British Quad amplifiers

[nick holme]

Yes you are right - I was hoping to avoid having to test everything, but that's going to be the only way. Thanks for now - I'll post the result if and when I find it.

Cheers

Nick

 

[MikeHunt79]

Hi MikeHunt79,
One thing I know helps the sound quality for sure is to get rid of all ceramic caps. Use mica or polystyrene to replace them. Don't buy those nutty expensive brands. Replace the low value electrolytics with film types (Wima makes 1 and 3u3 caps, 4u7 also). Bypassing the feedback electrolytic with 4u7 (use 3u3 or 1u0 if you can't get those) yields benefits. Replace the other with 85 ° caps at higher voltage ratings. The 85 ° have lower dissipation reading than the 105 ° types I have found. They may be marginally cheaper too.

Since you have it open, you may as well do this too. The lower value electrolytics may be bad anyway. If you measure them, you will see the dissipation value increasing over new ones.

-Chris

Well, it's still open now. I replaced the PSU caps with cerafines of the same value and voltage, but only one channel is working.
Also, the heatsink only gets hot on one side, so I'm thinking another smaller cap on the PCB is to blame. I'm tempted to buy all the capacitors in one go, do you know of a parts list or a schematic that would show what this amp uses?

 

[nick holme]

Hi Mike and everyone,

I've just finished fixing mine. The final fault that was causing the crackle at switch on turned out to be the small 120Pfcap connecting to TR3 base.

Mike, I spent quite a bit of time looking around on the net and I found a quad 405 service manual which has circuit diags for all the different boards and also a list of what to check and what effect various faulty components may have. I have attached it for your benefit.

My original fault was one channel not working. I found one of the fuses on the circuit board was blown and tracked the fault down to a faulty power transistor. Yours may be the same fault.

Thanks everyone for your help.

Nick

 

[anatech]

Hi Nick,
Good going sir. Well done.

-Chris

Edit: It's rare for ceramics to fail in signal circuits. Conduction around the outside of the wafer is the usual cause. To find this you must have been left with an idea that was unlikely, but the only possibilty.

 

[Jessjh]

Quad, again

Thanks everyone for responding to my original Quad posting, some of the
postings improved my understanding of how current dumping works.

It took me a long time reading all the material about Quads in the old Wireless World mag. to get SOME understanding of how it works. By the way, I surely miss Wireless World. When they changed to Electronics World and fired the current editor, I think the mag. changed and I lost interest.

I tried to model the current dumping portion of the Quad and write circuit equations but got bogged down in algebra and gave up. Has anyone tried and succeeded in writing these equations AND getting a sensible result, such as
getting a simple gain equation that stays pretty constant with freq.? (in the audio range)

And finally, I would like to again ask if anyone has any info on that Mattes amp which I believe used current dumping or something similar.

Thanks for your responses
Jess
Jessjh@comcast.net

 

[Jessjh]

Quad/Mattes

Hi:

Anybody who's interested in the Quad current dumper, might be interested in this. I had in previous threads asked if anyone knew anything about the Mattes amp. I had been more or less trying to find out about it for years.

Finally, I got hold of the Journal of Audio Engineering article describing this amp. It's in volume 13, July 1965, pps. 222-228.

I don't know when the Quad dumper was first introduced. If this Mattes amp preceded it, possibly the Quad boys got their circuit concept from the Mattes.

The Mattes also has its driver stage drive the speakers directly at low level and then the output pair, which is transformer coupled to the driver, picks up the load.

If anybody knows about the Mattes amp or looks it up, I'd be interested in any comments about the two circuits that he/she would care to make.

Jess

 

[Jessjh]

Quads

Hi:

Anybody who's interested in the Quad current dumper, might be interested in this. I had in previous threads asked if anyone knew anything about the Mattes amp. I had been more or less trying to find out about it for years.

Finally, I got hold of the Journal of Audio Engineering article describing this amp. It's in volume 13, July 1965, pps. 222-228.

I don't know when the Quad dumper was first introduced. If this Mattes amp preceded it, possibly the Quad boys got their circuit concept from the Mattes.

The Mattes also has the driver stage drive the speakers directly at low level and then the output pair, which is transformer coupled to the driver, picks up the load.

If anybody knows about the Mattes amp or looks it up, I'd be interested in any comments about the two circuits that he/she would care to make.

Jess