Pioneer SX-850 Common Faults and Fixes

Pioneer produced arguably some of the best stereo hifi receivers during the 1970s and you can still find useful examples available ‘preloved’ today. I remember seeing them at London Olympia’s Audio Fair in the early seventies and wishing I could afford them. After fifty plus years many need some attention to restore their former glory.

This post runs through several ways to fix common problems with the loss of audio or distorted crackles that affect these SX series receivers.

As with any mains powered equipment always switch off and remove power cables before opening or operating the unit with the case removed. Dangerous voltages are exposed and shock or serious injury can result if you ignore the warnings!

Pioneer’s SX 50 range of stereo receivers covered a wide selection of output power from the baby SX-450, with its modest 15w, to the huge SX-1250 at 160w per channel. Back in the 70s these watts were actually quoted in RMS rather than ‘total music power’ used in many of today’s marketing nonsense.

I was lucky to have an SX-850 in the workshop which still manages a respectable 85w into 4Ω. It weighs over 20kg testament to the solid engineering and size of the mains transformer needed to sustain that power level.

Faults and fixes

Paul, the owner, had reported the Pioneer SX-850 started crackling from the left channel after running for a while. Several faults were identified that all contributed to the poor audio quality. I’ll cover them in the order found and have listed below so you can jump to your area of interest.

1. Relay contact oxidation — causing intermittent crackly audio, most noticeable on the left but affected both channels
2. Input selector switch oxidation — causing distorted or crackly audio on FM, Aux and Phono inputs
3. Equaliser amplifier transistor failure — causing crackly audio on right channel via phono 1 & 2 inputs
4. Phono connector oxidation — causing intermittent audio on phono 1 & 2

Tools & equipment required

• Usual Philips screwdriver, Allen key for tuning knob removal
• Contact cleaner spray or isopropyl alcohol
• Contact burnisher
• Multimeter – to measure contact resistance (Ohm meter) and ideally voltage to check DC offset and bias
• Signal generator to check inputs or CD player / tape recorder / record deck (for phono 1&2)
• Ideally oscilloscope to check sine wave signal for distortion – or a good pair of ears via headphones!
• Capacitance tester – measures valve, ESR and V loss. See postscript.

Relay contact oxidation

So I began by powering up the SX-850 from cold and listening to FM radio via speakers and headphones until the fault occurred. I noticed from initial switch on both channels sounded rather harsh with distortion on vocal ,sibilance to my ears. Then after a few minutes the left channel audio crackled and cut out. The crackle seemed worse via speakers than headphone so I suspected the output relay contacts were too high resistance. I’ve had similar issues with the replay in Sony power amplifiers.

Pioneer SX-850 relay

Pioneer fitted a relay to disconnect audio output during power up and in the event of overload conditions to prevent damaging the loudspeakers. Unfortunately they selected relays without gold plated contacts and these tend to oxidise or deteriorate over time. By tapping the relay top with a screwdriver handle I found the left channel audio cut back in and out.

With the Pioneer powered off and using a separate 24v power supply to the relay coil, I measured the resistance of both sets of contacts — there are two per channel, wired in parallel, presumably to reduce resistance and add some resilience if one set fail. I found the left channel varied between 3-15Ω and the right 1-2Ω. They should typically measure a steady 0.1Ω on my digital multimeter.

Contact cleaning

I removed the two screws holding the relay to the chassis and carefully separated the clear plastic cover to access the contacts. I then soaked a thin card in contact cleaner, placing it between the contacts and applied 24v to energise the relay. By carefully moving the card to and fro with the contacts closed it’s possible to try and deoxidise the surfaces and reduce resistance. I measured the contacts again and although the resistance readings were lower, they still varied rather than show a steady resistance.

I remembered a pink paper card my father gave me for contact burnishing – he’d said it was made with jeweller’s rouge. I tried this, again with the relay energised, and noticed more black oxide marks were left on the card as had with the contact cleaner — see below. The meter confirmed, both channels now read a constant 0.1 ohms which allowing for meter lead resistance is reasonable.

Cleaning the relay contacts

If you can’t reduce the contact resistance by cleaning and burnishing find a replacement relay – there are several available from the US and Germany – but all differ from the original which looks to be an Omron copy.

With the relay back in place and the SX-850 powered back up, audio from both left and right channels sounded clearer with no drop out from either channel. Returning to the FM music source I noticed less distortion and both channels had equal volume. Sound levels were constant and not affected by tapping the relay casing. Leaving the SX-850 running for a few hours confirmed both channels working well via the pair of Q-Acoustics workshop monitor speakers.

