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On Wed, 05 May 2004 08:21:07 -0500, John Fields
wrote: On Wed, 05 May 2004 11:39:58 +0100, Paul Burridge wrote: Okay, John, just out of curiosity I will. I expect them to tell me to get f*cked, like HP or Tek would, but we'll see what their "extended aftersales care" is like. --- Excuse me??? Sorry. Tek, then. I must have been thinking of M$ - Oh, perhaps not; they don't even support their current range of 'products' :-( -- The BBC: licenced at public expense to spread lies. |
On Wed, 05 May 2004 15:28:23 +0100, Paul Burridge
wrote: Tell us. Were the readings close to 50, 75 ohms or what? 70-100 ohms depending on the positions of the coarse and fine attenuator knobs, but not nice and smoothly; jumping around a lot (I guess noisy pots that just want a squirt of contact lube. Does that sound right to you? I'd have expected a much greater range... From an attenuator a large range is undesirable. The opposite is what you want. Lets just call the RF output Z for this unit 75 ohms Nominal. :-) And you have crappy pots to put up with. Not good. This unit is a bit cheap in the attenuator department. Pots for both course and fine controls. I don't like that idea. I prefer a switched attenuator for course and a pot for fine. I'd much prefer a switch for fine also but that would be too expensive. Do a simple leakage test to see if this particular AVO sig gen is worth keeping. On the bench beside the signal generator place a radio and see if you can pick up the signal from the generator. If a signal roars into the radio then it is next to useless for serious or even hobby radio work. Shield the RF output connector or short it. ( attenuator set to minimum output) If it leaks RF and the frequency drifts a lot then you might as well forget it. A friend of mine did have a good use for a few old crappy signal generators. He was into restoring really old radio sets, record players with big horns and pianolas, those pianos that play by themselves. He was giving a demonstration of his gear at his house. In another room he had the old signal generators being externally modulated by modern tape recorders playing old 1920's and 30s type music. The visitors were amused and confused to hear the old style music coming from the old style radios as they could tune into the local stations also. The bull**** flew from those in the know. :-) Regards, John Crighton Sydney |
On Wed, 05 May 2004 15:28:23 +0100, Paul Burridge
wrote: Tell us. Were the readings close to 50, 75 ohms or what? 70-100 ohms depending on the positions of the coarse and fine attenuator knobs, but not nice and smoothly; jumping around a lot (I guess noisy pots that just want a squirt of contact lube. Does that sound right to you? I'd have expected a much greater range... From an attenuator a large range is undesirable. The opposite is what you want. Lets just call the RF output Z for this unit 75 ohms Nominal. :-) And you have crappy pots to put up with. Not good. This unit is a bit cheap in the attenuator department. Pots for both course and fine controls. I don't like that idea. I prefer a switched attenuator for course and a pot for fine. I'd much prefer a switch for fine also but that would be too expensive. Do a simple leakage test to see if this particular AVO sig gen is worth keeping. On the bench beside the signal generator place a radio and see if you can pick up the signal from the generator. If a signal roars into the radio then it is next to useless for serious or even hobby radio work. Shield the RF output connector or short it. ( attenuator set to minimum output) If it leaks RF and the frequency drifts a lot then you might as well forget it. A friend of mine did have a good use for a few old crappy signal generators. He was into restoring really old radio sets, record players with big horns and pianolas, those pianos that play by themselves. He was giving a demonstration of his gear at his house. In another room he had the old signal generators being externally modulated by modern tape recorders playing old 1920's and 30s type music. The visitors were amused and confused to hear the old style music coming from the old style radios as they could tune into the local stations also. The bull**** flew from those in the know. :-) Regards, John Crighton Sydney |
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On Wed, 05 May 2004 23:21:02 +0100, Paul Burridge
wrote: On Wed, 05 May 2004 18:45:36 GMT, (John Crighton) wrote: From an attenuator a large range is undesirable. The opposite is what you want. Oh, is that because the attenuator's maybe in parallel with the output signal and earth? You are not following me here, I am talking about accuracy here. Stick your ohm meter on your Marconi generators RF output connector and click through the ranges. You will see a nearly constant resistance reading. A small variation in resistance reading is acceptable. No variation is prefection. A large variation in resistance reading, such as short circuit or open circuit in any attenuator switch position means it is stuffed. Just for interest tell us what you measure. Someone may have transmitted into the generator or connected it to a large DC voltage (wihout a DC blocking capacitor) when testing a radio. That sort of thing happens all the time. In one place I worked at there was an HP 8640B sig gen. It had reverse RF protection and DC protection. You could hear a distinctive loud click when the protection circuit operated. The guy using the HP8640B would then look sheepish and embarrassed because everyone in the work shop, boss and all, knew he had