How to enforce pslatex to use Computer Modern font for math?

Question

How can I enforce pslatex to use Computer Modern font for math?

(I know, it would be much easier to use pdflatex, but my publisher uses pslatex.)

EDIT: Example:

\documentclass[twoside,11pt]{article}  
\usepackage{amssymb}  
\begin{document}  
${\cal P} = {\cal Q}$  
\end{document}

If you compile it with latex and pslatex the fonts are different.

Unfortunately, the recommendations of Herbert do not help for the following reasons:
1. As I said, the publisher uses pslatex and not latex.
2. The other two recommendations change both text and math fonts. I need to change math fonts only.

Answer 1

Use:

\AtBeginDocument{\RequirePackage{lmodern, times}}

(Thanks a lot for your help, Herbert!!)

Answer 2

\documentclass{article}
\AtBeginDocument{%
\SetSymbolFont{operators}   {normal}{OT1}{cmr} {m}{n}
\SetSymbolFont{letters}     {normal}{OML}{cmm} {m}{it}
\SetSymbolFont{symbols}     {normal}{OMS}{cmsy}{m}{n}
\SetSymbolFont{largesymbols}{normal}{OMX}{cmex}{m}{n}
\SetSymbolFont{operators}   {bold}  {OT1}{cmr} {bx}{n}
\SetSymbolFont{letters}     {bold}  {OML}{cmm} {b}{it}
\SetSymbolFont{symbols}     {bold}  {OMS}{cmsy}{b}{n}
\SetSymbolFont{largesymbols}{bold}  {OMX}{cmex}{m}{n}

\SetMathAlphabet{\mathbf}{normal}{OT1}{cmr}{bx}{n}
\SetMathAlphabet{\mathsf}{normal}{OT1}{cmss}{m}{n}
\SetMathAlphabet{\mathit}{normal}{OT1}{cmr}{m}{it}
\SetMathAlphabet{\mathtt}{normal}{OT1}{cmtt}{m}{n}
\SetMathAlphabet{\mathbf}{bold}  {OT1}{cmr}{bx}{n}
\SetMathAlphabet{\mathsf}{bold}  {OT1}{cmss}{bx}{n}
\SetMathAlphabet{\mathit}{bold}  {OT1}{cmr}{bx}{it}
\SetMathAlphabet{\mathtt}{bold}  {OT1}{cmtt}{m}{n}
}
\begin{document}

foo $y=f(x)$ bar

\end{document}

voss@shania:~> pdffonts test3.pdf
name                                 type              emb sub uni object ID
------------------------------------ ----------------- --- --- --- ---------
SBTDDY+CMR10                         Type 1C           yes yes no      12  0
OWQAHD+CMMI10                        Type 1C           yes yes no      10  0
YXBNUU+NimbusRomNo9L-Regu            Type 1C           yes yes no       8  0
voss@shania:~>