F4huy,toggle quoted messageShow quoted text
My best guess is a broken or shorted wire in one of the transformers: T8,9,10,11
The fact that you get 2 Watts of SSB at 30mhz is good news,
that shows that everything from the mike to the first mixer at D1,D2 is working fine.
When transmitting CW, CLK#2 is a square wave at the operating frequency.
Mixer D1,D2 is unbalanced by CW-KEY to allow that signal right on through to the final amp.
So this is not an issue with the BFO offset as previously suggested,
this would affect only SSB transmissions, and attenuate all bands equally.
If the receiver works at 3.5mhz, then the low pass filter at L1,2,3,4 should be working fine.
The problem is somewhere between Q90 and the antenna jack.
The signal going into Q90 should be around 0.1 volts pk-pk,
and the voltage increases by roughly a factor of four as it passes through each stage.
Since there are four stages of amplification between Q90 and the IRF510's,
the output from the IRF510's should be roughly 0.1*4*4*4*4 = 25.6 volts pk-pk.
If you have access to a good oscilloscope capable of seeing at least 3.5mhz,
probe the collector (or drain) of each of the four amplification stages.
See if the AC voltage increases with each stage when operating at 3.5mhz.
Compare that to what you see when operating at 30mhz.
If you don't have a scope, I suggest you build a diode RF probe.
Use either one of the first two circuits shown here, they should give equivalent results:
The first circuit shown is the one most commonly described elsewhere.
The second circuit is easier for most of us to understand.
Note that the smallest AC voltage either circuit can sense will be around 0.3 volts rms,
so it probably will see no signal if you look at the base of Q90.
Many webpages on diode RF probes recommend using a germanium diode,
but those are getting hard to find. A schottky diode such as the 1n5711 will do fine.
Many dishonest vendors of 1n34 germanium diodes will actually ship a schottky diode instead.
A true 1n34 will be able to see slightly lower level signals than an 1n5711.
When output varies more than it should between bands, my first suspicion
would be the output low pass filters selected by relays KT1, KT2, KT3.
However, all of those output filters are low pass, so all of them should pass
a 3.5mhz signal unmolested even if they got swapped around somehow.
If you do suspect trouble between the IRF510's and the antenna connector,
a good test would be to cut the trace between transformer T11 pin 5 and relay KT1 pin 12,
Then put your wattmeter directly between T11 pin 5 and ground.
The wattmeter will see the raw power coming out of the IRF510's, harmonics and all.
So readings will be a little high, but 3.5mhz should definitely be stronger than 30mhz
since each of the 4 amplifier stages will have more gain at lower frequencies.
Verify that your BNC antenna connector is working with an ohmmeter,
perhaps go around it by soldering a wire to the PC board till you find this problem.
There have been reports of some BNC connector center pins not making proper contact.
Jerry Gaffke, KE7ER
On Tue, Jan 14, 2020 at 01:30 PM, <f4huy.ham@...> wrote: