Blaupunkt Oxford M 26 car radio cassette player - UK Vintage Radio Repair and Restoration Discussion Forum

Techman · 18th Dec 2019, 2:14 am

I was asked to look at this unit at the weekend. The owner had bought it recently as 'un-tested' from an on-line sale. He wanted it to fit into an early VW Golf. He hadn't tried it himself, as he was unsure of the electrical connections, so brought it round to me to test. After some fettling and cleaning of volume control and band switching etc, the radio side worked well on all four outputs. However, the cassette mechanism although functioning, will only do so if the tape is held in engagement by hand. A bit of quick online research would seem to indicate that this could be a 'stock fault' with these, but with no answer as to the cause. The latching mechanism is held in by a solenoid, the question being what energizes the solenoid? Holding the solenoid in with a finger causes the mechanism to latch properly, but doesn't stay latched in. There strangely seems to be something like 10 volts across the solenoid when NOT energized, but this disappears when the cassette is pushed in - the solenoid coil seems to have continuity. I haven't delved too deeply yet, but I can't see any sort of switch contacts to energize the solenoid when the tape is pushed in, so perhaps it's electronic. I'm not going to spend too much time on it, but any ideas before I dig deeper into the unit? There may be some contacts in there that I can't see at the moment without doing a lot more dismantling.

AD360 Rob · 18th Dec 2019, 8:10 am

This looks similar to the mech used on some 80's sharp units. They also suffered this fault.
The solenoid is controlled by a sensor on the take up spool, there is usually a wire running from this sensor (black IIRC) to the pcb. This wire will have broken or be high resistance . It's caused by the tape mech flexing the wire every time a tape is inserted or ejected.

Rob

'LIVEWIRE?' · 18th Dec 2019, 10:19 am

Rob, it may look like the mech. used in some Sharp Car Stereos, but it's a Blaupunkt designed and built tape mechanism. The solenoid in the centre holds the deck up in the playing position. This in turn is controlled by the circuit on the PCB above the rear of the mech. I don't recall the details of the circuit or what ICs are involved, though I may have a diagram somewhere. The easiest thing to check is whether that solenoid is o/c or not. Other possible faults include the 'tape in' sensor, which is an optical pulse switch. IIRC, the take-up spool consists of a slip-clutch on top of which is a disc having alternate black & sliver segments. These produce a train of pulses detected by a phototransistor/diode assembly, which are fed to, and detected by, a circuit on the PCB. If the pulses stop, e.g. at the end of a tape, the cassette is ejected. The fault could be in any of these parts, most of which were custom made by Blaupunkt, exclusive to them, and long since obsiolete. (Techman,sorry if all this is putting you off attempting to repair the Oxford)

Techman · 19th Dec 2019, 2:09 am

Many thanks guys.

I was wondering if it was an electronic sensor picking up pulses. I haven't had a chance to look at it anymore, but will have a closer look tomorrow, so we'll see if it is down to either lack of pulses or lack of the pulses being processed.

The chap that owns it has another one already fitted in the car, but this is also faulty, but he was unclear on exactly what is wrong with it. He hasn't had the car very long and is going to check out the other radio cassette properly and probably bring that one down this weekend, so if it is the same model then perhaps one good one can be made up from the two - we'll see.

Livewire - I thought you would probably be the man that had experience with these semi-vintage car radio cassette units and I suspected that as you say, the parts are specific to Blaupunkt and are likely to be long obsolete.

AD360 Rob · 19th Dec 2019, 7:54 am

Livewire, I should have known Blaupunkt would have produced their own mech. However, the principal for tape in detection is the same, a sensor on the take up spool (Optical for Blaupunkt, mechanical for sharp) feeds a circuit on the pcb that powers the solonoid. As I said, with the sharp there was a wire from the sensor to the PCB that broke . How does the Blaupunkt get the pulses from the optical sensor to the PCB ?

'LIVEWIRE?' · 19th Dec 2019, 10:00 am

Basically, IIRC, the pulses are fed down a wire link to the control PCB, in the form of a varying voltage which can be detected with am analogue meter or 'scope. They are processed in one of the ICs.

