Blaupunkt Car Radios

 I agreed to accept a job lot of German car radio circuit boards recently (November 2020) and when they arrived I discovered the box contained some 27 items. There were 5 complete faulty radios plus one said to be working, and some 21 small circuit boards. All are vintage, dating from the 1970s and I guess quite sought after for restoration of similarly aged cars. "Blaupunkt = Blue Spot"

 

 
 

 Above you can see 14 loose circuit boards, all looking similar but with minor differences such as the version of the dual potentiometer, either Type A or Type B.

 

 

 

 

 

 

 

 

 

 

 

 

 

To the left is a 15th similar board to those above but with a broken potentiometer. Also here are 6 boards which look different in respect of the rear section carrying power transistors, with one having extra transistors and a stereo rather than a mono control..

 

Below are detailed pictures of an example of the most common circuit board with Type A pot.

 

 

 Below is a representative circuit diagram showing enough to make sense of all the board types. You can see the main on/off switch H1 which is coupled to the rear of the volume/tone potentiometer. Most of the boards (those for mono reception of Long and Medium wavebands) have a pair of small complementary power transistors BD433 (npn) and BD434 (pnp) connected in a push-pull configuration. The loudspeaker connects to the output transistor via C213, a 1000uF electrolytic capacitor. Input to the circuit, which is a straightforward audio amplifier, is from the RF board and a DIN connector, which also carries a low level output to an external equipment such as a tape recorder. Power is provided from a charging 12 volt car battery (=14 volts) via a 2 Amp fuse to the on/off switch. All nice and simple without the complications of modern car radios. To see technical details of a typical Blaupunkt car radio, the "Hamburg" model, click the circuit diagram below.
 

 So, why are there so many failed circuit boards? Clearly there may well be a common problem.. so what is it? The circuit was used for many years and looks petty straightforward and if components were correctly specified, and of decent quality, should be very reliable. Capacitors might be troublesome, but in the 1970s these were inherently pretty reliable, unlike modern components which can give endless problems. One source of trouble, which has been overlooked over the decades in favour of component MTBFs, is printed circuit tracking. Many circuit boards have parts that can get very hot and, if tracking is not designed with this in mind, metal fatigue will cause solder joints to crack. Once a solder joint fails you can get arcing which results in a higher than normal current. If this is within the fuse capability a copper track might get very warm and eventually detach from the circuit board material. If this happens the freed track will get warmer and warmer until it fails, sometimes vaporising leaving only a shadow of its presence. In this car radio with a 2 Amp fuse (assuming it hasn't been replaced with a heavier one by a car owner) damage should be limited. Take however, white goods such as a fridge-freezer or a dishwasher with a 13 Amp fuse.. a failed solder joint can result in catastrophe because, before the 13 Amp fuse fails, a circuit board might catch fire, burst into flames and destroy ones home, or in a recent case an entire high rise block of flats (I'm puzzled about why this wasn't given more weight.. many fridge freezers carry a device which deals with a mains surge from starting up, it might typically in fact be very similar to the device used in CRT television sets for auto-degaussing of the tube, and this device can fail due to solder joint (or wire crimp) failure and catch fire ??). In the case of the dishwasher mentioned above the manufacturer relied on "low risk=only a few houses burned to the ground" as their excuse, but to give them credit swapped the rogue part for a better one free of charge.

So why am I raising this topic? Most of the Blaupunkt boards above have dry joints, the usual term for failed solder connections. Before any serious damage can occur a user will be aware of a couple of possible events.. initially, crackling as the solder joint starts to fail, then a user will notice that waggling the volume control might help alleviate this. Soon though, waggling the volume cotrol will have made matters worse and the car radio will go off. Another waggle and the radio might come back on but in some cases the copper circuit board track gives up and fuses open circuit and no amount of waggling will bring the radio back on.. in fact there are several examples above where the volume control rivets have failed from too much desperate waggling. After all this is quite a normal reaction and I must admit to banging our hi-fi amplier to make it come on, and of course, being an electronics engineer I know exactly where to bang it!

In the case of this Blaupunkt car radio, something else is going to happen. The on/off switch is a fairly flimsy affair and if asked to handle a high current, it's contacts will burn and begin to develop a higher than normal resistance. That causes the switch to get even hotter and more burning will take place. The connections to the switch will get warmer and degradation will accelerate until the radio no longer turns on, or is so crackly and intermittent it gets chucked out... hence the heap of duff circuit boards above! I noted that early versions were fitted with a tiny on-board 2.5 Amp fuse but this was changed to a wire link in later boards and finally a narrow copper track was added (this would act like a fuse).
 

