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Old 10th Oct 2012, 5:34 pm   #1
HMV 1120
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Default Band 5 self-help amplifier?

I was wondering if anyone recognised the unit below? It's a UHF band five amplifier, part number 3616 466 58330. I'm guessing the BLX 98 reference means it uses BLX 98 1.5 W uhf output transisters (presumably beryllium-bonded to the enormous heatsink as the fins sport an unmissable TOXIC warning).

I assume it would have been used singly or combined as part of a self help system?

Robs
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Old 10th Oct 2012, 7:54 pm   #2
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Default Re: Band 5 self-help amplifier?

That part number (3913 466 58330) looks like a Pye TVT Philips-era 12NC part number. I cleared loads of junk from the factory when I was a schoolboy, and I remember a lot of things having part number labels starting 3913.

I don't know any more, I'm afraid...

Chris
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Old 10th Oct 2012, 9:06 pm   #3
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Default Re: Band 5 self-help amplifier?

You are correct! Taking the top plate off reveals PYE TVT etched onto the boards. The output device in indeed a single BLX 98, making the output ~ 1.5 W.

Rob
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Old 15th Oct 2012, 1:01 pm   #4
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Default Re: Band 5 self-help amplifier?

Yes, most definitely a BLX98 UHF broadcast power amplifier; the last time I saw these in active service was in multiple (Engelbrecht) configuration as the driver stage in PYE TVT klystron amplifiers e.g. BBC1 / 2 analogue at Emley Moor. They were also used as a final amplifier in their own right on low-power relay sites.

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Old 15th Oct 2012, 2:03 pm   #5
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Default Re: Band 5 self-help amplifier?

I am not familiar with this equipment, but would assume that the Beryllium (in the form of Beryllium Oxide) would most likely only be found within the transistor header itself and would only present a hazard if you actually broke into the transistor casing so as to generate dust. Handling an undamaged transistor, or even discrete beryllium oxide ceramic items, was not considered to present a hazard in the 1970's when I was using transistors of this construction when designing UHF power amplifiers. I don't know what the current regulations are.
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Old 15th Oct 2012, 10:48 pm   #6
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Default Re: Band 5 self-help amplifier?

Thankyou both for enlightening me as the use of this unit Guy. Do you recall when you saw these in service? I am wondering if it is an old piece of equipment since replaced or if it may have soldiered on until the DSO.

Regards the berylium, the hazard label states it is "contained in certain components" and to "consult handbook" before handling.

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Old 16th Oct 2012, 1:33 am   #7
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Default Re: Band 5 self-help amplifier?

The data sheet for the BLX88 states that it contains Beryllium Oxide, so I would assume that this is the [only] Beryllium Oxide-containing component. It will be quite safe as long as you don't go cutting the transistor casing open with a Dremel!
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Old 16th Oct 2012, 8:49 am   #8
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Default Re: Band 5 self-help amplifier?

Morning Rob

I was involved in Transmission Operations (and maintenance) Department at Holme Moss up to May 1988, Transmission Engineering (old Transmission Capital Projects) Department up to 1994.

Your BLX module may have been the subject of a normal Replacement Of Worn-Out Plant (RWOP) scheme ... but my bet is that it's techno-fallout from DSO.

I agree with emeritus in respect of the care and treatment of BeO devices. To misquote that old Elton John/Kiki Dee number: "Don't Go Breaking Apart"

Guy
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Old 16th Oct 2012, 9:08 am   #9
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Default Re: Band 5 self-help amplifier?

Rob, just out of interest, does your unit have any form of three-letter code on a label (or possibly hand-written) on it? Transmitter mobile maintenance teams sometimes marked up service and/or spare units with a station identifier (either their own base or sometimes the original site) to keep track of equipment movement. We had a policy of specialist centralised maintenance at large team bases e.g. Crystal Palace looked after Marconi carrier-modulated drive equipment, Holme Moss did Plessey transposers and so on.

Guy
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Old 16th Oct 2012, 9:19 am   #10
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Default Re: Band 5 self-help amplifier?

I work with 300Watt transistors with BeO in them. At the small end of the scale you find UHF devices wit a chunk of ceramic and the silicon mounted on it. It allows c, b, e to be brought out on connections and the heat sink path to be grounded. Some devices have internal bondwires linking emitter or source to the grounded can or flange or stud.

