RF HF Linear Amplifier Troubleshooting, Basics, Walkthru, 101 for dummies

Описание: Since I had the Heathkit HL-2200 all done up and ready to be packed away, but still had it partially stripped down with a decent clear view of the tubes and tank components, plus since the HL-2200 has full metering, I decided to do an off the cuff rundown on some of my thoughts on Linear Amplifier tips and troubleshooting. Nothing deep here, this is basic 101 stuff for non-techs and newbs. If you are a tech, you should know this stuff already.

1. Before I power up an amp, I first take the covers off and look at the electrolytic caps first. If they are old, bulging, have paper covers instead of plastic, blew goo, and/or generally just look bad, out they go. No, I don't test them. To me, old caps are like old tires with 125,000 miles on them. Sure, they may have served you well in the past, sure, they may hold air, sure, they may even have some tread left, but let them go. Same with old caps. They may look ok, some will even test ok, but inside, electrolytic caps have electrolyte in them. Over time, the electrolyte dries up and using dried out caps is just asking for trouble. When they go, they often short. Sometimes a dead short. Often, once one shorts, it takes out the power supply diodes as they are what sends power to the caps. Next, sometimes with a shorted power supply diode, it will take out the power transformer. Bad power transformer, cancel Christmas. All that, because someone rode on those old tires errrr I mean old caps.
2. Next, I check the diodes, especially the HV power supply diodes. Easy and quick non powered test using a multimeter on Diode Test. Power supply diodes will read around .4 to .5 volts forward and will read an open circuit reversed. A major problem with linear amplifiers is that a diode has shorted and is causing all kinds of havoc. Hence, before I power up, I check my diode string.
3. Look around for any other obvious problems. Yeah, many problems with components are internal and you can't see them at all. However, many problems can be physically seen. Broken wire, cold solder joint, burned component, something touching that isn't supposed to, arc's, heat problems. It often can save you a lot of aggravation and money by looking around first before founding out the hard way.
4. Stupid one, but.....Check Your Fuse's. First check to see if it is the right size. Some people with amps that blow the fuse, often replace the fuse with a bigger fuse. That's a No-No. If a fuse blows, there is a reason for that. Find out what size fuse your amp takes and make sure it is the right size. No bigger nor no smaller. Fuses need to be right.
5. Another stupid one, but pretty obvious, Clean Your Relays....if possible.
6. Electrolytic caps good, diodes good, fuses the right ones and good, and any physical problems addressed, relays clean, it's time to fire it up, but there is a proper way to do that too. I always use my Variac and slowly turn up the AC going into an amp. While I am doing this, I watch the Volts on the High Voltage and I watch how many amps the Variac is pulling as I slowly power it up. If my High Voltage is not going up, something is wrong. If my line amps are going up fast and high, something is wrong. At least I found out the easy way instead of plugging it straight in and letting the magic smoke out. Besides, even if the amp is working perfect, electrolytic caps that haven't been powered or used in a while, like it much better when they are slowly powered up. They don't like to have been sitting for long periods of times and then hit with full power.
7. At full power up, if the tubes are glass, first I check to see if the filaments are lighting up. Pretty easy on this one. Glass tubes should have that orange glow near the bottom. If it has the proper color glow, filaments are good. If it does not, troubleshoot the filament voltage before you go any further. Most tube filament problems are loose or dirty tube pin connections.
8. Second thing once powered up, I make sure my HV is good. Very easy with an amp that has a HV meter. Just look at the amps HV meter. However, many amps don't. If the amp doesn't have a HV meter and if it is an amp where the HV is less than 1000 (usually sweep tube amps) I connect my multimeter right to the HV. If it is over 1000 volts, I use my HV probe and the meter. The HV probe divides the voltage going into the meter by 1000. Hence, an amp that has 3000v's will read 3 volts on the meter using the probe. No HV? Troubleshoot it. HV first comes out of the HV Transformer windings as AC, to HV Diode Bank which rectifies it to rectified AC, then to HV filter caps which filter the rectified AC to DC, then to the HV plate choke, and to the tubes. Not hard to troubleshoot, but be careful, HV is LETHAL. Also, most HV problems can be troubleshooted unpowered, using continuity, resistance and diode checks.
I was going to post a few more, but alas, Youtube only allows me so many words per video and my words are full so: To Be Continued.