The term motorboating is applied to oscillations whose frequency is below the range of hearing, from 1 to 10 hertz, so the individual oscillations are heard as pulses.
Yep pretty sure it’s AGC related, it’s the AGC voltage ramping up and down. The AGC detector in some receivers is a simple low pass filter that will give you a DC signal proportional to the signal strength, ie the output of the IF AMP is effectively smoothed so with a large signal coming in, a large DC voltage is produced and that is used to feedback and reduce the gain of the IF AMPs and maybe the RF AMP. So if there is an issue with the AGC, I think it can be caused by supply decoupling capacitors going open circuit so when you get a certain amount of drain on the supply rail when you get a certain amount of audio output, it drags down the IF a supply too, the signal output drops casing the AGC voltage to change, the gain of the RF and IF stages are wound up, the end effect is the gain goes down, AF volume drops resulting in less drain on the supply rail so IF a supply comes up gain comes up , and so on… Yep coming back now… the fault isn’t necessary the AGC abut it’s the AGC and it’s time constant ramping up and down compensating for the AF that is effectively AM modulating the DC supply because the main supply decoupling cap is knackered. So if you ever get this symptom try tuning the volume down and does it reduce? then scope the AGC …. So in your case mate, you had a leaky HT decoupling capacitor and when you changed it that solved it.
Hey Buff, wow, you know your stuff and most of it is way over my head, I'm really still a newbie lol. Thanks Man, I'll re read your comments a few times and it should start to make more sense to me. Thanks for the info :)
@@exeterslab7781 lol sorry rereading it I haven’t really worded it right. So I’ll try and make it a bit clear So put aside what I said above for a moment. Imagine how the DC supply provided by the rectifier and smoothing capacitor is distributed throughout the receiver. If you go to 11:43 in your video where you show the schematic of the power supply? V4 is a double diode rectifier it has 2 anodes and a common cathode right, and remember that in order for current to flow through the cathode the cathode has to be negative with respect to either of the 2 anodes ok? So it’s exactly like soldering the cathodes of 2 solid state diodes together. As each of the top and bottom connection of the secondary winding of the transformer labeled L10 goes positive (in anti phase to each other) with respect to the centre tap each of the 2 anodes will conduct on one half of each cycle. R13 is connected to chassis earth and therefore references the chassis as 0V wrt to the bottom end of C18. So C18 is the main reservoir capacitor and I think you measured the voltage across it with your DVM before and after you changed C18 and the voltage was higher after you changed it? So without seeing the rest of the circuit the HT (I think the Yanks call it the B+) supply rail looks like the one above R12 which probably goes to the anode of your AF output valve via the primary winding of the AF output transformer? R12 looks like a dropper resistor and I would imagine that R11 probably connects to the top of C21 forming a potential divider? I’f this is the case then R12 and R11 form a potential divider and C21 is a supply decoupler and this supply will be lower than the HT and will probably be the screen supply to the valves. So that’s the supply distribution, now let’s imagine if C18 starts to go a bit leaky? What happens here is that a thing called ESR (effective series resistance) starts to go higher in value. What this means is as the capacitor ages it’s like having a series resistor inside the capacitor and even though it will still charge up and hold a charge, if you try to draw a reasonable amount of current from it then instead of the output voltage remaining at the full voltage it was when it was previously charged via V4, the output voltage dips instead of remaining high. That’s the job of the reservoir capacitor to keep the supply rail voltage across it as close to the charging voltage between the 2 positive halves of the output cycle? Hopefully you are still with me? So let’s imagine now if you are listening to a station with the volume really low and if your were to put an oscilloscope onto the HT rail (bottom of C18) then you would hopefully see a fairly smooth DC HT voltage (something around 150V DC ??).But now if you turn up the volume, the AF Output valve current will increase as you turn up the volume, yes? In fact the current through the valve and through the AF output transformer will vary with the say the voice of the person or the music that is being received as the signal. Now what we don’t want to happen is the audio signal as it gets louder, ie turn up the volume, to start to appear on the HT rail and that’s what happens when the reservoir capacitor C18 becomes leaky as we call it, like I said due to the ESR value in ohms increasing. So, what will happen if the main HT supply rail, starts to drop down and go up in time with the audio being received? Well for starters the screen supply may be affected if say C21 was also suffering with increased ESR but that’s not the main problem. What happens is that the IF amplifier valve HT supply also dips up and down. So hold that thought. Now let’s look at AGC, automatic gain control. So when we are say listening to an AM radio signal that is strong in signal strength ie we are just down the road from the transmitter station, we will need to turn down our volume as the signal will just boom out of the speaker. But when we retune to a weak signal, we have to turn