FYI Quad 303 owners - Check the condition of your driver board capacitor(s) (***Updated***)

 Hi everyone,

Please do check the condition of the electrolytic caps on the Quad 303 driver boards every once in a blue moon, especially the condition of C101.

C101 as per high-lighted, on the Quad 303 driver board pix from the service manual

I chance upon mine while investigating why only one channel was audible when using the Quad 33 from last month (since discovered had several problems).

Quad 303 #1

Please refer to the pix below.

C101 on each driver board - note the darken area towards bottom half of each capacitor

Even though the power amplifier performed flawlessly, the visually impacted C101 could be sign of a latent issue at large as these FC(s) were purchased from Element-14.

I troll the DIY forums for information regarding any similar observations. While I did not manage to locate any, I did note there were many reported issues originating from the PSU board. Since C101 on both boards were impacted, this had to be the common source of the issue!

Thus I obtained an updated Quad 303 refresh PDF from DaDa Electronics and followed the bias-ing procedure. 

Immediately discovered the issue - PSU was pumping out 74V and adjusting trimmer RV200 had no effect at all!!!

Testing TR201 using a el-cheapo tester from AliExpress shows that it was faulty and behaved as if it was 2 diode(s) which output to the 3rd pin - see following pix.

Result when testing TR201 on the transistor tester

Ordered a replacement BC441 as I already have the appropriate heatsink. While awaiting for the BC441 to arrive, I changed RV101 from 2K2 to 22K on both driver boards (since the S/N of the unit was well below 11000) as per recommend by the PDF.

After repairs

Since I have a few spare 470uF 35V lying around, will use them 1st to "test the waters" till I can purchase proper audio EC capacitors for C101.

Upon installing BC441 as TR201, the PSU was initially outputting 60V. I then followed the bias-ing procedure and obtained a result close to the recommendations.

Quad 303 #2

Checking #2, found the same C101 issues as well. 

The PSU on this unit was outputting 78V+!! Adjusting RV200 could only bring down the voltage by decimal points! Hence power-ed down the unit immediately and re-position the trimmer to mid-point.

The cause for unit #2 was not TR201, as checking the transistor via the el-cheapo AliExpress tester indicates it's working fine. 

Proceeded to test the various components on the PSU board via a Digital Multi-meter (aka DMM) which then reveal diode MR200 was in open status. 

Tester confirm bad diode detected during DMM inspection of the PCB

Replaced MR200 with a 1N4004 diode, as per suggested by the PDF.

The diode, MR200 on the Quad 303 PSU

Took the opportunity to change RV101 from 2K2 to 22K on both driver boards since the S/N of the unit was well below 11000. And used the spare 470uF 35V until I can purchase proper audio EC capacitors for C101.

Upon power up, voltage was 76V+ but could be reduced via adjusting trimmer RV200. Again, followed the bias-ing procedure and obtained a result close to the recommendations.

Quad 303 #3 (Update 27Jul2025)

Finally had the urge to take a look into Quad #3. Here's how it went ...

1. Upon power-up, there was a loud thud sound on the RHS channel, followed by a consistently mid-volume "burrr" with no audio during playback. LHS was performing as per expected.
2. Next, the black RHS banana jack receiver on the Quad broke😒as I was swapping the speaker connection for further testing.
3. Noted the entire heatsink was becoming quite warm as well.
4. Since I could not do much more, I desolder RV101 from both boards, took their resistance reading, set the replacement 22K to the previous reading(s) for the respective boards before resoldering them.
5. Ordered replacement banana plug receivers and was received the day a few days ago.

   

   Round version new

   
   
   
   
   
6. As the impacted RHS board was an older Issue 5, I could not locate a specific schematic for it. Had resorted to "1-for-1" part replacement for transistors. Unfortunately 1 could not read the markings of one transistor, closest to the driver input drill holes on the PCB - turns out to be the new BC560 - using process of elimination via presence of the other transistors.
   
   

   
   
   

   

   The transistors with heatsinks need to be replaced on the Issue 5 board - shown with transistors replaced on the sides

   
   

   
   
7. Removed the 2 transistors with heatsink before power on. Upon power on, no unexpected sounds eg no loud thud, no burr-ing & heatsink not heating up - indicting the bur-ring sound was not from an earthing issue. Hence power down the unit and swap with new BC441&461 with heatsinks.
8. Power up this time no loud thud and no bur-ring, but also no audio.
9. Power off and started to de-solder the remaining original transistor(s) for verification on the transistor tester. All passed the tester??? 😟
10. Using the DMM to test the resistor(s) and diode(s) indicate all these parts are ok.
11. No choice then, kept repeating to swap out the remaining transistors one at a time, then power on to test repeatly. Luckily I bought a few extras since had to look into possible issues for few Quad 303(s). In the end, had to replaced each and every transistor on the RHS board.
12. Eureka!!! Everything works now with the resultant imaging at roughly same spot as the previous(s) Quad😆. Bonus was heatsink now not warm at all😁
13. Will take the win & perform a bit of listening tests before performing the bias-ing a little later