SP 200 distorted audio with AVC on

N1KK · **Joined:** Jan Fri 13, 2012 4:12 am **Posts:** 79

I just picked this receiver up. It was dead other that static. One of the tubes
in the I.F. had a bent pin. Fixing that brought this receiver back to life.

The only problem I am having now (besides a slide hum in the audio)
is distortion when the AVC/Manual switch is in the AVC position.
The s meter didn't work and I found an open plate resistor in the AVC amplifier (2K) that
fixed the S Meter but not the distorted audio. It works perfectly with very good audio
in the AVC/Manual Manual mode with adjusting the Sensitivity control.
No distortion with good audio with the the right setting of Sensitivity.

I also found a screen resistor for the AVC amp that is suppose to be a 50K but
I measured 126K out of circuit. I changed that but still no fix.

Found another 50K measuring 125K going to the plate of the 1st audio tube.

If in the manual mode I can get good audio, should I assume I am not getting
enough AVC voltage to cut back the gain in the RF and I.F. stages.

I am surprised how sensitive this receiver is.

Ken

K7MCG · **Posted:** May Mon 14, 2012 3:04 am

If you haven't replaced the capacitors in the AVC subsystem, do it now. Don't miss paper capacitors hidden inside the AVC IF transformer can, T-6. For a better view of how the AVC works, see the military manuals: TM11-866 and TM11-4054.
Both of these manuals can be downloaded at: http://www.hammarlund.info/hamrmanu.html

--Chuck

BTW, there are also paper caps hidden in IF detector transformer T-4.

N1KK · **Joined:** Jan Fri 13, 2012 4:12 am **Posts:** 79

Thank you for the links. I will print them out today.

When you say "AVC subsystem" are you referring to the capacitors inside the T6 can
or different area.

In the T6 can I found the open 2K resistor (R51) that feeds the voltage through L34 and to the plate of V11 (6SK7) open and replaced it. That is the resistor that fixed the S meter.
I was also able to peak the S meter by adjusting C51 variable across teh L34 transformer
feeding the A-V-C RECT (V12)

When testing the tubes I noticed some of the tubes where not as indicated in
the manual.
For example,
3rd I.F. 6SK7 replaced with 6SS7
2nd I.F. 6SK7 replaced with 6SS7

1st RF 6K7 replaced with 6BZ6
2nd RF 6K7 replaced with 6BA6
Mixer 6L7 replaced with 5760
OSC 6J7 replaced with 6C4

AF Driver 6F6 replaced with 6V6
One of the 2 AF Output tubes (V16), 6F6 replaced with 6087

As far as sensitivity it seems pretty hot. I don't know
if any of these tube replacements would effect how the AVC
voltage behaves.

Ken
N1KK

K7MCG wrote:

If you haven't replaced the capacitors in the AVC subsystem, do it now. Don't miss paper capacitors hidden inside the AVC IF transformer can, T-6. For a better view of how the AVC works, see the military manuals: TM11-866 and TM11-4054.
Both of these manuals can be downloaded at: http://www.hammarlund.info/hamrmanu.html

--Chuck

BTW, there are also paper caps hidden in IF detector transformer T-4.

Ancient_Hacker · **Joined:** May Sat 22, 2010 4:42 pm **Posts:** 2424

Arggggh! Put the original tubes back in there! Do you think some random yahoo was a better electrical engineer than all of the staff at Hammarlund?

For the 6BZ6, if they changed the socket so you can't plug in the original octal, you can use a 6BA6, which is close to a 6K7.

Also you do have to replace every capacitor in the AVC circuitry-- just a few megohms of leakage can kill the AVC voltage. Also check all the resistors in the AVC chain-- they're usually very high values, the kind that tend to zoom upwards with time, it's not unusual to see 1 meg resistors that have doubled in value.