shoved RF or DC up the brand new (at that time) signal generator. All the other signal generators were being silently zapped in that Racal two-way radio factory. The man fixing the many signal generators in that factory got the ****s after a while. He insisted that an external pad be used. Meaning, all he had to do was open up a tiny external box and replace the burnt out resistors and not open up the sig gen. The point I am making is all attenuators are suspect because they are so easily damaged, I have done it my self. Pushed some stuff on the bench to one side and the mic button on a transciever connected to the sig gen got pressed. Lets just call the RF output Z for this unit 75 ohms Nominal. :-) And you have crappy pots to put up with. Not good. Not good but not serious! I can always just replace the pots with ones of equal values to the old ones - and better quality too. Fine. But why would you bother when you might be able to pick up a ten times better sig gen for 25 quid. The Marconi TF144H on ebay, near you, in your country, 2 hours away. http://cgi.ebay.co.uk/ws/eBayISAPI.d...4746 090&rd=1 Still 0 bids. This unit is a bit cheap in the attenuator department. Pots for both course and fine controls. I don't like that idea. I prefer a switched attenuator for course and a pot for fine. I'd much prefer a switch for fine also but that would be too expensive. My Marconi uses two rotary-switched attenuator controls, actually. What's the problem with pots? Switches can go noisy too... We are talking RF here! If you examine your marconi attenuator you will see it is probably a beautifully made cast aluminium assembly with a switch built into it. It will be a top quality switch with beautifull silver contacts. Each little resistor will have its own little compartment in that beautifully machined solid block of aluminium. SHIELDING! We are talking about shielding! A pot just can't compete with an attenuator like that shielding wise. Do a simple leakage test to see if this particular AVO sig gen is worth keeping. On the bench beside the signal generator place a radio and see if you can pick up the signal from the generator. If a signal roars into the radio then it is next to useless for serious or even hobby radio work. Shield the RF output connector or short it. ( attenuator set to minimum output) If it leaks RF and the frequency drifts a lot then you might as well forget it. Er, I *do* have a scope. Might that not be better or are you looking for something a scope wouldn't show up? We are talking microvolts here. Your scope, whatever it is, will have a maximum sensitivity in millivolts. If you see millivolts of RF by waving your scope probe near your sig gen then your sig gen is useless. All you will see is a noisy 50 Hz waveform. The RF signal should come out of the signal generator via the attenuator and RF connector and no where else. Two way radios, work down to one microvolt or less. General purpose scopes don't. If a radio can hear the signal generator and it is not even connected to it on the desired channel frequency or any of the IF frequencies. It means an RF signal is leaking from the case of the generator. The attenuator, should be the controller of all RF leaving the sig gen. A leaky signal generator as a service instument for measuring receiver sensitivity is useless. So before spending any money on the AVO sig gen, check to see if it leaks RF. If it leaks forget it. I only mentioned the Marconi 144H on ebay because it is easy to repair by hobbyists, performs well, is cheap, has industry standard 50 ohms output Z. circuit is available, the unit is for sale just up the M1 high speed motorway near you. I thought I would be helping a beginner who is struggling with filters and may need a second good sig gen. I'll get out your hair now. :-) Regards, John Crighton Sydney |
On Wed, 05 May 2004 23:21:02 +0100, Paul Burridge
wrote: On Wed, 05 May 2004 18:45:36 GMT, (John Crighton) wrote: From an attenuator a large range is undesirable. The opposite is what you want. Oh, is that because the attenuator's maybe in parallel with the output signal and earth? You are not following me here, I am talking about accuracy here. Stick your ohm meter on your Marconi generators RF output connector and click through the ranges. You will see a nearly constant resistance reading. A small variation in resistance reading is acceptable. No variation is prefection. A large variation in resistance reading, such as short circuit or open circuit in any attenuator switch position means it is stuffed. Just for interest tell us what you measure. Someone may have transmitted into the generator or connected it to a large DC voltage (wihout a DC blocking capacitor) when testing a radio. That sort of thing happens all the time. In one place I worked at there was an HP 8640B sig gen. It had reverse RF protection and DC protection. You could hear a distinctive loud click when the protection circuit operated. The guy using the HP8640B would then look sheepish and embarrassed because everyone in the work shop, boss and all, knew he had shoved RF or DC up the brand new (at that time) signal generator. All the other signal generators were being silently zapped in that Racal two-way radio factory. The man fixing the many signal generators in that factory got the ****s after a while. He insisted that an external pad be used. Meaning, all he had to do was open up a tiny external box and replace the burnt out resistors and not open up the sig gen. The point I am making is all attenuators are suspect because they are so easily damaged, I have done it my self. Pushed some stuff