Tascam 238 and 238S Repair and Service

denis​@​238pro​.​com

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:

https://www.mouser.com/ProductDetail/667-EEE-1CA100SR - C5, C13, C15
https://www.mouser.com/ProductDetail/667-EEE-1VA4R7SR - C7
https://www.mouser.com/ProductDetail/667-EEE-1HA010SR - C9
https://www.mouser.com/ProductDetail/581-12065C473KAZ2A - C2
https://www.mouser.com/ProductDetail/78-TZMB9V1 - 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 https://github.com/quietcat/tascam238s. 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:
https://www.mouser.com/ProductDetail/667-EEE-1CA220SR - C5
https://www.mouser.com/ProductDetail/667-EEE-1CA100SR - C12
https://www.mouser.com/ProductDetail/710-865090640004 - C13
https://www.mouser.com/ProductDetail/710-865090640002 - C15, C23
https://www.mouser.com/ProductDetail/710-865090640001 - C41, C44, C48

238S Dolby S Calibration Procedure

Calibration without Dolby S is the same as on dbx version with dbx out. For Dolby calibration you will need a Dolby level calibration tape. I am still uncertain what signal level Dolby level corresponds to on this machine, both on the tape and on the line input/output jacks; please share any insights.

First, calibrate the unit with Dolby S out. Then plug Dolby S in and play Dolby level calibration tone from tape. Use DEC_ADJ trimmer to set voltage on TP1 to 388 mV, then use DEC_OUT trimmer to set the voltage on the line output to Dolby level.

Next, feed 400Hz sine at Dolby level into the line input and use ENC_ADJ trimmer to set the voltage on TP1 to be 388mV. Then record for 15-20 seconds, rewind, play and check the voltage on TP1. Your goal is to use ENC_OUT trimmer to get this voltage to 388 mV on playback. You will likely need to repeat this record/playback several times until you get as close as possible to 388 mV on playback. Don’t touch any other trimmers during this process.

While you should try to get the voltage on TP1 as close as possible to 388 mV, keep in mind that if it’s between 350 and 425 mV you’re within +/-1dB margin, which is excellent.

Repeat the process for each channel. For the lack of a better option, I use the standard 400 Hz Dolby level calibration tape that plays back at 800 Hz on the 238, and assume that the line Dolby level is +2.7 dB on the meters (-7.3 dBV, 432 mV). This should work fine for new recordings but may not be the correct level for existing tapes recorded on a factory-calibrated unit. I will update this procedure if/when I find out more about the levels Tascam used for Dolby calibration.

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.

Signals on a correctly working capstan board

Bottom line of grid is 0V