Ancient_Hacker · **Joined:** May Sat 22, 2010 4:42 pm **Posts:** 2424

You can make an exception and keep the 6V6's in place of the 6F6's-- the 6V6 is slightly improved. The two output tubes should really be the same kind and age to minimize distortion. I'm hoping the 6087 is a typo as that seems to be a diode, not a pentode.

N1KK · **Joined:** Jan Fri 13, 2012 4:12 am **Posts:** 79

I will check when I get home tonight. Could have made a mistake on my notes.
If I have a 6BA6 I will swap it with the 1st RF tube (V1) which right now
is a 6BZ6. Would it have made a difference dealing with the AVC voltage?

The 2nd RF state has a 6BA6 and it also should be a 6K7

Will need to order the caps inside of T6. It's the values I don't have it's
the high 600 volt ratings.

Ken

Ancient_Hacker wrote:

You can make an exception and keep the 6V6's in place of the 6F6's-- the 6V6 is slightly improved. The two output tubes should really be the same kind and age to minimize distortion. I'm hoping the 6087 is a typo as that seems to be a diode, not a pentode.

N1KK · **Joined:** Jan Fri 13, 2012 4:12 am **Posts:** 79

I see what you mean. I looked up the 6K7 and 6BZ6 and they are different.
Hopefully the AVC would still work the same regardless.
I don't have the schematic in front of me but since I can control the gain using
the Sensitivity control I would think the AVC voltage should work with these tubes too?

Will have to find a place that has those caps at the 600 volt level

Ken

Ancient_Hacker wrote:

Arggggh! Put the original tubes back in there! Do you think some random yahoo was a better electrical engineer than all of the staff at Hammarlund?

For the 6BZ6, if they changed the socket so you can't plug in the original octal, you can use a 6BA6, which is close to a 6K7.

Also you do have to replace every capacitor in the AVC circuitry-- just a few megohms of leakage can kill the AVC voltage. Also check all the resistors in the AVC chain-- they're usually very high values, the kind that tend to zoom upwards with time, it's not unusual to see 1 meg resistors that have doubled in value.

Eickerman · **Joined:** Jan Fri 06, 2012 8:47 pm **Posts:** 2496

Sounds like you might have an SP-200 that suffers from the modifications of the late 50's and early 60's. From http://www.radioblvd.com/hammarlund_super_pro.htm

Quote:

L. E. Geisler Modifications - With the cheap, easy to find availability of the surplus WWII Super-Pro receivers in the mid-fifties and sixties, the "modification mania" did finally catch up with the line and the WWII Super-Pro was considered "fair game" for modifications. Most of the infamous Super-Pro modifications were derived from the first of the series, "Souping the Super Pro" by L.E. Geisler, published in the Dec.1957 issue of CQ magazine. Geisler was an engineer that worked out of Japan for a company that sold modified Super-Pro receivers. Today, Geisler's modifications are "tame" and basically replace the 6L7 mixer tube with a different octal mixer tube that is quieter, then he replaces all of the capacitors and does a full alignment - pretty much standard stuff. Geisler's earlier mods are conservative, make sense, improve performance and do no real harm to the receiver. One has to remember that Geisler's company sold these modified Super-Pro receivers so they had to perform better yet still retain the professional-commercial appearance in order to have marketability. As time went on, Geisler's modification ideas evolved. Later Geisler-modified Super-Pro receivers will have more invasive modifications.

Post-Geisler Modifications - The later modification articles go even further than the late-Geisler ideas with even more and more outrageous modifications. Included in the list of notorious "cut and hack" articles are "A Super 'Super-Pro'" and "SSBing the Super Pro" - both published in the "Surplus Conversion Handbook," part of the CQ Technical Series. These articles advocate the wholesale modification (destruction) of the entire receiver, including replacement of the front-end tubes with miniature tubes, an on-board solid-state power supply, removal of the 14 watt P-P audio section to install an anemic 6AQ5 single ended audio section (which also then provided room for the on-board power supply,) on-board converters to cover 10 and 15 meters, product and infinite impedance detectors - on and on. It's doubtful that a receiver could ever be returned to original after being the victim of these later modifications. I have only seen a couple of Super-Pros that attempted these modifications and they were wrecks. No doubt, the end product failed to impress the owners and the receivers were afterward relegated to the junk pile.

