HME HM 150 Bi Linear Amplifier Question

[JB383RI]

Hello everyone I've been reading the forums on here for a long time and finally decided to make a account and start posting. This is a great site and I look forward to being part of the community.

I just recently got this HME HM 150 Bi Linear and it didn't work. I re-caped it and changed all the carbon composition resistors to modern metal film ones and now it works great.

So because it's a BI Linear the Pre-Amp is always on and there's no switch to turn it off. It doesn't really work that good plus I don't want it on full time.

Is there a way to disable it internally?

Please let me know if you need more info on the Amp or need pictures. Thanks a lot.

 

[nomadradio]

I have seen people asking for a diagram for HME amplifiers on the web, and the net pre-web for decades.

A Google search has always turned up only requests, no sources.

There is bound to be a way to bypass a preamp. But the nitty-gritties of where things connect dictates how to go about it.

And if someone wants to step up and volunteer to reverse-engineer this kind of info, he (she?) would still need a working sample of that model to trace down and draw up a proper schematic.

Reminds me of a Monty Python sketch. A line of soldiers at attention are asked to volunteer for a dangerous assignment. All of them but one take a step backwards, leaving the one soldier out front of the line.

I just took one step back.

73

 

[JB383RI]

Maybe some pictures will help. Come on guys any ideas?

 

[Wire Weasel]

Hmmm. Well it says "bi-linear" but I have never seen such an amp that had a full time RX Premplifier that wasn't accompanied by a factory switch to operate it. You say it doesn't seem to work .... how do you know for a fact that it has one at all or that it's working at all if it even has one. Do the RX signals (plus sound) on your radio's meter increase when you turn the amp on? If they do .... then it apparently does have a RX Preamp. - And again you say it doesn't work well. Please explain what you mean by that exactly.

 

[nomadradio]

The preamp has a relay, but no switch.

An odd one. Has to have the preamp relay so you can leave it in line turned off.

I see two possibilities: Cut the trace feeding power to the preamp relay's coil. This only works if the relay's standby-side contacts are working properly.

-Or.. Just remove the preamp relay, and put a wire jumper across the center two of the six contact foil pads. If it turns out the relay's standby side also has problems, this gets around those.

73

 

[JB383RI]

The preamp has a relay, but no switch.

An odd one. Has to have the preamp relay so you can leave it in line turned off.

I see two possibilities: Cut the trace feeding power to the preamp relay's coil. This only works if the relay's standby-side contacts are working properly.

-Or.. Just remove the preamp relay, and put a wire jumper across the center two of the six contact foil pads. If it turns out the relay's standby side also has problems, this gets around those.

73

So I found the 12 volt supply to the pre-amp relay and desoldered the pin and it works great. No more full time pre-amp. Thanks again nomdradio I appreciate it.

 

[nomadradio]

Gotta love it when a plan comes together. Congratulations! problem solved.

73

 

[DTB Radio]

I know this is an older thread, but I recently had one of these come in DOA and given to the shop for parts. During a lol today, I pulled it out and did some trouble-shooting, and found that a bad ground and some loose screws were the main culprit, and got the main amp working. In this unit, the SSB delay switch had been repurposed for the receive boost relay, and no SSB delay was there. I'm about to add that, but wanted to post in case anyone else runs into one of these and wants an alternative idea. Of course, one could just drill a hole and mount a simple extra toggle for the receive boost relay....

 

[Shockwave]

That's the first time I've seen an input attenuator labeled as a Clarity switch. Do some positions produce distortion?

 

[nomadradio]

a Clarity switch.

Could label it a "Suicide Selector", with Slow, Fast and Faster.

73

 

[JB383RI]

That's the first time I've seen an input attenuator labeled as a Clarity switch. Do some positions produce distortion?

