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.

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 had 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

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