Do you have a Grundig City Boy 1100 radio that won’t tune or that hums when plugged in?
If so I’ll run through how to fix these faults on a Grundig City Boy 1100 radio from the 1970s that had been waiting in a jobs to be done pile beneath my desk. I found the service manual with the radio in a box that maybe useful as it has the circuit diagram / schematic – see link at end of post. Also found the original user’s manual which I’ll scan and add to the post when I get time.
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.
The City Boy is a cut down version of the Concert Boy that have both been featured on Radio Retro and covered in a previous restoration post – Grundig Concert Boy no sound fault repair. This City Boy radio is in excellent condition considering it’s nearly 40 years old but it fails to tune in to any station –the tuning thumbwheel has jammed. There’s also a noticeable hum when plugged in to mains power. I’ll cover fixing both these faults.
Jammed tuning thumbwheel
Opening the radio, by removing two small screws at the base, allows access to the drive cord and tuning capacitor – as shown far right in the picture below. When trying to turn the front control thumbwheel I could see the drive cord was trying to rotate the pulley that drives the tuning capacitor but would not turn.
Thick grease one end of the spindle at the centre of the pulley was visible so tried a small amount of switch cleaner lubricant on the bearing and shaft. By carefully moving the tuning capacitor fins at the same time as the thumbwheel back and forth, the pulley became easier to turn. Another quick squirt of Servisol and the tuning was free again. WD40 would probably have worked just as well. Avoid spraying lubricant on the dial cord as it can cause it to slip. Cover the tuning dial with a cloth to keep it clear of switch cleaner which could mark the dial and damage the lettering.

Grundig City Boy showing drive cord and tuning capacitor
I sprayed a very small amount of switch cleaner on the tuning capacitor stator contacts (adjacent to the 3 fin sections) to reduce audible crackle that can occur when the contacts become tarnished.
Audio hum
With the tuning free I replaced the back and tested the radio across the 4 wavebands. The hum that I’d noticed to begin with was evident on all wavebands and indicated a problem with the power supply as there was no hum on battery power. The volume slide control also crackled at the low end.
Having had a similar hum fault with a Grundig Concert Boy, I suspected the old light green coloured selenium bridge rectifier which can be seen on the left side by the red wire in the picture above. A close look at the rectifier’s edges showed corrosion so I replaced it with a silicon alternative – W005G-E4/51. Replacement bridge rectifiers are available in a variety of power levels. I chose one that can handle up to 50v at 1.5A well above the 20v 0.3A original. As a general rule select a higher voltage and current rating.
Access the PCB by removing the two hex slotted screws and pull gently away from the top and side where the tuning spindle slots into the pulley. There’s no need to disturb the tuning cord and pulley which remains in place.

City Boy 1100 bridge rectifier showing corrosion
While I had access I checked the large electrolytic capacitor (adjacent to the red wire) which smooths the AC voltage from the bridge rectifier as it could have been stressed by the faulty selenium rectifier, a Herrmann B 20 C 200, or have caused it to overhead.
A simple multimeter set to a 20K resistance range across the electrolytic capacitor terminals, black lead to negative, can indicate if it needs replacing. The meter should display a low resistance that slowly increases until it reaches maximum followed by the reverse (may show as ‘negative’ resistance) when the meter leads are reversed. With experience you can recognise the time it takes to charge and discharge from the multimeter. As an example on my multimeter 20K range it takes around 12 seconds to go from zero to 20K for 1000uF. A 470uF takes around half that time at 6.5 seconds and a 2200uF takes over 24 seconds. The exact timings will likely be different for your multimeter so compare the timings with new electrolytic capacitors if you have some.
When there’s a constant high or low resistance reading this indicates the cap has failed and must be replaced. Note there may be components in circuit that cause the constant resistance so you need to remove the capacitor to be certain, but it may give you a clue.
You could of course just replace the capacitor as a routine 100,000 km service as they are cheap but it’s helpful to know if a component has failed to look for related faults.
This City Boy radio only has a 1000uF 16v fitted so I plan to replace it with a 2200uf 25v as fitted in the Concert Boy. But for now I’ll leave it to check for other faults.
No shortwave bands working
As with this radio’s larger cousin this Grundig also needed the waveband switches cleaning. I used Servisol along the row of push button switches making sure the fine nozzle directed at the small opening at the front and rear of each switch. Covered the dial with a cloth to avoid switch cleaner on the dial that sits right above the switches. Repeatedly operating each switch up and down the wavebands helped dislodge years of tarnish. It’s one of the snags with silver plated switches as found on many radios of this era. Silver is a great conductor of electricity when it’s not been oxidised by years of exposure.
With the push button switches cleaned reception was good on all wavebands pulling in many stations across the two shortwave bands K1 and K2.
I noticed the volume slider still crackled, so I applied another small doze of switch cleaner and moved the slider back and forth several times to clear the wiper and carbon track. The slider contact looks silver plated and quite fragile – it’s easy to catch when replacing the volume control knob so ensure you remove these before attempting to reposition the main PCB chassis. Both sliders are tricky to reposition the centre slots and engage with the small plastic wipers on the volume and tone pots. So check the sliders are positioned correctly before you put back together to avoid taking the board out again if either tone or volume don’t work!
Poor bass response
After fixing the tuning, crackle, hum and waveband switching the overall sound quality on FM was lacking the usual Grundig rich tone quality I expected. The lack of bass compared with the larger Concert Boy was obvious. What could be the cause?
I checked the 5″ by 3″ speaker cone to see if it moved freely. Sometimes the cone is stiff due to problems with the voice coil overheating. But the speaker felt normal. Looking at the schematic I found the output coupling capacitor was only 470uF rather than the 1000uF fitted to the Concert Boy. So maybe Grundig decided to fit lower spec capacitors to both the power supply and amplifier I guess as a way to differentiate the sound quality for the two price points?
So I decided to replace the 470uF cap with a 1000uF. It sounds a little better with a little more bass suggesting the old capacitor had aged or should have been higher in the first place. Of course the City Boy will never match the bass sound of the larger Concert Boy as the latter has a larger elliptical speaker.
Mains lead
I noticed the radio had been put in a box on top of its mains lead that had left marks along the back plastic case. The insulation on the power cord had become soft and sticky and needs replacing.
Fault and fix summary
- Tuning thumbwheel jammed – apply switch cleaner to tuning capacitor main bearing to free seised spindle.
- Audio hum – replace B20 C300 bridge rectifier with silicon equivalent W005G-E4/51 and check C 428 1mF (1000uF) 16v electrolytic capacitor. Suggest replace with new 2200uF 25v which fits in the same diameter 3 pronged ring.
- Audible crackle – apply lubricating switch cleaner to volume and tone sliders and ideally clean along volume control tracks with cotton bud.
- AM wavebands not working – apply lubricating switch cleaner to row of pushbutton switches and operate multiple times to clean tarnished contacts.
- Poor bass – check speaker cone for free movement, replace electrolytic capacitor C425 470uF with 1000uF 25v
Service manual – pdf Grundig-City-Boy-1100
User manual – pdf Grundig-City-Boy-1100-User
18/12/2021 at 11:06 am
How much does this cost please?
01/10/2022 at 8:30 am
Is the radio for sale?
01/10/2022 at 4:05 pm
I do have a Grundig City Boy radio available along with several others including the larger Concert Boy. Send me a DM if interested.