If you have a Tandberg Series 15 reel to reel tape recorder that no longer plays it may be simple to repair so don’t throw it away. This post shows you what to check and how to fix your Tandberg that’s suffering from age related faults. It also uncovers a potential safety issue lurking inside that owners should be aware of.
Warning: Always unplug your mains powered tape recorder before removing the cabinet and never operate it with the cabinet removed. Dangerous voltages are exposed inside.
I’ve had a fascination with reel-to-reel tape recorders ever since a young boy. I guess growing up watching the sixties TV series Thunderbirds, which featured a miniature tape recorder – see Thunderbirds Taperecorder – along with Mission Impossible’s self destructive player started the interest.
Later at senior school reel-to-reel tape recorders were used in language lessons to hear real French and Spanish accents in the hope they would rub off. I still remember the sounds but more the timbre of the voices and how in particular a Tandberg produced a rich and vibrant sound.
The Tandberg was also proficient in music playback as I remember from one assembly where I was given the task of tape recorder operator. My one job was to queue the playing of George Harrison’s My Sweet Lord as the teacher signalled from the stage. I fumbled a little but soon the song played and I managed to stop it before the next track started.
So after 55 years, finding a Tandberg series 15 in need of some TLC, nostalgia took over and I have a new job to do and this post to write.
Warning – Never leave your mains powered vintage technology running unattended unless it’s been inspected and conforms to modern safety standards.
The faults and fixes
This particular series 15 arrived, apparently, just requiring a new set of belts – it didn’t of course. All four rubber feet were missing in what looked like a bungled attempt to gain access to the interior – the cabinet base is glued in and cannot be removed. Oddly, the red plastic packing spacers were still in place between the case and deck.
Removing the four black recessed screw fixings from the top deck, I found the speaker’s tag board had become separated from the loud speaker. This made disconnection tricky without damaging the fragile voice coil wiring. It’s best to carefully turn the deck upside down after removing the top fixings, ensuring you hold the cabinet and chassis in place, to lift the lighter cabinet up to disconnect the speaker leads. Three hands help!
It’s a pity the Tandberg service manual fails to describe how to access the internal chassis, but at least the manual is available on line – see useful links below.
With the wooden cabinet clear you can access the motor and the smaller tape counter belt on the left hand side. I reattached the speaker tag board either epoxy and left it to cure overnight.
Tape counter belt replacement
Replacing the old stretched tape counter belt is simple as it’s accessible with the outer case removed. Use a hook of wire to pull the belt under the transformer and around the counter pulley. I used a substitute 1.5mm square belt as I didn’t have a 80mm 3mm round v-belt to hand. The tape counter requires very little torque so the thinner belt works well for now, although I will try to source a correct size belt at some point.
Notice the exposed wiring between the motor and mains transformer which carry mains voltages so ensure the mains plug is removed before attempting any work inside.
Main belt replacement / restring
The motor drives the flywheel and tape capstan via an idler wheel. So when I first tested the Series 15 the tape moved forward in play even though the take up reel didn’t move. Neither rewind or fast forward worked so the main belt driving the turntables was indeed faulty.
Access to the main belt is not so simple. Again, the service manual offers no help although it is full of exploded diagrams. I recommend making sure you have s small pot or lid to collect all the parts in and take before and after photos to help recall where all the bits go, unless you have photographic memory!
Here’s what I did to remove the top plate and gain access to the main belt.
- Remove the 4 chrome cross head fixings holding the top plate to the chassis.
- Remove the upper and lower tape covers using the chrome thumb nuts. If tight, try placing cloth over the nut and use a close fitting box spanner. The cloth must be thick enough to prevent scratching the surface but provide enough friction to turn the thumb wheel.
- Remove the two brackets that support the lower head cover.
- Remove the left tape stud – note the black plastic mount has an index lug so don’t turn it.
- Remove all the chrome control knobs – these all pull off, there are no grub screws although they may be tight.
- Remove the two plastic tone control knobs. Again these pull off but as they are small can be difficult to grip. I gently levered from below using two thin cards with a screw driver blade to prevent damage. Take care not to apply too much pressure and break the plastic. A pull cord around the spindle can also be used.
