Marantz SD35 Cassette Deck
Servicing and Alignment

Received this 1989 Marantz SD35 cassette deck off a relative - it had been in his loft for a number of years, and initially we didn't know if it was working. It was thankfully, and so now I thought it was time to check this thing out in more detail.
 

After the usual dusting or wiping off of debris, I opened the SD35 up and found that all was good internally. There was naturally some dust and corrosion present as expected, but was only minor.
 

I needed to investigate whether the drive belt needed replacing. The cassette tape transport and motor mechanism are cheaply made, and a bit delicate, so I had to handle with care. After extracting the mechanism from the main unit, I realised that to replace the belt meant more careful disassembly. I checked the elasticity of the drive belt, and everything seemed to be fine - so no need to fix that issue!
 

Cassette transport and motor - top right.

On reassembly, the machine worked but then a little later decided not to work!? The next day, I opened up the deck and carefully disassembled, and reassembled again the cassette transport unit. All was working back to normal!

Later, all external and internal potentiometers were switch-cleaned with Servisol, and then the heads and capstan were demagnetised.
 

Next on the to-do list was the record/replay head alignment - using an ABEX 10Khz reference tape I aligned the head azimuth to the tape. The original setting was out by a fair margin, ~ 180° at 10Khz.
 

So now I could use my ant-audio.co.uk 400Hz full track width Dolby Level reference tape to calibrate the output at Dolby Level, making sure both left and light channels were equal - I eventually settled for an output of slightly under 500mV (RMS) at Dolby Level.
 

At this point the peak reading LED meters were only a fraction out, but I re-calibrated these regardless. Unfortunately, in order to set the peak-level meters, I had to take the LED display and control board off the chassis and adjust in situ - thankfully it was not difficult.

I later calibrated the internal record levels to match that with the sensitivity of the current batch of Maxell UR C90 tapes. That is: record at 315/333/400Hz at 0VU or Dolby Level (Dolby Level = +2.7dB above 0VU), and the tape now plays back at calibrated level.

The deck is now playing well - very stable sound, very pleased.

Back in 1989, this retailed for around £149.99 in the UK.


Article subject to alterations and corrections without notice. 21/02/2020

Sansui SC-1330

Cassette Deck

Maintenance/Servicing

Bought off ebay for little money, this was advertised as ´For Spares or Repair´. It was in good physical condition, and the heads had very little wear. The pinch roller looked aged, but after some cleaning with isopropyl alcohol, and later filtered water on a cotton bud, it is nearly as good as new!

So far I have replaced the old drive belt - a 72mm or less diameter is fine, 3.5mm width, although I have just ordered a 64mm diameter 6mm width belt, let us see how that works later when it arrives!?  

12/2/2020: The 64mm diameter, 0.7mm thickness, and 6mm wide belt was more difficult to fit. Overall, the tension was too high, due mainly to the thickness (0.7mm) of the belt. Also, this belt wouldn't always run well - not sure why!? So I reverted back to the 72mm belt.

I think a 68mm-70mm diameter, 3.5mm-5mm width belt would be optimal?

Replaced most electrolytic capacitors, and cleaned all potentiometers with Servisol. 

Rubber Renew

Bought 125ml of this toxic chemical, and so decided to use on my machines. It smells like rubber solution, only stronger! I decided to buy this stuff hoping that it would return the power and hence internal 'grip' for the tape rewind, fast forward, and the take-up spool during recording. It did the job, but I had already cleaned much of the idler gears with isopropyl alcohol, and diluted acetone anyway - I'm not sure if Rubber Renew is worth the money?

If you use Rubber Renew, be sure to follow the safety instructions.

Noisy ON/OFF switch 
The Sansui SC-1330 makes a small electrical arc when switching on or off. This interference can be clearly heard through both headphones and the loudspeakers. On investigate I saw that the internal mains switch does not have any form of arcing/noise suppression.

After studying the circuit, I decided to bridge the switching contacts with high voltage rating, ceramic capacitors - marked below in blue.

The diagram above was taken from the service manual. I also noted that only Japanese versions of the SC-1300 series had suppression support!?

I carefully soldered in some 3Kv (not 3.3Kv as suggested in diagram) rated capacitors - yes these are probably over-rated, but best to be safe than sorry?! These completely eliminated any sound blips due to on-off switch arcing issues. I didn't bother to add a small resistance in series with the capacitors - these limit the transient currents which only last for some micro-seconds anyway. This type of suppression circuit is know as an RC Snubber Circuit.

My alteration to the mains board is shown below.

Sansui SC-1330 Calibration
I have also calibrated the machine so that -

• Test Tape Input: 400Hz Dolby Level RefTape, gives Vout = 500mV (RMS)  or ∼1400mV p2p.
• Calibrated the peak level meters to read Dolby Level at RefTape Dolby Level input.
• Calibrated the internal Record Levels (for Maxell UR), this means .. record at 0VU, and achieve 0VU on playback etc.
• Bias adjustment: 1000Hz to 10,000Hz flat for Maxell UR tape. 
• Rec/Replay Head azimuth check with my ABEX 10Khz ref tape - the original setting was spot on!!!

21/02/2020

29/04/2020: I decided to give the shorter and stronger 64mm x 6mm belt another go. This time I had cleaned the pully wheel carefully, "roughing up" the surface for a better grip. I also allowed the flywheel to guide the line of the belt around the pulley - this was done by losening the pulley screws and turning the flywheel with my finger so that the belt would find a natural line, the pulley would then slide outward a little more. Finished this alignment by tightening the pulley grub screws.

Results: Both fast-forward (FF), and rewind (RW) are much better than before - no stalling!

So far, so good!
 

Common Emitter, Voltage Shunt Feedback