 Further checks revealed that several boards may have faulty power transistors, and some of these had a fused track from the BD434 emitter to ground. It looks like I'm going to have to remove all these BD433/434 transistors and test them as their forward bias checks differ widely, then I need to construct a jig so that I can test the boards to confirm their supply current is reasonable and they can amplify a test signal satisfactorily.

One point of note are the connections to the push-on tags at the end of the board. There are two tags at right angles of which the larger is Battery Plus, but the other is NOT chassis, but the automatic car antenna supply voltage (the chassis connection or Battery Negative is a tag on the radio chassis). The loudspeaker output carries audio to the speaker AND chassis ground. A slight puzzle is the DIN connector where a dotted line connects Pin 1 to Pin 2 which seems to represent that the mating plug is fitted with a link beteeen these two pins. That would connect the car radio audio output (marked "NF") to the amplifier input (also marked "NF"). I've also noticed that the connection between the plug shell (which connects to the shielding braid on a standard DIN cable) rarely makes good contact to the mating spring on the circuit board.

 

 Ref
 Notes (2 types of potentiometer A & B)

 Repairs

 Tested

ZA006
 B pot, dry joint at switch plus burnt out track Larger heatsink  Wires added, resoldered, track wired.

OK

ZA007
 A pot, dry joints at DIN. Twin LS plugs.  Wires added, resoldered.

 BAD

ZA008
 A pot, poor power connection Larger heatsink, solder link ground fuse open, note blue/green wires have to be linked Link re-made, dry joints soldered

OK
 ZA009  Rust and water staining, bad output transistors  Cleaned, new wires added, resoldered.

 BAD
 ZA010  A pot, dry joints at switch and DIN Larger heatsink, short-circuit capacitor, note blue/green wires have to be linked  2200uF cap fitted, dry joints soldered

 OK
 ZA011  A pot, looks ok, but several dry joints found later.  Wires added, resoldered.

 BAD
 ZA012  A pot, output transistors suspect  Wires added, resoldered.

 OK
 ZA013  A pot, dry joint at switch plus burnt track, loose heatsink, wire added to replace fused track, DIN dry joint  Wires added, resoldered.

 OK
 ZA014  A pot, dry joints at DIN and output transistors with green verdigris, Twin LS plugs.  Wires added, resoldered.

 BAD
 ZA015  Rivets loose on A pot, DIN dry joint  Wires added, resoldered.

 BAD
 ZA016  A pot, dry joints at switch, suspect output transistors (but in fact I later found a broken collector track) No DIN eth  Wires added, resoldered.

 OK
 ZA017  B pot, dry joints at switch, FAULTY POT  Wires added, resoldered.

 BAD
 ZA018  B pot, dry joints at DIN. Larger heatsink, bad BC328, burnt open track, only 2 wires  Wires added, BC328&resoldered

 OK
 ZA019  A pot, burnt track, suspect output transistors  Wires added, resoldered.

 OK
 ZA020  A pot, dry joints at switch, burnt track. Fused track to BD434 emitter  Wires added, resoldered.

 BAD
 ZA021  B pot, dry joint at BD434  Wires added, resoldered.

 OK
 ZA022  A pot, part broken off, output transistors bad. Twin LS plugs.  Wires added, resoldered.

BAD 
 ZA023  B pot, dry joints at switch and DIN  Wires added, resoldered.

 OK
 ZA024 Pot missing , bad output transistors, loose power lead, cracked circuit board. Larger heatsink -

 BAD
 ZA025  A pot Ground to output transistors bad  Wires added, resoldered.

 OK
 ZA026  Stereo, quad pots, output transistors dry joint suspected. Twin LS plugs. No phono  -

 BAD

 I tackled one of the boards chosen at random, being ZA016. Initially I fitted three new connecting wires viz. battery plus (red), negative (blue) and a lead connected to C200 (green) which goes to DIN Pin 1. I connected an 8 ohm speaker to the output plus an audio signal generator to the green wire, and, connecting a variable power supply with the current limit gradually increasing, I initially got intermittent results which worsened by flexing the board, so I removed solder from suspect dry joints then applied fresh solder.

