225 Pa. Code § 606 : — RULES OF EVIDENCEWITNESSES — Juror's Competency as a Witness

(a) At the Trial. A juror may not testify as a witness before the other jurors at the trial. If a juror is called to testify, the court must give a party an opportunity to object outside the jury's presence.
(b) During an Inquiry into the Validity of a Verdict
(1) Prohibited Testimony or Other Evidence. During an inquiry into the validity of a verdict, a juror may not testify about any statement made or incident that occurred during the jury's deliberations; the effect of anything on that juror's or another juror's vote; or any juror's mental processes concerning the verdict. The court may not receive a juror's affidavit or evidence of a juror's statement on these matters.
(2) Exceptions. A juror may testify about whether:
(A) prejudicial information not of record and beyond common knowledge and experience was improperly brought to the jury's attention; or
(B) an outside influence was improperly brought to bear on any juror.

225 Pa. Code § 606