back from the dead, even badly mauled.