Spectra A9 cooling upgrade project

[Elroy Jetson]

Since I have a milling machine and am looking for projects to keep it busy, I've decided to address the cooling problem
that A9 Spectras seem to all share. If you've ever had one you know how hot they get if you're transmitting very
much. Their heatsinking is inadequate for their power output.

I've obtained a few "not economical to repair" A9 Spectras that are really NOT very repairable, to use
as test mules. I've gutted them to the bare chassis and my plan is to completely mill off the heat sinking on
the chassis, cover the surface with heatsink compound, and screw new, considerably larger heatsinks to that
surface. I expect to triple or quadruple the heatsinking area of the radio this way. I hope to reduce the operating
temperature at full rated power output (110 watts VHF) by a good 20 to 30 degrees.

I've even considered adding thermoelectric coolers, activated by a thermostatic switch. But that may be overkill, and
can be left for a future project.

Elroy

[akardam]

Well, calling them "A9" spectras is a bit of a misnomer, since you can have a high power Spectra with any head type (A3-A9). So, calling it a Spectra High Power cooling upgrade project would be more accurate and less apt to confuse, I think.

Personally, although the high power Spectras and Astro Spectras I've had in the past have always gotten pretty warm when under a high duty cycle (keeping in mind, of course, that they're not rated for continuous duty), I've never felt it to be excessive. However, the cooler you can keep 'em the better, so as opposed to going to all the trouble of re-milling the case, why not just add some active cooling to the existing passive heatsink? Find a switched B+ source inside the radio chassis and use it to run a couple external fans, like what you might find on a computer or other electronic active heatsink. Stack a couple of those nice 120mm low RPM quiet fans side by side on the top of the heatsink, and you should be good to go.

[Elroy Jetson]

Yeah, I should properly call them high power drawer units. A9 is a misnomer but as it's usually associated with the high power remote models,
I guess it gets the meaning across.

I run fairly high duty cycles as the radio is in amateur radio service, and QSOs can run pretty long sometimes. I'm also communicating with fairly
distant stations so I don't want to have to throttle back the power output.

I thought about just adding fans, but since I've got spare empty chassis to work with, I think I'll try my experiment anyway just for the fun of it.


Elroy

[Will]

Elroy,

When you remove the high power PA TX board you will find one of the problems. The heatsink is cast and the flat spots where the transistors mount is NOT flat. So, step one mill the three transistor mounting pads flat. Then use a better heatsink compound like Arctic Silver used in the very fast computers. Two, use binder head screws to mount the transistors to the heatsink. This spreads out the holding force to keep the transistor flat, there by transferring more heat away from the transistors.

Another thing that works well in the many years working with Spectras, mount the radio vertical with the transmitter end up. This allows much better air flow on the heatsink. If mounting flat, use some spacers to get the radio up away from the mounting surface.

This works with the mid power Spectras also.

[Elroy Jetson]

I'm going to mill the heatsinks off from the OUTSIDE. I checked and the depth of the fins is uniform, so I should be able to clean up the
entire heat sink area without even being forced to remove the PA devices, if I wanted to do it that way. The non-flat internal mounting
pads for the PA devices shouldn't pose any problem as long as I don't mill too deep. Which I won't do. I've got scrappers to play with
to figure out how to do it without killing a "good" radio.

Using the silver compound probably is a good idea, but since I've got two one pound tubs of regular white heat sink compound just sitting
there looking for a job, I might as well use what I've got. For a first trial, anyway.


Elroy

[tvsjr]

Yeah, I should properly call them high power drawer units. A9 is a misnomer but as it's usually associated with the high power remote models,
I guess it gets the meaning across.

Not really... better not say anything to the mid-power A9 and W9 Spectras I have kicking about. "High-power Spectra drawer" would get the meaning across.

I run fairly high duty cycles as the radio is in amateur radio service, and QSOs can run pretty long sometimes. I'm also communicating with fairly distant stations so I don't want to have to throttle back the power output.

Talk about your ailing prostate less. QED.

[akardam]

I'm going to mill the heatsinks off from the OUTSIDE. I checked and the depth of the fins is uniform, so I should be able to clean up the entire heat sink area without even being forced to remove the PA devices, if I wanted to do it that way.

Well, IANATP (thermal physicist), but I would tend to think that milling off the existing heatsink fins, and bolting on a bigger heatsink, would add another thermal inteface, and that even if you liberally applied thermal interface material, you'd still have a lower efficiency in thermal transfer across that interface.

I'd also tend to think you'd want to remove all the electronics from the metal chassis during your milling process anyway, as it's liable to generate significant heat that could damage the boards.