I then found a hairline crack adjacent to the transistors where the board is weakened by the heatsink aperture, so I fitted three short lengths of btc wire across the cracked copper tracks and tried again. This time I heard a steady tone from the speaker with about a volt of audio input. Shorting the green wire to an adjacent capacitor (this goes to NF and then to the top of the volume control pot) resulted in close to 400mA current drain and a very loud output. Cranking down the audio input I found 10mV resulted in 188mA at 14 volts and 15mV input gave me 200mA drain with comfortably loud output at max volume. Next I'll measure the transistor voltages and see how close they are to those noted on my circuit diagrams. I measred the BD voltages and was puzzled because the collector of the BD433 was around 6 volts. How can this be when the collector is wired to the power supply? I carried out a few checks and discovered pins 11 and 12 of the hybrid device (which carries 4 resistors) was short-circuit. Could this explain the low collector voltage? Surely not, but I removed the hybrid and measured its 4 resistors and found R262, the 33 section measured zero ohms so I clipped the leg at pin 1, refitted the hybrid and wired a 33 ohm resistor in place of R262. Then, going back to the low collector voltage problem, I measured its resistance to the power lead and found it measured about 1 megohm. Peering at the copper tracks I noticed that for convenience I was feeding power to the red flying lead rather than the power connector at the end of the board. This lead connects to the circuit side of the switch and I'd measured the switch as working OK but the pad at the switch connection was lifted from the adjacent track and had a clean break isolating the track to the BD433 collector.

How the audio was actually getting amplified apparently so well is a mystery but at least the high supply current is now explained. Other boards I tested drew 28mA from the 14 volt supply, whilst this example drew 200mA because the short-circuit 33 ohm resistor fed the BD433 base with too high a voltage which caused the BD433 to draw excessive emitter current. Now that has been fixed the board drew 28mA just like the others. What had been happening, and this probably applies to most, if not all of these audio boards, is the user had discovered that the effects of dry joints could be alleviated by waggling the volume control, but this persistent waggling had worsened the dry joints and, in the example described, had broken the switch solder pad from the track leading to the red wire which, in an assembled radio, feeds the RF circuits.

Now that the boards are cleaned up and fitted with a set of fresh connecting wires, mostly red for plus, blue for neg and green for the DIN audio input, with bards carrying the larger heatsink having a blue wire in place of the green/white striped wire and an additional white wire. I think all the BD transistors are probably OK and it'll be fairly simple to connect a speaker to each in turn, apply power and use the DIN socket for testing. I've noticed that supply current is pretty low (less than 2mA under no signal condition (speaker disconnected), rising to around 20mA quiescent (with speaker and zero audio input) and 100mA for 5mV audio input. I've now added test results in the table. "OK" means a DIN input of 3 to 40mV results in about 45mA to 480mA of current drawn from a 14 volt supply, and a maximum in excess of 700mA, whilst "BAD" means the max current was limited or no current flowed. Several boards required a new link from either the ground to the speaker connection, or from the 14 volt tab to the switch on the pot.

 

 Ref

 Visual/Test

 Notes

 ZA001 (AA)

short circuit at power input 

No power 

 ZA002 (BB)

No power

 ZA003 (CC)

-

No sound 

 ZA004 (DD)

 badly cracked circuit board 

-

 ZA005 (EE)

burnt out power track 

-

 (FF)

 Draws 14 volt current depending on tuning/volume (>500mA max)

 Supplied as a reference working radio

 All the radios using a rear-mounted DIN socket (and that's almost all) are fitted with a blanking plug carrying a link between Pins 1 and 2 (as shown on the circuit diagram above). That means in order to test the audio boards a link needs to be added or a blanking plug fitted, otherwise a test signal needs to be applied to the top of R203, the volume control. Another point worthy of mention is the current taken by any of the loose audio boards will be only a few mA until a loudspeaker is connected. After that the current will vary from a few mA at the minimum setting of R203 to around 200mA with an input of say 10mV. A working radio will draw over 500mA because the recovered audio from tuning a strong signal must be around 50mV or more with background noise dictating the minimum audio and hence maximum supply current.

Having fixed the relatively straightforward boards I'm now working my way through the tricky ones.

Progress: 11 Good, 10 Bad.

 repairs pending

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