Heathkit SB301 question

[KK4MSA]

Hello all,

 I have an SB301 that a local gave to me.
I didn’t power it up when i first got it because the inside has a ton of dust and looked to be sand or something. so i replaced the electrolytics including the multi section filter cap. i powered it up after the cap job and all the tubes light up, i have 1 light out, the one behind the meter. i didnt have a power cord to i wired one in just behind the chassis on the existing 120vac plug, i also wired in a 8ohm speaker behind the chassis as well.
Problem is im getting a hum, like would normally come from a bad filter cap, so i rechecked polarity on the caps i installed and all were correct polarity. same problem, now when i put it in standby, the meter goes up some, but as soon as i switch it over to operate it slams to the left below 0, and i can hear the hum. after checking again i took the new filter caps out and reconnected the old can cap back. same problem, no change in the tone neither better or worse.
my question is, can someone help with this, i know i haven’t given allot of info here, but I have given what i know so far, im not sure if this is a "common" problem that someone can tell me about or not.
Im a new Ham and just picked up my general on the 21st, and i love the tube eq, and rather than throw this one out where it was going to end up, i would like to try and get it going.

thanks in advance

[W4NEQ]

I'm not sure how strong your tube-type electronic troubleshooting skills are, but I would recommend starting with checking the internal DC voltages against those in the manual on all the tube pins.  Very often this will show you at least the area where the problem lives.

[WBear2GCR]

Is there hum in the audio?
Does the rig receive anything?
I think it has a xtal calibrator, can you hear it?

Sounds like there may be one or more bad caps in the signal path.
Possibly causing DC leakage to the grid of one or more tubes.

Check the grid voltages on the pins of the tubes.
Check the cathode voltage on each of the tubes, compute the current through it, compare
to the expected current per the tube manual, and/or the pin voltage in the manual.

You may just have a bad tube, btw.

Or you can troubleshoot by pulling the tubes (you might want to
mark them so you know which holes they came out of, since I think they use
the same tube in more than one position) and re-insert them from the
audio section going back to the RF amp and see what it does.

For example if there is hum in the audio now, if you pull everything except the
audio amp tube, and the hum is gone, then you know it's not the audio amp,
so you go back one position... when the hum returns, look there to start. Check
the voltages on that tube, and the one before it that was already in place... etc.

A scope would be very helpful too... you can look at the waveforms and see hum
(for example) and see gain, etc...

I assume you have or have downloaded the schematic and manual?

My first real ham receiver was an SB301 that i built.

                         _-_-bear

PS. this is a nice SSB reciever - known as the "poor mans Collins" back in the day, due to it's intentional resemblance to the "S-Line" from Collins. Do NOT throw it out, it's a good reciever. It can be modified to receive AM too...

[KK4RF]

KK4MSA,
     Bear had some sage advice there. I also had a SB-301 many years ago. I traded it away. Wish I'd kept it. It's agood receiver.
     I would add trying to get some De-OxIt spray cleaner and to spray the pins on each of the tube sockets. Those pins get corroded and that prevents a good mechanical contact with the tube pins.
     If you had some spare replacement tubes, that would be good for swapping them out for a new tube.
     Hang in there and don't give up on the project. Keep us posted on your progress.
                          ---Marty, KK4RF---

[Pete, WA2CWA]

I would first verify all the tubes are in their correct sockets.

Next, I would test all the tubes. If you're going to dabble in tube boatanchors, having a decent mutual conductance tube tester is a great piece of test equipment.

Replace any weak ones and any that show a failure.

Next, I would use Figure 5-2 of the SB-301 manual, resistance points diagram, and verify the measured resistances are reasonably close to what is indicated on the diagram. Read the Note for Control and Switch positions before you start the measurements.

If there are discrepancies, repair and replace as necessary.

Next, I would use Figure 5-3 of the SB-301 manual, voltage points diagram, and verify the measured voltages are reasonably close to what is indicated on the diagram. Read the Note for Control and Switch positions before you start the measurements.

If there are discrepancies, repair and replace as necessary.

Given that this was a kit, cold solder joints, poor soldered connections, and possibly even unsoldered connections are not uncommon. You should always do a good visual inspection of all soldered connections and repair if necessary.

[KA8WTK]

 All of the above, and an easy thing to check.
 Slightly loosen all the hardware holding the circuit boards and re-tighten. These were known for corrosion happening at the hardware that gounds the boards to the chassis. A little bit of tweaking on the hardware will make sure that you have a good connection between chassis and board and that the hardware is tight.

Bill KA8WTK

[WBear2GCR]

I'll second the tube check, the visual check (up close) of all of the solder joints, and also the ground screws for the boards, IF they provide the ground...(don't over tighten and crack the boards...).

Also check with the giver to see if this unit was *ever* known to have worked properly. Some kits were built improperly and then parked, not fixed.

As a former Heathkit repair tech I can tell you that wires could be and were placed in all sorts of improper places and that soldering was often the problem.

So, it is a safe bet to take a good quality temperature controlled soldering iron, some eutectic solder (63/37 for example, NOT 60/40) and touch up 100% of the solder connections everywhere - especially on the boards...

