Sony TC-138SD Cassette Deck (1975) Restoration/Servicing
half promised myself not to buy another cassette deck, and although I
do tend to 'window shop' on ebay, many machines I view, I eventually
pass without a second thought. Then, sometime ago I saw the TC-138SD,
which intrigued me since it is similar to the TC-136SD, but appeared to
be of a much better build? According to the 1976 HiFi Year Book which I have owned since about 1977, these machines weigh about 7Kg, so it was a lot heavier than the TC-136SD. It also has a syncronized ac induction motor, unlike the 138SD.
There was one 138SD that caught my attention which got perpetually re-advertised and was finally down in price to £25 + £15 postage. I saw that the seller lived only 5Km from me, and so I contacted him and asked if he would sell it for £25 + £5 petrol money for delivering it to my door. And sure enough a few days later - I had another cassette deck!
Originally sold for - spares or repair, untested.
|The cassette door had broken, but luckily, |
the missing piece was inside the deck!
Within 20 minutes I had the machine basically working. I very first thing I did was to switch-clean (Servisol Super 10) all contacts and switches, paying special attention to the multi-functional switch that facilitates playback, and record modes. This is important because many adverts show their machines with one or more of the VU meters oscillating irrespective of mode. Over the years dirt, moisture, and low level oxidation - hence corrosion takes place and will 'bridge' circuit paths.
After lengthy and detailed service work on the Sony TC-138SD, I now have a fully working machine. I've listed to date, the work done on the TC-138SD -
- Deep clean of all switches and contacts using Servisol Super 10
(not WD 40 or equivalent!)
- Clean all internal areas and mechanisms.
- Lubricate the capstan
- Clean the heads, demagnetise capstan and the heads.
- Clean the pinch roller with diluted acetone, and later Rubber Renue.
- Fully re-cap all audio board electrolytic capacitors
- Replaced original motor run capacitor: now 1.5uF 400v rating.
- Fully replace all audio board transistors with suitable equivalents,
mainly C1815-GR, and C1845FTA NPN transistors.
- Add a bias (105Khz 'carrier' amplitude) circuit modification -
allows the user to alter bias continuously by a larger margin than originally.
- Add a bias trap circuit modification for variable adjustment -
better or worse rejection? Testing to follow at a later date.
Initially, the deck was in good condition, just a little dusty and dirty.
Playback Equilisation Adjustment
The EQ playback control (playback frequency curve, called de-emphasis)
is integrated into the feedback loop of the pre-amplifier circuit. I
strongly suspected the playback EQ response did not conform to the later IEC standard adopted and modified in the early 1980s - the IEC Prague 1981 standard? Some tape enthusiasts call this 1981 standard 'IEC 2', so I'll refer to it as IEC 2.
The playback EQ for Normal and Ferro Chrome/Chromium Dioxide tapes together with playback level (PB Level) conrol are accessed at the front of the deck.
|To give more flexibility and higher HF boost, |
I later exchanged the 2kΩ potentiometer for a 10kΩ,
although 5kΩ would have been sufficient.
This was always going to be an issue - matching the playback frequency response of the TC-138SD with an ideal IEC Prague 1981 'IEC 2' calibration playback tape.
Ideally, on playback from an IEC Prague 1981 tape, we should observe a flat response somewhere around 333Hz to 10Khz.
I am suggesting that pre-1981 machines (apart from Nakamichi?) generally adhere to an older pre-emphasis/de-emphasis standard, which at this moment, I am unfamiliar with. Is it a revised IEC/RIAA curve for 4.75cm/s? - I don't know. It seems that the general consensus within Tapehead forums is that manufacturers broadly abided to the old (pre-1981) IEC standard, but tweaked responses to match the ability of their record/playback heads having to make a balance between frequency response, noise, and distortion.
The Sony machines that I have seem to opt for more aggressive HF cut in their de-emphasis?, the Nakamichi - less so?
My solution, although not ideal was set some kind of compromise. The TC-138SD will be used as a recording machine, but mainly as a playback machine.
A Nakamichi IEC 2 tape?
Some time ago I made a Type I (TDK D C-46) frequency sweep tape at about -10dB (ref: dolby level) on my Nakamichi DR10. I used Audacity software to create a series of 100s of sweeps between 333Hz to 10,000Hz. This was transfered to an iPod and then used as a signal source for the Nakamichi DR10. The recording/playback response of the DR10 was flat from 333Hz to 10,000Hz. This flatness was also confirmed on my Revox B215, ±1dB at worst, and within ±1.5dB on a 1989 SD-35 Marantz deck.
Playback on the TC-138SD
Playing back the NAK DR10 (a now 'IEC 2' equivalent?) tape on the TC-138SD allowed me to observe the playback response of the old Sony. I could easily set the PB so that 333Hz and 10Khz were equal in amplitude, but at a cost - a large mid-band rise in output of about 3dB to 4dB somewhere between 1Khz and 5Khz.