the volume up and so therefore when we tune back to the first station it blows our eardrums out because we forgot to turn the bloody thing down! Also imagine driving in your car having to constantly be adjusting the volume control as you drive towards and away from the transmitter. Not to mention selective fading which occurs mainly above 2 to 3 MHz but can be lower than anyway, this will also cause the same problem. So we have AGC which is a magic little circuit built into the receiver that does the job of keeping the output volume as constant as possible. It does this my monitoring the level of the output signal from the IF Amp, if we got a strong signal, we get a huge signal out and if it’s weak we therefor get a weak signal. So if we turned our IF signal amplitude into a DC voltage that was proportional to the signal strength and we feed that voltage back onto the grids of the IF Amp valve stages, in theory we now can obtain a fairly constant signal level for a wide range of input signal strengths. Too much output, the AGC DC voltage may go up let’s say (some work the other way around) and this will alter the bias of the valves therefore reducing the gain of the IF a amps and if the signal strength drops, the AGC voltage drops and the gain is increased. Now one final point here, if we allow the gain of the IF amps to rapidly change then there is a chance that the whole receiver may go into oscillation so what we do is apply a time constant, in more expensive receivers we can have short, medium and long time constants often used to overcome fading at that particular frequency. So to wrap this up, let’s say then that C18 is faulty, it has a high ESR a then what will happen is that at low audio output with the volume turned down, the AF a output valve draws little current so the HT supply to all of the valves (AF, IF, MIXER, LOCAL OSCILLATOR and RF AMP). When we turn the volume up, the AF output valve draws more current and because C18 is faulty, the HT a supply will drop. What else will be effected? Well the IF amp will have a lower supply voltage and what will happen to the output of the IF amp with a lower HT? It will produce a smaller signal. What is the AGC going to think? Let’s drive the IF Amp harder, despite having a lower HT a it can still crank the output up. It gets to a point were the HT rail is pulled right down and the IF output can’t go any higher despite the AGC screaming give me more drive, at this point the signal going into the AF output stage will cut off with a thump, no AF output, oh look, low current drain by the AF output therefore C18 despite being leaky will gradually start to charge up again, the HT supply is restored, signal comes back with a thump and then the current drawn by the AF stage starts to pull the HT back down again because C18 cant store enough juice because of the ESR and the cycle repeats and the frequency of the beat is determined by how poorly C18 is and the time constant of the AGC. Sorry I turned that into a flipping lesson… hope it didn’t confuse you 😂😂😂😂😂😂
1:41 this is where on some old valve equipment because of the vent holes in the back when you remove any cover a MASSIVE bloody great SPIDER 🕷️ Jumps out at you and BARKS his head off at you 😂😂😂😂😂
Great video mate…now this has got me bugged … motor boating, like you have t heard this for years and you know what, I too have forgotten what used to cause this and I know that I figured it out many years ago. It can happen with transistor radios too, I have a very sneaky memory that it may be something to do with AGC…. I don’t know now and it’s bugging me now… perhaps someone in the comments can explain it. Funny how you forget things
I love the Rocky doll; Frank I was thinking you were much older. I was in the Arm Forces when the first Rocky movie was first released. Did you try to repair your solid-state generator?
Oh Ken I love Rocky as well. Well I'm turning 60 soon so I am getting on lol. I looked at the circuit board but to be honest, It was too much for me, as I have no idea how these things work. I'm going to use the case to house a signal tracer, which is my next project :)
hmm, looks a bit like a 209U, but AC only...and MW only..? dont think EBF35s were ever used here in the UK, certainly never seen one! probably 'similar' to a 6B8 ? these were used here by some...
@@exeterslab7781 octal ones?? EBF80 and 89 and UBF80 and 89 B9A types were extremely common here in the UK! the USA DDPs 6B8G or GT or 12C8GT were used here 'sometimes' i have a NZ radio i think? that uses an octal DDP but not sure of type number, certainly not EBF something,
The term motorboating is applied to oscillations whose frequency is below the range of hearing, from 1 to 10 hertz, so the individual oscillations are heard as pulses.
Great and simple definition Chris :)
Nice & easy one Frank, wish I could bloody find one this end !
Good vid mate 🙂
Yes, too easy, but I'm not complaining lol
Rock on, well done Frank 😁
Thanks Mike, you're a legend 😃
Hi Frank how about an episode on having a go at fixing your signal generator. Keep up the great videos cheers mate
Hey Chris, I'm not embarrassed to say I would have no idea how to fault find that gizmo. Honestly, IC chips leave me cold lol
That was easy. Gonna fly now. Yo, Adrian I did it!
lol
Yep pretty sure it’s AGC related, it’s the AGC voltage ramping up and down. The AGC detector in some receivers is a simple low pass filter that will give you a DC signal proportional to the signal strength, ie the output of the IF AMP is effectively smoothed so with a large signal coming in, a large DC voltage is produced and that is used to feedback and reduce the gain of the IF AMPs and maybe the RF AMP.
