[Boatanchors] Bird Termaline Question - Update (long)

[Brian]

Hello Doug,

The load resistor does not pop or blow like a fuse, as you so quaintly put 
it. The resistive coating on the outside of the ceramic cylindrical former 
expands at a rate greater than the ceramic - different TempCos - exacerbated 
by continued, ignorant overloading. The resistive coating may then crack and 
peel off; also the elevated temperature may stretch the terminal clamps - 
leading to higher resistance. The resistor can withstand 275 C, but the oil 
may start boiling below that temperature, leaving gas bubbles at the 
resistive surface, where the specific heat of the gas is much lower than 
that of the oil and so the temperature at the hot spots may well exceed 275 
C. Many oils vaporise around 170 to 200 C. And the gas may decompose, 
forming all sorts of nasty chemicals, some of which may damage the conic 
sections.

I suspect that so many faulty dummy loads come on the market because the 
present owners cannot afford to put them right. But, as there is no obvious 
outer cosmetic difference between a good one and a crap one, they can pass 
them off to unsuspecting novices.

Measure the length and diameter of the load resistor - now that you have 
dismantled your 8135. Tell us the size - some of us may have exactly what 
you want. The Bird offering, at $150, is moderately expensive. Surplus Sales 
of Nebraska has many non-inductive tubular 50 Ohm resistors - $275 for a 200 
W one a few years ago; however, their 60 W one (priced at $42 a few years 
ago) in oil may well handle 150 W, the rating of the 8135. Search on the 
internet for a replacement; visit ham events and field days; visit your 
local radio club.

Wash out the inside and the conic sections of your Termaline using a good 
oil-solvent soap to remove the last vestiges of the degraded oil - perhaps 
two washes and a water rinse. Then fill the case with water, pour the water 
into a measuring jug (then sun dry the casing and conic section), and you 
will know how much oil you require - probably around a pint. Visit your 
local electricity distribution authority, or better still, go to the 
transformer yard and ask for a pint of their synthetic transformer oil - 
probably no charge.

Bird has no need to comment on one of their products that has been severely 
abused. If the nomenclature plate riveted on the outside says 50 Ohm, that's 
what it was when it left the Bird factory. There is no way a 75 Ohm resistor 
could have crept in there. The fact that your impedance measurements show 
reactance, means that the resistor as it is now, is not working with the 
specific conic sections. (See Jacques Fortin's response in this stream)

The Heath Cantenna has the marvellous performance of 50 Ohm from DC to DC. 
Any higher frequency and it's reactive; it was clearly designed by an 
accountant. But don't throw it away - you may find the resistor inside is 
exactly what you seek for your 8135. The main use for a Heath Cantenna is as 
a doorstop. Really.

73 de Brian, VK2GCE.

On Wednesday, February 08, 2017 5:35 AM , Doug said:

<snip>The smell [of the oil] was strong and I expected more of a bland smell 
and what I got was a strong smell which may be a result of the over-heating. 
The construction is a single cartridge resistor being held in the center of 
a cone shaped perforated metal shield.  It looks relatively easy to service 
given you have the parts.

I then took my antenna analyzer, a Rig Expert 220 Zoom and it, more or less, 
agreed with my two DVOM's.  At 100 kHZ, Z & R were both 72.6 ohms.  X 
was -2.41 ohms.  At 21 mHZ, Z was 68.3 ohms, R was 67.7 ohms, X was -9.4 
ohms.   So my DVOM's were absolved of any suspicion.  The analyzer was 
connected through direct coupling of N female sockets via a two-way male N 
Gender changer.

Both Martin & Bird responded.  Martin said get the oil off ebay ($13 + 35 
shipping); get the resistor from Bird.  Bird said the oil was $55 and the 
resistor was $150.  Wow, that's a lot for a 150W load repair.

I asked both companies why the bad Bird units were all being advertised as 
75 ohm units and both responded, simply "Well, they are bad".  The deeper 
meaning to my question is how are so many units coming from so many 
different environments & users measuring the 70-73 ohm measure ? That must 
be the resistance across the cone, isn't it?  Otherwise, when the resistor 
pops, where does the measure come from?  The units could not all have been 
abused in the same way or for the same application.  I can understand the 
cartridge blowing like a fuse but I would like to know (for curiosity sake) 
where the 70-72 ohm measure is coming from.  Bird declined to comment, 
perhaps because it does not matter.  To put the unit back in service you 
have to change the oil, change the resistor, change the seal and hope you 
did it right.  At $205 plus shipping, I sure like the idea of just using my 
old Heath Cantenna !!


My investment is a total loss but if anyone can offer me something for it, 
it is available.


Thanks to all for the great responses.


At least my Patriots came through in the clutches.  That made my weekend !!


73, Doug W5JV





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