Of course, there's always the caveat that the engineers at Motorola who designed these things didn't exactly fall off the turnip truck, and probably knew what they were doing...

[Elroy Jetson]

Do you recall what the published duty cycle specs are for the high power Spectras?

5/5/90, I believe. Certainly it wasn't more than 10/10/80.

That's way too little for a 15 minute QSO at a 50 percent TX duty cycle or a half hour round table at 25 percent.

It seems to me that the Motorola engineers sacrificed thermal sinking capacity in order to make the chassis a bit smaller.
I'm hardly the only one who thinks the high power chassis' heat sinking is inadequate for high duty cycles.


(BTW, my prostate is fine, thank you very much. I'm the picture of health as long as you're not talking about MENTAL health. )


Although adding a mechanical interface isn't ideal for improving cooling, if carefully done I should get a net cooling gain that's
worth while.

As for temperatures achieved during machining of cast aluminum, I gather you've never done any machining? have no worries,
the part will remain cool to the touch at all times. Any generated heat will be removed in the chips that are cut off.
The machined surface will remain cool with reasonable cut feeds and speeds, provided the cutter isn't dull. And it won't be.

Elroy

[jackhackett]

Of course, there's always the caveat that the engineers at Motorola who designed these things didn't exactly fall off the turnip truck, and probably knew what they were doing...

Yeah.. umm... well... those can't be the same engineers they have today.

How good the heat transfer will be is going to depend partly on how flat and smooth you can mill the chassis. People doing high end audio and CPU cooling lap their heat sinks super smooth, not sure if that would be overkill here though. Optimally you want the two surfaces to mate as well as possible with the minimun amount of heat sink compound.

I wonder if milling off the raised area under the transistors and putting in a large copper sheet the same thickness as a heat spreader would have any benefit?

Another possible option would be to mill out an area in the heat sink to embed a thin fan, so you get the extra airflow without the fan protruding off the radio.

Good luck, let us know how things turn out.

[Elroy Jetson]

Yes, a smoother, flatter surface will improve heat transfer by means of a thinner film of heat sinking compound,
and I can achieve a fine smooth flat machined surface with my equipment, no problem.


Elroy

[Will]

Do you recall what the published duty cycle specs are for the high power Spectras?
Elroy

Yes, one min. transmit, four minutes standby/receive. That is a 20 % duty cycle.

[George]

This is one of the exact reasons why I use X9000 instead of Spectras. I don't have overheating problems.

I know a guy that uses an X at 225 watts out, running 18 volts into it with the power control turned wide open and current limit up all the way. He's running a DX packet noded in DFW and it's been running like this for the last 18 years.

[Elroy Jetson]

225 watts? That's INSANE!

It's also got to be near a record for a VHF mobile radio!

Until now, the most I'd heard of is that the old GE Rangr VHF radios could often be turned up to 160 watts.

I have a VHF Rangr that I want to put into service. Or I did...I can't find it. I'm really starting to think that
one of my idiot coworkers may have moved it or tossed it as it WAS partially disassembled and they may have
thought it was junk. But it was just getting some parts replaced due to age. (New capacitor time.) I'll be
ticked if I can't find that radio.

But I've got Syntor X radios, too. Now, the question is, can a Syntor X9000 be made to work with any of the
smaller sized Spectra control heads? A4, A5, etc?

Would anyone happen to know the answer to that question?


Elroy

[batdude]

no, the X9k will NOT work with any of the newer spectra heads...

matter of fact, i can say positively, as i did it on mistake once and smoked an A5 control head doing it...



doug

[Elroy Jetson]

That's too bad, because I think an X9000 is a substantially more rugged platform than a high power Spectra, but my installation in my car uses an A7 head
which is a perfect in-dash fit right below the CD player/radio. The smaller heads are all the same outline size as a standard DIN radio package, so they
lend themselves to really elegant in-dash installs if you happen to have a double DIN sized radio mounting option in your vehicle but your CD/radio is
a single DIN unit. That's my situation, so the A7 head occupies the second space, which previously was just a plastic box for holding your little essentials
of daily life...chewing gum, cigarettes, condoms, razor blades..things like that.

I like Spectras but I can get 100 percent identical functionality out of a more reliable X9000, as I don't actually need fancy things like digital modes. If I
could just get an A7 head working on an X9000 I'd have the best of both worlds.

I don't think I'm up to designing an adapter interface, unfortunately. But I know someone who might be, come to think of it, and he DOES owe me a favor...


Elroy