So if there is an issue with the AGC, I think it can be caused by supply decoupling capacitors going open circuit so when you get a certain amount of drain on the supply rail when you get a certain amount of audio output, it drags down the IF a supply too, the signal output drops casing the AGC voltage to change, the gain of the RF and IF stages are wound up, the end effect is the gain goes down, AF volume drops resulting in less drain on the supply rail so IF a supply comes up gain comes up , and so on…
Yep coming back now… the fault isn’t necessary the AGC abut it’s the AGC and it’s time constant ramping up and down compensating for the AF that is effectively AM modulating the DC supply because the main supply decoupling cap is knackered.
So if you ever get this symptom try tuning the volume down and does it reduce? then scope the AGC …. So in your case mate, you had a leaky HT decoupling capacitor and when you changed it that solved it.
Hey Buff, wow, you know your stuff and most of it is way over my head, I'm really still a newbie lol.
Thanks Man, I'll re read your comments a few times and it should start to make more sense to me. Thanks for the info :)
@@exeterslab7781 lol sorry rereading it I haven’t really worded it right. So I’ll try and make it a bit clear
So put aside what I said above for a moment. Imagine how the DC supply provided by the rectifier and smoothing capacitor is distributed throughout the receiver.
If you go to 11:43 in your video where you show the schematic of the power supply? V4 is a double diode rectifier it has 2 anodes and a common cathode right, and remember that in order for current to flow through the cathode the cathode has to be negative with respect to either of the 2 anodes ok? So it’s exactly like soldering the cathodes of 2 solid state diodes together.
As each of the top and bottom connection of the secondary winding of the transformer labeled L10 goes positive (in anti phase to each other) with respect to the centre tap each of the 2 anodes will conduct on one half of each cycle. R13 is connected to chassis earth and therefore references the chassis as 0V wrt to the bottom end of C18. So C18 is the main reservoir capacitor and I think you measured the voltage across it with your DVM before and after you changed C18 and the voltage was higher after you changed it?
So without seeing the rest of the circuit the HT (I think the Yanks call it the B+) supply rail looks like the one above R12 which probably goes to the anode of your AF output valve via the primary winding of the AF output transformer?
R12 looks like a dropper resistor and I would imagine that R11 probably connects to the top of C21 forming a potential divider? I’f this is the case then R12 and R11 form a potential divider and C21 is a supply decoupler and this supply will be lower than the HT and will probably be the screen supply to the valves.
So that’s the supply distribution, now let’s imagine if C18 starts to go a bit leaky? What happens here is that a thing called ESR (effective series resistance) starts to go higher in value. What this means is as the capacitor ages it’s like having a series resistor inside the capacitor and even though it will still charge up and hold a charge, if you try to draw a reasonable amount of current from it then instead of the output voltage remaining at the full voltage it was when it was previously charged via V4, the output voltage dips instead of remaining high. That’s the job of the reservoir capacitor to keep the supply rail voltage across it as close to the charging voltage between the 2 positive halves of the output cycle? Hopefully you are still with me?
So let’s imagine now if you are listening to a station with the volume really low and if your were to put an oscilloscope onto the HT rail (bottom of C18) then you would hopefully see a fairly smooth DC HT voltage (something around 150V DC ??).But now if you turn up the volume, the AF Output valve current will increase as you turn up the volume, yes? In fact the current through the valve and through the AF output transformer will vary with the say the voice of the person or the music that is being received as the signal. Now what we don’t want to happen is the audio signal as it gets louder, ie turn up the volume, to start to appear on the HT rail and that’s what happens when the reservoir capacitor C18 becomes leaky as we call it, like I said due to the ESR value in ohms increasing.
So, what will happen if the main HT supply rail, starts to drop down and go up in time with the audio being received? Well for starters the screen supply may be affected if say C21 was also suffering with increased ESR but that’s not the main problem.
What happens is that the IF amplifier valve HT supply also dips up and down. So hold that thought.
Now let’s look at AGC, automatic gain control. So when we are say listening to an AM radio signal that is strong in signal strength ie we are just down the road from the transmitter station, we will need to turn down our volume as the signal will just boom out of the speaker. But when we retune to a weak signal, we have to turn the volume up and so therefore when we tune back to the first station it blows our eardrums out because we forgot to turn the bloody thing down! Also imagine driving in your car having to constantly be adjusting the volume control as you drive towards and away from the transmitter. Not to mention selective fading which occurs mainly above 2 to 3 MHz but can be lower than anyway, this will also cause the same problem.
