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RM Station 550

Tallman

No its not incorrect.
My explanation and fix is exactly how it works.
I have no idea where you get your info from but its not from actual hands on measurements.

There is plenty of bias voltage with the stock components WHEN and only when you drive the amp above a certain drive level.

With the open resistor redlgh has zero bias. so just replacing it will improve the sound notably if all else is working

Transistors are not either off or on. That is nonesense.

They have a conduction curve that can clearly be seen with a volt meter measuring bias voltage at various drive levels.

Swapping r31 for a variable only allows you to adjust idle current
It won't do anything to stop the amp dropping back to class C between syllables on ssb.

RM released several revisions of the station 550 with the same fault.

The stock components bias the finals higher than ideal.

35v+ is higher than normal
my 550's are 30v off load 25v@300w fm.

35v will cause idle current to be higher for the same bias voltage.
The bias voltage will also be higher than normal because its fed from the same 35v.
 
Bob85, I do not want to get into a pissing contest with you over this. My information is directly from RMItaly engineering. Transistors in the area we were discussing are either on or off. It is a switching circuit to go from Receive to Transmit.
The variable resistor is used to set the Quiescent current for the conduction curve. If it is set properly it will not " Drop back to class "C" operation." The Quiescent current is like the Idle rpm setting on a carburetor.
The 35 volts is unloaded so+- 10 percent is good.
300 Watts FM is nominal for the 550.
 
I'm a little late to this discussion, but there are a couple of things I can think to mention.

"Sounds terrible on SSB" is a description that covers a lot of ground. More than one way to foul up the sound of a SSB signal. And if the problem is TWO of these, fixing just one doesn't get you the desired result.

Most-common thing I see is simple overdrive. Setting the radio's sideband modulation limiter "ALC" too high is the number one cause of nasty SSB audio, especially when switching on an amplifier.

Next-most common is the issue already raised, lack of quiescent "idle" bias current through the two transistors in the amplifier.

A 'scope will reveal a lack of ALC. The 'scope trace will show modulation peaks that aren't so much peaks, but flat-topped shapes. Hence the term "flattopping" used by hams for years to describe this problem.

If this is the issue, turning the radio down to a Watt or two on the wattmeter (while barefoot) will clean up all the rotten-sound problem. Not a cure, just a diagnostic result.

BUT!!! If turning the radio down this far makes it sound WORSE, you have a bias problem.

Just how clean does the radio sound on its own? The 'turn it up, turn it down" tests apply to the driver and final in the radio. Like the transistors in the Station 550, they also require idle current to sound clean on SSB.

A 'scope is the swiss-army knife of transmitter problems, but there is a way to set the ALC below the distortion point with only a wattmeter. Don't use peak reading mode, set the wattmeter to 'average' or 'rms' if it has a peak/average switch.

Turn up the mike gain, and set the radio's ALC pot all the way up as seen on the meter with loud voice audio. Now turn it down slowly. You should see a 'threshold', a point where the wattmeter kicks down drastically. Looks worse than it is, since you're reading average power. This point on the adjustment, where the average power takes a big dive, will be close enough to get decent audio from the radio.

I far prefer the 'scope method so I can see what's happening, and set the ALC so that there is no flattening at all of the audio peaks. This tends to be a slightly-lower setting than the wattmeter method above, but probably not enough to sound all that different at the other end of the conversation.

Never assume that there is just one thing wrong with sideband transmit audio. Setting the bias too low in the radio can exaggerate the sound of a "low-bias" problem in the amplifier. And if the radio's audio peaks are flattened barefoot, the amplifier will only make it sound nastier, no matter how clean it would be otherwise.

Maybe I should have read the whole thread more carefully, but how does the radio sound on its own? I'm curious how the high/low power test sounds on the radio barefoot.

73
 
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As for the 1 Ohm resistor for all intent is it not a short electrically? A fuse perhaps if so why not just use a fuse? Just wondering
 
You could just use a wire that would give you one ohm. If you can figure out how long and what gauge wire. The one ohm must be important otherwise they would put in a jumper.
 
Tallman if you don't want a pissing contest don't post tenchobull telling people they are wrong when you don't have a clue what you are talking about.

You claim that you are relaying info from your buddy andrew at RM.
You mean the guys that released 4 or 5 revisions of the station 550 and never fixed the bias switching gave you a mod that wont fix it.
Priceless lmao.

Im describing how it works in the real world when you take measurements of bias voltage & idle current at different temperatures and drive levels Plus a fix that does work

No need to make bias adjustable and risk inexperienced folk blowing their finals.

The circuit does not work the same with rf switching at low drive as it does when you manually turn it on with a jumper.
^THAT'S BASIC ELECTRONICS^.

You clearly don't understand the dynamics of the bias circuit nor have you ever taken measurements and fixed a station 550 bias issue.

The 1ohm that feeds the bias supply is a fuseable link.
They usually go OC but i have seen them go high and bias droop accordingly.

We must guard against disruptions from theoretical armchair technical bullshitters lest we allow the forum to end up like the uk forums.
 
