SpeakerTalk

[audiolabtower]

I managed to take some photos of the 105 Mk 1 led circuit










I guess

BC148 (20V) and BC158 (25V) used for driving the led?

1000uF 10V as the reservoir capacitor to power the led?

16uF 150V elcap as the input cap?

A thyristor for switching current through to the led circuit?

8 positions on switch plus listening position implies 9 resistors in the divider, although one position is open circuit so maybe only 8?

18 resistors total on the pcb, 2 diodes for the rectifier and 1uF 10% metalised polyester.

Anyone good for the circuit?

[speakerguru]

I don't have a schematic.
Use a graphics package (GIMP 2.8 is free) to mirror the track photo, give it some transparency and then superimpose it on the component side photo. You should then be able to trace the circuit very easily.

[audiolabtower]

Brilliant suggestion, thank you. I could try the same in photoshop, invert and put both in transparency and try to size them up to match when overlapped.

[speakerguru]

You're welcome. Perhaps you could post the schematic when you're done.

[audiolabtower]

I think I've got the rectifier circuit, the led drive circuit, the triac drive circuit, and the resistive divider around the switch.

But I'm stumped about which contact on the switch is the pole.

There should be 8 "wattage" positions, which agrees with my 7 resistors from one end to ground, plus a listening window, plus an "off" which I assume is o/c, 10 positions in total for 11 contacts in the photo.

If I assume the input 16uF goes to the pole there is no connection from the divider to the rest of the circuit for 8 positions. If I assume the input to the rectifier is the pole, then the input 16 uF goes to one contact and nowhere else. I'm only familiar with rotaries where the pole was in the centre, not on the perimeter.

[speakerguru]

I found this similar switch in my junk box if it helps:

http://img1.jpegbay.com/gallery/002166480/1_m.jpg
http://img1.jpegbay.com/gallery/002166480/2_t.jpg
http://img1.jpegbay.com/gallery/002166480/3_t.jpg

(can anyone explain to me, how to use HTML code in this forum input forms?)

Otherwise, you may have to remove the switch to work out how the wiper goes and then replace it. Don't attempt it unless you're good with a solder sucker.

Re the circuit, which is a (far too clever for me) Chris Moore design; I thought it had a thyristor switch mode PSU derived from the audio, then a 2 transistor Schmitt trigger with switchable threshold. I used to repair production failures on overtime. But that was a long time ago. All my brain cells have been replaced since then.

[audiolabtower]

Ah, thank you. As I thought the gap in the circular contacts (on the pcb) has the pole at one side, which your pic seems to confirm.

It's a long time since I've done electronics. Th1 is of course a thyristor, not a triac. The rectifier goes to a transistor driving the thyristor, the gate feeds the 1000uF reservoir, the pnp/npn pair drive the led.

I don't have a scanner at home, I'll try and scan what I've drawn out on Monday and post it to see if anyone can make sense of it.

[rx7rotary]

two of these circuits if anyone needs them...
pm me if you are after these.

[audiolabtower]

Here is the track side matched to the components



Circuit removed since wrong - see later in thread for correct circuit.

[clubsport911]

Curiosity has gotten the better of me... what does this circuit do ?

[audiolabtower]

If I've got it right it will flash the led when the peak power of an amplifier hits the (8 ohm) wattage in the list as selected by the switch to warn of the onset of clipping.

Mainly the led would glow at the low level signal setting and was arranged to be masked by it's housing such that if you could see both leds of the pair then you were guaranteed to have a direct sound of +/- 2dB of flat frequency response and hence top quality stereo image.

[speakerguru]

To double check the tracking, you could remove the "yell" label with a scalpel or razor blade and then re-stick it in place.

Are you sure about the 10 Ohm resistor? I seem to remember it was in series with the whole circuit, all of the time. I remember we had to change it for one with a higher current rating after some reports of it fusing open circuit.

Remind me what were the switch position markings? Apart from the clipping indicator at different power levels and the "listening window" position, what else was there?

[audiolabtower]

Hi, the speakers are all screwed up now with the space where the crossovers were packed up with foam. I took the pics before the last "spring clean" with new terminals etc, so not too easy to take it all apart again.

The "yel" label was mine when I originally rewired them years ago, the factory labels were placed so as to not cover stuff up!!

Switch positions, in order, are top right in the diagram, 8 power levels from 200W to 40W, then listening position and then off.

The 10 ohm is the middle resistor in the vertical row of 3 next to the elcap. If we assume the top contact of the rotary is the pole, it is connected to switch contact 9 of 10, the other end is connected to the junction of the 2 diodes, the 10k base feed, emitter of Q2 and the thyristor if you follow the track. If contact 9 is the listening window then it short circuits the rectifier diodes.

But if my guess of the contacts is wrong then the whole thing is wrong.

[Thierry]

[speakerguru]