Pioneer Amplifier Repairs
Pioneer Spec-4 Stereo Power Amplifier
This one had been stored in a damp garage and although it powered up, it didn't instill any confidence. Measuring the supply to the two channels one was +/-52V the other was +/-60V as they should have been around +/-71V there was something amiss, twas the relay contacts which were badly burned. This used two relays for the two transformers other variations used one. Cleaning the contacts sorted that, then the protection started kicking in. Eventually had to replace 2SC948 transistors in the protection circuit, two on the board behind the front panel and one on each of the power amp boards. Then... the LHS meter would jump to the right, needed more transistors replacing in the meter drive circuit. Then to top it all the temperature sense diode wire went open circuit, due to corrosion, taking out the power transistors, (hadn't been a bad week till then), 2SB554\2SD424 - replaced them and sounded ok.
This would drop the speakers out occasionally, the over-current transistors or one of them was faulty 2SC869, worked ok when replaced. Looking at the regulator PCB, a few capacitors looked in distress so I replaced them, the transistors on the small heatsink U-bracket were also loose, needed many dry joints going over...
This one had a weak channel, I thought it may be the relay at first but it turned out to be the tone board. Dry joints on the bass\treble controls and the defeat switch was a bit iffy.
This wouldn't operate the speaker relay as there was -45V on the right hand output. There were dry joints on the transistor at the RHS edge of the PCB, then there was either 1A or no quiescent in the LHS channel. This was caused by a faulty TO220 NPN driver transistor, I replaced all four just in case! Additional work included going over millions of dry joints on the power and PSU boards, it sounded okay afterwards.
The pre amp would produce sound intermittently so needed the relays replacing as they do all the switching for the various functions. The power amp had blown all the 4 output transistors in the right channel along with the associated resistors; 4.7, 47, 100ohm I also replaced the TO220 and TO92 drivers 2SC2705 etc sounded okay after.
Replaced +\- 52V regs with Zener regulators, so it may not be the most elegant but it works and does away with the need for the 2SK34 FET's when they become unavailable.
This needed the main capacitors replacing as one of them had dumped liquid into the chassis. Also many small 10µF 50V capacitors needed replacing (as the negative regulated voltages were low) on the PSU board along with several dry joints caused by hot regulator transistors. The switches needed dosing with WD40 and the Fuji transistors also needed replacing as they were giving varying DC voltages at the speaker outputs.
Phono noisy reportedly, protection kicking in randomly. 2SC869 in protection circuit was sending a spike to the rest of the circuit when it thought no one was looking.
The -19V regulator was short circuit in the PSU for the phono EQ, took out the capacitor on the EQ board and one of the 2SC1313 was noisy after having 50V shot at it.
The bass pot was making a whooshing noise as it was adjusted, there was DC on the track due to there being two bad 2.2µF 25V tantalum's. The speaker relay contacts also needed cleaning.
The volume pot was noisy and the speaker relay had been damaged having broken terminals. The contacts were black & tarnished as well.
No output from this large and heavy 1977 stereo receiver. The PSU needed several repairs and the 2SD313 13 V regulator for the tuner was o\c. The 100µF caps were replaced in various places on the PSU board. The left channel coupling cap on the tuner board was corroded and replaced, several lamps were also replaced. The tone switches needed soaking in cleaner.
The speaker relay was not operating from this late seventies amplifier and 20 V DC was measured on the right hand channel. The 2SC1451 was unserviceable and replaced with a BF391.
The selector switch was loose, the shaft was worn where its connected to the 'click' plate and noise was crackling through the RHS channel. I replaced the 2SA726 I\P's with 2SA970's and the 2SC1451's with C2910's. The selector switch shaft was drilled, pined and glued. I replaced the 47µF 16V capacitors across the Zener's as they were open circuit and replaced the other small capacitors on the PSU board as they looked a bit ill - 47µF 35\50 V.
Worked okay in direct mode, distorted when switched through the tone circuit. Bad contacts on the balance and tone pots caused the problem and the quiescent settings needed correcting.
The volume control was noisy and RHS channel was low, the speakers switched on after only ½ second. I replaced a capacitor in the protection circuit, looked as if the volume pot was faulty initially but further checking turned out to be a couple of capacitors on the tone board. The regulated +\- 48V needed setting to the preamp and lots of dry joints everywhere.
The 100ohm R241 was open circuit and looked to have corroded, in the base drivers, symptoms were no quiescent and distorted sound.