Hey Shockwave, sorry for the wicked late response but I don't check the forum very often. The amp is actually very clean regardless of where the clarity switch is. I actually use this amp as a back up for my bench radio and just recently had the relay stay latched up when I un keyed. It would only do this with the amp on Clarity 1 or (High). I found the keying transistor (2N3565) was bad. Replaced it and it works fine.

It also developed a new problem recently. This is how it is currently set up (Radio-HME 150-Bird meter reading reflect- Bird meter reading forward-Antenna.) Any other radio/amp setup I use the reflect is usually less than 0.25 of a watt. With the HME on clarity 3 (LOW) or clarity 2 (Medium) the reflect is also 0.25 of a watt or less but when the amp is on clarity 1 (high) when you first key up it is fine but as you talk with in about 10 seconds or so the reflect shoots up to around 15 watts. Im not exactly sure what the problem is yet but I'm working on it.

 

[Shockwave]

Hey Shockwave, sorry for the wicked late response but I don't check the forum very often. The amp is actually very clean regardless of where the clarity switch is. I actually use this amp as a back up for my bench radio and just recently had the relay stay latched up when I un keyed. It would only do this with the amp on Clarity 1 or (High). I found the keying transistor (2N3565) was bad. Replaced it and it works fine.

It also developed a new problem recently. This is how it is currently set up (Radio-HME 150-Bird meter reading reflect- Bird meter reading forward-Antenna.) Any other radio/amp setup I use the reflect is usually less than 0.25 of a watt. With the HME on clarity 3 (LOW) or clarity 2 (Medium) the reflect is also 0.25 of a watt or less but when the amp is on clarity 1 (high) when you first key up it is fine but as you talk with in about 10 seconds or so the reflect shoots up to around 15 watts. Im not exactly sure what the problem is yet but I'm working on it.

You are experiencing what's called self oscillation of the amplifier. Where it will begin to operate as an oscillator and produce a second output on another frequency that has a higher reflected power.

When the oscillation is strong enough, it will sustain enough RF to keep the keying relay latched, after you unkey the radio. The issue causing this problem, is a lack of negative feedback.

This is easy to add by placing a .1uf cap in series with a 100 ohm 2 watt resistor and placing these across the base - collector junctions of both output transistors.

 

[nomadradio]

Looks too old to have negative feedback. Adding NFB would probably settle it down.

Trouble is, we find that adding this to an old amplifier will settle it down and change the input impedance in a big way. Might boost the input match SWR to 3 or more.

Becomes a dual task. Add the NFB cap-plus-resistor to each transistor, then tweak the input circuit to bring the input impedance back up somewhere close to 50 ohms. Adding NFB usually reduces the amplifier's input impedance from what it was before. The input selector might compensate for this and make diddling the input match close enough (below two-to-one) just by selecting the low or medium setting.

An old amplifier with a new radio will usually sound better that way anyhow.

73

 

[Shockwave]

I honestly have never seen a case where adding negative feedback to a bipolar amplifier caused an input impedance to shift by a factor of 3 to 1. That means we've pulled the input impedance all the way down to 16.5 ohms. Whatever changes in input impedance occur, should be easily compensated by a simple adjustment of that Arco compression trimmer, in parallel with the input transformer primary.

Put the input attenuator in whatever position matches the exciters drive level the best, and adjust that compression trimmer for minimum input SWR. Doing that after adding negative feedback should straighten this amplifier right out. That's assuming that any screws grounding the circuit board to the chassis and the SO-239 connectors on the back, are all tight. This same problem can present itself from poor RF grounds.

 

[Shockwave]

We should also consider that if the addition of negative feedback does present a condition where the input impedance has also shifted by a factor of 3 to 1, this is likely revealing a clue that a self oscillation was present, prior to the addition of negative feedback.

Self oscillations don't just increase SWR on the output side, they reflect right back into the input side of a solid state amp and wreak havoc on the input impedance as well. Tetrode tube amplifiers can also exhibit this type of problem, when not neutralized. The grounded grid of a triode, reduces this type of coupling between the input and output of an amplifier.