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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only. |
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12th Apr 2019, 10:03 pm | #1 |
Pentode
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RF probe to work with amplified speaker
Hi can someone suggest a probe circuit for me that will work with an amplified speaker I have. I tried one I found on the internet with a dac 90a I have in an attempt to be a bit more selective over what components to replace on this set that only gets a couple of channels with decent reception and it would not work at all. The circuit used a germanium diode, a 4.7M resistor and a 10nf capacitor.
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12th Apr 2019, 10:06 pm | #2 |
Pentode
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Re: RF probe to work with amplified speaker
Sorry wrong circuit in my last post
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12th Apr 2019, 10:23 pm | #3 |
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Re: RF probe to work with amplified speaker
That's not a bad circuit, it's quite typical of the sort.
The snag to them is that they're not terribly sensitive, even if the audio amplifier after them is very sensitive. It takes a certain amount of signal to turn on the diode and any less signal than that produces nothing. Factor in a deaf radio and you are left trying to discriminate between different forms of nothing. You need a signal generator! But with a signal generator you no longer need the detector probe. Set the sig gen to make an audio tone. Apply to the output of the set's detector and see if you get plenty of audio. If not, fix the audio amplifier. Now use the sig gen to make a modulated IF signal Apply to the input of the detector. Hear plenty of audio? No? then fix the detector then work your way back up the IF strip, change to RF and do the mixer and RF stages. You reduce the sig gen output as wou work your way earlier in the set to account for the gains the stages should give. David
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12th Apr 2019, 10:50 pm | #4 |
Pentode
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Re: RF probe to work with amplified speaker
I guess I was looking for an easy way to probe around and find a problem, but that makes more sense. I am go to have to get to grips with using a signal generator and do it like you suggest.
Thanks for that, I'll give it a go and see how I get on. Mike |
13th Apr 2019, 12:08 am | #5 |
Dekatron
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Re: RF probe to work with amplified speaker
After getting the DC conditions correct and replacing all paper capacitors, if the radio is still not working well, I get out the signal generator and follow the alignment instructions. As well as correcting alignment problems, this procedure will quickly identify any tuned circuits or stages that are not behaving correctly and point you in the direction of the faulty component.
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13th Apr 2019, 2:19 am | #6 |
Heptode
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Re: RF probe to work with amplified speaker
Or, if you don't have a signal generator, a signal injector consisting of just a simple 2-transistor multivibrator running at 1kHz or so will give useable harmonics from audio to well past medium waves.
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13th Apr 2019, 6:21 am | #7 |
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Re: RF probe to work with amplified speaker
There is one of those counter-intuitive reversals of what seems obvious here.
The easiest signal generator to use is a posh laboratory grade one. You dial up a frequency and a level and switch on some modulation and it gets on with making just that, with decent accuracy. These were terribly expensive new, second hand ones are still a bit expensive, but bargains crop up occasionally. Marconi, Hewlett-Packard and Rohde & Schwarz were the big names. Nice, but a bit of a luxury. Sig gens for the radio repair trade were a lot cheaper. Their frequency accuracy will be a bit off, so you need to check it. You can do this by operating it near a working radio and checking the dial markings where it interferes with broadcast stations on known frequencies. They have output attenuators which will reduce the output level down to receiver input levels with switch positions for big steps and a pot for finer adjustment. The level accuracy will be poor, but adequate for the job. You may want to check what settings are receivable on a good radio for comparison. These sig gens will do the job of fixing a radio, but don't have the accuracy to measure sensitivity to check a radio meets its specification. This is usually not important. They do need a bit more effort to drive than does an all singing and dancing posh one. Advance were the main UK maker in this market. They tried to present their instruments as somewhat labby, but they weren't really. Good repair shop stuff. (AVO were in this business, too) There were cheap sig gens made for the hobbyist budget. Frequency scales were more cramped and the things drifted, but the biggest problem was the lack of a good enough attenuator. Lots of output, but even with the output pot turned down low there was still too much signal. Add an external attenuator and you find the amount of signal leaking out of the sig gen swamps it by getting to the radio directly. These things were found in the adverts in PW. Brands like Nombrex. One of these will do the job, but you have to nurse them. Skill and experience can make up for their limitations to an extent. Signal injectors can be made for pennies and give you very fast indication of whether there is a signal path. They are best when looking for completely dead circuits. They need more experience in interpreting their results on deaf sets. No use whatsoever for checking alignment. You need to practice on good and bad sets to learn interpretation. So there you have it, a direct trade-off between cost and skill. Most people would assume lab grade sig gens were for boffins with spotless lab coats. The military went for them because they de-skilled their service operations somewhat. Familiarity with a radio model and its usual problems means a lot were fixed with no gear other than a multimeter. Lots of people will tell you you need nothing more. They tend to forget that they had to build up a lot of experience, and they had experienced people with them to guide them. Without the direct guidance, and working on something different every time, learning is a lot harder and having the gear to do it the easy way can help a lot. Second hand test gear is now pretty much fully depreciated, so if you buy something and don't pay over the going rate, you can always sell it for what you paid. David
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13th Apr 2019, 9:09 am | #8 |
Pentode
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Re: RF probe to work with amplified speaker
Thanks for all the replies. The radio works of a fashion so I was trying to find a technique for testing the circuit hoping to be able to home in on a problem rather than change everything. I thought that I might also be able to gain a bit of experience following the signal path throu the radio. It's to gain a bit of experience as much as anything and get a feel for things. I do have an advance signal generator and a frequency counter. I had a go at radios a while back but got frustrated after a couple of disappointing results. I just fancy having another look at things so dusted everything off to have a look again.
