Grundig Concert Boy no sound fault repair

Has your retro Grundig Concert Boy 1100 lost its sound?

If your 1970s Grundig Concert Boy no longer works, with no sound from any of the wavebands, it may have faults similar to the ones I found and simple to repair. I’ve listed several symptoms found with two Concert Boy 1100 and how to repair them. I can’t promise it will solve your particular fault but if you are trying to repair your radio it’s worth checking. If you in a hurry and want to try the suggested fixes skip to the end. TLDR;

Warning: Disconnect the mains power before attempting to open you radio. Never operate your mains powered radio with the back open. Always use battery power or an external power supply if you need to test or make adjustments with the back removed.

Symptoms – No sound or fades to hum after switch on

Initially after plugging in, powering up and tuning in to an FM station the volume fades to be replaced with a constant low hum. Switching off and on restores the sound, which can last for a few seconds or several minutes or much longer. But after a while or after another power cycle the hum returns.

Intermittent faults like this are some of the most difficult to diagnose. Most of the time when the radio is switched on it works. Many electronic components fail permanently so the fault condition remains helping to identify the culprit(s).

So to help locate the source of the problem I needed to eliminate suspects (think like a detective). With a no sound fault the problem could be from aerial to speaker or the power supply so pretty much anywhere. There was a clue in the hum so I decided to check if it fades when powered from either mains or batteries. Inserting 6 D cells and moving the switch on the back from mains to to battery, I tried the radio again. It worked and did not fade. Maybe a power supply fault I though.

Removing the two screws at the base allowed me to investigate the small PCB that housed a blue bridge rectifier and transformer wiring. This 1970s radio uses a simple step down transformer to provide the 9v DC needed but the reservoir capacitor is mounted on the main PCB. I suspected the electrolytic capacitor but it looked like it had already been replaced with a modern equivalent (2200µf 25v). That left the slide-switch and the rectifier to I cleaned the switch and replaced the old selenium bridge rectifier with a modern silicon equivalent (W005G-E4/51).

Blue selenium bridge rectifier

Symptom – No reception on AM bands (Shortwave, medium wave and long wave)

After closing the back I switched it back to mains power and plugged in. It faded after a few minutes — failure snatched from the jaw of victory! So at least I knew it was not the bridge rectifier. While it was working I noticed only FM stations worked, all the AM – LW, MW and SW1 and 2 crackled but remained silent.

I opened the back again and applied switch cleaner, Servisol 10 cleaner and lubricant, to the row of push button switches and repeatedly operated the row up and down to help the cleaner. Switching back to battery, the AM bands now jumped into life. I applied a little more cleaner to the volume slider that also crackled along with the base and treble sliders.

Optimistically I replaced the back and plugged it back in to mains to soak test. Now with a smooth volume control and ability to try different wavebands I noticed a hum on the SW stations when it tuned into a signal. There was also some hum or base resonance on FM. Then just as I’m making some notes it fades to a hum.

You need a belligerent approach so radio repairs and it does help to laugh when you think you are making progress and it all comes tumbling down. It’s only a radio after all. But it’s a puzzle and need solving. At least I now had all the wavebands working part from the hum.

Hum is often due to ripple – 100Hz frequency caused by the bridge rectifier circuit failing to charge the reservoir capacitor – or excessive current drain. Okay get on with it what do I need to do to fix it I can hear you say. I needed more information. The SW bands did not hum as much on battery so it indicated excess ripple. So I hooked up some fine leads to measure the supply voltage while the radio was mains powered.

I waited patiently for the sound to fade while monitoring the supply voltage. In operation it measured between 12 and 16v depending on volume, much higher than the 9v battery power. This is normal as the simple circuit is unregulated and means the amplifier can provide a little more volume on mains power. A clue – when the sound faded in fault condition the Vcc drops from 16.5v to less than 10v.

Symptom – faint distorted sound on full volume

Power cycling mostly restored the supply voltage to 16v but on occasion the fault occurred again instantly and dropped to < 8 or 9v. What could be causing this? With the volume control fully on in the fault condition I could hear a faint distorted signal above the hum. So maybe it was the amplifier failing and draining the power?

