Baldwin-United Corp., the piano maker that grew to become a major finance and insurance conglomerate, yesterday disclosed that it has $1.3 billion in short-term debts, much of it past due, and a possible back tax bill of almost $100 million.

In a delayed filing with the Securities and Exchange Commission, the Cincinnati-based company said it will run out of cash in mid-July, but it can borrow no more because bankers have cut its credit lines.

The SEC filing revealed that the company's hottest product turned overnight into a significant drain on resources.

Frightened investors have been cashing in the so-called single-premium deferred annuities sold by Baldwin, which generated $1.5 billion in premiums in 1981 and $1.6 billion in 1982, one of its principal sources of revenues.

The annuities, which promise investors a future payout in return for a one-time premium payment, were sold mainly through brokerage firms such as Merrill Lynch, Pierce, Fenner & Smith. Last week, because of the company's problems, Merrill Lynch stopped selling the annuities.

The company said it had annuity sales of $442 million and redemptions of $26 million in the first 14 weeks of 1982. This year, for the same period, sales were $209 million and redemptions were $111 million. Ominously, for the three weeks ended April 8, the company sold only $27 million worth and investors cashed in $76 million.

Insurance companies are forced to maintain reserves to cover potential claims. Regulators from Arkansas to Hawaii are stepping in to make sure that Baladwin-United--a holding company whose primary business is to own other companies--doesn't use the resources of those companies to help it over its potentially fatal cash crunch.

Some of the regulators are asking Baldwin instead to inject more reserves into the insurance companies. Many of the companies hold Baldwin-United preferred stock and other securities in their investment portfolios. Baldwin, which said it is unlikely that it will pay dividends on common stock in the near future, said that, if it can't pay $36 million in dividends due on the preferred stock, regulators will make it even harder on the insurance subsidiaries.

"The need of the company . . . for cash has become acute," Baldwin said in the SEC filing. It noted that it has no assured source of that cash and no guarantee that its bankers--through whose forbearance the company is still in business--will continue to grant extensions on repayment of $440 million that was due last month and is now due July 15.

The most likely source of cash for Baldwin-United is the sale of some of its assets, the company said. But most of those assets are owned by its insurance companies. That means, even if the insurance companies sell the assets for cash, state insurance regulators will have to approve the transfer of the money to the parent company.

"There can be no such assurance that such approvals will be obtained," Baldwin said in the SEC document, the required annual report called a 10-K.

In 1967, Baldwin was primarily a manufacturer of musical instruments such as pianos. The company decided to branch out into financial services because it believed the potential growth of the music instrument business was meager.

The company bought insurance companies, banks (which it has had to divest under federal laws), a savings and loan association as well as the two top trading stamp companies in the United States: S&H and Top Value.

Baldwin paid for many of these purchases by issuing securities to its own insurance subsidiaries, who invested the premiums they obtained from writing life and other types of insurance and most recently the huge premiums derived from writing the single-premium annuities.

Such intracompany investments are legal if they are done at a modest level. But at the same time that Baldwin needed huge amounts of cash--$1.17 billion--to buy MGIC Investment Corp. in March 1982, its ability to raise cash at its insurance subsidiaries was constrained.

President Morley Thompson, who engineered much of the company's transformation, announced in 1982 that the MGIC purchase would be made out of internally generated cash. Instead it had to borrow $654 million from its bankers. It repaid $70 million last August, but when its debt came due last month, it could come up with only $144 million of the remaining $584 million.

Banks granted Baldwin several short-term extensions, then last week gave the company until July 15 to work out a repayment agreement. Baldwin, which has hired the investment banking firm Goldman Sachs & Co., said there is no guarantee it will get further extensions, even though it will be unable to pay those loans next July.

Even many of its long-term debtors may demand immediate payment. In a late maneuver last year, Thompson sold $13 million of bonds to banks owned by Jake Butcher, the Nashville financier whose empire collapsed in early March. The bonds are due now. The Federal Deposit Insurance Corp. has ordered the banks that own those bonds to demand payment. If Baldwin can't pay, as it says it cannot, and goes into default, many other debtors can cite "covenants" in their agreements and demand immediate payment