- Remove the Stop lever by releasing the spring that holds the top in place.
Carefully lift away the top plate making sure you don’t catch and bend the thin tape sense lever on the right above the volume control.
I found the main belt had become detached from the take up turntable but was still in place, as you can see above. Unfortunately, the service manual offers little help in showing how the belt is threaded. It does describe in words but why they omitted a picture as in previous series manual is a mystery.
I checked the belt expecting to find it had stretched like the tape counter’s. But it was fine with no evidence of deterioration. I rethreaded it and all worked well. The motor pulley has very little contact area with the belt so if it was too slack the turntables won’t spin. Maybe the previous owner had tried to fit a new belt at some point and given up?
If you do need to replace the main belt it’s 3mm round with a 200mm diameter or 300mm folded. Belt kits are also still available see useful links below.
Suppressor capacitor and smoke – Danger lurks here
After replacing and reattaching the belts and dropping the chassis back into its cabinet I applied power and switched on. After a couple of seconds the supply and take up reel turntables started spinning slowly although I had no tape loaded and the control lever was set to neutral! Then I smelt burning and smoke appeared from the rear vent.
Unplugging quickly, I inverted the beast and looked inside again. Using an IR camera to look for hot spots I discovered the Rifa suppressor capacitor, wired across the tape detector microswitch was hot and had cracked. The RC suppressor, shown below, has a 0.1µF capacitor in series with a 100Ω resistor which had overheated. Suppressor capacitors of this vintage are prone to fail – I’ve seen several bulged examples at the local Repair Cafe, so should have checked this earlier.
A multimeter across the suppressor proved the capacitor had failed short circuit – it measured 137Ω – and so the motor was being powered via the internal resistor now in series with the motor – this is why it ran slowly despite no tape being present!
Obviously this overpowered the resistor inside, that was designed only to absorb the small charge from the capacitor, and so its casing overheated and cracked.
Clearly the suppressor should be replaced on a machine of this age and must be replaced with a modern X2 safety rated type. The X safety rating ensures the capacitor can absorb high voltage spikes generated by inductive loads such as the motor.
Warning: If at all unsure have your Tandberg serviced by a local professional engineer to ensure it is safe to use. While the information provided here is correct to the best of my knowledge I’m no expert in the latest safety standards which also depend on country and region.
The dangers
X safety rated capacitors are designed to fail short circuit and require a fuse to ensure high current’s don’t cause overheating and fire. However, looking at the circuit diagram from the service manual, Tandberg decided to fit the suppressor (C3 – R10 as shown below) across the microswitch rather than across the AC motor. So a capacitor short circuit won’t ever blow a mains fuse as the motor is in series and forms the normal load. Instead the suppressor’s resistor attempts to power the motor.
Note the circuit diagram from the service manual shows a 47Ω resistor for R10 rather than the 100Ω fitted in my Tandberg. Rifa and others make a range of suppressors with various resistance values.
As resistors used in RC suppressors use a low wattage carbon type, they generally burn out quickly when over powered and act like a fuse. Being encapsulated in plastic, as the Rifa suppressor is, the whole casing expanded with the heat and cracked. So I wonder if the plastic casing could have ignited if powered for longer and that is cause for concern.
Tandberg did fit a safety fuse, but it’s on the mains transformer’s secondary winding, so won’t protect a suppressor fault. The only fuse protecting the suppressor or motor failure is in the mains plug for the UK. In Europe and other countries that don’t have plug fuses the overcurrent protection relies on a circuit breaker in the fuse box and that’s usually a much higher rating. This is a potentially dangerous design and worth bearing in mind so always remember to power down your vintage tech when not in use and don’t leave it running unattended.
Safer options?
One option Tandberg could have chosen is to use a Y class safety capacitor that’s designed to fail open circuit rather than short circuit.
As Y class safety capacitors are limited in value to 0.047uF two would need placing in parallel along with a 47Ω resistor to replace the Rifa suppressor without the fire risk as it doesn’t rely on a fuse. The only disadvantage of using a Y class capacitor is that if it fails you have no way of knowing until the microswitch contacts burn out due to arcing. Placing two capacitors in parallel reduces the chance so at least there’s some suppression if one fails.