Thanks Mike |
13th Apr 2019, 10:39 am | #9 |
Dekatron
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Re: RF probe to work with amplified speaker
You might want to take a look at this recent thread on probes, but just pass by any bits that get deeper than you want https://www.vintage-radio.net/forum/...light=RF+probe. As Wrangler says, simple probes lacks sensitivity, but for the minimal effort and cost of building one, you could give it a try.
There’s a recent thread about signal injectors here, but no circuit diagram is included https://www.vintage-radio.net/forum/...light=RF+probe. Should be easy to find a circuit on the forum of via Google. If you don’t have a sig gen, an injector would be useful. Having a set which is working to some degree is a big plus; at least you can tell if any change you make goes in the right or wrong direction. Sounds promising, but may take while as you go up the learning curve. B
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13th Apr 2019, 7:08 pm | #10 |
Heptode
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Re: RF probe to work with amplified speaker
Hi Mike,
If you're getting some signals through the radio (in roughly the right place on the dial), you ought to able to do a basic alignment by ear (I'm assuming this is a basic MW/LW set). At least you will be able to optimise what you've got or maybe deduce that the set is basically still in alignment and there are other problems. I used this method for many years before I had access to any test gear other than a cheap multimeter. You'll need a copy of a service sheet to see where various coils and trimmers are located. Choose a weak signal at one end of (say) MW. It's then a case of tweaking the correct RF tuned circuits by small amounts either side of their present positions - coils at the low frequency end and capacitors at the HF end. Only adjust ONE COMPONENT AT A TIME and if there is no effect, return it to its original position. In this way, if all else fails, you'll just be back where you started from. You can use the same technique on the IF transformers but always remembering the "GOLDEN RULE" above.
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13th Apr 2019, 8:00 pm | #11 |
Dekatron
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Re: RF probe to work with amplified speaker
Put a request for a cheap RF signal generator in the the wanted section. The benefit of buying from here is you can get one that is known working. A hobbiest signal generator is not going to be expensive.
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13th Apr 2019, 9:35 pm | #12 |
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Re: RF probe to work with amplified speaker
Erm, back in post 8 Mike says he already has an advance sig gen.
David
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13th Apr 2019, 10:36 pm | #13 |
Dekatron
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Re: RF probe to work with amplified speaker
Ooops!
The original request was help with the design of an RF probe to help locate faulty parts rather than do blanket component replacement. The general consensus was an RF probe was unlikely to help as the probe will only tell you the signal amplitude and will give no indication of why a stage is not be providing sufficient gain. The view was the best option is to use a signal generator and run through the alignment instructions as this will resolve alignment problems and reveal most other types of faults. Mike, why don't you have a go at alignment with support from the folks here. |
19th Apr 2019, 1:26 am | #14 |
Pentode
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Re: RF probe to work with amplified speaker
I've posted on here about alignment and met with a fairly firm don't do it. I have a post on at the moment concerning a dac90a. I'm going to do it mostly for the experience and to see what happens. It may not fix the problem but apart from breaking a core I don't see the harm in it. As long as I know where everything was before I start.
