Quad 405-2 help

[Studio263]

Originally Posted by soundray

I changed all the electrolytic capacitors, including the power caps, hoping that would cure the problems, it didn’t.

That's a great statement which I think should be displayed at the top of every forum page. It would certainly help prevent a lot of problems.

[gramofiend]

If you are uncertain or unsure as to how much work you wish to lavish on a piece of equipment such as this you have also a choice to send it back to the maker. In fact,as far as Quad is concerned, I know that as far as the 405 power boards were concerned, the maintenance unit in the region of the BBC, where I worked, stocked six boards and did not bother to repair at component level! Returning a faulty board in a well padded bag or protected box to Huntingdon, Where the service department is, being well worth it. By the way you very rarely get your own board back but a guaranteed one in spec and fully working. The people working in that service department were often trained there and may be almost as old and experienced as some on this forum. It must be one of the few originally british companies, whose new Chinese owner decided to keep a service department in britain and to maintain equipment going back to valve days.

From my reading of what you say in post 1. a QUAD 405-2 power amplifier should produce no hiss or any noise even if placing your ear next to the speaker! You say you get hiss when the pre-amp is increased which indicates a volume control which a 405-2 does not possess so which preamplifier is in front of the power module and which type of audio cable conects the two, 4pin Din or Phono! Therfore any noise is comming from the unit before the 405 as the 405 is a fixed gain block working at full binge into the speakers!

The fact that you get a crack from the amp when switching on seems to me to indicate a DC offset. What sort of noise does it make when you switch off?
I still think, in your shoes I would either take the thing to Huntingdon, which you can still arrange to do. Or courier the thing with a brief explanation of what the fault's are and you will receive back something which works as it did upon leaving the factory!
I have no connection with Quad other than being a very satisfied customer over a period of 35 years.

good luck
mike

[dave walsh]

Of course it's not cheap to get things repaired at Quad but you do get good service. I retired my 405 a few years ago Soundray as it started banging and crashing at switch on. I was advised to cease operations forthwith for fear of a damaging cascade that might take a lot of circuitry out

I've not had it sorted yet but I have read that the "protection" circuits in this design might be a bit over keen leading to the runaway effect mentioned. I believe that there are specialist replacement kits available but they need to be fitted by someone experienced-that's not me so I'll hang on for a bit and save up.

When the system was in use I did ring Quad and a woman [definetely from Cambridge] answered. I asked "Can you sort my Tuner and she actually said "We've got old boys here who can fix anything-some of them a've been around for sixty years". I said do you need all my details and she replied "No just pack it securely and send it over"

Dave W

[stevehertz]

My Quad 405-2 had a switch on thump. Then it went. Here's the thread:https://www.vintage-radio.net/forum/...d.php?t=121955

[dave walsh]

Hi Steve-memory lane eh. I see that I'm largely repeating myself. I'm glad you got sorted [sort of] but I'd be a bit nervous about reviving mine after three years, even through a variac. I think there was someone on here who fitted the replacement kit designed to stop the protective circuits doing the opposite but I can't recall who that was

Dave W

[soundray]

Originally Posted by gramofiend

From my reading of what you say in post 1. a QUAD 405-2 power amplifier should produce no hiss or any noise even if placing your ear next to the speaker! You say you get hiss when the pre-amp is increased which indicates a volume control which a 405-2 does not possess so which preamplifier is in front of the power module and which type of audio cable conects the two, 4pin Din or Phono! Therfore any noise is comming from the unit before the 405 as the 405 is a fixed gain block working at full binge into the speakers!

The fact that you get a crack from the amp when switching on seems to me to indicate a DC offset. What sort of noise does it make when you switch off?

Hi Mike, thanks for your input.

I may have misled you slightly with my original description of the faults. The hiss and hum described are there with or without any signal source connected. As the volume is raised the hiss on the left channel is masked by the programme material and doesn’t actually increase. The right channel with hum has low output level compared to the left and gets very distorted with any increase in input level

The large splat I described is on power down rather than power up. There is the usual power up thump, which seems standard on these amps, and I know from experience and my research that a small power down splat is also very common on these, but the one I’m getting is quite a lot larger than I have had before.

I may well send it to Quad if I can’t sort it out but, for the moment at least, I’m reasonably keen to see if I can do it myself, learning as I go. I do like the idea of sending just the boards rather than the whole amp though, I hadn’t thought of that.

Ray

[soundray]

Dave W,

I have had some experience of the Quad service department in the past and they are, as you say, very helpful. I may yet avail myself of their services.

stevehertz,

Thanks for the link, more info to add to my knowledge base.

David and Ted,

The bullet has been bitten and the Hameg scope acquired. All I have to do now is find somewhere on my small bench to put it and, more importantly, learn how to use it! Unfortunately progress is likely to be a bit slow as I have an elderly relative requiring a lot of attention at the moment.

Thanks for your encouragement, I’ll let you know how I get on when I’ve made some progress. In the meantime, any further ideas, suggestions or advice from members (specific or general) will always be gratefully received.

Ray

[Ted Kendall]

Originally Posted by soundray

The bullet has been bitten and the Hameg scope acquired.

Good show! You won't regret it.

If you don't already have one, an AF signal generator or a simple tone source is a great help, both in testing kit and putting a known signal into the 'scope whilst you get the hang of it.

