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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only. |
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7th May 2018, 7:19 pm | #1 |
Octode
Join Date: Jul 2014
Location: Petersfield, Hampshire, UK.
Posts: 1,043
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Grundig music boy universal 206 local osc problem
Hi
The radio works ok on vhf except below about 92 m/cls it stops oscillating I have changed the af125 and sub most of the components incl the v/c ( no it's not shorting ) I have tried other transistors with the same result It calibrates ok just won't oscillate below 92 megs ! When I tune it at 92 m/cls with a scope attached it's like a switch going on/ off Not graduly decreasing in amplitude Has any body with experience of these radios had this fault ! Circuit attached of osc section Cheers Pete |
7th May 2018, 8:22 pm | #2 |
Heptode
Join Date: Aug 2009
Location: Worthing, Sussex, UK.
Posts: 662
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Re: Grundig music boy universal 206 local osc problem
I have seen this before but with a different circuit. Turned out to be low gain on the feedback to the oscillator caused by a low power supply to the oscillator circuit.
As the radio calibrates correctly it is not likely to be the caps or inductors in the circuit. Check that the voltages on the transistor pins match what is on the circuit diagram and how these voltages are derived. |
7th May 2018, 8:50 pm | #3 |
Octode
Join Date: Jul 2014
Location: Petersfield, Hampshire, UK.
Posts: 1,043
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Re: Grundig music boy universal 206 local osc problem
There is about 7 v between the c and e
Bias seems ok too |
7th May 2018, 9:54 pm | #4 |
Heptode
Join Date: Aug 2009
Location: Worthing, Sussex, UK.
Posts: 662
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Re: Grundig music boy universal 206 local osc problem
Also check the base de-coupling caps. Try adding another cap in parallel with each as a quick test.
If any of these are oc it can result in a loss of gain. Around the osc circuit this could well have the same effect. |
8th May 2018, 3:14 pm | #5 |
Octode
Join Date: Nov 2005
Location: Surrey, UK.
Posts: 1,874
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Re: Grundig music boy universal 206 local osc problem
Check for a bent plate in the tuning capacitor - osc section.
Leon. |
8th May 2018, 3:17 pm | #6 |
Octode
Join Date: Nov 2005
Location: Surrey, UK.
Posts: 1,874
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Re: Grundig music boy universal 206 local osc problem
Just realised what "vc" refers to. Best not to use undefined abbreviations unless standard. A check on supplies, bias (check the bias diode carefully) and decouping is next.
Leon. |
8th May 2018, 7:23 pm | #7 |
Octode
Join Date: Jul 2014
Location: Petersfield, Hampshire, UK.
Posts: 1,043
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Re: Grundig music boy universal 206 local osc problem
I have added some de couplings caps to the base ,still the same
The bias is fed from a 1.4 v source which is correct I'm stumped ! |