G8MNY TECH 26.08.19 05:51l 94 Lines 4800 Bytes #365 (0) @ WW
BID : 18970_GB7CIP
Subj: AM TRF Radio
Path: OK0NBR IW0QNL HB9ON IW2OHX UA6ADV CX2SA N3HYM WH6FQE GB7YEW GB7CIP
Sent: 190826/0848Z @:GB7CIP.#32.GBR.EURO #:18970 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW

By G8MNY (New Nov 08)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
In a recent construction contest, we all had to build this QUASAR project kit No 3063 "One Chip AM Radio". Here I have redrawn the circuit in a more logical way.
┌─────────────────────────────────┬─────┬────┬──── +9V
4k7 5k6 │ │
├───┬──────────────┐ │ │/BC +│100u
1k │ │ ├───┤ 548 ===
┌────┬────┤ │ 10k _│_ │\e │
100k │ │ │ u47 │ \_/ 6R8 │
Rod │ ┌──┴──┐ ├───)─┤├──┬──150k──┤ ┌──────)─────┤ │+ 100u
┌──┬───)─┤MK484│ │ │ │ │ 820k _│_ ├────)─┤├─ LS
||( │_ │ └──┬──┘ │ │ │ ┌────┤ │ \_/ 6R8 │
||( │/| │ │ │ ├──┐ │ │/ │ ├─┤├───┤ │/e │ 8Ω
||( === │ │ === │ _│_ └─┤BC548 │ │n47 ├───┤ BC │
||( /│ │ │ u1│ │ \_/ │\e │ u47 │ │/ │\558 │
└──┴───┤ │ │ === │ │ 10k ─┤├──┴────┤ │ │
160pF │ │ │u47│ _│_ 100 │Log BC│\e │ │ ┌─ LS
=== │ │ │ \_/ │ │Volume 548 │ │ │ │
u47└────┴────┴───┴──┴──────┴────┴──────────────┴─────┴────┴──┴─ 0V

Construction was easy, about 30 min to do a good job. Most kits work OK.

The Tuned Radio Frequency Rx is fairly sensitive, but the high RF gain in this not flat & there is a tendency to get RF oscillation at the high frequency end of the band. Selectivity is surprisingly good. N.B. the IC runs at 1.2V maintained by the 2 diodes. 1.5 of an LED is a bit too high. The 1k load & 100k bias are said to be the optimum values for the IC's detecting & AGC action.

The AF amp is very poor, why they did not use an IC I don't know. The 1st class A stage in run on next to no voltage so it distorts. The following class A driver stage has too little standing current to drive the output NPN BC548 to deliver +ve current swings into the 8 ohm LS.

There is no LW option, Ideally a 2nd coil on the ferrite rod is needed.

MY IMPROVEMENTS
┌──────────────────┬──────────────┬─────┬────┬o\── +9V
│ │ bootstrap 1k │ │ ON
4k7 │ ┌──────┤ │ │
├───┐ │ +│ 3k3 │/BC +│100u
1k │ 10k === ├───┤548 ===
┌──────┬────┤ │ u47 │ │15u _│_ │\e │
100k │ ├───)─┤├──┬──150k──┤ │ \_/ 6R8 │
Rod Cd │ ┌──┴──┐ │ │ │ │ ├──────)─────┤ │+100u
┌──┬──┬──)──┬┤MK484│ │ │ │ ┌────┤ 820k _│_ ├────)─┤├─ LS ||( │_=== │ │└──┬──┘ │ │ │ │ 10k │ \_/ 6R8 │
||( │/|│ │ === │ │ ├──┐ │ │/ │ ├─┤├───┤ │/e │ 32Ω ||( === │ │ │n82│ === │ _│_ └─┤BC548 │ │n47 ├───┤BC │
||( /│ Rd │ oLW │ u1│ │ \_/ │\e │ u47 │ │/ │\558 │
└──┴──┴──┤ \ │ │ === │ │ 10k ─┤├──┴────┤ │ │
160pF ├──┘ │ │u47│ _│_ 100 │Log BC│\e │ │ ┌─ LS
=== │ │ │ \_/ │ │Volume 548 │ │ │ │
u47└──────┴────┴───┴──┴──────┴────┴──────────────┴─────┴────┴──┴─ 0V

Oscillation at HF end of MW.
By adding a light damping CR across the tuned circuit stops the oscillation, the Cd is kept very small, a 10pF, & Rd = 330k did it OK without degrading the performance.

1st AF amp distortion fault.
Running this AF amp from +9V gives lots more unclipped gain, but the unregulated DC feed can lead to instability with a high gain output stage, so I cut the track to the pot & added a 10k in series to attenuate the gain a bit.

Bootstrap.
I added a bootstrap to the output by changing the 5k6 for a 1k from +9V & a 3k3 to the NPN base, with the centre tap connected to a new cap to the output NPN / PNP. This provides a lot more +ve output drive current, without increasing the standing current & also increased the gain.

LS.
Using a higher 32 ohm LS reduces the LS current by 4, but I did not have one & had to make do with the 8Ω, which worked OK once boxed up.

LW.
Adding a switched 820 or 910pF gave tuning over 1 LW station @ 198KHz OK. I put it across the unused 60pF tuning cap section & switch both in circuit to gain the max tuning range. It is not very sensitive done this way, switching the coil to a bigger one would me much better!

ON OFF
I added an ON-OFF switch too.


Why Don't U send an interesting bul?


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