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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?

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    $\begingroup$ Is the output tuned near resonance frequency ? $\endgroup$ – Optionparty Jul 22 '19 at 15:44
  • $\begingroup$ Output was to an MFJ 300W/30 second dry dummy load (derating 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. Does the same thing on 80 m, 40m, and 10m that I've checked (as a recommended check against self-oscillation in the final stage). $\endgroup$ – Zeiss Ikon Jul 22 '19 at 15:59
  • $\begingroup$ To expand on @Optionparty's important question: Switch to the 80 meter band and rotate the loading capacitor so it is fully meshed. Now put it in tune and rotate the tuning capacitor. Does the plate current move when you do that? More importantly, is there a point where the plate current drops sharply? $\endgroup$ – Mike Waters Jul 22 '19 at 20:04
  • $\begingroup$ Can we assume that you have a wattmeter connected between the SB-102's RF output connector and the dummy load? $\endgroup$ – Mike Waters Jul 22 '19 at 20:07
  • $\begingroup$ Manuals, etc. found on this Google search. $\endgroup$ – Mike Waters Jul 22 '19 at 20:15
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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.

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  • $\begingroup$ I used to own an SB-102, and it is not a hot chassis. $\endgroup$ – Mike Waters Dec 17 '19 at 17:59
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    $\begingroup$ Good to know it’s not hot. Many back then were and I’ve seen too many radios to remember if that one was. Needless to say a good counterpoise is still important. As an old timer used to say,”Rf in the shack as it’s called will grill your cheese...” $\endgroup$ – Andrew Harman Dec 18 '19 at 15:45
  • $\begingroup$ Great advice, but I'm wondering if that could be causing this since Zeiss states here that he is using a dummy load. $\endgroup$ – Mike Waters Dec 18 '19 at 18:14
  • $\begingroup$ I don’t believe so. Read the tech bulletin I referenced. I believe the answer is there. $\endgroup$ – Andrew Harman Dec 19 '19 at 20:50
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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.

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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.

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  • $\begingroup$ Not sure I follow -- isn't IF a receiver feature? Why would you have an IF stage in the transmitter? $\endgroup$ – Zeiss Ikon Dec 16 '19 at 12:15
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    $\begingroup$ Yes, the SB-102 is a transceiver. There is an IF stage used in the mixer. $\endgroup$ – Andrew Harman Dec 16 '19 at 17:04
  • $\begingroup$ I advise you to also look at the tech bulletin on the radio. nostalgickitscentral.com/heath/Service_Bulletins/sb-sb102.pdf $\endgroup$ – Andrew Harman Dec 16 '19 at 18:33
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    $\begingroup$ @MikeWaters No, I have a genuine Heath HP-23A power supply, but had to build the cable myself -- pretty sure I wired it right, but the manual fragments I have called out wiring it so the PS won't operate without the radio connected or shorting a pair of pins. I think I now know where the final bias test point is, but adjusting the bias via the in-radio adjustment changed nothing. $\endgroup$ – Zeiss Ikon Dec 17 '19 at 14:40
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    $\begingroup$ It’s possibly a cable, I wouldn’t rule it out. As posted earlier by Andrew a first step is to verify power. This should be done under a load. Often those HP23A/Bs need to be re-capped. I’ll check and see if I have access to a cable or the specs. $\endgroup$ – Andrew Harman Dec 18 '19 at 15:38

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