Grundig Emden III Car radio

[Old Sparky]

Is anyone out there more familiar with this early 1970's AM/FM  mono car radio than I?  I have one 'n progress' in which the VHF band works OK, but the LW & MW ranges produce only noise. This set uses BF184/185 transistors in it's RF/IF stages, and, somewhat unusually perhaps, has a seperate AM oscillator (BF184), which isn't oscillating.  I've checked, but not changed, the BF184, replaced a couple of capacitors in the Colpitts Oscillator circuit, and checked the coils for continuity, but have drawn a blank.  I have a circuit diagram, but not a PCB layout drawing.  If anyone can suggest a cause of this fault I'd be grateful.  I may be able to post an excerpt from the circuit diagram later.

[Terry]

... I've checked, but not changed, the BF184 ...

Presumably you mean you've tested it under static conditions rather than as an oscillator at up to 2MHz ...

If the static tests are fine, it doesn't necessarily follow that it will still function satisfactorily as intended ...

[Old Sparky]

Morning, Terry.  Yes I removed the BF184, partly because I because I (wrongly) assumed there was a s/c between the base & screen, until I realized that  The designers had intended it that way i.e. there is a track connection between the screen & base (not grounded).  The transistor checked out OK 'cold', using the diode check facility on my DMM, but the thought had occurred to me that it might be OK as an amplifier, but not as an oscillator  I've probably got  a BF 254 or 255 somewhere, if not a BF 184, so I can replace it an see what happens!

[Old Sparky]

This afternoon I replaced the BF184 with a BF254. Made no difference! I'm probably missing something simple, but I'm going round in circles and getting nowhere with this radio!

[Mark Hennessy]

Speaking generally, when I've had an AM oscillator fail to play ball, it's usually caused by the decoupling electrolytic going O/C. Next, volts to low - perhaps caused by the same cap, but leaking this time. I've also had problems with the oscillator stopping as you go down the band, and that is a problem with a lack of Q in the coil. I've also had leads inside the can corrode away.

Naturally, switch contacts and dry joints are another factor, but I'm sure you'll have already thought about that.

There's a famous adage among analogue designers: Amplifiers oscillate, oscillators don't

[Old Sparky]

Haven't checked the decoupling Electrolytic(s ) yet, Mark, but the w/c switches function as they should, and there are no dry joints. I've been over those on the main PCB, checked and resoldered all the wires from the main PCB to the AM Tuner coilpack besides replacing the osc. transistor and a couple of capacitors. It's not a radio with which I'm familiar, which doesn't help. I have a circuit diagram, but no PCB layout, so it's a case of tracing out what goes where. Unusually for an AM set, this one uses a separate grounded base oscillator.

[Mark Hennessy]

I've found the schematic on RM.org. The usual Grundig style - I've always struggled with the way they draw their schematics...

I suppose the obvious question is this: how do the DC conditions compare to those given on the schematic?

[Old Sparky]

I, too, find Grundig circuit diagrams a bit difficult to follow, despite having worked for 23 years for a Blaupunkt Car radio dealer. Got used to their diagrams I guess! As to the DC conditions of the Emden, they're similar to those on the diagram, so I'm still puzzled, not least by the fact that two of the eight connections on the AM coil pack read o/c, but I'm not sure whether there are meant to be 3 or 4 coils present. There are only 3 cores in the 'variometer' as I've sometimes seen it described. Gues I'll have to 'bite the bullet', remove and open the AM coil pack to check. Pity there isn't a PCB layout on RM.

[Mark Hennessy]

Yes, opening up the coil pack sounds like a good plan. In the past, I've had O/C because of corroded wires.

In the Hacker Hunter - but no other Hacker sets, peculiarly - I've had several cases of capacitor wires silently corroding away inside the can. These are the polystyrene types. It looks like a reaction with the adhesive in the yellow tape that is used to wrap the coils before the can is placed over them. Perversely, the caps in question are in the AM IFTs, but they cause weak signals on FM

[Old Sparky]

Having removed the coil pack from the radio (with great care because of the permeability tuning cores) I am at a loss to see how the aluminium screening case is removed in order to gain access to the 'innards'. The radio is in my workshop, not here at home, but I'll take a photo or two. Maybe someone can see how to dismantle the coil pack, which seems to have been pushed on over the coils & PCB, then somehow crimped in place.