Motorola 114 Car Radio

[Mike Watterson]

They can SEEM to increase on some testers if there is leakage.

[Old Sparky]

Terry, that thought did occur to me, particularly as I don't know the history of this radio, other than at some point it had water intrusion, mainly in the rear LH corner where the power & speaker leads enter/exit the radio, but I can certainly try adjusting the oscillator tuning coil (it is an entirely conventional early 70s car radio with 3-stage permeability tuning, b.t.w., which was sent tom my a member of UKVRRR for repair.
As to the oscillator trimmer & padder caps., I haven't yet checked them for capacitance. They are probably within tolerance, but I was speculating as to the (few) possible causes of the upper freq. being about 300kHz LF of where it should be, after having replaced a faulty polarity c/o switch, faulty Electrolytics, and a 560 ohm resistor which read over 700 ohms, expecting the set to then work normally, only to be confronted with the tuning fault.

[Old Sparky]

I filaly found the fault! An intermittent connection between the coax. aerial cable, it's in-line choke and the aerial trimmer. Having checked and put back the oscillator trimmer & padder caps (no measurable faults in these), reception was still poor, but intermittent due to the above fault, though the stations are at approximately the correct positions on the dial. Did I correct a bad connection or dry solder joint removing & refitting the capacitors? I'll never know now!!

[Old Sparky]

The other day I went back to the 114, and discovered that the RF Alignment was out especially the RF & Mixer stages, so a quick tune up brought the radio up to par sensitivity and outpul wise. Whether the problem was down to water ingress I'm not sure, though there is no sign of water damage anywhere in the RF or IF stages. The radio will now produce about 6 watts o/p into 3 ohms @ 14v on a strong local station, and receives the four of five MW and one LW (Radio 4) that I'd expect to be able to listen to. The o/p power falls quite dramatically with reducing supply voltage, down to about 2watts or less @11v supply, but as Motorola didn't include any form of stabiliser in the design, this is only to be expected. Subject to a ;soak' test, this job is now finished.

[TNC]

Am I allowed to piggy-back on this thread?
I'm just beginning to work on a 70s Blaupunkt 6v transistoir car radio.

MW just about works LW has hiss but that's all, but when the FM is engaged
there is a brief, faint hum and then all sound disappears....haven't checked current
under that condition yet.
Any suggestions would be most welcome.

Thanks

Trevor

[Old Sparky]

What model Blaupunkt 6v Car radio do you have, Trevor? It's possible that among my large collection of Blaupunkt Service manuals and diagrams, I may have one for your set, but I'd need to know the model name (usually it's a German Town or City) serial no., including the first letter, and, if possible, the part Number, which begins 76xxxxxxxxxxx. as the same model name was used over a number of years. Incidentally I'd have thought a 6v Radio more likely to date from the 60s than the 70s. FM not working is most likely due to a front-end fault, though there could be a problem in the ratio detector circuitry. Pretty well all modes used the same transistors in a common FM/AM I.F. strip, with the 465kHz and 10.7 Mhz I.F.Ts in series, so yjr problem is unlikely to be there. As with all such faults, checking the bandswitcing, then working back from the last I.F, injecting a 10.7Mhz. signal (if you have the means to do so) is the way to go. Maybe start by injecting a 10.7Mhz FM signal at the tuner's output, since, IMHO, the fault is unlikely to be in the I.F. strip.

[Terry]

Probably originally fitted in a VW beetle. It looks as if some models moved from 6V to 12V in 1967 whilst others clung on to their 6V batteries until 1972.

Assuming this radio is solid state, that is just the right time span to include Philips group transistors afflicted with the dreaded tin whisker disease.

What transistor types are fitted in the IF/RF stages - including the FM tuner head?

[Old Sparky]

AFAIK, Terry, Blaupunkt never used any of the transistors prone to 'tin whisker' problems, always using the AF12x or equivalent series in RF & IF stages.