- 2025
- Jun
- 18
The Heathkit AG-7 Audio Generator part 4: Repairs
In the last post, something went bad in the oscillator portion of the Heathkit AG-7.
There were other things wrong as well, if they were caused by or were pre-existing to this failure, I don’t know. But one of those was easy to fix, that of the 10kΩ resistor in the power supply. This resistor decided it was going to be about ~300Ω or so, and had to go. Fortunately, I had some spare 2W units from a previous rebuild project, and put one in.
That, unfortunately, didn’t fix the issue. Since everything else seems to be mostly ok, I have to assume that I damaged the tube(s) during my testing of the device, or the incorrect power supply voltages damaged the tubes and it was just coincidence that it failed when it did.
Therefore, this unit gets set aside until I can replace the two oscillator tubes, verify it works, and rebuild the unit.
In the meantime, here’s the 10kΩ that was removed.
It looks pretty crusty at this point. Not sure what cause it to short.
But short (relatively) it did:
This one goes in my bin of oddball parts for later fun.
Next part of this series: Coming soon.
Previous part of this series: https://wereboar.com … or-part-3-diagnosis/
- 2025
- Jun
- 11
The Heathkit AG-7 Audio Generator part 3: Diagnosis?
When the AG-7 quit, I wasn’t really sure what caused it except that the oscillator now won’t. Tubes check fine on my cheapie tester, so I decided to start doing some measurements around the 6J7.
These didn’t seem right, much higher than they should be, but the individual components were (mostly) within tolerance after disconnecting from the circuit. It still didn’t seem right, things were bouncing around like there was another path to the filters.
I disconnected the line from the filter and that all went away. Ok…do I have a bad filter capacitor now? No, it checks ok, but that’s with the 10kΩ resistor across the filter disconnected.
And there it is. The 10kΩ resistor that goes from triangle to dash on the filter is bad. It’s not open, or high resistance, it’s 295Ω. Confirmed this with two meters…what? I’ve never had one of these old carbons short like this, but I suppose it’s completely plausible. It is 70ish years old at this point.
I have plenty of 10k resistors laying around to test with, so I’ll take some time and see if that gets me back to a semi-working condition so I can complete my troubleshooting from the last problem. I suspect this is going to need a complete rebuild in order to be functional with any kind of reliability…
Next part of this series: Coming soon.
Previous part of this series: https://wereboar.com … ator-part-2-testing/
- 2025
- Jun
- 10
The Heathkit AG-7 Audio Generator part 2: Testing
In the last post, we determined that this device was mostly operational, but had one of the problems you see with Wein bridge oscillators - one side of the output signal was clipped. That can be an issue with not enough or too much drive in the feedback circuit, so I decided to do some tests.
Here’s the schematic for this device:
An oscillator is an amplifier that provides it’s own input. You’ve probably run across that with feedback on a public address system - that’s the amplifier becoming an oscillator.
In this case, it’s controlled oscillations that we’re looking for. To do this, this particular oscillator model provides two kinds of feedback. Positive feedback, which is the actual oscillator drive and is controlled by the frequency selection capacitors and resistors. And, negative (servo mechanism) feedback that is provided by a lamp used as a positive temperature coefficient resistor - that is, the more current the circuit tries to draw, the more the lamp resists that and tries to keep the gain stable.
The reason you need the servo mechanism is because the oscillator will continue to draw more current until it reaches the limit it can draw from the supply - and probably clip and/or destroy things. The lamp is a quick and easy way to stop that, and was used in almost all oscillators of this type. Later ones used an FET or other current control method, and modern DDS generation doesn’t need this.
Before I tried anything, I swapped the bulb with a new one. No change there.
I’m going to start with checking the oscillator and feedback loop to see if the distortion is present in the oscillator. Pin 3 of the 6k6 Pentode is the output amplifier for this part of the circuit, so checking on Pin 3 should reveal most of what we need to know:
And the distortion is there, so we can probably assume that it’s present in the whole circuit.
(The schematic doesn’t show the suppressor grid - Grid 3 - on the 6K6, but it’s there. It’s tied to the cathode internally, so some manufacturers didn’t show it since you couldn’t access it.)
So that means the input to the 6J7’s control grid is probably distorted as well.
It is.
So it follows that the input to the 6K6 is also bad.
It is, and it’s quite high - perhaps this needs to be cut down some? (It’s inverted because each stage acts like an inverting amp, so the distortion is at the top now.)
Before moving on to the interconnect between the two stages, I decided to examine the feedback loop. Breaking the two resistors, I put a potentiometer removed from the Heathkit AF-1 in-between the stages.