Input selector switch oxidation

I listened carefully to several stations along the FM band from voice to music. I could still hear some sibilant distortion that could be due to the receiver or stereo decoder. I connected a CD player to the Aux input and selected a high quality audio CD source. This immediately sounded better than the internal FM radio so I increased the volume to listen in more detail at the bass frequencies. At moderate volume using the CD source I detected distortion especially from the left channel – most noticeable on the bass notes.

Switching between FM and Aux I noticed some audio cracking and suspected the push button selector switches were suffering the same problem as the relay – oxidised contacts. High contact resistance due to connector dirt and corrosion causes all kinds of poor audio quality and can be confused with amplifier component failure so well worth eliminating first.

Access to the switches requires a little effort as shown below. First, remove the front panel knobs – they simply pull off, except the tuning knob which needs a small hex Allen key. Pioneer originally supplied a key but likely its long since lost!

Next remove the top fixings to pull the front panel away from the chassis. This allows access to the fixings that secure each circuit assembly. The input selector switch assembly is underneath the flat amplifier board so remove three fixings holding the board to the sub panel. Free the wires behind the board to allow enough movement. Finally remove the two fixings that retain the switch assembly and carefully angle it to clear the other boards. You may need to unhook more wiring from the harness.

Access to input switch assembly

With the switch assembly free, place a tissue underneath and apply aerosol switch cleaner (non lubricating) to each selector switch in turn and operate the switch several times so the wipers displace oxide and tarnish contacts. There are two rows of slots in the top of each switch where cleaner can be applied. The tissue should collect any surplus cleaner. Use a multimeter to check the contact resistance of the last two pairs of contacts furthest from the switch front for each input. A reading of <1 ohm is good. Repeat the procedure with more cleaner if it still reads high.

Note each input has multiple pairs of contacts for left and right channels each with two pole (change over) along with contacts for the panel light indicators.

After thorough cleaning and reassembly the crackling and intermittent audio from the aux input and radio was fixed. AM also jumped into life and tuned into a various programmes. I hooked up an aging Farnell audio signal generator to the phono inputs as I did not have a record deck to hand. Phono inputs are more sensitive and only require a few millivolts of signal so can’t be driven from an external source like a CD or tape player. I could hear the signal generator’s sine wave along with considerable hum – a not uncommon problem when connecting equipment from a mains powered source. Unplugging the generator removed the hum so all switch assembly inputs seemed good, the hum was due to the generator.

Tape monitor

The SX-850 supports two tape recorders via the monitor toggle switches so audio is not routed via the push button switch assembly. I used a Denon mini hifi which has tape inputs and outputs to simulate a tape recorder. This allowed me to check both the Pioneer’s tape functions. It also allowed me to use and compare the Denon’s FM tuner which I’d routed to the tape play input and vice versa.

Comparing the FM stereo quality from the same programme driven via an aerial splitter with identical signals was helpful in assessing the sibilant distortion I’d heard in earlier tests. Listening via headphones, the FM from both the Denon, via tape monitor, and internally sounded very similar. There was no crackling or distortion when switching between tape 1 or tape 2.

Equaliser amplifier transistor failure

After the relay and selector switch deoxidation we noticed the phono input began to crackle as we clicked through the push button selection. Only the phono stages (1&2) produced the noise and just on the right channel. This was after I’d been running the Pioneer on a soak test all morning to my dismay!

Eager to find what had gone awry I opened the base to access the phono input’s equalisation amplifier as this had to be causing the crackling audio since it was not present on radio or tape input . A gentle knock along the equaliser PCB seemed to increase the noise and tapping one transistor a 2SC1313 produced the tell-tale crackle.

I tried reflowing the solder around the transistor in case of a dry joint but the crackle persisted indicating the device must be to blame. A quick check of its collector voltage measured -17v, rather than 1.6v for the other channel. This was causing -3v on the output side of a 3.3uF electrolytic capacitor which may have damaged the cap so I replaced the capacitor to check it was not causing fault.

Google will tell you Pioneer’s SX 50 range has a reputation for noisy transistors and this appeared to confirm the problem – see links below. I replaced the faulty C1313 with a BC547B which has very similar gain. If you do use an alternative transistor, check its pin out as this may differ.

This fixed the crackle. I checked the levels were equal using a signal generator source which again introduced a hum as described in the postscript below.

Equaliser board with faulty transistor

You may choose to replace all C1313 transistors as a precaution given one in the batch has failed. I don’t advise this as a rule without specialist tools as it can introduce more faults and problems due to frail PCB tracks that can be easily broken if too much heat is used.

