Runaway capstan fix

In most cases the fix is to replace electrolytic capacitors C5, C7, C9, C13, C15, and the ceramic C2 (See schematic) to get it back to normal. Once it’s working, use the speed calibration process from the service manual. Also check the voltage on the zener diode (between C2 and C15 on the board) to be about 9V.

If replacing the capacitors listed above does not resolve the issue, that probably means that one of the traces had corroded and disconnected. Check voltage on the ground and power pins on IC1 and IC2 with a voltmeter, and continuity of all traces on the board with an ohmmeter, in particular on the left side of the board. You can also compare signals at the ends of traces with an oscilloscope while it’s running. I’ve seen two cases where the trace between C7 and R4 has corroded right at the edge of the pad of C7, one case with the trace between IC1 and C16, on the edge of IC1 pad, and one case with the ground bus between C7 and the zener diode.

Parts that I use in repair: - C5, C13, C15 - C7 - C9 - C2 - Z1

To pull out the transport for service you will need to disconnect two cables running to the speed control board on the front left side of the machine. The capstan will not run without the three- or four-pin connector plugged in, but there is a way to simulate the connection without having to pull out the speed control board if you need to test the repair or perform other service that requires the capstan to be running. Use a piece of wire to connect red and orange wires on the connector (same on a four-wire connector, orange and red will be the middle two wires):

Capstan not running fix

Most likely cause is corroded or cracked/damaged trace(s) on the capstan board.

Another possible cause is a disconnect or a failed component on the speed control board.

238S Dolby S Repair

Dolby S daughtercards in Tascam 238S suffer from the same problem as the capstan - faulty electrolytic capacitors. Unlike the capstan board, the daughtercards are double-sided with a number of vias, in addition to much thinner traces. These vias and traces fail in an unpredictable pattern and requre a complete continuity test of each trace, especially around the input components (Q1-Q4, D1-D4, U2) and the Dolby chip U1. As a result, a reliable repair is time-consuming and is cost-prohibitive in most cases.

The alternative that may work better is replacing all daughtercards with a rebuilt version. I performed reverse-engineering of the R/P board and the Dolby S daughtercard and created KiCad schematics available at Under dolby-hic there is a PCB layout and Gerber files for the Dolby S daughtercard.

If you have a Tascam 238S with faulty Dolby S, I think the best course of action is to order a set of replacement PCBs from a low cost fab such as JLCPCB or OSH Park and transfer all components from the original boards.

Replacement electrolytic capacitors: - C5 - C12 - C13 - C15, C23 - C41, C44, C48

238S Dolby S Calibration Procedure

First, calibrate the unit with Dolby S out.

Set all channels to recording mode, then press Pause and Record.

Feed -10 dBV/0 dBVU 400Hz (316 mV RMS) signal to the line inputs, and use ENC ADJ trimmer to adjust AC voltage on TP1 to be 388 mV.

Use ENC OUT trimmer to adjust AC voltage on REC LEVEL trimmer input lead (closest to the front) to be the same in both Dolby IN and OUT modes on each channel.

With Dolby S OUT, make a recording of -10 dBV/0 dBVU 400Hz test tone on all channels.

Set Dolby to IN, play back the recording and use DEC ADJ trimmer to adjust AC voltage on TP1 to be 388 mV on all channels.

Use DEC OUT trimmer to set voltage on the line out to -10 dBV (316 mV) on all channels.

Intermittent stopping

Most likely cause is the aging of cassette presence and record protection sensors. To fix, spray some Deoxit under the sensor levers.

Excessive wow and flutter

Manifests itself as distortion or “warbling”:

Most likely cause, if it’s not mechanical, is either bad C16 or trace corrosion around C16 and R3.