QYT CB-58 Bad AM modulation repair for second transceiver

QYT28(CB58)AM変調不具合があるトランシーバーの修理（2台目）

前回同様に変調トランジスタMJE2955Tをネジ留めしました。

ファイナルFET(FQP13N10)もネジ留めしました。

(2022.6.26追記）電源シャットダウンで過去（2022年3月)に修理したトランシーバーのオーナー様からAM変調不具合があると連絡があり、早速トランシーバーを送って頂き確認したところ、周波数が低い側でAM変調が汚いのが確認できました。 前回の修理でファイナルFETをネジ留めするときに変調トランジスタMJE2955Tもネジ留めしてあるので、追加の対策が必要なことがわかりました。 又、最近修理したトランシーバーも外部アンテナをHB9CVに接続すると変調が汚くなる現象が出ることが判明しました。 更に何か他に改善方法がないか引き続き実験、調査してみます。

(Added on 2022.6.26)  Owner of transceiver repaired in the past (March 2022) in the power shutdown problem informed me that there is AM modulation problem. Immediately after I received it, I checked and confirmed AM modulation is dirty on the low frequency side of 10m band. In the previous repair, when screwing final FET, modulation transistor MJE2955T was also screwed on, so it was found that additional solution is necessary.  In addition, it was found that the recently repaired transceiver also has a phenomenon that the modulation becomes dirty when the external antenna is connected to HB9CV.  I will continue to experiment and investigate whether there are any other ways to improve it.

海外ユーザーも多いようなので英語にて書きましたがGoogle自動翻訳で簡単に日本語になります。

I received second transceiver that is not good for AM modulation.

Solution : I screwed modulation transistor MJE2955T to aluminum die cast body as same before.

Compare two videos before and after screwed modulation transistor MJE2955T.

Bad modulation video from my tweet

Improved AM modulation video from my tweet

It is best to screw FQP13N10 of final FET as well.

Disassemble and remove final FET FQP13N10 and Modulation transistor MJE2955T

Removed MJE2955T can be reused if leg is enough length for reuse.

MJE2955T collector connected to mounting base.

Removed FQP13N10 can not be used. 

Input capacitor is big (not good for RF use).

Recommend to replace with low input capacitor type.

Drill and tap

Screwed and soldered for both of FQP13N10 and MJE2955T

If replaced with new FET which is high Vt = 4.2V (Gate voltage), it requires to change resistors.

Added 10 ohm over existing 150 ohm

Also added 2.2K ohm over existing 2.2K (this becomes total 1.1K)

Power is adjustable by volume

3.9W at 29.300MHz

4.2W at 28.235MHz

I reported this AM modulation problem to QYT.

I hope they will improve in the future.

By JO1ACW