Yours sounds like a little more than 1 and a little less than 2.

Curtis Eickerman

N1KK · **Joined:** Jan Fri 13, 2012 4:12 am **Posts:** 79

Maybe not, this receiver has a built in power supply. I don't know enough about
these oldies to know what is normal. As I go through the manual I do not see anything
about an internal supply. Yet this one has a line cord in the connected to a couple
of terminals.
The color of the face is a bright blue. Everything I have seen so far looks black.
The case is black, just the front panel is a bright blue color.
I guess I don't care if it's been moded. It seems to have a lot sensitivity with
pretty good audio when using the AVC/Manual in the Manual mode and adjusting
the sensitivity control. I was also amazed how close the dial freq alignment was.

I want it just for shortwave listening and maybe 160/75/40m AM.

Ken
N1KK

Eickerman wrote:

Sounds like you might have an SP-200 that suffers from the modifications of the late 50's and early 60's. From http://www.radioblvd.com/hammarlund_super_pro.htm

Quote:

L. E. Geisler Modifications - With the cheap, easy to find availability of the surplus WWII Super-Pro receivers in the mid-fifties and sixties, the "modification mania" did finally catch up with the line and the WWII Super-Pro was considered "fair game" for modifications. Most of the infamous Super-Pro modifications were derived from the first of the series, "Souping the Super Pro" by L.E. Geisler, published in the Dec.1957 issue of CQ magazine. Geisler was an engineer that worked out of Japan for a company that sold modified Super-Pro receivers. Today, Geisler's modifications are "tame" and basically replace the 6L7 mixer tube with a different octal mixer tube that is quieter, then he replaces all of the capacitors and does a full alignment - pretty much standard stuff. Geisler's earlier mods are conservative, make sense, improve performance and do no real harm to the receiver. One has to remember that Geisler's company sold these modified Super-Pro receivers so they had to perform better yet still retain the professional-commercial appearance in order to have marketability. As time went on, Geisler's modification ideas evolved. Later Geisler-modified Super-Pro receivers will have more invasive modifications.

Post-Geisler Modifications - The later modification articles go even further than the late-Geisler ideas with even more and more outrageous modifications. Included in the list of notorious "cut and hack" articles are "A Super 'Super-Pro'" and "SSBing the Super Pro" - both published in the "Surplus Conversion Handbook," part of the CQ Technical Series. These articles advocate the wholesale modification (destruction) of the entire receiver, including replacement of the front-end tubes with miniature tubes, an on-board solid-state power supply, removal of the 14 watt P-P audio section to install an anemic 6AQ5 single ended audio section (which also then provided room for the on-board power supply,) on-board converters to cover 10 and 15 meters, product and infinite impedance detectors - on and on. It's doubtful that a receiver could ever be returned to original after being the victim of these later modifications. I have only seen a couple of Super-Pros that attempted these modifications and they were wrecks. No doubt, the end product failed to impress the owners and the receivers were afterward relegated to the junk pile.

Yours sounds like a little more than 1 and a little less than 2.

Curtis Eickerman

K7MCG · **Posted:** May Mon 14, 2012 7:49 pm

Responding to your earlier question about the sensitivity control working, but AVC not working..... Both controls work by adjusting the control grid bias on the RF and IF amplifier tubes (in this radio). But (!) the sensitivity control provides a solid, low impedance voltage source obtained from the -50volt bias supply while the AVC voltage derives from detected signal - a very high impedance signal source. So, while the sensitivity control may overcome bypass capacitor leakage in grid circuits the AVC may be loaded down by the same leakage. AND this sensitivity of the AVC voltage to loading is why you need to use a VTVM to measure voltages in the ACG circuit and the grid circuits of the controlled stages. A dvm with only 1 megohm input impedance will load the ACG circuits and give you low readings.

