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Old 24th Aug 2011, 11:56 pm   #1
Top Cap
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Default RM KL400 Linear Amp - servicing

I have recently repaired two KL400 Linear amplifiers, both suspected of having blown output transistors but in fact both had dry joints on their boards. I have written a complete step by step instruction for the complete check out of these popular amplifiers. If you have one in the junk box you thought had blown its output transistors then it might prove useful to check it out with the guide. It was put together with a bit of a rush and includes circuit diagrams and layouts so I hope I do not have too many errors in it. The link is below (hope it works)
Les

http://g4cnh.com/public/Servicing%20the%20KL400.pdf
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Old 25th Aug 2011, 12:02 am   #2
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Default Re: RM KL400 Linear Amp - servicing

I have never had anything to do with those but I think you have done an excellent article.

Lawrence.
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Old 30th Aug 2011, 8:52 pm   #3
Dave UXB
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Default Re: RM KL400 Linear Amp - servicing

That is the most comprehensive document of it's type I seen in years; almost military in its breadth.
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Old 9th Dec 2012, 4:01 pm   #4
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Default Re: RM KL400 Linear Amp - servicing

Many thanks that is most useful. I use one of the lower powered HLA150 things for my low power digital (JT65) operation.
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Old 10th Jun 2016, 11:32 pm   #5
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Default Re: RM KL400 Linear Amp - servicing

That's a good and interesting write up, thank you.

Although I try to avoid repairing solid state linear amplifiers I did reluctantly take on a KL500 that had accidentally been given the full output from a HF transceiver. Those 10 ohm resistors had badly burned up and the printed board was charcoal. I did successfully repair it for its owner, but generally I'll only touch the older valve units made by such as Zetagi and Bremi - well, no one else will touch them round here as they're old technology, and that's only if I'm in the mood or have the time. However, I don't mind working on my own stuff.
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Old 11th Jun 2016, 11:06 am   #6
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Default Re: RM KL400 Linear Amp - servicing

Hi Techman, yes I have repaired a couple of those valve units in the past. I remember one that had four EL509's in it and they all had holes in the envelopes. This is where the anodes had reached such a high temperature that the glass had melted and was sucked in by the vacuum. Crazy abuse some CB-ers gave their rigs in those days
Les
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Old 15th Jun 2016, 11:06 am   #7
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Default Re: RM KL400 Linear Amp - servicing

Top Cap..thank you for a most informative write up.
These amps are cheap and effective but unfortunately run off in to thermal meltdown...they get that hot.
So good cooling is a must..computer fan..or what else floats your boat.
Thanks again for the write up.
Best regards Roger-M0TDM
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Old 15th Jun 2016, 11:23 am   #8
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Default Re: RM KL400 Linear Amp - servicing

Quote:
Originally Posted by Top Cap View Post
Hi Techman, yes I have repaired a couple of those valve units in the past. I remember one that had four EL509's in it and they all had holes in the envelopes. This is where the anodes had reached such a high temperature that the glass had melted and was sucked in by the vacuum. Crazy abuse some CB-ers gave their rigs in those days
I've seen this when they set the thing's mode-switch to "SSB" (so AB2 biasing) and then fire a continuous-carrier mode (AM or FM) into its input. Either the anodes melt their way through the glass envelopes, or the transformer overheats and releases the magic smoke, or both.... and then I pick the thing up for £1 as non-working

The tuning capacitors are OK for HF-band 'pi'-section ATUs.
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Old 20th Jun 2016, 1:41 am   #9
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Default Re: RM KL400 Linear Amp - servicing

Ah, I see this thread has now been edited and your post with the 'new' working link has been deleted and new link has now replaced the old link in the first post, which is a good idea. Folk will probably wonder why I seem to have suddenly answered a thread from 2012, but now they'll know. As it happens, I don't remember seeing this thread the first time round, but that was probably because I hadn't re-awoken the interest in such things at that particular time.

The one you mention with the four EL509 valves will probably be the Bremi BRL500 (I think Zetagi made a similar version). They actually have five EL509s with the 5th one as a 'driver' for the other four. I've certainly seen the glass sucked in on a 6KD6 on an early Zetagi BV131, but not seen it happen to the EL509s in the big ones, as the forced air cooling usually stops this happening. I've certainly seen PL509/19 valves with the glass sucked in to a hole in colour TV set line output stages, although that's another story. The fans in these big amps look like a very large computer fan, but they're 240 volt driven and have metal blades and sound like a vacuum cleaner working, so no amplified base microphones with these, as the fan will come over louder than your voice!

The big Bremi will tick over all day so long as it's not over driven and the air intake for the fan is not obstructed. The problem with the smaller ones, particularly the Zetagi BV131, is that there's too much heat from the single valve in a confined space with no cooling fan. The Bremi BRL200 also runs hot with its two 6JB6A valves, but seems to be happy at this, other than the anode feed choke melting a bit from heat from the nearby valve/s - as it seemed to do in various other models without to much detriment to the overall operation.

