Saturday, September 12, 2020

Craig 5502 Receiver Restoration

Unit: AM/FM Stereo Receiver
Manufacturer: Craig
Model: 5502
SN: 26107601

A friend of mine bought this vintage receiver on a yard sale for $10 but didn’t know that it has a problem. The right channel has a constant “hum” noise and no sound at all. The left channel is highly distorted. So, he asked me to repair it if it wouldn’t be too costly. He wanted to use it in his garage. I agreed to take a look and fix it if he pays just for parts. We made a deal.

Craig 5502 Receiver Restoration

I don’t know much about Graig’s receivers and he didn’t have any documentation for it. I tried to search online for a service manual but was not able to find even a schematic. A very popular website where you can download service manuals for free is https://www.hifiengine.com/. But it doesn’t have a service manual for this model. So, it was a little bit tricky for me to service it without a schematic. But I like to be challenged!

Initial check with Dim Bulb Tester

I used my Dim Bulb Tester (DBT) to initially power up the Graig receiver and check if it has any short circuits somewhere. For those who don’t know the DBT will protect your vintage gear limiting the maximum current to the device under test. It is especially important when a vintage gear with unknown history is tested or after the repair is done. I always use my DBT as a quick test between different restoration steps. So, the light bulb flashed for a second on bright and then dimmed out almost completely. It’s a good sign meaning that the Graig receiver has no short circuit. I also checked the DC voltage across speaker terminals to make sure that I can connect the receiver to my speaker system. However, the DC voltage across the right speaker terminals was ~5.3V which is way too high and would definitely damage the speaker. The DC voltage across the left speaker terminals was ~4.0mV which is normal. Now it’s time to remove a wood cabinet and look inside.

Power Amplifier Board

Well, the power amplifier board has signs of extensive heat damage and somebody tried to repair it. The power transistor Q215 was previously replaced but as can be seen from the photo below the wires close to the transistor leads were overheated and just wrapped by electric tape. All emitter resistors R237/R239 (left channel) and R238/R240 (right channel) were overheated. Two of them R239 and R240 were completely burned out. Resistors R229 & R235 (left channel) and R230 (right channel) were overheated as well. I tested all power transistors and all overheated resistors from this board, and the results are below.

Test results on all power transistors and overheated resistors from the power amplifier board:

Q213: 2SD180, NPN, left channel – Okay
Q215: 2SA626, PNP, left channel – Okay (previously replaced)
Q214: 2SD180, NPN, right channel – Okay
Q216: 2SA626, PNP, right channel – shorted collector-emitter
R237: rated resistance – 0.47Ω, measured – 0.5Ω
R239: rated resistance – 0.47Ω, measured – open circuit
R238: rated resistance – 0.47Ω, measured – 0.5Ω
R240: rated resistance – 0.47Ω, measured – open circuit
R229: rated resistance – 220Ω, measured – 221Ω
R235: rated resistance – 6.8Ω, measured – 6.8Ω
R230: rated resistance – 220Ω, measured – 218Ω

Power Amplifier board - power transistor Q215 (second from the left) was previously replaced

Craig 5502 - Power Amplifier Board

Wires close to the transistor leads were overheated and just wrapped by electric tape

Green emitter resistors (left channel) and resistors R229 & R235  are overheated

Green emitter resistors (right channel) and resistor R230 are overheated

Power transistor Q216 (right channel) - shorted between collector and emitter

Repair and Adjustments

I replaced all emitter resistors with 3W wire wound resistors to improve power dissipation. All ½ W resistors were also replaced with metal film resistors. Two power transistors from the right channel were replaced with ON Semiconductors MJ transistors. Two power transistors from the left channel were kept original. Finally, I tested all diodes and driver transistors on this board and no other issues were found.

After the repair was finished I powered up the receiver and adjusted the idle current to ~30mA on each channel. The idle current is controlled by trimmers VR203 (left channel) and VR204 (right channel). The resistance of each green emitter resistor is 0.47Ω. So, to adjust the idle current to ~30mA the voltage across the green wire wound resistors should be adjusted to ~14mV (0.03A x 0.47Ω = 0.014V).

Power Amplifier board - after repair

Power Amplifier board - after repair, left channel

New ON Semiconductors power transistors installed on the right channel

Idle current on the left and right channel after repair

The repair is finished and the receiver is in perfect working condition again. The final result can be seen in the photos below. Thank you for reading.

