Here’s a Fostex X-15 4-track cassette deck in for repair – owner had tried swapping some parts over from a donor machine to fix an issue with the head block plastic breaking (known issue with the X-15 portastudio), and got in a bit of a mess, no playback etc.
First thing noticed, the four record selector switch fingers didn’t look right:
The switch ends were not in their respective pegs, meaning all 4 channels currently in record mode (not good for a tape you don’t want to overwrite!)
However noticed that even after correcting this, playback still wasn’t working, and the REC LED not lighting up when in record mode.
Found Q105 was dead (probably a result of the overcurrent experienced when all 4 channels try to record – this transistor is responsible for the bias current etc). This meant that recording not functioning (no REC red LED was also symptomatic).
This was a 2SC1878 often used for muting functions, so replaced from stock:
Could have left it there, but asked the owner if they wanted a service and alignment done as they’d disturbed heads etc), which was agreed.
This was quite timely as it turned out the speed was quite a bit slow by about 1-2% and there was an issue with channel 3, EQ was over volume at higher frequencies, chased tail a bit and even swapped out the LA2730 dolby IC as the fault seemed to be localised to that. Can see the recording switches located correct now too:
The culprit, in the end, was a badly crimped leg on a 50K preset pot responsible for EQ adjustment – quite hard to find as the usual resistance checks didn’t particularly show up the fault in circuit, only once removed it was found.
A gentle squeeze with some pliers restored the connectivity to the carbon track.
Stripped, cleaned and lubricated the mechanics (grease had turned very sticky and sluggish):
and was able to complete the service and alignment adjustments to all 4 tracks. Record and playback perfectly!
In summary:
Electronic repairs to burnt out transistor and faulty potentiometer
Cleaned + demagnetised heads, capstan and pinch roller
Checked and adjusted Record sliders after correcting the assembly
Head azimuth adjusted
Tape speed adjusted
W&F measured at 0.09% RMS
Playback alignment (Dolby level and EQ) with reference level, azimuth and frequency response test tapes
Meter calibration checked (ok)
Erase current adjusted to 32mV
Bias current adjusted to 50mV initially
Bias trap adjusted to minimal leakage (record amp)
Bias trap adjusted to minimal leakage (playback amp)
Record alignment (Level adjustment and bias currents tweaked for frequency response)
Signal to noise measurements checked
THD measured at 0.26% average