However, this does not remove the concern of a missing mains voltage side fuse. A short circuit fault in the motor’s windings would rely solely on the plug’s mains fuse – or the circuit breaker in European homes. And, as I checked with this machine, it had been fitted with a default 13A fuse! That’s suitable for a 3kW kettle but totally useless in a tape recorder that’s rated at 50 watts maximum. I replaced it with a 1A fuse to reduce the available power to 250 watts, still not ideal but better than 3kW.
As mentioned above if you have any doubt have your machine serviced by a professional — it’s a small price to pay for safety. Remember design and safety standards have evolved since the 1960s and 1970s when much the vintage technology we see today was manufactured.
Deoxidation – The usual suspects
Before reassembly I applied switch cleaner to the usual suspects – the rotary potentiometers and switches. I’d noticed a rather odd looking switch mounted on the preamplifier board which used printed copper wiring. The schematic shows this provides the multiple contacts used to flip between playback and record. I cleaned the copper tracks with a small stencil brush soaked in IPA. (Not the beer!) It still appeared a little tarnished but removed the green corrosion around the contacts.
Listening tests
With the belt and suppressor faults addressed I reassembled the top deck plate, connected the internal speaker, tag now solidly in place, and rested the chassis back into its cabinet ready for testing. I had several 7″ BASF tapes with stereo recordings made from my old Akai reel-to-reel — see Akai 4000D Repairs
So my first attempt at playback sounded rather strange. I’d thought the Tandberg’s 2 track mono head would simply play back my stereo recording in mono. What I actually heard was a mishmash of one track from one side of the tape with another track in reverse. Oh, how much fun these reel-to-reel players are!
I reached for a 5″ reel from the Uher 4000 Report S Portable Tape Recorder that I knew had a mono recording on. This played back normally and even sounded rather good. It was a Shirley Horn track from the excellent album The Main Ingredient. I find jazz music useful as a test source for tape recorders to reveal any speed variation and tonal distortion.
I plugged a Bang & Olufsen CX100 into the rear DIN socket to compare sound quality. Immediately the bass became clearer and top hat treble more refined than the single oval speaker fitted internally. It’s hardly surprising of course, but still reminded me of rotund sounding language lessons from the similar school Tandberg.
Fed from a modern CD source, the series 15’s recording quality lived up to expectations. Yes, tape noise was audible at higher volume albeit masked by the top hat sounds from Shirley’s percussionist. Keep in mind there’s no Dolby noise reduction or compression artefacts so does benefit from low noise magnetic tape when you can find it. Will try with my Maxell UD tape when I find it.
What makes this Tandberg interesting to listen to is its valve like audio quality. The power amplifier uses a matched pair of AUY51 germanium transistors fed from a smaller pair. So while it is a transistorised amplifier, it sounds more rounded than modern silicon bipolar equivalents. I tried hooking up a Tek 1052 to examine the harmonics using its FFT function. These were all well below the noise floor at -50dB
At just 10 watts the output power is sufficient to drive the internal speaker to fill a large room. And with a a good external 4 ohm speaker both bass and treble frequencies are clear and audible. Well worth rescuing if you can and to indulge in that analogue audio experience.
Tandberg series 15 fault and fix summary
- Perished tape counter belt – replace with new 80mm diameter belt accessed by removing cabinet
- No drive to supply or take up reels – rethread or replace main belt accessed by removing top plate.
- Motor drives reels with no tape loaded – replace faulty RC suppressor with safety rated alternative.
- Noisy switches / rotary controls – apply switch cleaner / deoxidise contacts
Useful links
Some links to items mentioned above.
- Hifi Engine – Tandberg Series 14/15 Service Manual and Instructions
- Tandberg Series 15 belt kit – https://www.oddbits.co.uk/product-page/tandberg-series-15-belt-set
- Safety capacitor types – https://www.allaboutcircuits.com/technical-articles/safety-capacitor-class-x-and-class-y-capacitors/
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