I have now had the same problem with both my Bronica SQ and my Mamiya RZ: a roll of film will have several good exposures and a bunch of wildly underexposed frames.

There didn't seem to be any rhyme or reason to it until I studied the 8 rolls I processed last night and found that the first frame of every roll was good, followed by however many of the same subject that were underexposed. Whenever I found a new subject (which might take ten minutes or an hour), the same pattern repeated itself: one good frame, the rest bad. Obviously, the pause refreshes, to coin a phrase...

The battery checked out okay, and viewing the shutter speeds on the RZ through the lens also checked out okay. Then I remembered that, even though even though the old battery read fine on the status light, the problem on the SQ only cleared up when I installed a new battery. Viewing through the lens on the SQ, it was very apparent that the shutter was wildly inconsistent.

So, thinking battery problems, I dug around and found this, which was pulled from the now extinct Bronica website via the Wayback Machine:
Bronica Learning Center :: Batteries

Today, virtually everything man made that measures time uses an electronic timer. Small,
portable timers (watches, pacemakers,etc.) use batteries. All cameras have timers for shutters, and all modern cameras use electronic timers, and therefore, use batteries.

Batteries differ in size, shape and cell type. The type of cell, and its chemistry determines its voltage, voltage stability, useful temperature range, shelf life, and discharge character. When you're looking to put a battery in a Bronica, there are three available cell types

* silver oxide
* alkaline manganese dioxide
* or lithium cells

SILVER OXIDE

It is the recommended battery for all Bronica;ETR-Si, SQ-A, SQ-Ai, SQ-B, and GS-1

The silver oxide battery. It has excellent voltage stability, the best longevity, wide temperature latitude, good shelf life, and a predictable (and comfortable for the power requirements of a camera) discharge character.

ALKALINE MANGANESE DIOXIDE

Next is the alkaline manganese dioxide battery, known simply as the alkaline. It has good voltage stability, reasonable longevity, wide temperature latitude, good shelf life, and a predictable discharge character. It is also the least expensive cell, and works well in all Bronicas.

LITHIUM

The performance of lithiums vary greatly, as there are several types. They can be engineered for high amp draw, and lower shelf life; or low amp draw, but longer shelf life. They have the potential for the highest energy density per volume of any low cost cell, but the trade off has been made, in "consumer" applications, for long shelf life. This allows these cells to be terrific for hearing aids, and pacemakers....but poor for an intermittent high amp draw requirement like a
Bronica.


The following chart outlines the recommended cells for all of the recently manufactured Bronica cameras.
ETR-Si

Best: Silver Oxide 6V(544,PX-28)

Good: Alkaline 6V(544A, Etc.)

OK: Lithium 6V(544L)

SQ-A/SQ-B

Best: 4 Silver Oxide (MS-76)

Good: 4 Alkaline (MS-76A)

Forget it: Lithium

SQ-Ai with AE Finder

Best:
4 Silver Oxide (MS-76)

Good:
4 Alkaline (MS-76A)

Forget it:
Lithium

GS-1

Best:
Silver Oxide 6V(544,PX-28)

Good:
Alkaline 6V(544A, Etc.)

Forget it:
Lithium

* The AE Finders for the SQ-Ai cameras represent an unusual situation. The silver oxide, the tenacious cell that it is, allows there to be a brief period, when it is near exhaustion, where the low voltage threshold of the AE is skirted due to the power demand of the electromagnetic release mechanism. The result is insufficient voltage to
set the correct shutter speed in the Auto mode only. It can occur for a roll of film, or two...so the precaution has been issued to prevent this. This situation is not present in the older SQ-A, SQ-Am, SQ; or the ETR style cameras. It also doesn't affect the GS-1. Keep extra batteries in your camera case.
Note that Bronica does not recommend the use of Lithium batteries.

So, I dug out my SQA, which I wore out last year without a single problem, and naturally I had replaced the previous battery with an alkaline cell. And in my RZ, which spent about a year idle, was a lithium.

I'll be replacing all the lithium batteries in my SQ and RZ, and I'll be sure to report back if I have any further problems. I hope this thread will pre-empt anyone else blowing a bunch of good film on bad exposures!