Interesting - more resistance caused the device to go into severe clipping:
Less resistance did nothing much. One thing I did notice is that too much resistance will cause the oscillator to become unstable - this explains the problems I’m seeing with the BW200 oscillator that was on the bench recently.
I was going to move on to the interconnect between the stages (plate of the 6J7 to the control grid of the 6K6) but something happened. I had no output at all, and I’m not sure what happened. Did something pop, like one of the capacitors? The tubes seem to check good:
I need to do some more checking on this thing before making my decisions about what’s going to happen to it.
Next part of this series: https://wereboar.com … or-part-3-diagnosis/
Previous part of this series: https://wereboar.com … part-1-observations/
- 2025
- May
- 30
The Heathkit AG-7 Audio Generator part 1: Observations
If you’ve been reading this site, you’ve probably seen this guy. It’s a Heathkit AG-7 Audio Generator that was purchased at the Breezeshooters Hamfest in Butler, PA during their 2023 event. It’s not something I needed, but was interesting because it had a decal from General Telephone’s Erie, PA office, and had drill points that looked like it had been set-and-forget. Some sort of tone generator for the telephone switch, perhaps? I’m not sure, save that this device has some interesting history behind it.
Here’s the decal on the side:
The chassis shows some signs of work over the years. J-hooked parts show someone probably did some investigative repairs, and that resistor pair that’s tied together certainly wasn’t factory…
What’s the initial diagnosis here?
Power supply seems to be working, and filter capacitors aren’t dried out. They’re not getting hot and nothing else is smoking.
The device has an output:
You can see this device has the same issue that a lot of Wien bridge oscillators have - the lower half of the signal is partially cut off. Later devices took care of this by putting adjustments in the oscillator circuit itself, but this one didn’t have that:
So there’s probably some resistors that have drifted in the oscillator circuit itself. No surprise there, and the repairs are going to be the first suspect.
It does have another, less apparent issue - the oscillator will stall and restart when you rapidly change the dial. There are trimmers in the unit to help alleviate this, and an adjustment procedure for them. This isn’t really that big of an issue, so I’m not concerned with it right now. The main thing is going to be figuring out which parts needs to be adjusted to bring this device back to a pure sine wave, at which point a partial or full rebuild may be in order.
Next part of this series will be determining where the sine goes bad. Stay tuned!
Next part of this series: https://wereboar.com … ator-part-2-testing/
Original post: https://wereboar.com … -7-signal-generator/
- 2025
- Apr
- 30
A PACO G-30 RF Signal Generator - Part 7: Wrapup
Now that the PACO G-30 is working, and is probably in a bit better shape than it was than before I purchased it, it’s time to take a look back on what happened with it.
When I purchased the unit at Findlay 2022, it was sold working. And it was - mostly. I didn’t check the audio portion, but the RF worked fine. The previous owner had removed some of the old capacitors and replaced a few of the carbon composite resistors. He hadn’t replaced them all as they were well within tolerance - surprising for high-value CC resistors. He had replaced the filters with a strange array of electrolytics. But what hadn’t been replaced was the across-the-line capacitors, as well as the selenium rectifier.
The across-the-line cap was of the three-leg type, a device that has two units back to back with a common leg. They’re well known for blowing apart, and that this one hadn’t done so was surprising. The other device was the original selenium rectifier.
Selenium devices were a stop-gap measure between vacuum rectifiers and silicon rectifiers. They worked fine when new, but as they age the forward voltage drop of the device increases. This happens used or unused. Get enough resistance there, you get more voltage drop, more drop means more heat, more heat means that eventually you’re going to wind up with a device that burns open and releases a lot of toxic smoke into your space. You don’t want that, so replacing it is mandatory. Sure, it’s probably still going to work (for a while,) but it’s a case of when, not if it goes bad.
For the most part, this was just a replace the parts that are known to go bad (mostly done,) replace the carbon resistors because they will eventually go bad (mostly done,) and fix some of the issues left by the previous owner. The across-the-line capacitor was easy enough to replace with modern safety caps, and the old line cord was replaced with a new polarized cord.
As stated, the selenium rectifier is a part that was destined to fail from birth. It needs to be replaced, and for a small rectifier like the one in this device, it’s easy to replace with a 1N4007 silicon. I used this type because I have them on hand, but you could use any diode that exceeds the miniscule current rating, and at least 200V. (You should go higher than this for safety margins.)
