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MACO / BRUTE Seventy-Five
- Thread starter Recon
- Start date
It was made in several different versions.
Most of them had a bad habit of breaking into oscillation, holding itself keyed up after you unkey the radio. Tends to burn up the tube.
It originally used a large tube called the "M2057". It was a custom version of the 8950 tube, wider with a higher heat rating. Both of these are now rare and expensive, 30-plus years after they quit making them.
It was built both as a grounded grid (cathode driven) amplifier, and also as a grounded-cathode grid-driven "modulator" design.
So yeah, it's not just one amplifier. It's a whole family of them.
The one single, simplest thing that settles this model down has to do with the coax that carries the tube's output power from the Load control on the front panel to the relay alongside the rear panel.
The factory did not connect the shield of the coax that makes this connection. Causes feedback issues. Simply grounding the shield braid of that coax at each end will stop a lot of bad behavior.
73
Most of them had a bad habit of breaking into oscillation, holding itself keyed up after you unkey the radio. Tends to burn up the tube.
It originally used a large tube called the "M2057". It was a custom version of the 8950 tube, wider with a higher heat rating. Both of these are now rare and expensive, 30-plus years after they quit making them.
It was built both as a grounded grid (cathode driven) amplifier, and also as a grounded-cathode grid-driven "modulator" design.
So yeah, it's not just one amplifier. It's a whole family of them.
The one single, simplest thing that settles this model down has to do with the coax that carries the tube's output power from the Load control on the front panel to the relay alongside the rear panel.
The factory did not connect the shield of the coax that makes this connection. Causes feedback issues. Simply grounding the shield braid of that coax at each end will stop a lot of bad behavior.
73
Thank you for the information. Being a nostalgic type of guy, I recently acquired an original Brute Seventy-Five. As per the factory tag, the unit is set-up for 21 MHz. The M2057 tube has not been installed and it is wrapped / enclosed in a protective box. It appears that this unit was never used because the plastic protective wrap is still attached on the front panel. The plan is to convert the unit from 21 MHz to 27 MGz and see what happens.
I have a Brute Seventy-Five with a good pair of 8950 tubes. Output is real low, maybe 10 - 20 watts, and the grid on one of the tubes starts to turn red while it is keyed. It only does it on one socket, either tube you plug in that socket starts to turn red. Also the tuning capacitors are erratic. One of them has to be turned as far as it will go to get output. Anybody have any suggestions?
Attachments
KJ4CYH,
I would look at the bottoms of the tube sockets and look for differences in the 2 sockets. It appears from your picture of the bottom of the tube sockets that the coax cable running across the tube to the front of the chassis may be tight across the buss wire connectors between the tubes. Has it broken the outer insulation?
Are the bypass capacitors on the tube socket pins good? Do they measure different from one socket to the other?
I did notice the coax shield is not exposed and likely not grounded as per Nomads recommendation.
Good luck!
73
David
I would look at the bottoms of the tube sockets and look for differences in the 2 sockets. It appears from your picture of the bottom of the tube sockets that the coax cable running across the tube to the front of the chassis may be tight across the buss wire connectors between the tubes. Has it broken the outer insulation?
Are the bypass capacitors on the tube socket pins good? Do they measure different from one socket to the other?
I did notice the coax shield is not exposed and likely not grounded as per Nomads recommendation.
Good luck!
73
David
KJ4CYH,
I do not. But search the forums for the Maco Brute. It seems to me that Nomad posted a schematic in another recent Maco Brute thread. I believe it also included a "marked up" schematic needed to make the Brute operate correctly.
Nomad IS THE "Amplifier Guru"!!
73
David
I do not. But search the forums for the Maco Brute. It seems to me that Nomad posted a schematic in another recent Maco Brute thread. I believe it also included a "marked up" schematic needed to make the Brute operate correctly.
Nomad IS THE "Amplifier Guru"!!
73
David
Tuning will continue to be erratic until the shield of the coax from the Load control to the relay gets grounded.
At both ends.
The shield is left "floating", not connected to anything at the factory. This turns the shield into a feedback antenna, radiating the output signal back into the input circuit.
Both sockets should be wired identical, one to the other. Either the bad socket has a damaged pin contact or there is a wiring difference (like maybe a toasted part?) in the 'hot' socket that overheats the tube.
73
At both ends.
The shield is left "floating", not connected to anything at the factory. This turns the shield into a feedback antenna, radiating the output signal back into the input circuit.
Both sockets should be wired identical, one to the other. Either the bad socket has a damaged pin contact or there is a wiring difference (like maybe a toasted part?) in the 'hot' socket that overheats the tube.
73
I think this is the post that dMans referred to.
[URL]https://www.worldwidedx.com/threads/maco-brute-75.248469/page-3[/url]
73
[URL]https://www.worldwidedx.com/threads/maco-brute-75.248469/page-3[/url]
73
Those caps are microfarads, or uF.
If they are shorted, they will show a short or low resistance on the ohms scale of a meter.
There has to be something different on the socket that works from the socket that cherries the tube.
A break inside the socket between the spring contact on the top and the solder lug under the socket?
Not common, but that would do it.
Checking the DC voltage on each lug under both sockets with the tubes removed and the power turned on should reveal some difference between the one socket and the other.
The calibrated eyeball is still the number-one tool for this kind of question.
73