A defective R/W head is sadly one of the most common reasons why Commodore’s 5,25″ floppy drives fail. In this post, I explain how to test the R/W head as there are quit a few misconceptions around it.

A bit of background

If you have bought untested or defective Commodore-floppy drives, you may have come by a drive that returned a 21, read errors. Although a read error may have trivial causes such as a clogged up R/W head, a fair share of them are caused by a break in one of the tiny coils that rest inside the R/W head.

A dead R/W head is quite frankly as close to an 8-bit disaster as you will ever get. Manufacturers stopped producing them decades ago, and the only effective way to source a spare in 2025 is to take it from a working drive, i.e. in effect replace the entire drive mechanism.

Luckily, not all drives are susceptible to the dead R/W head disease.

Commodore used a broad range of third party 5,25″ floppy drive mechanisms, and in my personal experience, the risk of finding a dead R/W head outside a Mitsumi / Newtronics mechanism is extremely slim.

Drive mechanism	1540	1541	1541C	1541-II	1570	1571
Alps	x	x	x		x
Mitsumi / Newtronics		x	x	x		x
Chinon				x
Kwanasia				x
Safronic / Japan Corp.				x

How to measure

Even if a Mitsumi / Newtronics drive with a 21, read error is concerning, it is important to keep in mind that there could be other reasons that the R/W head. And since R/W heads are becoming such a scarce ressource, you certainly don’t want to risk discarding a drive mechanism with a working head over the suspicion of a dead R/W head.

So, having detected a 21, read error, the next step in your trouble shooting process should be to test the R/W head.

First, you need to need to find your multimeter and set it to RESISTANCE mode

Next, open the drive and locate the R/W head cable connector. The cable is black, and the connector is a flat, black 6-pin connector, although one of the pins is missing. On a 1541 mainboard, it is connected to the pin header at designation P8. On a 1541-II board, the pin header designation is CN2.

The full head assembly with cable

The R/W head seen from above

The lower side of an R/W head

You need to perform measurements across the R/W head.

Ideally, you should place the multimeter probes directly between the solder joints seen in the third picture above. The problem is that, in order to do so, you will need to take the drive mechanism apart. Although the R/W head cable could be faulty without the R/W head being faulty, I usually start out measuring on the cable connector instead, as this is much simpler. If the R/W head is fine, there is no point in taking the drive mechanism apart.

If I don’t get the expected measurements, I move on to measure directly on head instead.

So, where exactly should you put the probes?

You need to perform three measurements:

• R/W end to end (blue to red – or – pin 1 to 5)
• R/W end to center tap (blue or red to white – or – pin 1 or 5 to 4)
• Erase coil end to end (yellow to white – or – pin 3 to 4)

You do not need to measure all pin combinations, as some of the measurements are overlapping.

The illustration below shows can be found in the 1540-1541_Service_Manual page 36, and it provides a useful overview of what we need to measure and why:

Interpretation and results

In the 1540-1541_Service_Manual page 19, you will find the expected resistance readings:

I did however measure resistance on some working R/W heads, and as seen below, the ohm readings from those actual, working heads deviate from the Ohm-readings listed in the service manual. In other words, it shouldn’t worry that the values are a bit off.

The defect heads that I have seen, either gave resistance readings in MOhms or KOhms or simply reported open line. Additionally, and this may be a coincidence, all the defective heads that I tested for this article, had issuees when reading from pin 5 (red).