Often resistor and cap leads come through the board and are *in solder* but not actually connected.

               _-_-bear

[KK4MSA]

Hello everyone,

thanks for the help guys, i have no plans to trash it, i want to get it going for sure.
i have been on the road since i posted this, however i got in there today and put the new filter caps back in.
and its better now for some reason? strange, i still have a low buzz in the speaker, but i can hear the calibrator loud and clear on ssb. and i can hear the static clearly. again, i still have a small buzz/hum in there when the volume is down but not bad. the RF gain works as it should also.

Now AM is another story, i cant hear anything but the buzz on am ( its a low constant buzz, nothing to loud) no static at all, dead, and the buzz gets a little louder when i turn the af gain down in the AM mode. so ssb seems to be working, but not AM, and the signal meter still isn’t working correctly, it will slam to the left when the unit is in operate no matter what mode its in. so still a ways to go, but at least its working somewhat now
I will make some voltage checks tomorrow before i have to leave out again and let you all know how that turns out. So thanks again for the help, hopefully with everyone’s help ill have this unit going 100% soon.

[WQ9E]

Unless it has the optional AM filter installed it won't operate in the AM mode.  Check the chassis top, the filters are located on the left side near the front.  The standard SSB filter is in the middle, the slot for the optional CW is closest to the front panel and the slot for AM is the slot on the other side of the SSB filter.

Without the AM filter you can operate it in SSB mode and tune for zero beat.

[KK4MSA]

well that’d be the reason, no AM filter in there at all.

 I will make those voltage checks this coming weekend when i get back and make sure all else is ok, i also have to figure out what’s going on with the s meter. But at least im going in the right direction now.

thanks again, ill keep everyone updated, and im still open to any input.

KK4MSA

[WBear2GCR]

heck, you *can* operate it in AM mode, you just have to figure out the input Z and output Z of the filter, then make a pad out of resistors, one for each side, then put a small ceramic cap between the legs of the pad. The pads should equal the  insertion loss of the stock filter and the cap passes the full width of the IF...

If you want a narrower bandwidth in there, without the stock filter you could build up either a passive LRC bandpass filter or an active bandpass filter and stick it in the slot!

Iirc the frequency that the xtal filters on the SB-301 are at fairly high frequency, so the parts will be small for a passive LRC...

You could actually just stick a cap in there and it should work...
 
                             _-_-bear

There is a pot adjust for the S meter on the chassis, iirc...

[WQ9E]

Although a simple passive coupling for AM works fine in receivers like the Drake R-4C that has some selectivity both from the "roofing" filter and the final 50 Khz. IF or a Collins 75A-4 with its 455 Khz. IF strip the SB-301 would be very broad.  The first IF is in the SB-301 is designed for a greater than 500 Khz. passband and the 3.395 second IF is also very broad without a filter.

The Heathkit AM filters do show up on ebay and might still be available from Inrad.  The SW oriented SB-310 receivers included a nominal 5 Khz. @ 6db down filter while the SB-301 amateur oriented AM filter was specified for 3.75 Khz. @ 6db down.  The Kenwood 599 series receivers use the same final IF frequency so a filter from one of those could be made to work without much difficulty.  I believe that all 4 filters were standard in the Kenwood receivers so a parts unit should yield one.

[Steve - K4HX]

Yea, there was even a 20 kHz filter in those R-599s! Talk about wideband hifi audio! The "normal" AM filter in 599s was 5 kHz BW.

I used to have a 599D back when nearly all the AM activity on 75 meters occurred between 3770 and 3890 (probably more often than not 3875 and 3885). I would use the 599D as an activity monitor by tuning it to 3880 with the 25 kHz filter selected. Then I would turn up the squelch on a dead band.

I see Inrad has some 3395 kHz filter but SSB bandwidths only. I one was desperate, it might be worth giving them a call. Too bad the filter would cost almost as much as the receiver is worth. 

[WQ9E]

Yea, there was even a 20 kHz filter in those R-599s! Talk about wideband hifi audio! The "normal" AM filter in 599s was 5 kHz BW.

I had just upgraded from novice to general when I had my first experience with a Kenwood receiver, it was a R-599A (basically a silver face version prior to the D series) at the club field day site.  It was at the 10 meter setup and the receiver was so quiet it sounded dead compared to my SB-102.  But once you tuned across a signal it popped out of the quiet background and sounded great.

The R-599A and D are electronically equivalent and are really under-appreciated receivers.  They cover all the pre-Warc ham bands from 160-10 plus 11 meters and a WWV position.  As Steve noted filters for all modes including FM are standard and any filter can be selected for any mode.  The NB, calibrator, and all mode squelch are standard and the only options are an external speaker and internal converters for 6 and 2.  With the matching T-599 transmitter it can run split, transceive from either, or reverse split.  AM mode works well on the transmitter, it uses real transmitting tubes (6146B instead of sweep) with a built in cooling fan and the only real drawback to this compact pair is the transmitter doesn't cover 160 meters but a Dentron 160XV takes care of that issue.