There are some considerations to look at here. You’ll need to remember that the silicon rectifier drops much less voltage than a selenium, so some sort of dropping resistor will be needed. There was already a 2.2kΩ resistor in the power supply circuit between the two filter capacitors, so changing that was easy. You’ll need to make sure to measure the current to do some basic calculations and see what you need to raise that value to, but in this case I already knew ~3.3kΩ would work as others have taken the time to do the calcs.
Another consideration you’ll need to make is that full peak voltage of the line will appear on the capacitors until the set is warm and drawing current. Therefore, any capacitors you use will need to handle at least this voltage. The old ones may not have been rated for this, but those old paper capacitors could handle surges much better than modern stuff. Modern parts are easier to get with higher voltages, so don’t be afraid to use 300WVDC or 450WVDC capacitors here.
Resistors are the last consideration you need to look at. Remember that you’re working with higher voltages than a battery device - you need resistors rated for at least your maximum B+ voltage, be it the rectified voltage or the Peak voltage that you see on the filters. Most resistors are rated for at least 300WVDC these days, so you’re good for this device - but if you’re working with 400, 800, or higher - your resistors need to be rated for this higher voltage. In carbon resistors, you’ll see ones that are longer than normal and that gives you a clear indication that it’s a high voltage part. Modern parts give you no such indication, so buy your parts from a reputable house.
Resistors also have to be rated for your surge currents. Carbon resistors can handle surges, and that’s especially important here where you have full voltage on your circuit at the start. Rate it small, and your metal film resistor is a fuse. Don’t be afraid to step up the power rating on resistors in power supplies and other areas where you’re going to be supplying current and voltage to the rest of the system.
What did I specifically run into?
First - I forgot how selenium rectifiers work in a vacuum circuit. It’s been far too long since I’ve seen this type of device, so it took some remembering.
When I first turned on the set, I had a 150WVDC capacitor after the dropping resistor. As the entire of the peak line voltage appeared here (180VDC) I went crap and turned it off. It took a while to remember that the rest of the set isn’t hot and wanting current, so the voltage drops in the power supply aren’t there yet. A quick swap with a 450WVDC part and a bit of waiting, and B+ dropped right back to where it should be, which is ~110VDC.
Second - I didn’t have any audio on the audio side. This turned out to be a part the previous owner installed. The grid of the audio oscillator was supposed to have a 100kΩ resistor, The previous owner had a 15kΩ resistor here, which is what I replaced it with as I replaced each component individually with it’s modern equivalent. Once I put the proper value in the circuit and a new tube (the old one didn’t seem to work well) it came right up. It’s a bit off in frequency, but it’s not really of importance as it’s there for you to hear, not compare to the sounds of nature.
This unit is a good example of why you examine devices closely when you’re working with them - especially when someone else has been in there first. A schematic is essential for this, so make sure you have all your docs in order.
That’s about all - other than the wrong part leftover from the former owner, this was a simple “bring it into the modern age” device. Next up is probably an Eico 24x VTVM, but it’s also another “just needs resistors” device. It just needs a lot of resistors as the divider ladders in the measurement circuit are all out of tolerance. Stay tuned!
A PACO G-30 RF Signal Generator: https://wereboar.com … rf-signal-generator/
A PACO G-30 RF Signal Generator - Part 2: Revisiting https://wereboar.com … r-part-2-revisiting/
A PACO G-30 RF Signal Generator - Part 3: Parts https://wereboar.com … erator-part-3-parts/
A PACO G-30 RF Signal Generator - Part 4: The power supply: https://wereboar.com … -4-the-power-supply/
A PACO G-30 RF Signal Generator - Part 5: Resistors: https://wereboar.com … or-part-5-resistors/
A PACO G-30 RF Signal Generator - Part 6: Troubleshooting: https://wereboar.com … t-6-troubleshooting/
A PACO G-30 RF Signal Generator - Part 7: Wrapup: You’re reading it now!
Previous part of this series: https://wereboar.com … t-6-troubleshooting/
- 2025
- Apr
- 28
A Tricraft Products Corp SM-101 Filament Checker
I picked this little gadget up at the ACARA hamfest. Not because I plan on using it, but because it’s an unusual little thing and had a brand new book with it.
It’s a rather unassuming box with a number on the side.
What’s inside is this little gadget, a power cord, and a book. The gadget:
It claims to be a Tricraft Products Corp SM-101 Filament Checker. I’m not sure how it works because there’s no instructions for the device. It appears to be used, but only very little. I can see some wiping on the contacts. The power cord was unrolled and rolled back up with bread ties.
I can’t imagine this was terribly useful, your VOM would have been quicker than getting out a power cord and plugging all kinds of little pins together in an attempt see if the filament was good.
