I've received a number of reports of "no RF output" from the PSK-20. The alignment reading at TP1 will be reasonable; the reading at TP2 extremely low. Builders with a 'scope or RF probe will note low signal levels at Q10 collector and Q11 base.
This problem has been traced to a defective lot of PA transistors. These devices are marked:
If you have one of these devices with the '9D73' lot code , please contact me for a prompt replacement. This change has been reflected in current production.
PA Devices marked "Sanyo C2078 0108" are bad. Please contact me for a replacement,.
(If you do not have one of these devices, please read on........)
Low RF Output- other causes:
1) Did you perform the receiver alignment? These steps must be performed prior to transmitter alignment to establish the correct operating frequency
2) Final Amplifier transistors marked as 'C1678' may exhibit low gain. A maximum of 1 to 1.5W output in the 'Tune' position is typical. Contact me for a replacement.
Low RF Output- the other other causes:
Both the driver collector choke (L10) and driver transistor (Q10) characteristics deteriorated since design and initial production. These devices have been replaced with different types in current production; e-mail us for support.
Here are two schematics with troubleshooting measurements included. These are in .pfd format- you will need an Acrobat reader (free download from Acrobat.com) to view these documents.
The first chart shows DC voltage measurements you may make using a multimeter set to a 'DC volts' scale. This chart also includes AC voltage measurements (pk-pk) useful if you have access to an oscilloscope. (If you do not have a 'scope, this AC information will not be helpful- multimeter AC response at RF will provide misleading results.)
Click here for DC/AC measurements chart:
The second chart provides information on constructing a simple (4 parts) RF probe, or alternatively, using the built-in circuitry of 'TP2' to perform RF measurements. This allows builders without a scope to measure RF levels for comparision with the chart. This chart is most applicable to reports of low transmitter output power.
Click here for RF Probe measurements chart:
A number of builders have commented on the difficulty of peaking the receiver gains due to the slow movement of the waterfall' display. Here's how I peak the receiver gain:
When you perform the 'Frequency Alignment' at the end of p.23 of the manual, you should note that the presence of the strong signal source causes the display background to darken. This is caused by Receiver AGC action- we'll put that to good use.
While the strong signal source is 'key-down', measure the DC voltage at the right end of R24 (AGC control voltage) and tune T1 and L3 for maximum voltage. (This reading is typically ~6V in the presence of a strong signal and will vary by about .05V with the adjustments.)
A number of builders have mentioned difficulty aligning the 14.072 marker signal to the correct position onscreen- it will come within 200-300 Hz of the correct frequency when adjusting C48.
'Expedient fix': Once C48 is adjusted to bring the marker signal as far to the right onscreen as possible, readjust C44 to bring it the rest of the way into alignment
or: padding C48 with a 10 pF NPO disk (or equivalent) cap should provide adequate coverage range.
Note: The correct settings for the two local oscillators for the PSK-20 are approximately 9004.5 KHz (schematic point 'E') and 5068.5 KHz ( point 'B'). These two numbers add up to 14073.0 Khz when alignment is completed.
Wide filter passband: (PSK-20)
Crystal parameters shifted with the last crystal shipment received in August. Several builders have noted poor passband rolloff characteristics. If you've received a kit after early August, the correct value of filter capacitance (C8-C12 and C25-29) is 33 pF. This modification requires removing and replacing 10 disk capacitors, I don't recommend undertaking this change unless you're comfortable with parts removal / replacement on the PSK-20's double-sided board- it is not a 'showstopper' technically. Alternatively, padding each of these capacitors with a parallel 10 pF cap accomplishes the same thing- this was recently documented on the PSK31 reflector.
This change has been reflected in current production.
Lower gain at the high-audio end has been traced to incorrect values of L5 and C7. Original production values were 6.8 uH/ 22 pF, the correct values are 10 uH and 33 pF. This change has been reflected in current production
The value of C23
has been increased from 1 uF to 10 uF.
The value of C24 has been increased from 10 uF to 47 uF.
/ These changes 'flatten out' the TX drive, allowing for fewer readjustments at lower tone-frequencies. Thanks to Stan Wilson, AK0B, for the suggestion. This change has been reflected in current production.
Current component substitutions:
June 23, 2001