Rep. John Dingell (D-Mich.), chairman of the House Energy and Commerce Committee who also chairs the subcommittee in charge of oversight for the Corporation for Public Broadcasting, apparently hasn't lost interest in CPB's proposal to study "bias" in Public Broadcasting Service documentaries . . .

Dingell has written a second letter to CPB President Martin Rubenstein asking for additional materials related to the proposal . . .

Both sides Friday refused to discuss details of the second letter, but there are hints the powerful House member wasn't satisfied with materials sent last month in response to his initial inquiry into the proposed survey . . .

In addition, Dingell apparently questions reports that CPB staffers are considering competitive bidding on the proposed two-year "content analysis study," . . . his point being that CPB should settle the question of whether such a study is really necessary before talking about bids . . .

A decision by the CPB board on the study, opposed by many PBS executives, is expected at its September meeting . . .

Bill Moyers' CBS News Special, "The Burger Years," did not do well in its Wednesday night appearance, averaging a 4.9 Nielsen national rating and a 10 percent audience share (each rating point represents 859,000 TV homes) . . .

In San Fernando, Calif., former "Diff'rent Strokes" star Todd Bridges Friday was sentenced to one year in the county jail after pleading no contest to making a bomb threat against a man he hired to customize his sports car . . .

Superior Court Judge Robert Fratianne imposed the jail term as a condition of the three years' probation he ordered the 20-year-old actor to serve . . .

The judge then postponed the jail sentence until Jan. 7, pending a review of psychiatric and probation department reports. If the actor obeys all conditions of his probation, the judge will then postpone the jail term further or may even cancel it entirely, according to the judge's clerk . . .

Bridges was also ordered to make restitution up to $6,000, pay a $2,500 fine, continue psychiatric counseling and perform 300 hours of community service. He was also ordered to give up his gun collection . . .

Bridges was arrested March 28 at his Northridge home on suspicion of making two bomb threats over the telephone against Greg Tyree, whom the actor had hired to make personalized alterations on his Porsche. One bomb, which caused minor damage, was set off in Tyree's car, according to the prosecution . . .

In 1983, Bridges was arrested by Beverly Hills police when a loaded automatic pistol was found in his car. He paid a $240 fine in that incident after pleading guilty to a charge of carrying a concealed weapon . . . Moving Right Along

The news that Phyllis McGrady is leaving her job as executive producer of ABC's "Good Morning America" to become producer of Barbara Walters specials . . . means that Barbara has signed a new contract with ABC News. Got a nice raise, too, we hear . . .

Friday it was revealed that McGrady, who will replace Beth Polson as Walters' producer, will stay on the job at GMA until Labor Day . . .

Phil Beuth, the ABC vice president in charge of early morning television, said Friday he was "not in any rush" to pick McGrady's successor. "It could be at least a couple of weeks" . . .

He said the replacement could come from within the network or outside. "I've had lots of calls already. There are a couple of in-house producers I know, but the trouble is they're so effective on the jobs they already have" . . .

Polson has resigned after three years with Walters to form her own independent production company, where she will make TV movies and series . . .

Walters said Friday that Polson's was "the right move for her. I wish her every success." Polson, who produced two network movies last year, operates on the West Coast, as has the Walters show in recent years. With the change in command, the Walters specials will be produced out of New York now . . .

McGrady first joined GMA in 1977 as a writer and had a couple of stints as a producer with "20/20" before returning to GMA in 1982 as a line producer and then senior producer . . .

Friday, with the story of her departure official, McGrady said that GMA host David Hartman "has been unbelievably wonderful. Despite what some people have written, he's nice to work with and I've liked working with him. It was a real sad thing to call him (Thursday) and tell him I was leaving" . . .

Steve Friedman, executive producer of NBC's "Today" show, touched McGrady Friday by sending her a bottle of Dom Perignon along with a note that said he would miss her . . .

Friedman's program finally caught up with GMA in the ratings and then blew past the longtime front-runner earlier this year . . .

Also in the NewsC-SPAN, citing the fact that it has already aired 247 hours of House and Senate debate on the tax-reform bill, has asked the key chairmen involved in the upcoming conferences on the legislation for permission to televise those debates, which could begin as early as tomorrow . . .

Brian P. Lamb, chairman and chief executive officer of C-SPAN, sent letters to Rep. Dan Rostenkowski (D-Ill.), chairman of the House Ways and Means Committee, and Sen. Robert Packwood (R-Ore.), chairman of the Senate Finance Committee, asking gavel-to-gavel coverage, " 'live,' when possible" . . .

The request was forthcoming after Chairman Rostenkowski closed the earlier markup sessions on the bill to the TV cameras . . .

Channel 7's gamble of moving "ABC World News Tonight" to 6:30 and running "Jeopardy!" at 7 against "CBS Evening News" on Nine and "NBC Nightly News" on Four continues to pay off . . . and has even caught up with Five's strong sitcom combination between 6:30 and 7:30 . . .

In the last four weeks, "Jeopardy!" has finally moved into a tie for first place with "CBS Evening News" in the 7 p.m. time period . . .

Meanwhile, "ABC World News Tonight" at 6:30 continues to beat the numbers for the 7 p.m. "NBC Nightly News" handily . . .

To compare: During the high-viewing month of February, CBS News on Nine at 7 averaged a 16/26; "Three's Company" on Five did a 16/25; "Jeopardy!" on Seven did an 8.6/14; and NBC News on Four an 8.4/14 . . . while ABC News on Seven at 6:30 did a 9.2/16, compared with a 15/25 for "Too Close For Comfort" on Five . . .

In the last four weeks, running through July 4, "Jeopardy!" and "CBS Evening News" have been tied with a 9.8/22; a combination of "Three's Company" and "One Day at a Time" on Five slipped to 8.0/19; and "NBC Nightly News" on Four has fallen to a 6.0/13 . . .

"ABC World News Tonight" at 6:30 has averaged an 8.0/18 during the same period, compared with a 9.0/21 for "Too Close for Comfort" on Five . . .

There is life after the "Partridge Family": Susan Dey, who played Laurie in the ABC series of the early 1970s, has joined the cast of "L.A. Law" . . . dubbed the hot newcomer in NBC's upcoming fall schedule . . .

Meanwhile, the East Coast units of two major actors' unions -- the Screen Actors Guild and the American Federation of Television and Radio Artists -- have joined their West Coast brothers and sisters to approve the mailing of strike ballots to the 90,000 combined membership . . . which could mean a strike vote by the end of the month, which could tie up fall TV series production . . .

The Radio-Television News Directors Association will honor FCC Chairman Mark Fowler with its RTNDA Distinguished Service Award at a luncheon at the Ramada Renaissance Hotel on Wednesday, July 23 . . .

RTNDA is calling it a "tribute to Chairman Fowler's long and devoted service to the First Amendment values we all support" . . .