Grundig 110a GB radio

[boater sam]

I picked this up from somewhere a while back, not my usual radio, its far too modern and full of 3 legged fuses.
It works a bit on all wavebands but the AM sound is heavily modulated with mains hum. Probably the smoothing capacitors have dried out.
But I am having difficulty in understanding the power supply arrangements.
There are 2 bridge rectifiers but only the output of one seems to be smoothed. The other has nothing at all to smooth the DC.
Electrotanya provided the schematics, there is also an early version, 110 as well as the later 110a. Same power arrangement.
I felt the tuning cord jump off something so it will have to come to bits to get at it. Why is it that the more modern stuff is shoddy compared with the '30s and '40s stuff? I suppose its all down to cost engineering.

[radioman]

.....
But I am having difficulty in understanding the power supply arrangements.
There are 2 bridge rectifiers but only the output of one seems to be smoothed. The other has nothing at all to smooth the DC.

Hi Sam,
I had a quick look at the circuit diagram and it's certainly a bit hard to follow....
The output of the second bridge-rectifier, eventually reaches C67 (1000uF), so it is smoothed, it's just been drawn in the vicinity of the output stage instead of near the bridge....
Good luck in getting it going.

Andy
   

[boater sam]

.....
But I am having difficulty in understanding the power supply arrangements.
There are 2 bridge rectifiers but only the output of one seems to be smoothed. The other has nothing at all to smooth the DC.

Hi Sam,
I had a quick look at the circuit diagram and it's certainly a bit hard to follow....
The output of the second bridge-rectifier, eventually reaches C67 (1000uF), so it is smoothed, it's just been drawn in the vicinity of the output stage instead of near the bridge....
Good luck in getting it going.

Andy

Thanks Andy,
 I see that now. Not the usual way to draw a circuit.
Considering the pi network to smooth the other bridge rectifier, that seems most odd to me.
I will attempt to get the cord drive restrung today, after I have glued the floating tag strip back onto the speaker basket.
Grundig quality certainly seems to have taken a dive in the '60s. 

[Mark Hennessy]

It's not just badly drawn - par for the course for Grundig, IMO - but it is an unusual topology.

The radio is positive earth, and everything apart from the output stage runs from -12V. That is generated by the middle of the two windings, and stabilised by the zener diode. That much is easy to see.

What is different about this is the winding for the output stage, which yes, is smoothed by C67. This is around 12V.

I had to sketch it out to be sure, but this is a variation of the Transova circuit. It has been used by several manufacturers over the years, including Haffler and QSC. UK monitor speaker manufacturer ATC have used it (probably still do?) in their active speakers - they call it "grounded source" in their marketing. I understand the concept was engineered for them by Tim Issacs. But I think this radio might be the earliest commercial example - at least, it's the earliest I've seen so far...

With such an amplifier, the output stage is powered by a separate supply. This supply has to be isolated from all others, which means a 3 way active speaker needs 3 separate centre-tapped windings in addition to the winding for all the small-signal electronics. This makes it inconvenient for DIYers who don't want the expense of a custom transformer or multiple transformers in the same box.

Why does the supply for the power stage need to be separate? Because the whole thing moves up and down with the signal. Contrast that with a regular amplifier where the PSU is effectively grounded and the signal moves within the confines of the rails. Essentially, instead of grounding the negative side of the speaker in the usual way, the output side of the speaker is grounded (hence "grounded source").

What's the point? So far, it's extra transformer windings, and the need to wrap your head around a new topology. Standard topologies have been around forever, and work well.

Naturally people like to experiment, and innovation is useful, perhaps if only indirectly. But here, there are some advantages.

Think about the circuitry that drives a conventional unity gain output stage (usually emitter followers, as in this example). The voltage driving them is the same voltage that ends up across the loudspeaker, so 80V peak to peak for a 100W/8R amplifier. The small-signal stages prior to the output stage must work at this voltage, obviously.

But with the ground moved (and the power supply to the output stage allowed to bob up and down), the signal driving the output stage is now driving the base-emitter junctions of the same output transistors. By moving the earth to the other side of the loudspeaker, the output stage has gone from emitter follower to common-emitter. This means that, with respect to earth, the preceding stages only need to generate a voltage swing of a couple of volts, in round numbers, rather than 80V peak to peak. The output stage provides the voltage gain, rather than the usual VAS ahead of it.

