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Heathkit SW-7800: Comments & Mods (Read 12285 times)
wireaddict
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Heathkit SW-7800: Comments & Mods
06/14/10 at 05:07:16
 
The Heath SW-7800 has potential to be a decent LW receiver with double conversion & continuous tuning range starting at 150 KHz [to 30 MHz].  It also has some liabilities such as lack of features like noise blanker/limiter, notch filter, variable bandwidth, etc., digital display noise & freq drift so turning this radio into an Icom or Drake beater is a little challenging.
 
After reading a few on-line evaluations & mods for Heath's final general coverage receiver offering, the SW-7800, I decided to get one.  Heathkit stopped selling this receiver in 1985 but they're still fairly plentiful on e-Bay & ham radio forums.  I prefer the latter because of a few experiences with e-Bay where I bid against members with more dollars than sense, to whom cost was meaningless & bid accordingly, no doubt to the seller's delight.
 
I found one in working condition for sale on a ham radio forum for $120 less manual & promptly bought it even though I couldn't do much work on it until I got a manual.  There are several manual printers on the internet; Manualman is as good as any.
 
Two sites that have a lot of info on this radio are: www.schmitzhouse.com/Johns_Electronics_04.htm and www.schmitzhouse.com/Heath%20SW7800%20Mods.txt for info on mods.
 
I applied Schmitzhouse's mods & found that most worked great & others, as originally posted, were rather ineffective or, in one case, detrimental.  The following are my comments on the author's mods.
 
Rcvr. Board
[1] Increase capacitance between audio stages to improve LF response; probably better but hard to tell because of distortion.
[2] Increase "fast" AGC delay; good mod.
[3] Change Audio Preamp biasing to reduce distortion; mods reduced gain excessively, working on it; some improvement but distortion's still high, will probably have to wait til winter to find cause & fix.
[4] Front-end filtering improvement; Good mods, IMO.
[5] S meter calibration; I installed 25K lin. pot on rear panel & connected it & 10K resistor in place of R493.
[6] Decrease quiescent IF amp bias to increase sensitivity; good mod.
 
Controller Board
[1] Replace C331 & C332 with negative temp coefficient caps to reduce freq drift; I replaced with micas which have higher "Q" & high dimensional/freq/temp stability but no change so, after some research, built compensator circuit with NTC thermistor, transistor & varactor.  This reduced drift by about 67%; drawing attached.
 
Synth. Board
[1] Add 0.1uF disk RF bypass cap across C221 to help reduce noise radiation; good mod.
[2] Shield [2] ribbon cables to digital display board for same reason; good mod except neater & more effective to separate wires from ribbon cables into 2 twisted bundles; shield per instructions & wrap with tape; Scotch brand worked for me.
[3; My mod] Mount 5V reg., U206, to left side of chassis to reduce device heat buildup; pigtail leads & use heat sink compound.
[4; My mod] Install 10uF [min.] cap on 5V output line from U206; slightly improved 5V stability.
 
Others
[1] Replace S meter lamp with LED[s]; good mod; I used white LED with 390 ohm, 0.5W series resistor.
[2; future] I plan to add RF & first IF amps but probably not til next winter.  Will advise when completed.
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« Last Edit: 06/21/10 at 15:43:15 by Administrator »  

Dave
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Re: Heathkit SW-7800: Comments & Mods
Reply #1 - 07/15/10 at 20:29:30
 
First, I should include some construction & adjustment details for the controller board freq drift compensator mod.  Referring to the attached drawing for the compensator, I mounted all the components on two small pieces of perf board [with 0.10 inch hole spacing] except for the thermistor, TR31.  I installed the board containing C31,32,35, P31,32 & Q31, on a slightly larger perf board outside the shield can & the other piece containing C33,34, D31 & R31 inside the oscillator [center] section of the shield can.  I mounted the boards via two 6-32 screws through two vent holes in the shield can.  It may be necessary to file the holes slightly bigger to accommodate the screws.  I used both screws [with spacers & nuts] to mount the larger board, one about 0.75 inch long & the other about 1.5 inch or so.  I mounted the internal board via the longer screw & a hole in its center.  One other important construction detail I should mention is the thermistor, TR31.  It needs to sense the [slight] heat given off by the 15V regulator, U305 in order for this circuit to work.  I replaced the mounting screw for U305 with a longer one & mounted TR31 via this screw & an insulated spring clip.  I made the clip from a piece cut from metal flashing & glued some plastic cut from an old credit card to keep the ungrounded side of TR31 from shorting to ground.  This clip doesn't need to exert much force on TR31, just enough to hold it firmly against the inside wall of the shield can directly next to U305 on the other side of the can wall.  Use heat sink compound on both devices; the grounded side of TR31 goes next to the shield can wall & bend enough offset or standoff in the clip to accommodate TR31.
 
