Quadrevision Live: Peter's 303 Explosion
"Helaas is mijn 303 gisteren met veel brom en audio-explosies (de woofers lagen zowat in de kamer) overleden. Het bleek dat één van de voedingselco's was bezweken. Ik heb inmiddels de twee voedingselco's van 2000 mu vervangendoor één van 4700 mu en de doorgebrande zekering vervangen. Bij inschakelen ontstond een lichtflits op een versterkerbordje en toen ben ik maar gestopt...."
Unfortunately my 303 has deceded yesterday with a lot of rumble and audio-explosions (the woofers were almost laying in the middle of the room). Obviously one of the power-supply capacitors was broke. I have replaced both capacitors by one 4700 mF and I replaced the fuse. When switching it on there was a flash of light on one of the amplifier-circuits and so I decided to give it up...
This morning Peter's 303 arrived in Antwerp. A first visual inspection of the 303 does not show a nice picture, but we've seen worse than this, haven't we?
Revision Step 1: Checking the power-supply
The first step is to disconnect the power-supply from the rest of the amp and to measure the voltage. Wow !!! It shows tho be 83 Volt instead of 67 Volt as it should be.
In fact 83 Volt is the unstabilised voltage on the capacitor so it is clear that the stabiliser-circuit no longer does it's job. This has probably blown up the output-circuits as well ;-(
Removing the stabiliser-circuit and connecting it to an external stabilised power-supply soon showed that the faulty component was the small trimmer-potentiometer on the board.
It is always fascinating to find out what components in old vintage Hifi cause the problem... Usually it are the capacitors and things like potentiometers that are aging in a very bad way.
The Printed Circuit-board itself shows some burn-spots - it's had it too hot lately. But for the rest this won't influence the good functioning as far as all the components are OK.
After replacing the trimmer with a new one and calibrating it, we measure 67 Volt. Tomorrow we'll check the amplifier circuits.
Revision Step 2: Checking the amplifier-boards
It's even worse than I tought.
On the PCB-board a complete track has gone up in smoke...
This is not difficult to fix with some wire but this means there has been some very serious overload on the circuit.
The other channel still works as it should, obviously (there is 37,5 Volt on the output-line before the output capacitor).
Checking the transistors shows that, indeed, TR1, TR2, TR105 and TR107 are blown up.
TR1 & TR2 Wont' be a problem, they can be replaced by a (good quality) 2N3055, which is the most common power-transistor ever made. TR107 won't cause any problems neither, this is a simple BC109.
But what about TR105 (the PNP driver)?
In the original design "38495" (NPN) and "38496" (PNP) were used as drivers. Of course they don't exist anymore.
Some surfing on Google learned that they were replaced by BC441 and BC461 resp. Actually BC441 has been replaced by 2N5320.
So I ordered 8 of each. It's not that I like to blow them up but I want some reserve if it works;-)
And BTW if I change them in one channel I'll have to change them in the other channel too, we want exactly the same sound in both channels of a 303, don't we?
Spending my time in a good way, waiting for the transistors to be delivered, I already replaced the output-capacitors with 4700 mF 100V.
Revision Step 3: Solving logistic problems ;-(
The new BC461 that replaces the original 38496 (TR105) has a different casing, of course... So we'll have to order adapted heat-sinks.
This is the usual problem when repairing Vintage amps or vintage cars the like: finding the replacement-parts ....
(Picture Copyright Farnell.com)
Revision Step 4: Mission accomplished ;-)
Apparently the TO-5 Heatsinks don't exist anymore, my suppliers tell me... It's a problem we see more and more when looking for vintage components: "Product-lifecycles" get shorter and shorter.
With some heath-conducting glue I managed to put the new TR105 in with the old heatsink. I checked it with my Fluke thermometer for an hour playing disco @ maximum volume and it did fine (40° C).
I also had to change TR107 and MR105/106.
Then came the final calibration:
- The power-supply-voltage (with RV200) to exactly 67 Volt
- The output DC before the cap (with RV100) to exactly 33,5 Volt
- The rest-current to 33 mA (with RV101), this means 20 mV between pin 4 and 6 of the driver-board
After 2 hours of playing music @ maximum volume those calibrations were repeated.
The 303 sounds very transparent now and no trace of crossover-distortion can be measured on my scope.
Up to the next project...
PS. Joost added:
The transistor equivalents are as follows:
TR105 39496 BC461 2N5322
TR106 39495 BC441 2N5320
PPS. Chris (UK) says that the TO5 hetasinks are still available @ www.cpc.co.uk