Cracks and pops from both channels, mainly the right, coming from the power amps, replaced the 2 I\P transistors both channels, turned out not to be those! Re-soldering dry joints eventually got rid of the noise.
Lamps replaced 6.5 V, there was slight distortion which turned out to be dry joints on the power amp board, also dry joints on protection board & replaced the two 2SC869's with BC182l in the protection circuit as it switched off intermittently. Adjusted the quiescent to 30 ma, the lamps seemed quite bright with 7.2 V across them so wired a 0.47 ohm resistor in series.
This had stopped working after getting rather hot. Found several components around the regulator transistors 'loose'. The solder had crystallized; re-soldered and removed regulators 2SD313 and 2SB507 then refitted with heat sink compound, worked okay after. Odd amplifier but sounds excellent.
This needed a repair as again the speaker relay wasn't energised. One of the power amp supply rails had disappeared, the -48V, the reg transistor was open circuit, I used an MJE350 the original was a weedy TO92.
80 Watt power amplifier from 1975 needing repair. This beast had no o\p due to one channel holding the speaker relay off, the o\p transistors, drivers and voltage amp required replacing along with the positive regulated supply transistor on the PSU board. Fitting decent heat sinks on the PSU transistors will ensure it will work for another 30 years.
This was not energizing the relay for the speakers. All the voltages looked okay on the protection board but there was a volt on the collector of Q5 2SA572. This turned out to be leaky or something, replaced it with a BC212, quiescent needed setting as each channel was passing over 150 mA.
This large 80 W digital receiver from 1980 was looking dead - the heat sink was at gas Mk6! Re-soldering the +\- regulators on the PSU board brought it back to life and setting the quiescent current from 200 mA to 150 mA stopped the overheating. This was the transition model from the 70's analogues to the all digital 80's models. Note - there is a special bias setting procedure for this model.
This was in protection, there was 46V on one channel o\p, the 220 ohm resistors in the emitters of both channels voltage drivers were either o\c or high value, these are fusible resistors, and one of the o\p transistors had 0.35 V across the b-e junction, changed all the o\p transistors in the faulty channel and the resistors but wouldn't zero the o\p voltage (all remaining transistors checked okay as did all other components), one of the drivers 2SC1918\918 was faulty even though it checked out okay, cleaned relay contacts as the sound didn't sound right connected to speakers, the capacitors in the PSU were changed as one of them is close to a Zener that runs hot, some other small repairs were done but sounded fine after on test.
Pioneer Spec 4
This had destroyed one channel and needed a repair, the temperature diode had gone o\c, this needed all the transistors replacing after the input section and several resistors had burned up, the resistor in the speaker relay supply was also open circuit this is on the board behind the meters.
Eggs could be fried on one half of the heat sink on this amplifier from 1979, the o\p transistors had been replaced on the offending channel and suspect the bias setting hadn't been reset. In the process of inspecting it the other channels o\p transistors came to an untimely end. These are the large rectangular package type and difficult to find (ordered replacements from Singapore!). When they had been replaced the bias was reset-there are 2 pre-sets per channel, they are all set to ccw to begin with, then the inner ones adjusted to 53 mV across the emitter resistors, the outer pre-sets are then adjusted to give 60 mV. This amp uses a cct to clip the reverse biased signal to the o\p transistors to about 0.6 V on +/- variations and produces an impossibly low distortion level.
This tuner\amp from 1969 was in good condition considering its age. One of the resistors on the o\p capacitors was badly soldered (0.7ohms?) it should have been a 0.7 ohm but had been replaced with a 1 ohm. Setting up the DC volts on the o\p to 35V and repairing the speaker output wiring, wires had been connected to the socket internally as the sockets are Pioneer's own (non standard). I managed to 'fabricate' a couple from solenoid terminals\connectors used in industry and Araldited together to form a block. The dial back lights used 6.3V plug bulbs, sounded excellent afterwards.
Pioneer SA-9100, 1974 65W\ch amplifier
This would work fine for 10-15 minutes then it would trip the DC protection, a couple of the driver transistors on the power amp board Fuji 2SC1451's run at gas Mk6 in these amps and were suspect. I am noticing that some transistors have tarnished leads and these are turning out to be faulty, perhaps the leads are oxidising. I replaced them and the other transistors and it worked reliably.
Another problem which didn't manifest itself until some probing was called for in the amp. There is a regulator PCB under a screen that runs the length of the front panel just behind it. Inside are the regulators for the drive stage in the power amp. There are small holes in the cover to allow heat to dissipate this is more of a token gesture than any practical use as a result all the capacitors were o\c and needed replacing.