Triang Child's Record Player - Cartridge

[af024]

The addition of a 100uF capacitor really cranks the gain up. Depending upon which cartridge (or home brew contraption) I fit, will, I suspect, depend upon whether I need to divide the resistor or not.

I'll keep everyone posted.

Andy

[Paul Stenning]

Originally Posted by af024

Hey, that's clever Paul - you've even done the mod in my writing!

I just moved bits around in PhotoPaint - I didn't draw/write anything!

[af024]

Well it now looks like my problems are multiplying. Not only have I now managed to wreak my Acos 101 cartridge (too much filing etc), but the amplifier now suffers from thermal runaway to the extent that the current drawn from the PP9 is enough to stall the motor! It runs ok for 2 mins, then that's your lot. Both output transistors then get very hot indeed. Amazingly, removing the power and waiting for 5 mins or so causes the thing to work again. I thought that maybe I'd caused this by messing about with the gain, but even with the 100uF cap removed, it does exactly the same thing. The two T4 transistors in question are not fitted with a heatsink. Perhaps that's my next move?

Oh, in view of the fact that I killed my only small-ish cartridge, I decided to order some of those 'Bimorph Vibration Elements' from RS. I've not really thought too much about how I might deploy them, but it sounds a fun thing to mess about with all the same. The data suggests that 'cantiliever vibration' is what it responds to, so presumably, if I want to re-create a mono cartridge, I need to mount the thing under the stylus so that side-to-side vibrations cause the cantilever action in question. Just for interest, the data reckons that its impedance is 1M ohm.

Andy

[Trevor]

it may well be that the thing is oscillating at a supersonic frequency if you remove the 100 uf capacitor try it again and if it plays for more than 5 mins thats a likely cause
do you have a scope ? to look at the output
REGARDS TREV

[af024]

That's what I thought Trevor, but it does just the same with the cap removed. Interesting eh.

Andy

[Trevor]

Welll the next thing to do is put a low value pot between the bases of the out put devices and adjust the bias starting with the pot set to 0 ohms adjust while playing a record untill the xover distortion clears
and than fit a resistor of that value .
If this does not work its more than likely that 1 of the output devices is defective
regards Trev

[af024]

Thanks Trevor.

I guess I need to sketch out the rest of the amplifier circuit to be honest.

Andy

[af024]

I closely inspected the two final drive transistors and found that one was very poorly mounted (wasn’t easy to see) – so much so that two of its legs were shorted together! I removed it and re-seated it accordingly. Amazingly both transistors have survived. I wonder how long it’s been like that?

Anyhow, I have made another interesting observation, one of the ‘T4’ transistors (the one that was shorted), has the white spot on the wrong side. I always thought that the spot (regardless of colour) denoted the collector. Well in this case, on this transistor, and this one alone, it was alongside the emitter! I wonder how that happened? Anyway, why are some transistors marked with a white dot and others with a red?

Anyhow, it doesn’t cook anymore and seems to behave ok thank goodness.

Just so you can see how things are coming along, I have loaded up some photos.

The first few show how I re-stuffed two sick caps and the last couple show my attempt at making a new +ve battery clip (both the clip and the black card bit was missing). I used a clip off a dead PP9 and made the black bit out of two bits of card which were glued together, shaped with scissors, wire clippers, files etc and then painted satin black. This pretty much matches the look and feel of the one remaining original.

The last shot shows the amp back in situ under the deck. Can you tell which caps were replaced? I hope not.

All I need now is to get that cartridge sorted out.

PS I’m still trying to work out the circuit for a sketch.

[af024]

Remaining photos ...

[Nickthedentist]

Very nice work, Andy, I admire your attention to detail

Intriguing to see that it's got a diecast chassis - not what you'd expect on a "toy" record player.

I wonder how the cartridge came to be so corroded, yet the rest of it is pristine.

Nick.

[Paul Stenning]

The output and driver stages are probably something like this, though component values will vary.

http://www.vintage-radio.com/images/...-output-2a.gif

[af024]

Thanks for your compliments Nick - a good question about the cartridge, mind you, the battery clamps (spring steel and not shown in the photos) are also in a mess.

Thanks for your circuit Paul I am trying to make a comparison now. Here is what I think mine looks like this ... (not sure if I understand the result to be honest).

Andy

[Paul Stenning]

The transformer will have three windings, the same as the one in my diagram. Because it has three pins each side, one winding will be between a pin on one side and a pin on the other side. So the bottom left and bottom right points on the transformer in your drawing are actually a separate winding and not tappings on the other windings. You could confirm this with a meter, but might need to remove the transformer form the PCB to do so.

Also you have the emitter and collector of the top output transistor the wrong way around.

With these corrections it should look similar to the one I posted. In fact apart from the lack of emitter resistors on the output transistors and some different resistor values I would say it is the same as the circuit I posted.

[Trevor]

I agree your circuit is wrong !
its more than likely similar to Pauls !!
The spot was always used to mark the collector
Different colours were used top mark differant frequency ranges in the past .
Allthough differant coloured spots have been used to denote pnp/npn
regards Trev

[af024]

OK thanks all, I'll take another look and see where I went wrong.

Andy

[af024]

OK, I think I've got it now. The top transistor was indeed incorrectly drawn. The white spot being marked on the wrong side of the transistor caused me some confusion there.

I took the transformer out in order to determine once and for all what the arrangement was.

I am happy to report that Paul's theory was correct.

I am attaching the final circuit and corresponding component layout here for everyone's interest.

I still have to sort something out for the cartridge though. Mind you, the Bimorph elements should arrive on Monday, so I'll explore that next.

Thanks for everyone's help.

Andy

[af024]

How strange, I wondered what happened to the component layout picture? Let me try again ...

[Paul Stenning]

I was a bit bored this afternoon, so here's a CAD drawn version....

[af024]

Good gracious Paul - that's brilliant! I can hardly believe my eyes. How magic is that?

Many thanks - that's much much better than my untidy mess.

I think I'll print that out and keep it with the player for future reference.

I must say that I was surprised to find that the transformer was wound in that particular way - just goes to show how you can get sent up the garden path.

What about that T4 transistor with the white dot alongside the emitter eh? Now that was a turn-up for the books!

Thanks again Paul!

Andy

[af024]

Well I got my bimorph vibration elements. I tried various orientations in the insides of the remains of the original jelly material (that was part of the original cartridge) and they sort-of worked. I wouldn't say that it was particularly successful however. They are just a bit too long and fowl the rear peg. As a consequence I had to slide the element in at a bit of an angle. I can hardly see the fine wires!

The level was well down, so I decided to try two in series (but it was difficult to know what the phases are so I tried it wired up differently to see what happened). It appeared a bit better (perhaps), but still not really there.

Eventually I decided to strap the remains of the old stylus directly to the element using some rubber sleeving and I positioned the whole contraption, using tape, so that it looked roughly in the right place to play a record. This worked MUCH better. The trouble is I can't really work out how to mount it properly and in a manner which allows the stylus to be changed by anyone post-me.

Then the amp started getting hot again. No, nothing was shorted this time (not sure whether there was anything shorted before now!). Geez.

I also managed to crack open the Acos 101. Well actually I didn't need to hack it about at all. I found a tiny pip of plastic to the rear, which, when pressed, released a slider which allowed me to extract the innards buy sliding them out. Guess what ... its innards are also too long to slip into the remains of my original jelly material!

So two problems still remain. Maybe I just need to sit back and have more of think about the bimorph elements and how to support them. I must say that I was quite impressed considering my lash-up.


Andy