So what? Small signal transistors are cheap and widely available, right? Not so much today. Which is why I've always been quite interested in this topology - it potentially makes for a cheap and simple amplifier that might just use op-amps to drive the output transistors. If you want more power, just scale up the supply to the output transistors - everything else still runs on the low value rails, which might be +/-15V or less. For that reason, it is often seen in PA amplifiers (though most have gone to class D now). Great for an active speaker where you might want 200W for the woofer, 75W for the midrange and 25W for the tweeter, say. Yes, it means a custom transformer, but for a manufacturer mass-producing these, that's not expensive.

But no real advantage for a lower power amplifier, where suitable small-signal transistors are still plentiful and cheap.

So why on earth have Grundig used it here? I've honestly no idea. As I say, it's great to experiment, but quite how it left the R&D lab is beyond me, as I'm not seeing any of the advantages in a ~2 watt table radio, and the added cost of an extra transformer winding and extra rectifiers and caps. The fact they dropped it for the RF111 says quite a lot, really...

There's a similar discussion here: https://www.vintage-radio.net/forum/show...p?t=186598

In the last post I've attached a sketch of the output stage which might make the concept slightly clearer.

[boater sam]

Thank you Mark, I am understanding the circuit better now but would ask WHY? seems to be an unnecessary complication for a cheap radio.

On first checking the electrolytic caps they seem to be in the ballpark, I will disconnect one end of them and test properly later.

The dial cord is a devil to do, its behind the backlight panel and is the thinnest cord I have ever seen. 
Its as thin as a wire drive and rotten. I tried joining it but it snaps again when you wind to the end of the scale. 
I will have to find some very thin super strong cord, my stock bead cord at 0.9mm is way too thick.

[Mike Watterson]

Nylon fishing line or guitar string (steel wire or nylon). I've used both on sets that used either fine cord, in in case of one Ever Ready and one Phillips, actual steel wire was original.

[PerdioPal]

You can get very thin stranded stainless steel wire from fishing supplies.

[boater sam]

I have never had much success with fishing line, I prefer fine bead cord. I have found some fine cord. 
I managed to remove the illuminating plastic panel after unsoldering all the pilot lamp wiring. Noting as I went that the tiny 7v 35mA wedge bulbs are both blown, the MES 7v bulbs are OK.  Anyone know where these tiny bulbs could be sourced?

Having had several attempts to restring the drive which looks quite simple on paper I have taken a break before the patience dissolves.
Getting the spring in the right place is not simple. The difficulty is compounded by the tuning capacitor being geared so that the drum rotates in the opposite way and turns two turns to one of the capacitor. 
The diagram is confusing. The notch in the edge of the pcb should line up the spring in the correct place but it shows the drum fully rotated in the opposite direction if the end of the cord is in the groove. On this pcb the groove is filled with solder anyway.

I will get it right, eventually but at present it is a mind bender. I may get the thing working electrically first anyway as the cord runs across all the electrolytic cap solder joints and knowing my shaking hands I will likely melt the new cord whilst soldering!

[boater sam]

I seem to be getting nowhere with this set.
I have disconnected one end of these capacitors and measured them on the magic box as follows:-
                   
                    Nominal uf      Actual      Loss%      ESR ohms
C61 (tant)         47                 69           0.6            4.3
C62                  100              125           1.3            3.3
C64                  100              149           2.2            3.4
C65                  50                66            2.0             5.2
C66                  1000            1257         0.6             3.2
C67                  1000            1203         1.2             2.9


Which to my way of reasoning they should all be working well enough but it still hums like crazy.
I checked that tant because I don't trust them.

[Mark Hennessy]

I'd say those seem OK. Not in brilliant form, but good enough. You can always double-check by tacking a new low-ESR cap across the back of each.

And, as the hum is only on AM, that probably rules out all of the audio amp caps. I'd be looking further downstream, such as C48 and C49.

Have you had a poke around with a 'scope yet? It might be worth making sure there isn't too much ripple on the -12V rail, just in case the zener is playing up (unlikely, but not impossible).

And is it definitely mains hum, locked to the "line" trigger source on your 'scope? I've had instability that sounded like hum, only it just happened to be near-ish to mains frequency, as luck(!) would have it.

In lieu of a scope, the hum disappearing the instant the mains is pulled - assuming the radio plays for a few seconds before dying - can be a useful indicator.

Another test might be to lift the left hand end of R55 (the end coming from C64) and apply a voltage from an external bench power supply - whatever voltage is needed to result in -12V at the zener. If you don't have a bench PSU, a PP3 across the zener might do - the radio likely would be happy enough at the lower voltage.

(Sorry - can't remember what you've got in your workshop. But then, I can't remember what I was doing yesterday Do you still have those Telequipment scopes?)

Cheers,

Mark