To calibrate, P31 only needs to be set so that the signal from TR31 never causes Q31 to saturate.  When the radio is first turned on the base-emitter voltage should be less than -0.5V & become more negative as it warms up & TR31's resistance drops.  Turn P31 CCW to make this reading less negative & vice-versa.  Next, as the radio warms up watch which way the frequency drifts; if the displayed freq drifts lower turn P32 CCW slightly to compensate & vice-versa.  My radio originally drifted over 12KHz but I got it down to about 3 or 4KHz.  Expect to tweak this about a dozen times or more to get it optimized.
 
Other Receiver Board mods [continued from first post].
[7] Replace detector diodes, D415,416 with 1N5817 Schottky diodes.  The original germanium diodes had a forward voltage drop of about 0.27 & 1N5817s read about 0.14V on a Fluke 77 multimeter & have a much higher reverse resistance.
[8] The original 1N4002 1A PS diodes ran a little hot as evidenced by discoloration on the PC board next to them.  I replaced them with 1N5404 diodes which are rated at 3A.  I had to ream the lead holes on the board with a 0.040 in. drill bit.  Also form the leads so there's about 0.75 in. between the diodes & the holes for better ventilation.
[9] Drill holes in PC board on either side of main filter cap, C511, for small cable tie to help secure it.  Be careful to avoid damaging foil traces on the board.
[10] Replace 390K resistor, R444, with small piece of perf board containing 100K resistor in series with a 100K, 1-turn pot to control AGC threshold.  This & the next mod are shown on a separate drawing which will probably be attached soon.  Adjust this pot for maximum volume without overloading & distortion on strong stations.
[11] Replace C475 & R464 with small piece of perf board containing a 1Meg resistor & 22pF cap in parallel with each other & in series with a 500K, 1-turn pot.  This pot adjusts the sound level in AM wide mode only; adjust for equal volume between AM wide & narrow modes.  Incidentally, I found that the source of the audio distortion is U404d when in AM wide mode; I'll probably dig deeper into this after it gets too cold to play outside.  Will advise if I find any more worthwhile mods.  I hope some of you find these mods as much fun & useful as I have.  73s.
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« Last Edit: 07/18/10 at 14:54:25 by wireaddict »  

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Dave
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Re: Heathkit SW-7800: Comments & Mods
Reply #2 - 10/25/10 at 04:07:05
 
First, I need to correct an error in my last post [from July]; at the end of my the posting I mistakenly said that the the source of the audio distortion was U404d but further work proved that it actually came from the audio preamp, Q402.  I found that the bias was wasn't right even though it looked OK on a protoboard before I implemented it in the radio.  It turns out that Mr. Schmitzhouse's choice for the biasing resistor, R468, was about right; he said to remove the original 1M resistor & install a 220K ohm resistor between Q402's base & +11V.  [I  tried using a 680K resistor here or 270K in place of the original R468 (between base & collector).  I found that if this resistor is connected between the collector & base its optimum value is about 40% of the value needed when connected between the base & +Vcc.]  With that said, here are the latest mods; all are for the receiver board.  Sketch showing these mods to follow.
 
