Quad 303 repairs.

[seanjamesterryb]

I posted this on another forum but have not got any replys. i just changed the top two power transistors in my quad 303 amp, they were overheating. as soon as i turned the amp on it blew and there was a small smell. i dont know if it was the thermal paste or if a transistor pin touched the chassis that caused it to blow. when i change the amp fuse it blows straight away. where is the best place to look for a blown part.

thanks.

[paulsherwin]

No offence meant, but it sounds as if you don't have a lot of electronics expertise or experience. You are very unlikely to fix this by randomly changing components. It would be better to hand it over to a professional repairer.

[Nickthedentist]

Hello and welcome.

I'm afraid I agree with Paul. Maybe better to post here: http://www.vintage-radio.net/forum/f...play.php?f=145

Transistor amps are tricky to fix, even for those of us with lots of general electronics experience.

Just because the transistors were overheating doesn't mean they are faullty. It's more likely that something else is causing them to heat up.

Good luck with it whatever you do.

Nick.

[Radio Wrangler]

Have you kept the original power transistors?

This is very important because it is not easy to find replacements for them and it is most probable as Nick has said that they were OK. There are two problems. Firstly though these transistors are still being made, the modern ones have been 'improved' a lot and are often unstable in the quad circuit. Secondly, these particular types have been very extensively faked and unless you really know where they came from you will most probably get something which has no resemblance to what it purports to be.

Repairing a Quad 303 with a hope of a good outcome requires a serious amount of knowledge. I agree with Paul that you need someone with the right experience, and he'll want the original transistors.

Have a search through this forum and you'll see plenty of material on Quad 303 power transistors.

The 303 design was very good in its time, but it relied on some characteristics of the output transistors to keep it stable, and it's now hard to find transistors slow enough to work safely in this circuit. Back in the 1980s I used to use this as an example when I gave lectures on design oversights!

David

[Grubhead]

I slightly disagree with Paul here. I don't assume a person has no experience of electronics from one short post. I bet there are a lot of trained engineers out there who would randomly replace an electronic part just on a hunch and probably get it right, by chance than training. However it seems to me you have some grasp of the equipment needed to service an amp so here's some general advice.
Fuses blow due to an overload. The most common fault being a shorted semi-conductor.
Acting like it was a piece of wire. An ohmmeter will show which part is shorted. So if you have one of them go hunting around for the short.

[PJL]

The difficulty with repair of solid state amplifiers is in determining the root cause of the failure. Replacing parts and switching back on will more often than not result in the new part failing from the same cause. The 303 then adds a further complexity as it is prone to instability when the output transistors are replaced.

[julie_m]

My personal experience is that about four times out of five, when an amplifier blows its fuse instantly at switch-on, the output transistors will be the only parts needing replacement. If one of the output transistors fails short-circuit, it usually takes its partner and the power supply fuses with it before anything else has a chance to get damaged.

But if there is any other faulty component besides the output devices, that can upset the voltages enough to kill all the other transistors.

[seanjamesterryb]

paulsherwin, I am not a electronics expert but I have built my own Marshall blues breaker amp clone, and sorted other guitar amps with problems.

Radio Wrangler, I have kept the old transistors. the two new RCA ones are supposed to be old, and the same number as the old ones I took out. I read a lot of posts where bad power transistors cause this amp to overheat. I tried them with a multimeter. I asked here because I thought someone might know what causes the fuse to blow when you replace them.

Thanks julie m, The other night I put one of the old transistors back in, I will put the other old one back in tonight to see if it still blows the fuse. The fuse was not blowing before I took the old two out.

Thanks.

[Grubhead]

The most obvious cause of a power transistor overheating if it's not itself, would be the power supply circuit that feeds it. Voltage measurements should track down the cause.
Are the transistors overheating on both channels or just one?
If it is just one, test and compare the voltages of the good channel with the bad one.
If both channels are overheating then you have a power supply problem, or something that is common to both channels.

[seanjamesterryb]

Grubhead thanks for the reply. The two transistors on one channel are over heating, the other channel is OK. The sound on the overheating channel gets distorted when it happens.

Thanks.

[Restoration73]

A multimeter may not give anything more than a static check on a semiconductor. As already mentioned the fault may be due to problems with the driver or power supplycircuitry. If the amplifier still functions, you may be able to fault find without the destructive effects by reducing the mains voltage or limiting the DC current to the outputstage using a wirewound resistor, e.g. 47 ohm 11 watt or similar. Faults caused by instability may need a signal source and oscilloscope to observe.