Graig 5502 - after repair

Thursday, September 3, 2020

Pioneer SX-828 Receiver Restoration

Unit: AM/FM Stereo Receiver
Manufacturer: Pioneer
Model: SX-828
SN: SF1600962

One of my customers asked me to test and repair this beautiful vintage receiver. He bought it almost 20 years ago and it has been sitting in his storage since then. The receiver is in excellent cosmetic condition with a very nice-looking engraved faceplate. It is a very well-built receiver but not the easiest to work on. However, it was worth the effort. According to the Pioneer database, this unit was manufactured in June 1972.

The Pioneer SX-828 was manufactured from 1972 to 1974. It produces 54 watts per channel into 8 ohms with no more than 0.5% total harmonic distortion. The list price in 1972 was $429.95 (Ref. Stereo Review, March 1972, page 1). It quickly jumped to $469.95 in the following year. The SX-828 model was “one of Pioneer's most prized packages, with a wealth of outstanding performance in an AM/FM solid-state stereo receiver. The SX-828 is a big power AM/FM stereo receiver with advanced circuit design, outstanding FM selectivity and sensitivity, and linear scale tuning dial”.

Below are some photos I took during my restoration process, complete with comments and recommendations. This receiver had some issues with electronics which I will describe in detail below.

Update: a few months later, I was working on another SX-828 and serviced all boards except a tuner section. Refer to the following article for details: Pioneer SX-828 receiver restoration. 

AF Pre-Amp Board

During my initial evaluation, I noticed a significant “pop” sound in both speakers when the low filter switch was turned on. The protection relay was engaged for a few seconds and the sound was killed for a moment. I checked the AF Pre-Amp board (#AWK-012) and found that all original coupling capacitors installed on this board are sky-blue Sanyo electrolytic caps. These e-caps have a bad reputation to get electrically leaky over time. As a result, they eventually fail shorted and allow DC to pass through the circuit. I tested all original capacitors from this board and the results are below. Notice that the factory capacitance tolerance is usually +/- 20%.

Test results on sky blue Sanyo capacitors removed from AF Pre-Amp board:

C15: rated capacitance – 1.5uF, measured capacitance – 1.9uF, deviation: +26.7%
C16: rated capacitance – 1.5uF, measured capacitance – 2.0uF, deviation: +33.3%
C25: rated capacitance – 3.3uF, measured capacitance – 4.6uF, deviation: +39.4%
C26: rated capacitance – 3.3uF, measured capacitance – 4.8uF, deviation: +45.5%
C31: rated capacitance – 2.2uF, measured capacitance – 3.0uF, deviation: +36.4%
C32: rated capacitance – 2.2uF, measured capacitance – 3.2uF, deviation: +45.5%
C37: rated capacitance – 1.0uF, measured capacitance – N/A, resistance – 15.9 kΩ
C38: rated capacitance – 1.0uF, measured capacitance – N/A, resistance – 1.6 kΩ
C45: rated capacitance – 3.3uF, measured capacitance – 4.4uF, deviation: +33.3%
C46: rated capacitance – 3.3uF, measured capacitance – 4.5uF, deviation: +36.4%

As one can see, the test results show a large deviation from the rated capacitance for all sky blue Sanyo caps. Two caps C37/C38 actually act as resistors in the circuit instead of coupling capacitors. These two bad caps are a root cause for a significant “pop” sound in both speakers when the low filter switch is turned on. The DC is not blocked to pass through the circuit due to those bad caps. I replaced all sky blue Sanyo e-caps with low leakage Nichicon UKL caps and the “pop” sound has gone. The remaining e-caps on this board were replaced with Nichicon UFG. Notice that the AF Pre-Amp board is a double solder PCB which is a major problem to work on. I recommend being very patient and careful if you need to service this board to prevent any overheating while unsoldering electronic components.

AF Pre-Amp board (#AWK-012) - before and after

One of sky blue Sanyo e-caps - the deviation from rated capacitance is 45.5%

Power Supply Board

The second problem with this receiver was with the power supply board (#AWR-010). Two transistors Q1 and Q3, and the resistor R12 are running very hot under normal operating conditions often damaging the PCB. The nylon screw holding the heat sink to the transistor Q3 was overheated and broken. As a result, the heat sink has fallen off. The insulation on one lead of resistor R12 was melted due to extensive heat and the PCB was discolored. I measured the voltage drop across resistor R12 and it was almost 25V. Using Ohm’s law, the current flowing through this resistor is 25V/330Ohm = 0.075A. The electrical power dissipated by a resistor is 0.075A x 25V = 1.89W. The resistor R12 has a rated power of 2W which is very close to its working limit. I replaced the affected resistor with a flameproof 3W metal oxide resistor to improve its heat dissipation. I also replaced transistors Q1 and Q3 with Fairchild MJE transistors due to better thermal characteristics in comparison to the original transistors. All electrolytic capacitors on the power supply board were replaced with low impedance and high-reliability Nichicon UPW caps. I tested all original e-caps removed from this board and found that two e-caps C5 and C13 were completely out of spec.

