What a pig to fix.

[nigelwright7557]

I recenntly bought two 1980's Maplin 50 watt Hi Fi amplifier modules off ebay.
One was supposed to be working and the other had transistors missing.

The one that was supposed to working was osicllatiing so I fixed that with a capacitor in the VAS across c-b.

The other had obsolete transistors missing so I found modern equivalents just to get it going.
The amplifier just wouldnt work with the new transistors.
I eventuall tracked down a high voltage on Q5 base that shouldnt be there.
I tested the transistor and it DVMed fine. BC and BE low resistance one way and high the other with high resistance from c-e. I checked Q4 too then it struck me they both chekced out fine in teh same phase yet one is NPN and the other is PNP. Q5 was the wrong transistor !

So I changed Q5 for a modern equiv just to try it and the amp burst into life.

Amps can be hard enough tto fault find without people putting in worng components.

I just need to tidy up the pcb now and it will another addition to my collection of old amps.
Here is a picture of the first working one.
http://www.ckp-railways.talktalk.net/m50.jpg

[Ian Finch]

Hi, just for completeness, it would be nice to see a schematic for this beast, if you have one, of course. Perhaps also a comment on how "high" the fidelity seems to you.

[EssB]

Ian,
here's two of the magazine articles. Apparently it was reprinted a few times over the years.
50W HiFi Power Amplifier LW35Q Maplin Kit
50W HiFi Power Amplifier Maplin

[kasey197]

Man i miss my old huge maplin catalogues ... I wonder if they still print them anymore ....

[Ian Finch]

Thanks EssB, Ah yes, I have seen this and thought about the basic zener CS, output coil, CFP output stage and compensation that make it different, even in that era. You might think the SQ would be awful from that recipe.

[EssB]

Don't know about their SQ, never heard them.
I'm still using a pair of Crimson Electrik CE1708 modules (from about 1979) that superficially don't look any more sohisticated and they sound rather crispy. I traced the Crimson circuit out once and decided they were like a Quad 303 circuit on steroids. I could be all wrong about that, it was a long time ago when I knew even less than I do now!

[nigelwright7557]

Originally Posted by Ian Finch

Hi, just for completeness, it would be nice to see a schematic for this beast, if you have one, of course. Perhaps also a comment on how "high" the fidelity seems to you.

I had forgotten how good these little amps sound.
It seems to have a good range and sounds very clean.

I originally made one in 1980 for use on a disco.
It sounded great but wasnt loud enough for a disco so I bought a Maplin 225WRMS amplifier module.

[nigelwright7557]

Originally Posted by kasey197

Man i miss my old huge maplin catalogues ... I wonder if they still print them anymore ....

Yes they still do the thick catalogues.
My local Maplin is less than a mile away and is always handy for the odd component I am short of. They are pretty expensive tho, 16p for a resistor when I buy them from RS for a penny. But in an emergency Maplin are useful.
I have been a customer of Maplin since 1980.
They used to do some good gear. I built their mobile disco with record decks. preamps, amplifiers and sound to light in 1980.

[nigelwright7557]

Originally Posted by nigelwright7557

I just need to tidy up the pcb now and it will another addition to my collection of old amps.

Tracked down some of the obsolete transistors to ebay of all places.
BD711, BD712, BC141 and BC161.