After much experiment and thought, I decided on a playback drop of -3.5dB to -4dB at 10Khz from the NAK DR10 ('IEC 2' equivalent?) self-made test tape. This significantly reduced the mid-band 'hump' to +1.2dB/+2.6dB. I could have reduced the R-ch 'hump' to 1.2dB but this caused a further drop in response at 10Khz. As can be seen, there is a lack of symmetry here.
Bias settings were tweaked to give the TC-138SD a record/playback difference at 333Hz/10Khz of about -1dB, that is 10Khz is down by -1dB with reference to 333Hz. This of course gave a rise of about +3dB at 10Khz if played back on the 'IEC 2' spec Nak DR10, or Revox B215.
I may at a later date modify the record EQ to dampen any further mid-upper band rise in frequency before dropping to -1dB at 10Khz as stated above.
Since the TC-138SD has still, but now a small rise in output between 1Khz and 5Khz, I deliberately reduced the internal record level calibration back by about -2.5dB at 333Hz for post 1990 TDK D60 tapes. This ensured the meters on playback won't jump into the red zone too often.
Recordings made with the TC-138SD genrally sound good, balanced, and match up well on the meters of other old pre-1981 Sony, and post 1981 machines. It's not ideal, but it works and the mismatch isn't obvious.
High Frequency Saturation
If I do have one criticism at this moment, it is that recorded high frequency distortion or saturation can be heard on some tapes when listening back to speech sibilances, ie the high frequency transients in speech. Even my TC-134SD doesn't exhibit such record/playback issues, and yet uses the same record/replay head!? There is no HF distortion/saturation issues for the 138SD on playback from another 'better' machine. I need to look into this matter.
Recording Bias, and Bias Trap Control
Once again, there is going to be some discrepancy between the ideal IEC-Prague 1981 'IEC 2' recording EQ (pre-emphasis) and the recording EQ ('IEC 1'?) curve adopted by Sony back in the 1970s. I do have some control over this, but for now - let's see where I can go without modifying the recording pre-emphasis.
I wanted better control over the bias - effectively, this is a high frequency 105Khz bias signal 'carrier'.
In brief - controlling the bias, controls the amplitude of the 105Khz bias signal.
Bias adjustment also influences HF recording levels, output levels, noise, and distortion. The trick is - to get the balance about right.
The audio then 'sits' on this bias 'carrier' as both signals are used to produce a varying magnetic field at the head face gap.
I removed the original 8pF trimmer, and desoldered the capacitance bridges from the capacitance bank. A new 10~60pF trimmer (with high voltage rating) was inserted into the same place and no capacitor bridge was re-soldered.
I am now able to modify the 105Khz bias amplitude at the head from about 15v (or less?) to about 46v peak.
The purpose of the 'trap' is to block the bias high frequency signal, and more importantly - the threatening high voltage that can leak back into the audio section. The trap comprises of a simple bandstop resonance circuit, which at resonance (about 105Khz for the 138SD) acts as a very high impedance to the bias signal, but not the audio.
In theory the rejection at resonance is infinite, but in reality due to inductor resistances (typically 10Ω - 30Ω?) and capacitance leakages/losses, the rejection is finite.
Oscilloscope traces show that on the record head side of the bias trap (bandstop) LC filter, the 105Khz voltage is about 35v peak, and on the other side of the LC filter - less than 0.5v/0.7v (L/R) peak. I would have liked to have seen a better rejection - I neglected to compare the original rejection capabilities.
As a comparison, I later measured bias frequency amplitudes either side of the bandstop filter on a Sony TC-K45 deck - they were 15v and 0.36v.
So comparing this TC-138SD's 35v:0.6v (average), to the TC-K45's 15v:0.36v, it can be seen that the two ratios compare well, that is - rejection is similar.
The intention of adding a bias trap trimming capacitor was to tweak for best resonance and so best rejection. Oscilloscope signal observation allowed me to minimise this bias leakage.
Bias Trap Modification: I eventually replaced the default 91pF for two high voltage rated capacitors in parallel. They are a 68pF 1KV rated, and a 10~60pF 250v rated trimmer. Before soldering into circuit I measured both so that in the parallel configuration they became ~ 90pF. Turning the trimmer allowed me to observe minimum leakage voltage fed back to the audio section.
Bias Trap - later revisions: Later, I may experiment by inserting a lower variable inductance and a higher value capacitor. Better bias signal rejection may be obtained from lower resistance in the inductor choke coil?
|Access to -|
Meter Calibration (out of shot at rear),
Record Levels (as indicated),
Bias, and Bias Trap access is easier than earlier Sony models.
|Small 10pF .. 60pF (high voltage rated) |
trimmer capacitors were adapted to ...
(i) add variable bias control,
(ii) add variable, and better bias trap rejection?
The Working Sony TC-138SD
The cassette lid will be repaired and fitted later.
This blog/article is still subject to corrections or minor modifications if the need arises.