Mike |
19th Apr 2019, 8:31 am | #15 |
Dekatron
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Re: RF probe to work with amplified speaker
If you have a proper signal generator, there is no harm in having a go at alignment. Make sure you know how the signal generator works first!! This may sound stupid but if you've never used one before then playing around with it using a working radio is a good idea first just to get to know spot frequencies etc. With a working radio you have a good starting point knowing that it (should be) adjusted correctly.
One thing to remember is that the output of the signal generator should not be set too high. The alignment instructions for most radios advise to set the volume control of the radio to maximum and the output of the signal generator is then adjusted to give a barely audible output. Initially the output of the generator is set to a higher level and as the tuned circuits of the radio are brought into alignment, the output is reduced. There is a good reason for this and it's to avoid AGC action of the radio which might give a false 'peak' to the signal. Make sure you have the alignment instructions for the radio and read through them first. It isn't rocket science but does need to be done carefully. With practice it will become easier and with experience you will be able to carry out a basic alignment without a signal generator (using broadcast signals) which can be useful if service information for a particular set is difficult to find or someone has been there before and messed it all up.
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19th Apr 2019, 10:46 am | #16 |
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Re: RF probe to work with amplified speaker
An RF probe (and AF probe) can be useful with a signal tracer/injector to pinpoint faults by checking at points in a circuit where we'd expect to find signals. Some time ago I found a circuit on internet for an automatic RF/AF probe which I built for use with a Velleman K7000, tracer/injector, with which it works well. I've attached the circuit below, which I built on a small PCB that I designed, but it could just as easily be built on perfboard, stripboard or a piece of plain Paxolin or whatever and tucked into whatever tube is to be used as the probe container. I built mine into a length of plastic conduit and turned a nylon bung for each end.
After building it I tested it on a working TR82 radio and it worked well. I claim no originality for the circuit, the explanation of which states that if an audio signal is being measured, the path of the signal is via C1. The audio signal can’t pass C2 since as capacitance is very small and has a very high impedance for audio frequency signals. When measuring RF signals, C2 has low enough impedance to pass the signal to the rectifier D1 and D2, which not only rectify the signal, but also double the DC level at C3. If the probe tip is placed at a high voltage point such as the anode of an IF valve rather than the grid of the AF valve, C1 & C2, rated at 400V will protect the diodes. (I actually used 630V and 1,000V caps as I had them to hand). As to signal tracing techniques, if you click on this link below and go to the circuit of a valve receiver, you'll see various points on the circuit marked with capital letters in brackets EG ('A') etc. If you hover your cursor over those points in turn, it will tell you what to expect, and what should be happening. For example, it explains that you should not expect to hear any signals at points C, F, K & V - the Screen Grids. http://www.nostalgiaair.org/Referenc...ost/post01.htm Hope that's of interest.
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19th Apr 2019, 11:46 am | #17 | |
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Re: RF probe to work with amplified speaker
Quote:
Al. |
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19th Apr 2019, 12:31 pm | #18 | |
Dekatron
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Re: RF probe to work with amplified speaker
Quote:
You can check the alignment without any "tweaking" using a signal generator, then depending on the results you can then make a call as to whether or not the alignment should be adjusted. If you do decide it needs re-alignment then slug breakages etc aside, it's not hard to do, if you mess it up in signal terms, it can always be un-messed, even FM IF/ discriminator alignment is easy to do as in most cases the manufacture will give an alignment method using an ordinary signal generator and a meter. Getting to know how to align and the relationship between the signal, local oscillator and IF will never be wasted, when I started in the Radio & TV servicing trade the first thing I learned to do with test gear was voltage/current/resistance analysis of the circuit using a multimeter, closely followed by doing RF/IF alignment using a signal generator. When doing re-alignment always check the mechanical alignment first in respect of the tuning scale pointer to the tuning scale with respect to the max and min positions of the tuning gangs rotor. Lawrence. |
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19th Apr 2019, 2:05 pm | #19 | |
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Re: RF probe to work with amplified speaker
Quote:
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19th Apr 2019, 2:37 pm | #20 |
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Re: RF probe to work with amplified speaker
Sorry - don't know what went wrong there. I've re-posted it here, which seems to work:
http://www.nostalgiaair.org/Referenc...ost/post01.htm Basically, it shows a radio circuit and traces the signal from the aerial to the speaker outlining what you should hear at key points. Hope it's of help.
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