TL-922A

TL-922

This is a really good looking, compact, powerful RF amplifier based on a pair of 3-500Z tubes. Some would say ‘if its so good why are there so many 922 on the used market?’ There are different levels of used market. There are used items that are still very useful but where the owner has lost interest. There are used items with minor problems, and there are used items that have been used up. The majority of 922′s offered for sale have been used up and require a significant investment in time and treasure to effect resurrection..

If a seller admits to a power output of less than 1000 watts on any band, you can bet that the tubes are soft. Replacement tubes cost $360. Well, 3-500z’s are not all that great anyway. Actually the tubes are not the problem. The Kenwood design runs the tube filaments at a voltage higher than recommended by the tube manufacturer. Tube life in a 922 is only about 30% of what it should be. In other words you will be replacing tubes three times as often as you would had Kenwood obeyed the tube manufacturers recommendation. Put another way, at todays prices, It will cost you $720 more to run a 922 as compared to running a well designed amp with the same features. History shows that this cost will increase in the future as tube prices rise.

Wait, but that is not all!

Inrush current is great enough to eventually cause on/off switch failure. Total inrush current when turned on is 48 amps. Not sure how much of that is due to HV electrolytics charging as compared to cold filament surge, but I do know it is not good for the on/off switch.

Instability is invited by the high resistance path between tube ground and output tank assemblies ground.

T/R relay is slow enough to cause hot switching. Normally you switch the antenna before turning on the amp by removing cutoff bias. Not so in the 922. It is more fun to watch the sparks fly? The main features of the 922 T/R switch are: hot switching, mechanical noise, and a voltage level guaranteed to fry the PTT line of any modern exciter.

The Kenwood user manual actually has a section on a Spark Suppressor. The spark suppressor is designed to be a sacrificial spark gap to prevent arching the final tank capacitor. They reassure the user that the arching they will hear while running VOX is perfectly normal!!!! While this is definately not normal, leave the spark gap alone. It also protects the bandswitch.

I am not sure if this is a first but it is the first time I have seen a manufacturer officially design in a problem and ask customers to accept that problem as normal. The TL-922A I own had its short comings corrected as well as 10 meter capability restored.

The 922 design is over 20 years old. Those electrolytic filter caps in the HV supply are most likely dried out and have lost a good portion of their ability to store electrical energy. Replacement cost is around $150.

All these problems have solutions. Parts cost to implement solutions to all these problems is about $700 and if you are a less than accomplished technician, you can add $300 in labor costs. Wow, that is $1000 right there! Add the $900 people are asking for their used up amps and we are close to $2000.

Now, if I had nearly $2000 to put down on an amp, would I spend it on trying to correct mistakes of the past or look for something of value like a well designed and maintained Alpha?

If you are a good technician and a fair scrounger, you can resurrect an old 922, ending up with a good 3-500z amp, for under $500. Now if you figure in a used purchase price of $700 for the amp, we are at $1200 total. Can you buy a new (or used) amp in this power class for $1200? I think so but you might not find one as well built mechanically as the 922.

A good 922, one that has been de-fanged and all its problems solved is worth $1000. That means you should not pay more than $500 for an old TL-922 and then only if you are good at fixing things and don’t mind working for free.

This is the reason you see so many TL-922′s on the used market, and since people are asking twice what they are worth, you will probably see more and more of them in the future.

If you can wait five years, you might be able to pick up a used unmodified 922 for under $300. Assuming you can still get a matched pair of 3-500z’s for under $500 at that time, it might be a good inexpensive way to run some serious power.

After using this amp for several weeks I decided to check the input SWR on all bands after I discovered that it was necessary to use a six and a half foot coax cable between the exciter and amplifier to ensure a good match. All bands except 80 meters had SWRs levels below 1.5:1. The SWR on 80/75 meters was 3:1. The schematic of the input circuit had a coil padded with 220pf mica caps on both ends of the coil. The caps in the amp were 120pf mica. Evidently some at Kenwood decided parts is parts when they built this amp. I added mica capacitors to bring the total capacitance up to what was called out in the schematic and the SWR dropped to below 1.5:1 on 80/75 meters.

I understand that modifying this amp voids the warrenty. I guess that s because the manufacturer wants you to use the equipment the way they intended, all screwed up. I am really worried about the warrantee on a factory messed up amp that is over 20 years old!

While considering mods lets remember that over modification is never a cure for over or under engineering. I recently found an article advocating the removal of the interlock switch controlling primary power to the HV transformer. The reason given for this modification was that the switch might fail and limit current to the transformer and cause damage!!!! I guess that person just did not like interlock switches, almost as much as I don’t like modifications that don’t make sense.

Beware of claims like ‘this modification does not effect the amp.’ If that is true then why do the mod?