[1] On the audio preamp, Q402, replace R468, which is connected between the base & collector, with a 100K resistor.  Also, C41, the emitter resistor bypass cap, should be increased from 1uF to 4.7uF to improve low frequency response; anything much higher makes the audio "muddy" sounding.  Both changes improve audio quality noticeably.
[2] Add a 100pF cap to the inverting input, pin 2, of the AGC amp, U407, to stabilize the signal from the detector [& also delay AGC action slightly].
[3] Add a 0.1uF cap to the +11V supply for the AGC & audio sections of the receiver board.
[4] I devised a better way to adjust the AGC level.  Undo/disregard change #10 in my July post & reinstall the the original 390K resistor for R444.  Remove one of the [2] germanium diodes, D418, from the noninverting input, pin 3, of U407 & add C44, C45 & P41 as shown on the sketch.  If the [2] detector diodes, D415, 416, are not replaced with 1N5817 [Schottky] diodes it will probably be necessary to keep D418 in the circuit due to the higher positive charge on the inverting input of the AGC amp.  AGC adjustment pot, P41, is set for maximum signal strength without overload & distortion on strong stations.
 
Next mods will probably be RF & first IF amp stages but, even without them, I was able to hear five NDBs during the day & more at night including DDP-San Juan, PR, using only the built-in 4 foot whip antenna after the upgrades shown in these postings were completed where before, I could only hear FN-269 in Flint, MI, about 25 miles distant, during the day.  [Also, for those interested, I found that rosin soldering flux can be removed from PC boards with denatured alcohol or 91% isopropyl alcohol.]  73s.
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« Last Edit: 11/10/10 at 05:51:53 by Administrator »  

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Re: Heathkit SW-7800: Comments & Mods
Reply #3 - 03/13/11 at 20:17:29
 
Since last fall I've made a few more improvements to this receiver in three areas; what follows is a description of these wiring changes.
[1] BFO Fix.  First, there was a problem with the BFO that I didn't notice until recently probably because I didn't use it very often: the BFO frequently wouldn't oscillate in LSB or USB modes.  I compared BFO voltages to those shown on the schematic & they were OK. I tried tweaking the bias with only slight improvement.  Next I tried installing a 1K resistor between the +11V common & the junction of R484 & R508 so that the BFO [& product detector] receive partial power while the radio's operating in AM but this didn't help much either.  Finally, it occurred to me that the 7.5-7.6-volt BFO supply after zener diode D429 & R508 might be a little low for stable operation so I replaced D429 with a 1N4739 [or NTE139A], 9-volt zener.  Now, the BFO seems to work great after recalibration.  I'm attaching a sketch showing these & other changes but, if anyone has a BFO problem, start by replacing D429.
[2] AGC.  I also added an RF amp stage which I'll cover shortly but the extra gain from it causes overload because the AGC circuit as designed, & even after my earlier changes, can't compensate enough.  This is because the signal from the detector diodes, D415 & D416 grows more negative [with respect to ground] as the signal strength increases & is applied to the inverting input of the AGC amp, U407.  However U407 is fed from a single-ended [11VDC] power supply so its output can't respond to an input going negative beyond zero volts; instead the output jumps to about 10.5 volts & shuts down the [second IF amp, U403].  I thought of two ways to correct this; first, basically is to reverse the polarity of the detector diodes, D415 & D416, & swap the signals to the inverting & non-inverting inputs of U407.  I chose not to do this because of the daunting logistics of extensive PC board mods.  The other way is to add about 3VDC of bias to the inverting input of U407 & increase the voltage to its non-inverting input so as to approximately equal or balance the two with no RF signal present.  That way even the strongest signals will never drive U407's inverting input to zero volts but will allow the inverting input to always track the signal from the detector.  I didn't want to use batteries for the 3-volt bias supply to U407's inverting input because of the hassle of replacing them.  Instead I added another secondary winding to the power transformer, T1.  I found just enough space between the doughnut or winding package & the two outside "posts" or end legs of the core for 30-gage kynar or PVC-insulated [wire wrap] wire.  Each turn of wire yields about 0.06 VAC so, ideally, there should be about 50 turns of wire, 25 on each end, connected in in phase so as to produce about 3 VAC.  This new secondary winding is then connected to a diode bridge made from Schottky diodes to minimize forward voltage drop across them.  If there isn't enough room to add all 50 turns to the transformer then put on as many as possible; the AGC action will be controlled by a user-accessible pot.  Also note the 3.3V zener across the DC output from the diode bridge; its purpose is to regulate the bias voltage to the inverting input as is the zener on the non-inverting input circuit so a momentary AC supply voltage change won't cause the input signal to temporarily overload or disappear.  Notice there's no current limiting resistor between the bridge & the zener; none's needed here because of the resistance from the inefficient magnetic coupling to the added secondary winding.  While this works great there's one more thing to point out: adding this new secondary winding to the AGC circuit precludes this radio from being 12-volt battery-powered without using a 12VDC to 120VAC inverter.  I made the bias voltage to U407's non-inverting input adjustable by means of a 100K potentiometer which I mounted on a bracket or panel on the back of the radio facing the front.  I made the mounting panel wide enough to accommodate additional controls.  Since the SW-7800 has no switch to disable the AGC for weak stations this pot is handy for setting the AGC level for both strong & DX signals.
 