However, if you are faced with a persistent intermittent fault where no specific component can be identified, even after trying freezer spray on suspect devices, then replacing each electrolytic capacitor and transistor may be the only way.

Phono connector oxidation

After another morning’s soak test, Paul returned with a deck to try the phono inputs with a real source. We plugged in the PL-400 and spun the Doobie Brothers — a classic from the period — but there was no sound from the left channel! I’d checked levels using a signal generator so the fault had to be connection related. Cleaning phono plug and sockets with wire wool removed debris and oxide and restored the audio to both channels.

Many hifi manufacturers in the 1970s chose not to use expensive gold plated connectors and Pioneer were no exception as you can see below. The rear panel is packed with nickel plated phono sockets that can oxidise and build up dust and dirt over time. This often affects the least used sockets and especially if the receiver has only ever been used as a radio. Note that gold plated sockets still suffer from dust and dirt built up so need to be kept clean.

SX-850 rear input / output connections

Poor conductivity resulting from the dust and oxide affects sound quality and in this case causing intermittent loss of audio from phono 1 & 2. Phono inputs are particularly vulnerable due to low signal strength and the low source impedance from moving magnet cartridges.

Phono input oxide build up

I noticed some high frequency distortion — the Doobie’s 12 string guitar playing a giveaway — so we increased tracking weight by 0.5g and sound quality improved.  Finally the Pioneer was performing as intended and sounding as good as it looks.

Fault and fix summary

Many faults in vintage audio equipment can be fixed by simply cleaning the dirt, grease and oxide that builds up on mechanical contacts. Applying the right cleaner and cleaning approach will ensure years of trouble free service so it’s worth the effort and time spent. Other faults, such as semiconductor failures, requires some basic knowledge and soldering skills but can be tackled with care.

1. Relay contact oxidation — Fault: intermittent crackly audio, affecting one or both channels. Fix: deoxidise relay contacts.
2. Input selector switch oxidation — Fault: distorted / noisy audio on AM /FM / Aux or phono inputs. Fix: deoxidise switch contacts.
3. Equaliser amplifier transistor failure — Fault: crackly / noisy audio on one channel via phono 1 & 2 inputs. Fix: replace C1313 transistor. Compare collector voltages between left and right channels to help identify failed devices. Also see list known suspect transistors in link below.
4. Phono connector oxidation — Fault: intermittent audio on L or R phono 1 or 2. Fix clean / polish phono sockets. ideally replace with gold plated sockets for phono stage inputs.

Useful links

• Service Manual – HiFi Engine – https://www.hifiengine.com/manual_library/pioneer/sx-850.shtml
• Top 10 faulty transistor suspects – https://audiokarma.org/forums/index.php?threads/top-ten-worst-transistors-noisy-failure-prone-whatever-and-replacements.731653/

Postscript

Mains powered signal generators like the Farnell ESG1 are not ideal for diagnosing phono preamplifier input stages due to low levels involved. Moving magnet cartridges generate up to 5mV and moving coil much lower levels. The ESG1’s signal, being mains powered, has considerable hum at low input levels. Use a battery powered audio signal generator for best results and this avoids earthing / earth loop issues that can occur with mains powered generators.

After a couple of weeks of working Paul said the right channel was cutting out again and he suspected his speaker was to blame. I lent Paul a multimeter to check the speaker and wiring resistance and suggested listening via headphones to check if it was the SX-850 and swapping leads to try and identify what was at fault. The speakers measured okay at around 6 ohms and so he returned with the SX-850. I gave the receiver a full bench test using a pair Q-Acoustics 2200is. All sounded fine with no fading crackle or cutout. I checked the relay contact resistance to see if it had increased again but it measured <0.1Ω.

As I had a new Omron 24v relay in stock I suggested changing the relay in case the load from his Pioneer speakers was causing issues. After replacing the relay I discovered one pair of contacts on the original relay were discoloured consistent with overheating or oxidation.

We had noticed there were some dark marks on the inside of the wooden case above the power amplifier section. A small resistor looked to have over heated in the past but measured correctly. One of the driver transistors had been replaced indicating the receiver had been repaired previously.

I double checked both power amplifier’s DC voltage levels and idle current were in spec which they both were.

After a further morning’s soak test, the SX-850 was returned and all was reported to be working well. Then after another few weeks the right channel started to cut out again and a headphone test appeared to confirm the Pioneer speakers were to blame!