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bob85 said:
Tallman if you don't want a pissing contest don't post tenchobull telling people they are wrong when you don't have a clue what you are talking about.
Click to expand...
It must be so dark where your head is. Setting the Quiescent current needs to be done in a static condition without rf signal applied. I will not even dignify the rest of your falderal with response. I would take Andrews advice over anything you would have to say because of history and experience. Sounds so typical of the "Brits" that don't know what they are talking about.
 
Idle current does need to be set with no rf drive.
You can set it with tr3 shorted as you claim mr armchair expert.
It won't fix the class C distortion at low drive.
It won't make the bias supply be only off or hard on.

You can't prove the transistors in that area only switch off or on like a switch because they don't
They just appear to work that way from your armchair.

I posted an explanation and a fix for anybody with the same issue on ssb.
 
bob85 said:
Idle current does need to be set with no rf drive.
You can set it with tr3 shorted as you claim mr armchair expert.
It won't fix the class C distortion at low drive.
They just appear to work that way from your armchair.
I posted an explanation and a fix for anybody with the same issue on ssb.
Click to expand...
You need to go back to school. If transistors don't turn on and off how do you explain Switch mode power supplies? I am not an arm chair expert I have systems I designed and assisted in the design that work perfectly in the harshest military environment. and have been in the electronics industry since 1970 started and sold two companies and all of those products are still working to this day. So I must be doing something right. If you're trolling me to get the credit for a RF Burn, don't bother me with your trivial B.S. and go back to school and learn how a transistor works.
Your fix must be fixing some other problem, a transistor amp with the idle current properly set will never go to "C" class distortion. If your does then your radio is crap.
 
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Tall man.
Im all about getting to the truth.
when clueless armchair experts disrupt threads with technobull telling me im wrong i will try to educate them

You cannot prove transistors in that circuit act like a switch because you are wrong. I don't care how they appear from your armchair.

Me & you know you have never fixed a station 550.
your an extra class ham that has a rudimentary understanding of electronics trying to be a smart ass on a radio forum.

I will say the same to you as i used to tell Doc.
Please don't disrupt threads with technobull.
Post links to respected sources of learning that prove what you claim about transistors.

You cannot because your an armchair expert and real life aint like it looks from that chair.

You claim any amp biased class AB will never fall back to class C.
That is INCORRECT
For any AB amplifier to stay in AB the source impedance of the bias supply must be low enough to not current limit at maximum rf drive

The bias supply must also turn fully on at all drive levels.

Everything you type proves you are an armchair expert with no practical skills or clue how circuits operate.
 
Last edited:
bob85 said:
We must guard against disruptions from theoretical armchair technical bullshitters lest we allow the forum to end up like the uk forums.
Click to expand...
bob85 said:
Tall man.
Im all about getting to the truth.
when clueless armchair experts disrupt threads with technobull telling me im wrong i will try to educate them

You cannot prove transistors in that circuit act like a switch because you are wrong. I don't care how they appear from your armchair.

Me & you know you have never fixed a station 550.
your an extra class ham that has a rudimentary understanding of electronics trying to be a smart ass on a radio forum.

I will say the same to you as i used to tell Doc.
Please don't disrupt threads with technobull.
Post links to respected sources of learning that prove what you claim about transistors.

You cannot because your an armchair expert and real life aint like it looks from that chair.

You claim any amp biased class AB will never fall back to class C.
That is INCORRECT
For any AB amplifier to stay in AB the source impedance of the bias supply must be low enough to not current limit at maximum rf drive

The bias supply must also turn fully on at all drive levels.

Everything you type proves you are an armchair expert with no practical skills or clue how circuits operate.
Click to expand...

It must be very very dark where you head is. Transistors do act like switches. That is how logic circuits are formed. Apparently you are stuck in your own little world of analog only is the abnormal existence of your choosing.
Some of what you are parroting is what I said early on in this thread and now you are trying bolster you groundless attack on me which you have now made personal.
I do seriously suggest you go back to school and learn how transistors and electronics. Maybe you could go to "Gofundme" for tuition assistance. I will not respond to you in the future.
 
In the real analogue world outside your extra class armchair small rf signals that are fairy weakly coupled to a rectifier driving the base emitter junction of the keying transistor tr3 can pull in the relay so the amp is in line but its not been turned on hard enough to shunt enough voltage from the base of tr8 to turn it fully off.

As a result you end up with the relay activated at low drive but no or very low bias to the finals.
A simple measurement of the bias voltage will confirm this.
The volt meter will also confirm that as you slowly increase drive the bias voltage rises untill at moderate to high drive you have full bias voltage.

Your logic analogy is very much flawed.
The sniffing rf rectifying and turning on the bias supply is very much analogue in its action complete with the conduction curves of the devices as it turns on and the dipping towards class C at high drive due to bias supply source impedance been too high and been fed from a droopy psu.

If you would only leave that armchair and take some measurements you would learn something.

One thing you can be sure about when you meet an armchair expert disrupting threads on internet forums
They can never post proof of their stupid ideas from any respected source.

Im not here to be popular or keep my brown tongue % counter high.
If i wanted to be popular id post in the banal threads joke around with folk and not defend my position when dick heads like you jump in telling me im wrong. Id play the victim .
 

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