Heathkit SB-102 Plate Current out of control

Question

I bought a "technician special" Heathkit SB-102 transceiver. Everything seems to work well until I attach a dummy load and try to go through the "initial tune up" procedure in the manual.

At the point where I switch the "Mode" to "Tune", instead of showing 50-60 mA plate current, I'm seeing an initial switching spike and then the needle settles around 150-250 mA and begins to creep upward. When it gets past 300 mA (as little as 10-15 seconds), the upward creep accelerates, and when it gets past 350 mA the breaker in the HP-23A power supply pops (presumably to protect various components). During this time, grid current appears to be negative -- the needle goes past the zero point on the meter (which I've zeroed with the zeroing screw when power was off).

Output was to an MFJ 300W/30 second dry dummy load (derating graph shows 100W up to 90 seconds, and hasn't even gotten warm in any test). If there's any resonance going on, it's inside the transceiver's housing. Further, it does the same thing on bottom of 80 m, 40m, and top of 10m that I've checked (as a recommended check against self-oscillation in the final stage).

I have very limited diagnostic tools -- a digital multimeter (cheap unit from Harbor Freight) and kit-built frequency counter (which I'm sure would need attenuation for use in this situation, even if it were likely to be useful). I could build an oscilloscope probe to plug into the headphone jack of an Android device, but I doubt that would be any more useful than my multimeter.

The manual doesn't ask for more than a VTVM -- but makes it clear this isn't the same as a VOM, specifically for resistance testing circuit segments with diodes, and as I understand it, for this type of testing, my DMM is more like a VOM (low test voltage) than a VTVM. Comments have suggested that this modern digital meter is more like a VTVM than a VOM from the era when the SB-102 manual was printed, so should be okay for these tests.

I've replaced the mismatched 6146 and 6146A final tubes that were in the rig when I got it with actual 6146 tubes -- I now have three with varying date codes, and I'm using the closest-dated (on Internet advice), about a year apart. No change.

Not sure it's relevant, but turning down the variac (feeding the power supply ~100 V instead of ~120 V), thereby reducing plate voltage from 800+ to just under 700, has reduced this effect, lowering the initial settling point and slowing the upward creep in plate current.

Question is, at this point, what kind of failure do I need to look for? Bad grid leak resistor on one of the 6146 tubes? Failed capacitor in the final stage?

Answer 1

The creap could be a bad capacitor or the circuit is over loaded due to excessive voltage on the plate. This sounds systematic of a component failure and requiring component isolation and L-C meters. You could have any number of bad components. You should be using a VTVM however the main reason is the level of the reading, milli. Sounds simple but first look for broken or discolored components and solder fractures. Make sure you are grounding the radio properly as well. The mixer is critical of that. If it’s a hot chassis don’t use the house wiring as a counterpoise. Many of the older manuals note this and how it may trip breakers.

Answer 2

In response to tuning the finals and grid dip: I’ve tuned a lot of IF strips as a Navaids Tech. VORs and Tacans mostly. Ham later. The procedure was always to start at the first IF and work to the final. Dip the plate and max the grid. All done with VTVM. These days I use a audio sig gen and a usb dongle o’scope. Like you said not much more effective for voltage but it is another way. I do find the o’scope useful for looking at the the audio. That and a usb spectrum analyzer. Yes attenuators! Id look at spectrum purity before i ever transmitted on an old radio like that.

I’m sure you started with the simple. The most likely to fail in those older radios (other than tubes) are the capacitors. Then dirty tube sockets, trim components and rotary contacts. Power off, unplug and ohm out the rotary contacts, milli. Be very careful what you clean with. Some contact cleaners eat the wafer material. Not sure of your knowledge base so I wanted to add that.

Look closely at the first IF tank circuit and try to isolate the components and verify values. Then work out to the final. There are LC meters on eBay for under $30.

Answer 3

Zeiss, the first step in fixing any radio is usually to check the power supply levels. If the power supplies are wrong then nothing will work properly. I would do that first before doing anything else. Also, when the plate current goes up does one or both of the valves glow brighter and brighter ? These two things will give clues to the fault. The fact that the dummy load doesn't get hot of course means that the high plate current isn't getting transferred to the output.

Your fault is never caused by faulty valves. In radios with valves you can almost always use the rule of thumb that the problem isn't the valves as it hardly ever is. The fault is probably also not caused by bad tuning or 'resonance' and it is very highly unlikely that it's caused by dirty contacts in the tube sockets, switches, relays or dials.

The fault is almost definitely caused by one or more capacitors or resistors in the transmit output stage which have HV on them that have failed due to age. The fault you describe cannot be caused by any stages preceding the transmit output stages such as in the IF as a fault in these stages cannot cause high plate current in the TX output stages while at the same time causing no output at the antenna socket, unless something is pulling down a supply rail.

I would replace all the high voltage coupling capacitors around the transmit driver and output stages first, as these capacitors can and very often do go leaky and turn into a temperature dependent resistors or go short circuit which can cause the symptoms you are seeing.

Also replace all of the resistors in the biasing circuits around the transmit driver and output valves which have high voltage on them.

I bet if you do all this the problem will go away. I can add the component numbers to this post if you need.

I also point out that if there are components failed due to age causing your problems in the transmit output stages, there is a high chance that there are components in other parts of the radio that also need changing, and another good rule of thumb when fixing old radios with valves in them is to change all the capacitors in the radio which have HV (> 20 V) on them.