Some of the X-pattern stud mount jobs have a disc of BeO on top of the stud, then the die goes in the centre of that, four connection taped go outwards in a cross-shape, with bond wires to the die, then an alumina cap is cemented over the whole bang shoot. Usually you get two emitter connections. Some transistors I use are designed for grounded-base class-C so I get two base leads.

It isn't safe to try to take the caps off, because the cement may be good and crack may go through the beryllia. The danger is inhaled dust. Seemingy you're supposed to be safe if you eat chunks of the stuff, but the dust is slowly deadly. Cracking, sanding, grinding, drilling are right out.

We bag faulty devices and send tem back to the supplier. Gold recovery pays for proper disposal.

Take sensible precautions. Never think of opening an RF power semiconductor, but don't panic.

David
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Old 16th Oct 2012, 10:48 am   #11
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Default Re: Band 5 self-help amplifier?

Rob, you have a PM.

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Old 16th Oct 2012, 4:07 pm   #12
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Default Re: Band 5 self-help amplifier?

Thankyou all with regards the safety advice. As usual it appears to be a case of "don't trouble it and it won't trouble you".

There is a unit serial number (No. 1612) on the amp and what I take to be a quality control/acceptance stamp of "TVT 33" within a circle. Nothing else externally, although it might be worth having a peak inside (once all 14 screws have been removed from the top plate - little chance of phantom twiddlers here...).

Guy, PM sent.
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Old 17th Oct 2012, 9:21 am   #13
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Default Re: Band 5 self-help amplifier?

I remember at work 30 years ago we had a valved PYE 1kW HF transmitter. Any components that contained Beryllium (drivers, filters etc) were clearly marked with a purple band around their circumference. Some years ago, I even came across a water damaged Schlumberger oscilloscope from about 1978 which had deflection transistors containing beryllium.
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Old 17th Oct 2012, 9:33 am   #14
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Default Re: Band 5 self-help amplifier?

I worked at TVT from 1977 to 1985 but, sadly, do not remember seeing one of those.
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Old 20th Oct 2012, 10:29 pm   #15
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Default Re: Band 5 self-help amplifier?

Quote:
Originally Posted by HMV 1120 View Post
...Taking the top plate off reveals ...The output device in indeed a single BLX 98...
This type of amplifier was very widely used in the transmission side, back in the analogue days. I doubt if any have survived to the present day, in service.

This form of heatsink assembly was used to house quite a variety of differnt configurations. In BBC Transmission Dept, at least, they were simply known generically as 'BLX modules'.

The variety that you have, containing only a single BLX98 transistor, was one of the less-common varieties. The more usual type used a pair of BLX transistors, connected in Engelbrect configuration, and having separate inputs for the DC supply brought out to separate connectors (that's why the choc-block for the supply has two inlets). This was done so that each 'half' of the amplifier could be fed from separate DC supplies - this greatly increased the redundancy of the system: you could 'lose' onr transistor, or one DC supply, and the amplifier would continue to provide a service for the viewing public (albeit at 6dB less power level).

The two-transistor versions would provide about 2.5 - 3 watts of peak output power, at intermodulation levels of the order of -60dB on peak power. To get these figures, selected transistors had to be used, and you may find that your transistor is actually marked BLX98T, the 'T' signifying that the device has been passed for TV transposer use. Some versions on these chassis used BLX93T transistors - slightly higher gain, lower peak-power handling.

If I recall correctly, the BLX93/98 series of transistors were constructed with multiple-emitter technology, to improve UHF performance (all of the multiple emitters were connected internally to the package). Although this improved performance, it reduced lifetime - individual emitters would 'pop' at irregular intervals, and the package gain would reduce slightly. At service intervals, one had to go round these units checking the power output, and replacing any units that had fallen below spec.

That feed-through capacitor on the top of your picture, unlabelled and seemingly never having ever been connected to anything, is in some units internally connected to a small diode detector coupled to the RF output - a moving-coil meter connected between there and chassis would give an indication of power output level.

When changing the BLX transistors: in addition to heeding the above vital information regarding internal beryllium oxide, it is worth remembering that when you tighten up the mounting screw you should use a torque-wrench and aim for a specific figure. Too low, and the heat conduction will be inadequate. Too high, and the screw may snap (it's very soft stuff) or the internal header may be over-stressed and crack the semiconductor die. Ticklish beasties, these BLXs.

That should be enough for you to be going on with, for the time being...
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