OHR Classic Dual

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From: W. Daniel (pandora!daniel@Think.COM)
Date: Sat Jun 04 1994 - 14:52:52 EDT


Hi Gang,

        I need help!

        I am building my OHR Classic Dual Bander and have run into some
snags. The receive side seems to be working but the transmit side is dead as
far as I can tell. Could anyone else who have built the same please advise:-

0. Input voltage at U100 is 13.11 V and output is 8.91 V. This is
        provided for comparison. Scope has input capacitance of about 20pF
        and probe has input capacitance and resistance of about 72.5 pF and
        237 ohms at the x1 probe setting and 17.5 pF and 9 Mohm at the x10
        probe setting. The bandwidth is 15 MHz and 60 MHz respectively for
        the probe. Scope has 50 MHz bandwidth.

1. The signal at the "Test Loop" on the Oscillator board for 20M is
        substantially smaller than that for 40M, under the scope. This
        situation can be rectified by playing with the L101 and L100 but the
        setting is quite critical. When it is set properly, I am getting
        about 0.54 V p-p at the test loop for 20M and about 5.6 for 40M.
        Could anyone verify this? Measurements was made with the probe on
        the x1 (no attenuation) setting in RX mode.

2. The alignment instructions said to measure frequency at C141 but I
        was unable to do this for the 20M band selection because the
        oscillator would stop, perhaps due to excessive loading. However I
        did check that the oscillation for 20M was substantially lower than
        for the 40M band here. The signal here was about 110 mV (0.11 V) p-p
        for 20M and about 1.32 V p-p for 40M! This is almost 10 times
        bigger! Measurements were made using the x10 probe setting, ie. the
        above values I was the result of 10 times what I measured off the
        scope. Using the x1 setting, the 40M signal measure at C141 was only
        0.38 V p-p. The 20M readings using x1 probe setting is not available
        because the oscillator would stop if I stuck the probe at C141.

* Note that the x1 and x10 setting is not in scale of 10 because of
        the different probe capacitance of the two settings thus providing a
        slightly different loading to the circuit under test.

3. T/R Mix oscillator signal measured off pin 6 of U203 (NE602AN) on
        the receive board was about 1.8 V p-p. This is same for any setting.
        Measurements made using the x1 setting on the probe.

4. On Tune mode (TX mode) the signal measured at the junction of D309
        and D310 on the T/R board was a real mix of signals and the p-p
        voltage (x1 probe setting) was about 0.65 V for 20M and 40M.

5. On Tune Mode (TX mode) the signal measured at the junction of D301
        and D302 on the T/R board was a clean sine wave and the p-p voltage
        (measured using the x1 probe setting) for 20M was about 95 mV
        (0.095 V) and for 40M was about 140 mV (0.14 V).

6. On Tune Mode (TX mode), p-p signal on the collector of Q300 on the
        T/R board (measured off the casing of Q300) (x1 probe setting) was
        about 50 mV for 20M and 40M.

7. On Tune Mode (TX mode), p-p signal on the collector of Q301 on the
        T/R board (measure off the casing of Q301, 2N3866) (x1 probe
        setting) with power setting set at maximum, was 0.18 V for 20M and
        0.44 V for 40M.

8. There was no signal on the collector of Q302 on any mode. This was
        measured off the heatsink tab of the 2SC2075.

        Due to the measurements at item 8, I am suspecting that Q302, the PA
final is either bad or being under driven as the 2N3866 driver is putting at
best 0.44 V at into Q302's base which is not biased (Class C).

        I would really, really, really, appreciate it if someone could take
the trouble to verify the above for me. Any advise also welcomed. I am
getting desperate. For the past 3 days I have not slept!!! Thanks.

73,
Daniel

-- 
+-------------+-------------------------------------+
| Daniel Wee  | daniel%pandora@csah.com             | ** Man needs more
| UUCP1.12b   | daniel.wee@f516.n600.z6.fidonet.org |  than a new start, he
| SNEWS 1.91  | csah.com!pandora!daniel             |  needs a new heart! **
+-------------+-------------------------------------+


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