The real gem is the book. The RCA Tube Manual RC-19 from 1959. It appears to be unused, if a little dirty.
That’s worth the few dollars I paid for it right there. The other piece? A cool gimmick gadget for display.
- 2025
- Apr
- 28
A PACO G-30 RF Signal Generator - Part 6: Troubleshooting
I’ve discovered the audio side of this unit doesn’t work. When you get a device like this, you can assume one of three things (and this applies to the state this device was in when received.)
1: There’s something wrong, i.e. something is broken.
2: You put a part in wrong, or made a mistake in replacing parts.
3: The previous owner did something wrong, and you followed in his (incorrect) footsteps.
I started this by checking the plate and grid voltages.
A word about this device: I can’t seem to figure out where they measured from here - Chassis is ground, but the device also is raised about 8V above ground to provide a negative voltage. Some measurements seem to be from chassis, some from raised ground (return). I wasn’t able to get some of the marked voltages, so…who knows. Bob from Accounting probably drew the schematic for this at lunch so it’s possible something isn’t exactly right.
Regardless, B+ was correct on the 6C4 audio oscillator. Grid was not, so ah-ha, there’s my problem. Well..not necessarily in this case because you could go to return and get a different voltage for each measurement. But in this case, it was the problem.
I discovered this because I looked at each part I took out and did a like-for-like comparison with what I put in. Everything matched. The problem became apparent once I really started looking at the schematic.
So, you’ll notice there’s a 15KΩ resistor coming from Pin 6 of the 6C4. That’s what the previous owner had in there.
The schematic, however, states that we need a 100kΩ resistor:
That’s probably the problem. I had some 100kΩ left from one of the previous rebuilds, where I wisely just bought a bunch. One of those goes in:
It still didn’t work, so I grabbed the tube from a parts unit I got at Columbus…
And, there’s our audio.
It’s 500Hz instead of 400Hz, but who cares. It’s still going to be heard just fine. The bad grid resistor destroyed the original tube, but…oh well.
Since the device is now working, it’s time to button up for the wrapup post. Stay tuned!
Next part of this series: https://wereboar.com … rator-part-7-wrapup/
Previous part of this series: https://wereboar.com … or-part-5-resistors/
- 2025
- Apr
- 28
A PACO G-30 RF Signal Generator - Part 5: Resistors
After confirming the power supply was operational (and remembering how that works!) I started in on the resistors.
Some had been replaced before I received the unit (foreshadowing!) and some were original. It was a mix of modern film, old film and old carbon comp resistors. I have to wonder if these were the special military units, as the remaining carbon comp parts were well within tolerance. Considering I’ve seen multiple of this particular device and every one of them has a different style of parts inside, it’s hard to tell.
I decided to just do them one at a time. Unsolder, remove, and replace with the same value. At least, the same value as what’s in the unit now.
Here’s the bottom after replacing the resistors, diode, and capacitors (yay bad lighting!)
This is what the top looked like before replacement:
And after:
After a quick inspection, I fire it up. RF looks good - for the given value of good this device offers. The audio side? Not so much, there’s nothing there. Uh oh…stay tuned for troubleshooting.
Next part of this series: https://wereboar.com … t-6-troubleshooting/
Previous part of this series: https://wereboar.com … -4-the-power-supply/
- 2025
- Apr
- 23
A PACO G-30 RF Signal Generator - Part 4: The power supply.
First things first, this device needs it’s input and power supply corrected. This involves adding a new polarized power cord, replacing the across-the-line capacitor, and replacing the selenium rectifier and capacitors with a silicon diode and new capacitors.
The across-the-line capacitor is two capacitors back to back, with a common leg.
It’s really amazing that this thing isn’t cracked in half. Two new safety capacitors will go in it’s place.
The unit had new capacitors installed by the former owner, but I didn’t like they way they looked.
I understand the person probably used what they had on hand, but still. This is kind of sloppy, but it worked. Note that the capacitors are 450WVDC…
The first thing to do here is start removing parts. I’m going to use the screw for the tuning capacitor to hold a new terminal strip, and put a new hole in for a second, so I can string the new capacitors out a bit. This also gives me room to add a new dropping resistor, since I have to account for the voltage drop of the selenium diode. The old safety capacitor was removed, as was the old line cord.
All of the new parts were added in save for the dropping resistor in-between the two capacitors. I intentionally left that out just in case I needed to experiment. I’ve seen others doing this same task, and they’ve come up with a 3.3kΩ resistor, so I decided to start with that.
