Re: [Amateur-repairs] Test leads and other things needed ?

Juha

 

Well, this can be a very lengthy discussion since it is more about technique than it is about tools.  Test probes do not have to be exotic or expensive, but they do need to be selected with consideration of the circuit you are using them in.

 

There are two cardinal rules I reccommend you adopt.

1. Study the circuit first, then probe.   - determine what the signal and DC voltage levels are at the intended test point. 
2. Don't insert problems while looking for problems. - don't use a probe the induces resistive load at the test point or induces resonance.

 

For most general circuit analysis, my probe of choice is most often a common 1:1 O-Scope probe.   Handy, easy to use, minimal in-circuit loading. However, keep in mind that they have series impedance, typically 1MΩ or 10MΩ, so will impart attenuation in the signal level displayed.  Typically, when performing alignments, the goal will be to obtain maximum level of the desired signal, and the true acurate level measurement is seldom critical.  When accurate level measurements are required, the gold standard is an Active Probe (e.g. Fet Probe), but a simple wire probe is usually sufficient.  However, always remember you are observing low level RF signals, and inserting a long wire at a test point is like attaching an antenna, it can become part of the resonant circuit and potentially de-tune the target circuit.  I seldom encounter this problem in most IF circuits, but when directly sampling tuned circuits like oscillators, beware.  Probe lead resonance can be a significant problem when attempting to observe very low level HF signals.  This is why commercial probes like those from Pasternak are expensive, they are designed to limit in-circuit loading.  Keep in mind that in radio receivers, most RF paths are either 50Ω impedance.  

Always keep in mind that your TinySA is a very sensitive instrument designed to detect and display RF signals are very low levels. Remember, at signal that will ultimately result in an S-9 reading on a the meter of a calibrated receiver is actually a very low level signal on the antenna wire, -73dBm (50 uVolts).  In most IF circuits, signal levels will seldom be higher than @ -25 dBm (@35mV).  The TinySA specifications state "Suggested maximum input power of +0dBm with internal attenuation in automatic mode", so it is important to always ensure you protect it from potential damage. Always study the circuit you are going to probe to determine what the potential signal level may be.   It is also a good idea to always use "Automatic Attenuation" to ensure the instrument can at least attempt to protect itself from overload. 

 

The entire purpose of a Spectrum Analyzer is to observe signals in the frequency domain.  Obviously, this assumes the most important observation is the level of the signal at a specific frequency.   In the IF circuits of radios, the RF signals may or may not be isolated from DC bias voltages depending on the circuit design.  Some signals will be gated, e.g. "Switched" via DC levels.  When working with most solid state circuits, the levels of DC present are seldom significant and not a threat to your TinySA, but in Tube radios, bias voltages can be very high and it is necessary to protect the instrument from a high level input. 

 

With respect to a "DC Blocker", yes, this is always a good idea, especially when working with Vacuum Tube radios due to the exceptionally high DC voltages present. But all you need is a method of blocking that DC voltage from entering your delicate instrument. That is what capacitors do.   When probing RF signals, a simple solution is to insert a small capacitor, e.g. 25-50 pF,  in series with the probe tip.  Be sure the capacitor has a DC rating high enough for the target cirtuit.  This will effectively block the DC while allowing the RF (AC) signal to pass with minimal attenuation.  A simple 1:1 Sope Probe is ideal for this, you can clip on lead of the capacitor in the probe tip, and use the other lead to probe the test point. 

 

For your Lafayette Receiver, again, I recommend you study the circuit first.  The method they use for alignment could be performed with a reasonably accurate signal generator and a RF level meter, e.g. AC Volt Meter (True RMS DVM) or an O-Scope.  They introduce a fixed RF frequency to a tuned circuit (455 KHz or 2.608MHz) and optimize the circuit for maximum output level.  So, the input level is not critical, it just needs to be sufficient to excite the circuit but not too strong to avoid saturation.  I would start out with a relatively low level e.g. -60dBm and then increase the level until you can observe response at the test point.

For the remainder of the alignment, they simply introduce an RF signal at the antenna port.  No modulation is necessary but won't matter much anyway.  Each band is tuned again for maximum signal strength, and again, they do not specify the level.  I suggest using -72dBm as the input level since that normally represents and S-9 level signal. However, examining the HA-800B manual,  I noted that the S-Meter alignment uses 100uV (-67dBm) as the calibration value.  

WRT the 1st IF Trap, understand what the circuit is for.  Since the 1st IF frequency is 2.608 MHz, the receiver front end needs to reject an outside signal at the same frequency to prevent IMD and overload of the receiver.  They use a relatively strong signal modulated at 1Khz to emulate an AM signal.  This adjustment ensures that that frequency is sufficiently attenuated when it is applied to the Antenna terminal.  

 

Good Luck, this is where the fun begins.  BTW, when you get to the Kenwood, don't forget the DC blocking cap to protect the TinySA, and keep your fingers out of the spitzin sparken circuits. 

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73's
Dave - KB7JS

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