As for transformers burning out, I've never heard of a burnt out transformer in a Bremi. It seemed to be a Zetagi problem and particularly with the early ones with the 6KD6 valve. The later ones with the EL509 seemed more reliable, although I have known the trasformer burn out in these. The causes of this were several. Firstly, the transformers were wound for 220 volts. Just measure the heater voltage on the valve - it was a bit more than 6.3 volts - no wonder they worked so well ! Even at 220 volts, the transformers were only just up to the job. Then there was all that radiated heat from the valve straight onto the transformer, not to mention the heat melting the front panel watt meter, causing it to stick at first and eventually falling out in the worst cases. John Birkett in Lincoln used to sell a meter exactly the same, other than the scale, that was an exact replacement for something like 50p, you just had to fit the scale from the original meter into the replacement.

The tranformers in the Bremi linears seem to be wound for 240 volts. The heaters of the five EL509 valves are in series across a 32 volt line in the BRL500 - makes me think that if they were going to do a series heater chain then they may as well have used PL509 valves. Having just said that, try getting hold of PL509 or 19 valves these days - it's bad luck if you're trying to restore an early colour TV set, as I think all the hams have used them all in homebrew linears! Meanwhile, EL509s are ten a penny (slight exaggeration), but luckily the Russian military used them in their thousands, so there's still many thousands out there.

The 6KD6 is getting very hard to find at a sensible price and its American 12 pin base, which burns out on the heater pins in the Zetagi, is virtually impossible to find these days - best convert to EL509, although they don't seem to work quite as well as the 6KD6, or is that just my imagination. I think there's a design fault with the 6KD6 in that the heater current of getting on for 3 amps is far too much for the pin size and its base on this particular valve - was this valve ever really used as a 'sweep tube' in American colour TVs? There were two other higher heater voltage versions of this valve which would not have this problem due to the lower heater current.

The 6JB6A valves used in the Bremi BRL200 and the Zetagi BV130P (I think) have now become expensive and hard to find - last quote from Watford valves I heard was £120 for a pair. This would in most cases make the linear not worth repairing if it needed two - which they often did. Again, were these little valves really used as 'sweep tubes' (line output valves) in American TV sets?

Finally, just getting back to the question of failures in these valve linear amplifiers - it's often the low voltage 'solid state' section of the unit that causes the most trouble. Perhaps it's the mixture of hot valves and the transistor that don't mix, although they mix well enough in the old hybrid TV sets - or did they? The transistor circuit that controls the relay seems to cause problems with either the transistor itself, or the small electrolytic blowing its guts out, probably caused by the rectifier diode going short and burning out. Although I haven't known glass melting in the large four output valve linear amplifiers, I've found several shattered glass envelopes. I suspect that this could be caused by the fact that the commonly found Sylvania type valves seem to have very thin glass and the ceramic valve bases are not always quite level on the chassis, and the valves are forced into position on the four way top connection circuit board. I used to think that this soldering of the top caps to that circuit board was a dreadful idea. I actually now stand this board off the top of the valves and fit a flexible connection to the four top caps. I did at one time think that this 'rigid' soldered on connection method restricted the expansion and contraction of the valves as they heated and cooled, thus causing the weak glass to shatter. In reality, there's probably enough 'give' in the top board to accommodate this movement, and it'd more likely be the fact that the valves are all forced into line at the top, against the angle of the pins as they relate to the VERY strong, but slightly out of level ceramic bases. As a note, there's another thread running on the forum regarding the force needed to remove or insert valves into these type of bases, and the bases in the BRL500 are no exception. Thinking about these 'soldered on' top cap connections to a board that's rigidly fixed to the upright anode feed choke, it perhaps wasn't such a bodge when you consider that these amplifier units would have been shipped in cargo holds from Italy and would have likely been dropped upside down while being transported, there's never going to be any problems with valves working their way out of holders - packing valves separately and fitting by dealer or user would NOT be an option.

Below is a picture of the chassis of a Bremi BRL500 that was being repaired (anode choke slightly melted due to suspected air starvation caused by probably being placed hard up against a tight corner in a 'shack' somewhere) showing the four valve bases mounted on the chassis. Also showing the fan in relation to the valves. The 5th driver valve is mounted separately on its side

I'm a bit conscious that this thread could be drifting slightly 'off topic' with regards to discussing the valve versions of the solid state RM KL400, although these valve versions are more 'vintage' - the one in the picture having a date sticker on the transformer of 12th June 1981. Perhaps there should be a thread split to another one on the 'older' valve linear amplifiers, although this may be rather difficult to do easily. Also, discussions on the various valves (tubes) used is another complete subject area in itself.
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Old 20th Jun 2016, 2:01 am   #10
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Default Re: RM KL400 Linear Amp - servicing

Love the pi tank
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Old 20th Jun 2016, 2:18 am   #11
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Default Re: RM KL400 Linear Amp - servicing

Yeah, great 'innit'!
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Old 20th Jun 2016, 2:36 am   #12
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Default Re: RM KL400 Linear Amp - servicing

Just remembered another fault, open circuit cathode. The ribbon connection to the cathode within the valve blown open circuit - probably caused by a H/K short?
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