18th Dec 2019, 2:14 am	#1
Techman Dekatron Join Date: Nov 2009 Location: Lincolnshire, UK. Posts: 5,000	Blaupunkt Oxford M 26 car radio cassette player I was asked to look at this unit at the weekend. The owner had bought it recently as 'un-tested' from an on-line sale. He wanted it to fit into an early VW Golf. He hadn't tried it himself, as he was unsure of the electrical connections, so brought it round to me to test. After some fettling and cleaning of volume control and band switching etc, the radio side worked well on all four outputs. However, the cassette mechanism although functioning, will only do so if the tape is held in engagement by hand. A bit of quick online research would seem to indicate that this could be a 'stock fault' with these, but with no answer as to the cause. The latching mechanism is held in by a solenoid, the question being what energizes the solenoid? Holding the solenoid in with a finger causes the mechanism to latch properly, but doesn't stay latched in. There strangely seems to be something like 10 volts across the solenoid when NOT energized, but this disappears when the cassette is pushed in - the solenoid coil seems to have continuity. I haven't delved too deeply yet, but I can't see any sort of switch contacts to energize the solenoid when the tape is pushed in, so perhaps it's electronic. I'm not going to spend too much time on it, but any ideas before I dig deeper into the unit? There may be some contacts in there that I can't see at the moment without doing a lot more dismantling. Attached Thumbnails

18th Dec 2019, 8:10 am	#2
AD360 Rob Hexode Join Date: Jun 2012 Location: Chester, Cheshire, UK. Posts: 421	Re: Blaupunkt Oxford M 26 car radio cassette player This looks similar to the mech used on some 80's sharp units. They also suffered this fault. The solenoid is controlled by a sensor on the take up spool, there is usually a wire running from this sensor (black IIRC) to the pcb. This wire will have broken or be high resistance . It's caused by the tape mech flexing the wire every time a tape is inserted or ejected. Rob

18th Dec 2019, 10:19 am	#3
'LIVEWIRE?' Rest in Peace Join Date: Mar 2006 Location: N.W. Oxfordshire(Chipping Norton) Posts: 7,306	Re: Blaupunkt Oxford M 26 car radio cassette player Rob, it may look like the mech. used in some Sharp Car Stereos, but it's a Blaupunkt designed and built tape mechanism. The solenoid in the centre holds the deck up in the playing position. This in turn is controlled by the circuit on the PCB above the rear of the mech. I don't recall the details of the circuit or what ICs are involved, though I may have a diagram somewhere. The easiest thing to check is whether that solenoid is o/c or not. Other possible faults include the 'tape in' sensor, which is an optical pulse switch. IIRC, the take-up spool consists of a slip-clutch on top of which is a disc having alternate black & sliver segments. These produce a train of pulses detected by a phototransistor/diode assembly, which are fed to, and detected by, a circuit on the PCB. If the pulses stop, e.g. at the end of a tape, the cassette is ejected. The fault could be in any of these parts, most of which were custom made by Blaupunkt, exclusive to them, and long since obsiolete. (Techman,sorry if all this is putting you off attempting to repair the Oxford)

19th Dec 2019, 2:09 am	#4
Techman Dekatron Join Date: Nov 2009 Location: Lincolnshire, UK. Posts: 5,000	Re: Blaupunkt Oxford M 26 car radio cassette player Many thanks guys. I was wondering if it was an electronic sensor picking up pulses. I haven't had a chance to look at it anymore, but will have a closer look tomorrow, so we'll see if it is down to either lack of pulses or lack of the pulses being processed. The chap that owns it has another one already fitted in the car, but this is also faulty, but he was unclear on exactly what is wrong with it. He hasn't had the car very long and is going to check out the other radio cassette properly and probably bring that one down this weekend, so if it is the same model then perhaps one good one can be made up from the two - we'll see. Livewire - I thought you would probably be the man that had experience with these semi-vintage car radio cassette units and I suspected that as you say, the parts are specific to Blaupunkt and are likely to be long obsolete.

19th Dec 2019, 7:54 am	#5
AD360 Rob Hexode Join Date: Jun 2012 Location: Chester, Cheshire, UK. Posts: 421	Re: Blaupunkt Oxford M 26 car radio cassette player Livewire, I should have known Blaupunkt would have produced their own mech. However, the principal for tape in detection is the same, a sensor on the take up spool (Optical for Blaupunkt, mechanical for sharp) feeds a circuit on the pcb that powers the solonoid. As I said, with the sharp there was a wire from the sensor to the PCB that broke . How does the Blaupunkt get the pulses from the optical sensor to the PCB ?

19th Dec 2019, 10:00 am	#6
'LIVEWIRE?' Rest in Peace Join Date: Mar 2006 Location: N.W. Oxfordshire(Chipping Norton) Posts: 7,306	Re: Blaupunkt Oxford M 26 car radio cassette player Basically, IIRC, the pulses are fed down a wire link to the control PCB, in the form of a varying voltage which can be detected with am analogue meter or 'scope. They are processed in one of the ICs.