Shogun - volume off noise
Here is a case encountered on an Emperor Shogun (TS-5010). The volume being zero -or the squelch being active- and while the reception is a bit noisy, we hear a slightly but unpleasant crackling in the outside speaker (especially in SSB mode). If a modulation is present, we also hear it very degraded.
Second problem invoked, the volume is difficult to adjust when you want to listen to low level. It works a little in "all or nothing" mode: when you start the device, the sound suddenly comes as soon as the volume button exceeds the position at 9 o'clock.
On this model precisely, I noticed two things also found on the HR2510 (Lincoln), the latter having the same architecture and common elements:
First of all, the AF amplifier has too much gain. It should be remembered that these CB devices have a "PA" function (public address) in order to allow them to be used to make public announcements, a use comparable to what we would do with a megaphone. The AF amplifier is therefore configured for this versatility. The preamp itself could have less gain.
Then, the "crackling" audible in the speaker when the reception is active (parasites or signals) while the volume is zero, reaches precisely through the PA line (public address). And the funniest thing is that, even if we isolate this line from Q119 by removing C176, for example, the defect is still there!
This parasitic signal is in fact transmitted via the capacitive coupling between the tracks and thus reaches point R234/R233 without having crossed Q119.
The solution adopted
It implies no longer needing PA function in the future.
First, it is a question of cutting the track from R221 (coming from Q119) as close as possible to point R233/R234.
We can already see that there is no longer any noise in the speaker when the volume is zero or that the Squelch is activated.
Then, so that the volume is more flexible to adjust, you can lower the preamp gain as well as that of the amplifier by replacing a few resistances:
-
1) GAIN OF IC104
Decrease R228 (100k) to 47K
We go from an amplification report from 454 to 213 - 2) Gain of Q123
Increase R236 (270) to 560 ohms.
We go from an amplification ratio of 5.5 to 2.7
Result
The volume to zero, no noise or breath.
The volume setting is much more flexible. I measured 6VCC * of noise on the HP with the volume at 10:30 o'clock before and now the same level is reached between 11:30 and 12 o'clock * (with the same noise source). And above all, there is no longer this "all or nothing" effect when you start the device.