So we have AGC which is a magic little circuit built into the receiver that does the job of keeping the output volume as constant as possible. It does this my monitoring the level of the output signal from the IF Amp, if we got a strong signal, we get a huge signal out and if it’s weak we therefor get a weak signal. So if we turned our IF signal amplitude into a DC voltage that was proportional to the signal strength and we feed that voltage back onto the grids of the IF Amp valve stages, in theory we now can obtain a fairly constant signal level for a wide range of input signal strengths. Too much output, the AGC DC voltage may go up let’s say (some work the other way around) and this will alter the bias of the valves therefore reducing the gain of the IF a amps and if the signal strength drops, the AGC voltage drops and the gain is increased. Now one final point here, if we allow the gain of the IF amps to rapidly change then there is a chance that the whole receiver may go into oscillation so what we do is apply a time constant, in more expensive receivers we can have short, medium and long time constants often used to overcome fading at that particular frequency.
So to wrap this up, let’s say then that C18 is faulty, it has a high ESR a then what will happen is that at low audio output with the volume turned down, the AF a output valve draws little current so the HT supply to all of the valves (AF, IF, MIXER, LOCAL OSCILLATOR and RF AMP). When we turn the volume up, the AF output valve draws more current and because C18 is faulty, the HT a supply will drop. What else will be effected? Well the IF amp will have a lower supply voltage and what will happen to the output of the IF amp with a lower HT? It will produce a smaller signal. What is the AGC going to think? Let’s drive the IF Amp harder, despite having a lower HT a it can still crank the output up. It gets to a point were the HT rail is pulled right down and the IF output can’t go any higher despite the AGC screaming give me more drive, at this point the signal going into the AF output stage will cut off with a thump, no AF output, oh look, low current drain by the AF output therefore C18 despite being leaky will gradually start to charge up again, the HT supply is restored, signal comes back with a thump and then the current drawn by the AF stage starts to pull the HT back down again because C18 cant store enough juice because of the ESR and the cycle repeats and the frequency of the beat is determined by how poorly C18 is and the time constant of the AGC.
Sorry I turned that into a flipping lesson… hope it didn’t confuse you 😂😂😂😂😂😂
@@buffplums Thanks for the lesson Buff, it will take me some time to get around all of it. Great explanation on AGC. :)
1:41 this is where on some old valve equipment because of the vent holes in the back when you remove any cover a MASSIVE bloody great SPIDER 🕷️ Jumps out at you and BARKS his head off at you 😂😂😂😂😂
That's one of my greatest fears Buff, they give me the creeps big time lol
Great video mate…now this has got me bugged … motor boating, like you have t heard this for years and you know what, I too have forgotten what used to cause this and I know that I figured it out many years ago. It can happen with transistor radios too, I have a very sneaky memory that it may be something to do with AGC…. I don’t know now and it’s bugging me now… perhaps someone in the comments can explain it. Funny how you forget things
Who else here thought motor boating had a completely different meaning unrelated to motor boats?
Good Show :-)
Thanks for watching JE :)
Can you imagine Electroboom working on a bit of kit like this? 😂😂😂
Yeah, I've seen some of his videos, he could turn it into a death ray lol.
@@exeterslab7781haha yeah… saying that he’s a very clever and funny guy but he knows his stuff … imagine BANG ‼️ OH F***K ‼️ 😂😂😂
I love the Rocky doll; Frank I was thinking you were much older. I was in the Arm Forces when the first Rocky movie was first released. Did you try to repair your solid-state generator?
Oh Ken I love Rocky as well. Well I'm turning 60 soon so I am getting on lol.
I looked at the circuit board but to be honest, It was too much for me, as I have no idea how these things work. I'm going to use the case to house a signal tracer, which is my next project :)
hmm, looks a bit like a 209U, but AC only...and MW only..? dont think EBF35s were ever used here in the UK, certainly never seen one! probably 'similar' to a 6B8 ? these were used here by some...
update, just looked at the data,, different pinout, so not a drop in equivalent, BUT an EBF32 IS same pinout as 6B8... tut tut, Philips!
EBF's are very common in Aussie Radios
@@exeterslab7781 octal ones?? EBF80 and 89 and UBF80 and 89 B9A types were extremely common here in the UK! the USA DDPs 6B8G or GT or 12C8GT were used here 'sometimes' i have a NZ radio i think? that uses an octal DDP but not sure of type number, certainly not EBF something,
What do you use when you record this, everytime you don't talking noise will amplifying, looks like AGC, i'm write?
I use a JVC Everio. I do know the Mic is quite sensitive. Not sure if it has AGC though
@@exeterslab7781 Okay, thanks for the answer, it is not a big question, or problem.
Pull it apart anyway. 🙂
I did and have no idea how it works lol. I'm going to use the case for my signal tracer
J ust watched a bit more of video nothing wrong with generator . Ha ha cheers