[dave cox]

1st step, you'll need to identify which revision of the board you have.

If you are using speakers while you are testing, it would be a good idea to connect a series resistor, maybe 100R, to save a speaker meltdown (not to mention your ears). In any case I wouldn't use any speakers you care about Alternatively, buy some power resistors (I use cheap 15R / 5W) and connect them in parallel with the speaker + 100R resistor so your amplifier has something to push against. Wound power resistors are very robust so no worry about only being 5W, they will probably be glowing red before they die!

From the sound of it you have at least 2 different faults, 1 channel with 'hiss' but otherwise working and 1 channel with almost no output. On the other hand, you have no exploding fuses or smoke being emitted which is a good place to start.

I would start by looking for any dc offset at the outputs and ripple on the power lines.

dc

[soundray]

Thanks to all for your input and encouragement.

Ted

I have a simple tone generator that gives me a selectable range between 40 Hz and 15 kHz at -30, -10 or 0dB. I’ve squeezed the scope onto my bench and have managed to get a 1 kHz tone at -10dB onto the scope. That’s about all I’ve had time to do with it so far with other things going on.

dc

Thanks for your advice, as you say, I’ve got two problems on the go here. I’ve put a meter across the outputs and there doesn’t appear to be any significant DC on them. Ripple on the power lines is something I’ll have to look into how to find. It’s all a learning exercise at the moment. As you say, no blown fuses, burnt components or smoke so far so I guess I can count myself lucky. Thanks for the tip on the power resistors, I’ll get a couple of those and put them to use.

Ray

[Top Cap]

I have had the misfortune of getting one of these for repair
Yes our dear friend the 3k3 resistor R7 had gone open circuit, probably crippling the op-amp and taking all the power devices and fuses with it. I have been able to order suitable replacement parts but I suspect one of the current limiters has gone on vacation too. I have not been able to find any of these even Dada have no stock so plan B is to build one (maybe two?) on isolated pad prototyping board, necessary to keep physical size down. Just when I thought my Pin and Wire days were long gone.
I just hope the ubiquitous 2N3904 and 2N3906 will be OK.
Just in case anyone else has this problem I will attach my sketches so far but bear in mind they have not been built yet but confidence here is running high

Suggested build is to fit all components as shown in parts placement, hopefully there is more room than indicated! Wire for pins can be 20 SWG tinned copper such as RS 355-063. Note how it is threaded from component side then routed back (Pads F & G) to give good support and prevent pad lift.
Short links can be made using solder at:-
1A to 1B, 1E to 1F & 1G, 1D to 2D, 2E, 2F & 2G,
3G to 4G & 4F, 3D to 3E, 4C to 4D,
5F to 5G, 6E to 6F & 6G.

Fine wire links can now be added at:-
1A to 4A, 1B to 2C, 2C to 1E,
1C to 2A, 2A to 6C, 2B to 5D,
3A to 5A, 3F to 4E, 4F to 5C,
5E to 6D, 5F to 6A.

Give me a good Valve amp repair any day - Current dumpers indeed!

[Craig Sawyers]

The dumping principle is a good one. Unfortunately the dreaded underrated 3.3k and dried out electrolytics (above a power resistor) are its nemesis.

And of course the output clamp. What a daft idea - all it does is kill a perfectly working amp before the fuses do their work.

The protection circuit was designed by an old friend of mine during a few years he worked for QUAD in the early 80's. At least you have the schematic - which was from the earliest versions that used discrete parts. It was later put into a hybrid-like potted part.

Teaching my grandmother and all that - but have you thought about laying out a little PCB rather than hard wiring?

Craig

[Top Cap]

Yes Craig, I have thoughts of a PCB and I have a friend in Sweden who is very good at laying them out. Regarding my previous pin and wire design, I had to reverse (literally) the whole thing such as to place the components away from the power transistors as space is so limited. I would like to make one so that I can work out the best way to test the circuit, I have a 50v supply with current limit and hopefully I can apply the required external components and some simple stimulus and monitoring to checkout the circuits operation. If anyone has already done this then I would be grateful for any help on this.
My attempts at contacting Quad have proven fruitless, it seems you have to have a special code number to get to their service department. Like days of old with Murphy and B&O were like that too, wonder if they behave like it today, strictly dealers only for technical advice. I was once a Technical Officer at GEC Radio & Television and any inquiry from a member of the public was always dealt with the same as a Dealer. In fact I went out on many occasions to service TV's directly, on one instance taking a New Old Stock 2028 chassis complete with new CRT for a customer complaining that the tube had failed after 15 years of service. Don't think any Manufacturers Technical department would do that today but to ask for a couple of thick film circuits? Sad how things have become nowadays and yes I would not want those hybrid-like potted parts to fail :-(

[Craig Sawyers]

From the Quad website:

Our UK Service department is available Monday - Friday.

IAG Service department,
Units 13/14 Glebe Road,
St Peters Industrial Estate,
Huntingdon,
Cambridgeshire PE29 7DL

Telephone : 01480 452561

E-mail : rob@iaguk.co.uk

[Top Cap]

Thanks for that info Craig, it took a while to get an answer but worth the wait.
The items are available for just over a tenner and you can collect them personally too.
Opening hours: 7.30am to 5.00pm Monday to Thursday and 7.30am to 3.30pm on Friday.