This was the clue – it was not a power supply problem. I check if there was a DC voltage across the loudspeaker indicating the 1000µf coupling capacitor could be failing with the higher mains power. This could explain why the radio seemed well behaved on battery.

I removed the two screws holding the main PCB in place and gently pulled away from the tuning control. Fortunately Grundig designed the tuning pulleys to remain in place while the circuit is removed but it may need a touch of switch cleaner on the metal spindle to free it. I replaced the orange REO 1000uF 10v output coupling capacitor with a 16v equivalent.

This made no difference the sound still faded after several minutes. I looked at the circuit diagram in the service manual (https://elektrotanya.com/grundig_concert_boy_1100.pdf/download.html) and it shows there’s a resistor from the bias driver connected to speaker, not via the usual AC coupled capacitor. Oh well last least the old capacitor won’t be a problem in future. See The Beolit Speaker Problem for the similar faults with these ROE capacitors in Beolit.

As the fault indicated the amplifier was drawing too much current I tried adjusting the small skeleton potentiometer that sets the quiescent current through the output transistors. A slight tweak and the sound faded. I tried switch cleaner on pot and rotated it several times. The pot felt loose. I powered up the radio and it worked but again after a short time it faded. I tried moving the pot to reduce the current flow and the sound returned without having to power cycle the radio. A turn in the opposite direction and the sound faded.

Success at last, it must be an intermittent wiper on the pot. Looking at the circuit if the pot failed with an open circuit wiper the quiescent current would be too high pulling the power supply down.Not good for a germanium transistor output stage that is subject to thermal runaway when the current increases as the temperature increases and vice versa. (It’s one of the reasons germanium semiconductors were surpassed with silicon) I connected a 1k resistor between base and collector of the transistor connected to the wiper so if the pot failed the transistor would be switched on rather than off and decrease the quiescent current.

When I find a 47Ω pot I’ll replace the faulty device but for now it’s working without fading and has done so for several days.

Audio amplifier schematic

Suggested fixes

1. Use lubricating switch cleaner (such as Servisol super 10) on all moving parts such as wave change switches, sliders and preset potentiometer (R650). This solved the no sound on AM bands particularly SW (K1, K2). Expect these buttons had rarely been used so their contacts had tarnished.
2. Check if R650 (50R) pot is the culprit by gently moving it with a small screwdriver or trimming tool. The pot’s normal position is around midway if you don’t have a meter to set the output stage current to around 8mA. This solved the sound fade to hum fault on all wavebands.
3. Replace C806 2200µF 16v electrolytic capacitor – this is very likely to be low value if original as nearly 40 years old and can cause hum on AM bands. This had already been replaced on my Grundig Concert Boy. You need to wire the three negative terminals on the PCB together as modern replacements have only one negative lead instead of the three fitted on the original capacitor. Make sure the replacement capacitor has a low ESR as needed for the power supply reservoir. You could use a 3300µF replacement that would lower the ripple further but suggest fix 4 is done at the same time. Don’t go too high as the surge current may damage the bridge rectifier.
4. Replace the bridge rectifier B20 C 500 – this can cause hum. The original often corrodes along the edges which become rusty and overheat. As these mains powered radios have no mains on off switch the rectifier runs continuously when plugged in even if the radio’s switch is off or switched to Battery and is best replaced with a silicon equivalent such as W005G-E4/51.
5. Replace the output capacitor C655 1000µF 10v – the original on my Grundig had an orange REO electrolytic capacitor that often fail at this age and should be replaced with a 1000µf 16v or 25v. Most modern caps are much smaller and a higher voltage specification will fit and offer better resilience. You could use a 1500µF to improve bass frequencies a touch. Don’t go too high as the surge current may damage the output transistors.

Hope one of the suggestions works in your case.

Good luck – it’s worth the effort restoring these radios as they do sound so much better than the majority of today’s DAB radios.