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Old 13th May 2022, 10:46 pm   #1
MotorBikeLes
Octode
 
Join Date: Dec 2012
Location: Kirk Michael, Isle of Man
Posts: 1,993
Default Nordmende Globetrotter 808 output stage problem.

I have had this radio from (almost) new, certainly 40 years. I bought it directly from the NordMende UK importers. It had been sold, and was returned as faulty, and that was how it came to me.
One or both of the output transistors was/were faulty. I replaced them with a beefier pair: BD241/BD242 to replace the OEM BD239/BD330.
I have always used the optional dry lead accumulator (8v, comprised of a four cell battery). However, over the last 12 months or so, if I have plugged it into the mains to recharge the battery, the radio has frequently "locked up". There is a click and the audio disappears, though the tuning indicator tells me it is still tuned in OK. Switching off the mains, then the radio OFF and ON again normally restores things to normal, or even pressing various band change press buttons. It then behaves until next recharge, which I tended to do overnight to get around it as a problem. I assumed it was the battery needing to be replaced.
As reported elsewhere, I have just converted it to take lithium ion cells. Nominal 3.7v xs 2 = 7.4 v. However, they will reach around 9.2v under control by a dedicated charge controller. Whilst fitting the lithium ion cells, I decided to replace the various electrolytic capacitors in the PSU and output section.
The fault is still present, so NOT due to failing Accu. or electrolytic capacitors!
Before I started the battery conversion exercise, I connected the Accu connectors (battery removed) and ran it up on my Farnell power supply. I increased the voltage from around 5v ( = drift of tuning + distortion) steadily up to 15v. No misbehaving.
I had on my list to check quiescent current (10mA at 9v battery output (4 off "U2" types) which showed about 3.5mA, so I tweaked the control. It shot up to 37mA, and no adjustment of the pot would change things. I had ensured it was on FM, no station, min volume, but it occurred to me it may have once more "locked out". Turning up volume and altering FM tuning confirmed this. Would anybody suggest a first line of attack?
Les.
Attached Files
File Type: pdf Output circuit.pdf (47.4 KB, 52 views)
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