on the bench to one side and the mic button on a transciever connected to the sig gen got pressed. Lets just call the RF output Z for this unit 75 ohms Nominal. :-) And you have crappy pots to put up with. Not good. Not good but not serious! I can always just replace the pots with ones of equal values to the old ones - and better quality too. Fine. But why would you bother when you might be able to pick up a ten times better sig gen for 25 quid. The Marconi TF144H on ebay, near you, in your country, 2 hours away. http://cgi.ebay.co.uk/ws/eBayISAPI.d...4746 090&rd=1 Still 0 bids. This unit is a bit cheap in the attenuator department. Pots for both course and fine controls. I don't like that idea. I prefer a switched attenuator for course and a pot for fine. I'd much prefer a switch for fine also but that would be too expensive. My Marconi uses two rotary-switched attenuator controls, actually. What's the problem with pots? Switches can go noisy too... We are talking RF here! If you examine your marconi attenuator you will see it is probably a beautifully made cast aluminium assembly with a switch built into it. It will be a top quality switch with beautifull silver contacts. Each little resistor will have its own little compartment in that beautifully machined solid block of aluminium. SHIELDING! We are talking about shielding! A pot just can't compete with an attenuator like that shielding wise. Do a simple leakage test to see if this particular AVO sig gen is worth keeping. On the bench beside the signal generator place a radio and see if you can pick up the signal from the generator. If a signal roars into the radio then it is next to useless for serious or even hobby radio work. Shield the RF output connector or short it. ( attenuator set to minimum output) If it leaks RF and the frequency drifts a lot then you might as well forget it. Er, I *do* have a scope. Might that not be better or are you looking for something a scope wouldn't show up? We are talking microvolts here. Your scope, whatever it is, will have a maximum sensitivity in millivolts. If you see millivolts of RF by waving your scope probe near your sig gen then your sig gen is useless. All you will see is a noisy 50 Hz waveform. The RF signal should come out of the signal generator via the attenuator and RF connector and no where else. Two way radios, work down to one microvolt or less. General purpose scopes don't. If a radio can hear the signal generator and it is not even connected to it on the desired channel frequency or any of the IF frequencies. It means an RF signal is leaking from the case of the generator. The attenuator, should be the controller of all RF leaving the sig gen. A leaky signal generator as a service instument for measuring receiver sensitivity is useless. So before spending any money on the AVO sig gen, check to see if it leaks RF. If it leaks forget it. I only mentioned the Marconi 144H on ebay because it is easy to repair by hobbyists, performs well, is cheap, has industry standard 50 ohms output Z. circuit is available, the unit is for sale just up the M1 high speed motorway near you. I thought I would be helping a beginner who is struggling with filters and may need a second good sig gen. I'll get out your hair now. :-) Regards, John Crighton Sydney |
On Thu, 06 May 2004 04:12:58 GMT, (John Crighton)
wrote: You are not following me here, I am talking about accuracy here. Stick your ohm meter on your Marconi generators RF output connector and click through the ranges. You will see a nearly constant resistance reading. A small variation in resistance reading is acceptable. No variation is prefection. A large variation in resistance reading, such as short circuit or open circuit in any attenuator switch position means it is stuffed. Just for interest tell us what you measure. Okay, well the vast majority of the positions are bang on 50 ohms. About 7 are 51 ohms and after that point, the very end of the range goes one ohm at a time up to 56 ohms as the worst instance. I guess that's acceptable? [cautionary tale snipped] Fine. But why would you bother when you might be able to pick up a ten times better sig gen for 25 quid. The Marconi TF144H on ebay, near you, in your country, 2 hours away. http://cgi.ebay.co.uk/ws/eBayISAPI.d...4746 090&rd=1 Still 0 bids. How the hell do you find this stuff? Don't tell me, that was a rhetorical question. :-) The thing is, John, I rather like the *look* of my AVO. What's stopping me from just hooking up an external, switched attenuator? And you say the use of pots is cheapo, but this is an ex-military piece of kit withe MOD record stamped on the front. The military don't buy junk - well, apart from Nimrod, that is. We are talking RF here! Oh yeah. I'd forgotten about that. Plus the AVO goes up to 250Mhz, too, so it's even more important. Not bad for its day, though! If you examine your marconi attenuator you will see it is probably a beautifully made cast aluminium assembly with a switch built into it. It will be a top quality switch with beautifull silver contacts. Each little resistor will have its own little compartment in that beautifully machined solid block of aluminium. SHIELDING! We are talking about shielding! A pot just can't compete with an attenuator like that shielding wise. Okay, okay. Er, I *do* have a scope. Might that not be better or are you looking for something a scope wouldn't show up? We are talking microvolts here. Your scope, whatever it is, will have a maximum sensitivity in millivolts. If you see millivolts of RF by waving your scope probe near your sig gen then your sig gen is useless. All