Test results on 8 original capacitors removed from the Power Supply board:

C5: rated capacitance – 1000uF, measured capacitance – 92uF, deviation: -90.8% (the capacitance is almost 10 times lower!)
C6: rated capacitance – 100uF, measured capacitance – 120uF, deviation: +20.0%
C7: rated capacitance – 100uF, measured capacitance – 111uF, deviation: +11.0%
C8: rated capacitance – 100uF, measured capacitance – 122uF, deviation: +22.0%
C9: rated capacitance – 100uF, measured capacitance – 115uF, deviation: +15.0%
C10: rated capacitance – 220uF, measured capacitance – 254uF, deviation: +15.5%
C12: rated capacitance – 220uF, measured capacitance – 276uF, deviation: +25.5%
C13: rated capacitance – 470uF, measured capacitance – 9.1uF, deviation: -98.1% (the capacitance is almost 50 times lower!)

Power Supply Board (#AWR-010) - PCB discoloration due to excessive heat

Power Supply Board (#AWR-010) - old transistor Q3 and resistor R12 removed, all e-caps replaced

Power Supply Board (#AWR-010) - new Fairchild MJE transistors Q1 & Q3 and resistor R12 installed

Capacitor C13 was removed from the Power Supply board - the measured capacitance is almost 50 times lower!

Power Supply Board (#AWR-010) - before and after

Dial and Indicator Lamps

To replace 5 dial lamps I pulled off all knobs from shafts, removed nuts and washers from their shafts, removed a faceplate, and then carefully pulled off four clips holding a dial scale. Those four clips are very brittle and can be easily broken. I strongly recommend using an appropriate plastic tool and being gentle when removing those clips. I used a plastic tool with a very thin blade from an iPhone repair kit and it worked great. All original incandescent bulbs were replaced with warm white LED lamps to maintain the original look and decrease heat. I also replaced two original bulbs behind the signal strength and FM tuning meters with warm white LED lamps. However, it was a tricky place to work on due to the lack of space. Ideally, the dial cord should be removed to give more access to the bulb holder. But who wants to replace a dial cord string on those vintage gears if it works perfectly?!

The stereo indicator bulb burned out and I replaced it with a new incandescent bulb. The original rubber (?) holder for the stereo indicator bulb has dried and degraded due to age and eventually became very stiff. I used an X-ACTO #2 Knife to remove the original bulb from this fragile holder carefully trimming around the bulb. Then I put a heat shrink on a new incandescent bulb to slightly increase its diameter, inserted it into the holder, and secured it with super glue. I would say it was a very delicate work but fortunately the holder was not destroyed.

Original incandescent bulbs behind the signal strength and FM tuning meters

Warm white LED lamps behind the signal strength and FM tuning meters

DC offset and Bias Adjustments

Finally, I checked and adjusted the DC offset and Bias on the main amplifier unit. For some reason, this procedure is not described in the original manual but it is pretty much straightforward.

Trimmer VR1 on the main amplifier unit (#AWH-010) controls the DC offset measured between pin 7 and the ground. Trimmer VR2 controls the DC offset measured between pin 8 and the ground. I adjusted the DC offset as close as possible to 0V.

The bias measured between pins 11 and 17 is controlled by trimmer VR3 for one channel and between pins 12 and 18 by trimmer VR4 for another channel. I adjusted the bias to ~15mV on each channel.

As usual, all the knobs and the faceplate were gently cleaned in warm water with dish soap. Also, all pots have been cleaned with DeoxIT 5% contact cleaner and lubricated with DeoxIT FaderLube 5% spray.

The final result can be seen in the photos below. The receiver works perfectly now, the sound is very nice, smooth, and fairly well detailed. All issues with electronics have been solved and the receiver has a perfect look again. Thanks for reading.

Pioneer SX-828 - before restoration

Pioneer SX-828 - after restoration