[Due to size limits on postings the remainder of this article will appear in my next post.]
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« Last Edit: 03/26/11 at 01:19:50 by Administrator »  

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Dave
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Re: Heathkit SW-7800: Comments & Mods
Reply #4 - 03/13/11 at 20:21:47
 
[Here's the balance of my article on Heath SW-7800 mods, dated today.]
 
[3] RF Amp.  The RF amp stage goes between the filter section & the first mixer, U401, & is pretty self-explanatory.  I experimented with different circuits & biasing schemes at different frequencies & got the best results with a circuit similar to the audio preamp, Q402.  Results, however, were mixed with best performance at higher frequencies, some improvement at the AM BC band & absolutely useless at frequencies below 500 KHz because of  continuous strong intermod.  The reason for the LF intermod, I believe, is in the receiver's design; band 1 tunes from 150 KHz.-1.0 MHz. & has a low-pass filter designed to restrict signals at frequencies above 1 MHz.  This does nothing to keep lower BCB signals out of the RF amp;  what's needed is a low-pass filter with a cut-off frequency of around 520 KHz. that can be switched out & bypassed when not needed.  Steve Ratzlaff has a low-pass filter for low freqs on his web site although I haven't built or tried it.
Since the RF stage won't always be needed I used a switch mounted on the new rear panel added for the AGC level control on the back of the set to operate an "ice-cube" relay with a 12VDC coil to connect or bypass the amp.  While this could be also be accomplished with a DPDT switch it would require wires carrying low level &/or high freq currents be a lot longer than to a relay installed near the receiver board so I opted to use the relay as RL Drake did in their R7 & R7A receivers.  The relay I used had the same pinout as an NTE R16-11D3-12P although any 12VDC DPDT [dual "C" form contact] relay will work.  Incidentally, I designated the relay as "K" in fig. 2 so the contacts & coil have the letter "K" & the pin numbers next to them.  The JIC [Joint Industrial Council] standard symbols for relay contacts resemble those of capacitors; the contacts with the diagonal line through them in the bypass line are normally closed & the others at the input & output of the RF amp are normally open. I mention this to avoid confusion.  Also, the wiring between the receiver board, RF amp & the relay contacts should be shielded.
 
[4] Conclusion.  I mounted most components for the AGC bias supply & RF amp on a piece of double-sided copper clad blank PC board using a combination of Manhattan/"ugly" & wire-wrap circuit board layout styles.  I drilled holes for component leads through the board then used a 1/4 inch circle cutting tool I made up to cut insulating grooves around the holes where needed [on both sides] then soldered the leads to the resulting pads.  Before soldering I connected the components together with #30 kynar-insulated wire using a wire-wrap tool  to wrap the wire around their leads where they extend through the board.  To simplify removing the [existiing] receiver & new circuit boards I made quick connectors from Philmore P/N 61-190 crimp pins.  The wire sockets that Heath used on their boards are neater but I couldn't find any.  I still plan to build a first IF amp stage to go between the first & second mixers but I'm not sure just when; if this results in any performance improvement you'll be the next to know.  73s.
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« Last Edit: 03/26/11 at 01:20:44 by Administrator »  

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