[julie_m]

Do you know how to test a transistor with a multimeter?

[Radio Wrangler]

Just getting hot could be something as simple as the slider in the quiescent current trimmer losing contact with its track. The quiescent current in the output transistors then goes too high, but not dramatically high and too much heat is created in the output transistors.

The first port of call is to check some voltages: The power supply voltage (67v), the DC level at the positive end of C1L and C1R, and the voltage across each of R124 and R125.

These will give an initial view of what's going on, and show where to go looking next.

You've worked n valve amplifiers before, and in those the valves are normally AC coupled to each other with blocking capacitors between stages. If one valve goes bad, it's DC conditions go bad, but it cannot affect the DC conditions of the other stages. This both localises damage and makes it easier to determine what to replace.

Early transistor designs mimicked valve circuitry and used coupling transformers and DC-isolated stages, but in that era, the germanium transistors gave plenty enough trouble.

Your Quad is one of the first transistor designs to be considered really good. It is a forerunner of the style of design used today. This style uses many transistors and they are all DC coupled together. There are DC blocking capacitors only at the input and the output to allow the Quas to run from a single power rail.

What this means in terms of fault fixing is that a failure of one part can easily damage other devices. If you check parts and replace the first bad one you come across, there is the risk that your new part gets immediately destroyed by whatever killed its predecessor. You of course are now thinking that the part you put in is certainly OK, so you continue onwards, finf the next dud and replace it. It is now killed by the recently deceased part you put in before.

It doesn't always happen, but you can get stuck going round in a loop replacing the same things over and over again. When it does happen, it confuses the living daylights out of people.

So, with transistor amplifiers, the best approach is to identify all the bad parts and to replace the lot of them at once. Sometimes you can't tell if some parts are bad or not, but small parts are cheap enough to swap if there is any doubt about them being OK.

Transistor amplifiers have acquired a reputation for being hard to fix. Yes, they're harder to fix than valve designs, but they still are fixable. They need a different approach, though, and you probably wind up replacing a number of innocent parts. This isn't terrible for cheap parts. In the 1920s, one valve cost a week of someone's wages.

David

[nutteronthebus]

Hi this is the cct diagram if it helps




Dave

[seanjamesterryb]

I've just found a blown resistor on the middle board, it is next to the left heatsink. I can't read the colours as its too burnt. When I desoldered it, it broke in half. It looks like a 10 ohm 1/2 watt resistor. I will change that and put new sleeves on the transistor pins as they look short. They might have shrunk over the years. I have put the two old transistors back in.

Thanks for all your replies.

[PJL]

Before you switch back on you need to know why the resistor failed and what else failed at the same time. It sounds like one in the bias chain in which case you should test the driver transistors. As you have a working driver board, you can do some in circuit checks by comparing resistance measurements between the good and failed board. Don't switch power back on till you are confident all transistors on the board are good.

[Grubhead]

Bad speaker wiring and faulty speakers can also cause burnt up resistors. So check them out too.

[G4_Pete]

Hi.

Note some of the later Quad test sheets show 2N3055 devices on the service sheet. ( Drawing No M12160 iss 1) I queried this in 1998 as the voltage ratings looked tight. I quote directly from the letter I received from QUAD service.

" The 2N3055 was a selected device. We now use the MJ15003 at £2.68 each plus VAT and postage."

No mention was made in the letter of any other component changes made for the device change and I can't recall any.

Pete

[SteveCG]

A useful thing to check is the state of the large electrolytic capacitors that the 303 used. If orginal, they are mounted with the solder tags on the underside. These very often ooze electrolyte and can go short circuit (a possible cause of your problem). Quad changed the design later (and in servicing returns) to mount them with the tags upright.

[seanjamesterryb]

I posted on here 5 years ago about a quad 303 amp I bought. I was having trouble with it. I changed 4 of the output transistors for 2N3055 and left the middle one which is a 40411. Could I put a 2N3055 in the middle spot or would I need a different one. I think it is the driver. are the base and the emitter pins reversed on the 2N3055 from the RCA 38494. The thread I posted on 5 years ago is now closed. I have not bothered with the amp since then till now. After i changed the output transistors it blows a 68 ohm resistor under a heat sink on the middle board.

Thanks, Peter.