you will see is a noisy 50 Hz waveform. The RF signal should come out of the signal generator via the attenuator and RF connector and no where else. It does. There's no leakage whatsoever from the case. Nonetheless, the attenuator appears to be shot to bits. I either get full output or nowt but jitter. But I can't bear to junk it so I'm going to (in due time) try to replace the pots. You can't really blame me, can you? It'd be a sin to chuck one of these onto the trash heap. It would also make a very nice looking ornament for the mantlepiece; looks a lot nicer in real life than that photo I took. Two way radios, work down to one microvolt or less. General purpose scopes don't. If a radio can hear the signal generator and it is not even connected to it on the desired channel frequency or any of the IF frequencies. It means an RF signal is leaking from the case of the generator. The attenuator, should be the controller of all RF leaving the sig gen. A leaky signal generator as a service instument for measuring receiver sensitivity is useless. It's not leaking one iota. So before spending any money on the AVO sig gen, check to see if it leaks RF. If it leaks forget it. Er, it doesn't. See above. I only mentioned the Marconi 144H on ebay because it is easy to repair by hobbyists, performs well, is cheap, has industry standard 50 ohms output Z. circuit is available, the unit is for sale just up the M1 high speed motorway near you. Hey, "near you" in Australian terms, sure. But this is a very overcrowded little island indeed! The M1 ain't no "high speed motorway" except maybe at 3am. And to get on it in the first place I'd have to circumnavigate the London orbital M25 "The Road to Hell" in the Chris Rea record (and if you want to know why, just listen to the intro to part 1 of the record). The only alternative is attempting to drive straight across London, which would be even worse still. Yeah, it's a great deal, but the guy won't deliver or post, so he might as well be on the dark side of the moon as in Bedfordshire, I'm afraid, notwithstanding it's less than 150 miles away. :-( I thought I would be helping a beginner who is struggling with filters and may need a second good sig gen. I'll get out your hair now. :-) I was a "beginner" 35 years ago. Obviously I'm not making much progress. Thanks for the advice, anyway, it's always valued. p. -- The BBC: licenced at public expense to spread lies. |
On Thu, 06 May 2004 17:27:52 +0100, Paul Burridge
wrote: On Thu, 06 May 2004 04:12:58 GMT, (John Crighton) wrote: You are not following me here, I am talking about accuracy here. Stick your ohm meter on your Marconi generators RF output connector and click through the ranges. You will see a nearly constant resistance reading. A small variation in resistance reading is acceptable. No variation is prefection. A large variation in resistance reading, such as short circuit or open circuit in any attenuator switch position means it is stuffed. Just for interest tell us what you measure. Okay, well the vast majority of the positions are bang on 50 ohms. About 7 are 51 ohms and after that point, the very end of the range goes one ohm at a time up to 56 ohms as the worst instance. I guess that's acceptable? Yes, that is fine. You are laughing with that unit. Great! :-) The thing is, John, I rather like the *look* of my AVO. What's stopping me from just hooking up an external, switched attenuator? Nothing! That is an excellent idea. And you say the use of pots is cheapo, but this is an ex-military piece of kit withe MOD record stamped on the front. The military don't buy junk - well, apart from Nimrod, that is. Heh heh heh, I can see where you are coming from. Military buying junk is a whole topic on its own. It does. There's no leakage whatsoever from the case. Nonetheless, the attenuator appears to be shot to bits. I either get full output or nowt but jitter. But I can't bear to junk it so I'm going to (in due time) try to replace the pots. You can't really blame me, can you? I like to see old stuff preserved and working too. It'd be a sin to chuck one of these onto the trash heap. It would also make a very nice looking ornament for the mantlepiece; looks a lot nicer in real life than that photo I took. Yes it would be a sin to chuck it. Just don't expect a lot from it. I thought I would be helping a beginner who is struggling with filters and may need a second good sig gen. I'll get out your hair now. :-) I was a "beginner" 35 years ago. Obviously I'm not making much progress. Thanks for the advice, anyway, it's always valued. I should have said helping a fellow beginner and hobbyist. I am in the same boat as you, continually struggling. So I know what it is like being in the dark and feeling my way. :-) Getting back to your external attenuator idea. I often buy dud damaged boat anchor sig gens, for a few dollars at ham meetings/trash and treasure buy and sells, in the hope of finding a nice attenuator. I have humped a few monsters home only to find rubbish inside. I have been lucky a couple of times. There are plenty of ready made attenuator units for sale usually for big dollars from a hobbyists point of view. Have a look at this one. http://cgi.ebay.co.uk/ws/eBayISAPI.d...2959 880&rd=1 How far is Farnam from you? Postage is 4 quid. No information as to the maximum frequency but if you can pick it up for a tenner it might be a good buy even if you use it for low frequncies for something else. I am sure you spend more that that at your local. Take a chance! Bid 10 max. :-) Regards, John Crighton Sydney |
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