Regarding the dropping resistor: You’ll note that the original was a carbon comp part rated for 1/2W. While I’m going to more modern oxide and film resistors, I stepped the power rating up. Why?
CC resistors had the ability to withstand a good current surge, especially if it was quick. Modern film resistors do not have this ability, and can act like a fuse - especially in circuits like this where capacitors are going to be charged immediately when the set is turned on. You get that small surge of current, and that can destroy things. Since modern parts are smaller for the rated values, I chose a relatively inexpensive 5W oxide part for this section.
After removing all parts and installing their replacements, I set up for some testing:
I used a 47μF in place of one of the capacitors, mostly because the previous owner put 30μF in for both sides of the filter. It’s not that much of a stretch to go from 20μF to 50μF, especially with a modern diode in there, and the effect of the dropping resistor limiting current at startup.
After a quick wiring check (make sure those wires from the transformer are correct, the red color on the red/yellow line has long since faded to nothing!) I clipped the resistor in, and applied power.
B+ was 180VDC.
On a 150WVDC capacitor. Whoops. Turn it off. What went wrong here? Well, it was staring me in the face, but I did a test with the original selenium rectifier. Same thing, only about 170VDC. The selenium obviously had a voltage drop, but it wasn’t enough. What was going on here - the original parts were only rated for 150WVDC.
I think I know what’s going on here, and it’s related to the rectifier type of the circuit. A solid-state device performs differently than a tube device, and the OEM took advantage of old part characteristics when designing the unit. I changed the 47μF with another 22μF @ 450WVDC part, applied power, and…
Upon powering the unit, the B+ went to 180VDC - and then dropped as the tubes warmed up and started conducting. Just as expected, once the circuit was operating the B+ fell right in line at 109.1VDC (s/b 110) with an input of 120VAC.
So why did this happen? In a pure tube circuit, where the rectifier is also vacuum, you have a circuit that warms up together. That is, the full B+ probably won’t be available until the rectifier is hot - by that time, everything else is hot and wanting current, so you have a proper load.
A selenium circuit, however, has B+ available the minute you turn the set on, as the selenium stack is essentially a primitive semiconductor diode. You’ll have full line (120 * 1.414 - Vf) peak there - until the rest of the circuit is hot and wanting current. Only then do you have the proper loads and can expect B+ to be the nominal value.
But the original part was only 150WVDC! Yes, and that’s because those old paper capacitors could handle surge voltages for a short time, and they didn’t care. Sure, it may have shortened the life some, but 20 years later it’s dry as a bone anyway, who cares. You need to rate your parts for the full peak voltage you’ll see on the device! A 250WVDC part would have been fine here, but I have 450WVDC parts - so in they go.
I can always add more capacitance if there’s an issue, but it’s back to where the OEM had it.
So, that’s on me. I forgot things I learned 35 years ago and never used until today. But now I know, and if I (or you!) run into a selenium powered circuit that you want to retrofit, you have a better idea of what to expect.
Next is resistors. While some have been changed, and everything else is right on tolerance, I’m going to replace them anyway because they’re almost all carbon comp resistors. I was only planning on changing a few, but there’s no need to leave the old ones in there - resistors are cheap, and making this thing ready for it’s next 50 years is what I’m trying to do here. I have a few on order, as soon as they get here I’ll solder the dropping resistor into place and get started on replacing other parts.
Stay tuned!
Next part of this series: https://wereboar.com … or-part-5-resistors/
Previous part of this series: https://wereboar.com … erator-part-3-parts/
- 2025
- Apr
- 22
Two hamfests this weekend, April 26 and 27.
There are two shows happening this weekend.
(Both of these have passed. Pictures on the way!)
The first is the TUSCO Amateur Radio Club hamfest at the Tuscarawas county fairgrounds in Dover, OH. This is a small show, and usually is good for an hour of wandering and looking at everything thrice. I’ve been attending this one for a few years, and there’s always a trinket or doodad that comes home with me.
The second is the ACARA Athens Hamfest in Athens, OH at the Athens Community Center. I’ve never been to this one, but it’s close enough that I’ll check it out just to see what’s there. It may become part of my regular show schedule.
TUSCO ARC Hamfest
Commercial Building at the Tuscarawas County Fairgrounds
295 South Tuscarawas Ave
Dover, OH 44622
April 26
8A - 1PM
https://www.w8zx.net/hamfest
ACARA Athens Hamfest
Athens Community Center
701 E State Street
Athens, OH 45701
April 27
8A - 12P
https://www.ac-ara.org/
Next up is Dayton. See you there!