More than 300 residents of Grosvenor Park Condominiums signed a complaint filed last week with the Montgomery County Office of Consumer Affairs charging that one of the nation's largest condominium converters had reneged on a promise.

American Invsco, residents charged, has refused to dredge a 500-foot pond on the 30-acre complex despite assurances last year that the firm would do the work.

The residents action comes at the same time that American Invsco's plans to convert the Promendade Apartments in Bethesda to a cooperative have sparked a dispute with tenants and a proposal for emergency legislation to govern cooporative conversions.

At issue in the battle of the neglected lake is a dispute over the definition of an "addendum."

Condominium owners claim that a property report filed by the firm last April with the county consumer office, stating an intention to clean the lake, legally binds the company. American Invsco representatives have told residents a second property report -- an "addendum -- filed last June frees the company from this obligation.

Few residents, however, accept the company's explanation.

"Its plain-out chicanery," said resident and complainant Peter Anas. "When they (American Invsco) discovered how expensive the dredging might be, they scoured their files looking for a way out."

American Invsco officials would not comment on the residents' charges, but a letter sent last month to Grosvenor Park owners explained the company's position.

The letter, signed by Invsco representative Harold Judelson, said the addendum filed with the comsumer office last June and distributed to prospective buyers, clearly stated that the original property report was only a basic outline. The amended report, the letter said, provided a "comprehensive description of the work to be accomplished during the program" and did not include dredging the pond.

Most residents, however, contend they were led to believe the second report, or addendum, was merely an update and did not negate any previous promises.

Last summer, after a lengthy and emotionally charged process, American Invsco converted the Rockville Pike complex into condominiums. At the time, residents claimed they believed a promise to dredge the lake would be honored.

During the past decade, residents claim, excess sediment from upstream construction has caused the man-made pond to deteriorate from an aesthetic expanse fit for flora and fowl to an unsightly body of muddied water. The once four-foot-deep pond is now shallow, and scattered groups of ducks paddle around numerous mudbanks.

Residents are not asking American Invsco to dredge the pond immediately, because construction upstream would cause the problem to recur soon, explained resident John Wells. Instead, the residents want the company to put the dredging money in escrow.

The complaint was filed this summer because American Invsco will turn over management to condominium owners in October.

Consumers Affairs officials said a letter was sent last week to American Invsco demanding a response to the charges within 10 days. If a satisfactory response is not made, the matter will be referred to the county attorney, said consumer affairs director Barbara Gregg.