--Chuck

Eickerman · **Joined:** Jan Fri 06, 2012 8:47 pm **Posts:** 2496

N1KK wrote:

Maybe not, this receiver has a built in power supply.

A couple of pictures (especially inside) of this receiver would be nice. I can't imagine where you would put a power supply inside of an SP-200 without pulling out something. In those modifications cited earlier I think they gutted the push-pull output stage to get some room for a power supply.

I'm looking at the 1948 manual from http://www.hammarlund.info/download/TM11-866-1948.PDF

Curtis Eickerman

N1KK · **Joined:** Jan Fri 13, 2012 4:12 am **Posts:** 79

OK, will take a few photos tonight and upload them.

Ken

Burnt Fingers · **Posted:** May Mon 14, 2012 10:01 pm

That sounds like a completely modified radio and I WOULD NOT replace any of the tubes that are in it unless defective.

The 6BZ6 is an excellent RF tube that was used in many radios of the 50-60's from Hammarlund, National and others and is what will determine the front end noise figure. Depending upon the socket wiring a 6BA6 may not be a direct swap as the pinouts are different. Leave it alone.
The 6BA6 is the first 7 pin remote control pentode and roughly equivalent to a 6SG7 octal and should be setup for just a bit over enough gain to overcome coil and wiririg losses as done in the HRO-60 which has excellent signal handling.

The 5760 is simply a 5 Star 6BE6 which is a bit noisy but a hell of a lot better than the 6L7 and the 6C4 was the favorite oscillator for most all manufacturers.

The 6SS7 is a lower gain 6SK7 which makes good sense based upon the other tubes used.

The 6V6 is a more efficient and lower distortion tube and was used by Hammarlund early in the SP-400 production.

All in all I see nothing but good thought behind all the tube choices and not what was in the ham rags mods pages.

The built in PS was often done by removing the audio transformers, first 6F6 and socket, going to a resistance coupled input from the 6C5 directly to Class AB1 6V6's and relocating another smaller ouput transformer under the chassis. Not as much output but less distortion and the original 14W is never needed anyway.

Since someone went thru a lot of work and if it is well done it would be a shame to "butcher" it to return to original. The SP-200 and its military models are far from scarce while a well done conversion is quite rare. The stock receiver is also far from sensitive on the higher frequencies....all the gain is in noise....and overloads easy. These sets also drifted badly and Id be interested in knowing if some work was done with TC as was the SP-400.

I have a SP-100, SP-200 (BC-1004 which is stock and cost $25 at Nearfest last year), and a later production SP-400 ($40 the year before). Im very tempted to update the BC-1004 using the latest thoughts in tube receiver designs, components and gain distribution.

Carl

Ancient_Hacker · **Joined:** May Sat 22, 2010 4:42 pm **Posts:** 2424

And why stop there? The radio sure could use a real product detector, and a digital readout. You could probably slip in a SMPS from a microwave oven in there to save the weight of the power transformer. Put in white LED's instead of those failure-prone #47's. Wire the tube heaters in series and power them from the line. Lots of ingenious mods one could make.

Well, IMHO making that many changes to a radio makes it a whole nother radio. Sort of like "Grandpa's axe", which is original, except it's gone through three handles, and oh yeah, the head's was changed too around 1963.

Is this really a SP-200 anymore? When did it go over the line into a Frankenstein's monster of the electronic arts? How do you keep the angry mob with the torches away?

N1KK · **Joined:** Jan Fri 13, 2012 4:12 am **Posts:** 79

Well guys that was something....

It's working so I really don't see any need
to change it. I could use the help with the AVC, unless this receiver's AVC was never really
designed to deal with really strong stations in the AVC position. In Manual mode
it works just fine but I do notice a trace of Hum that only appears to stand out
when the AVC is in the manual mode and when I have the sensitivity set right
at the spot where the audio sounds good.

I have attached a few pictures of it. I am curious where this beast fits in.

This receiver has 2 inputs on the rear. One picture shows this.

The picture of the front right below the band change knob has 4 holes that
look like a tag or emblem might have been there.

The freq range is 1250KHz to 40 Mhz.

Ken

Burnt Fingers · **Posted:** May Tue 15, 2012 2:26 am

Its a BC-794 and Ive seen that front panel photo before somewhere, maybe Nearfest. The military ID tag holes are very visible.

All those bathtub caps need replacing plus the the paper caps inside the AVC IF. Unless they were changed a bunch more is inside the RF cage. Any or all can affect the AVC as well as the AVC transformer alignment, plus ALL resistors on the AVC buss. The electrolytics look 50-60's and that blue paper cap is even older, get rid of all.

The audio output and driver transformer was removed and a PS transformer and choke were added. Looks like just a 6C5 driving a single 6F6/6V6 into the little transformer on the rear panel for 2-3 watts which is fine.

While the overly excitable will whine and knash teeth its actually a pretty nice conversion considering what many look like. By the front panel color the owner must have been a French Canadian from NH or ME :lol:

Id like to see some detail shots of that BS dial as the mask is missing and its certainly not stock. Id like to add calibrated BS to mine.
Or add a digital readout to further annoy the purists

but the drift would have to be cured first.

Carl

N1KK · **Joined:** Jan Fri 13, 2012 4:12 am **Posts:** 79

I am getting ready to order some caps. All the caps inside T6 have 600 volt rating and I
don't have any.

The AVC seems to work on week signals with no distortion. In fact the audio is much
cleaner with the AVC on and listening to a weak signal. With the AVC off and sensitivity
adjusted there is a hum in the background that peaks when I hit the right spot
for the sensitivity control getting rid of the distortion.
I really like the audio. Plenty of High frequency response. Just
what I need for these old ears of mine.

If you where a betting man and wanted to trouble shoot the problem which
cap would you start with? I did change a resistor inside T6 (2K) that feeds plate
voltage to the AVC amp. I was thinking of the cap that hangs off of it to ground.
If it where leaky maybe it caused it to open, yet so far all is working
with the replacement resistor but no change in distortion on strong signals.

Ken
N1KK

N1KK · **Joined:** Jan Fri 13, 2012 4:12 am **Posts:** 79

AVC alignment, If you are referring to the adjustable cap inside T6, I did try it.
The manual states the s meter should peak. I ran the cap 360 degrees with
the s meter doing a major swing but even at peak the distortion is still there
on strong signals.

Ken

Burnt Fingers · **Posted:** May Tue 15, 2012 2:14 pm

I wouldnt bet as each bad component adds its share to the distortion. Even a tube on the AVC chain with leakage will be a problem.

T4 and T6 both have paper caps and several resistors inside.

Again.....ALL paper caps should be changed and that includes the bathtubs bolted to the chassis walls. There is no halfway with this. Same with ALL out of tolerance resistors. They dont have to be on the AVC line to affect it.

Carl

Ancient_Hacker · **Joined:** May Sat 22, 2010 4:42 pm **Posts:** 2424

N1KK wrote:

... but even at peak the distortion is still there
on strong signals.

Ken

Some web pages say that many of those series of radios distort on strong signals. They claim that the user's manual explicitly states that it will distort and yuo should back off on the RF gain. So it may be "normal". Which is weird, as you'd think if they went to all the trouble of putting a 14 watt audio amp in a radio, they'd make sure it didn't distort (I'm thinking it's clipping in the last IF amplifier stage ).

If, and only if, it was designed that way, it's probably possible to re-engineer things to have it NOT distort-- basically, applying more AVC to earlier stages, and less to the last stage, and maybe run the last IF stage at a bit more plate current. While I'm generally not in favor of rejiggering an original design, if the original design was a bit off, and you'd rather have clean audio than 100%.

SP 200 distorted audio with AVC on

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