If you have found this post, you are likely to have a radio like this Philips Babette from the 1950s. If so, your radio is suffering from poor-quality, distorted sound. I have found some ways to improve it so it can be used again.
Warning: Do not operate your mains powered radio with the case open. Dangerous voltages are present inside.
I tried two different approaches to tackle the Babette’s distortion. The first, my favoured, is to repair the existing audio amplifier, to restore it to its original sound. I describe what I tried and the results in the section Audio amplifier repair. Spoiler alert: my restoration did not succeed, although yours may do, so the second approach, see A new hope, details a replacement amplifier module using a TBA810 integrated circuit. Grundig also used this chip on several later radios. There are many alternative audio ICs that could also be used so long as the positive earth modifications I describe are followed. If you’re in a rush I’ve summarised the faults and remedies here.
For the purists in pursuit of perfection, this blog post is not for you. I would have preferred an authentic Babette restoration, so when time permits, I’ll open Pandora’s box again and report back.
How it started
The Babette like their sisters, Evette and Annette were designed by Philips Germany and its retro styling caught my wife’s eye at a local vintage fair. Supplied with a car adapter cord, I’d assumed the set was an early all transistor model and was surprised to find a combination of valves and germanium semiconductors inside. In fact the transistors are quite well hidden, only the paper schematic diagram inside showing a solid state audio stage gives a clue to the radio’s technology.
Back in the 1950s transistors were only just being introduced into portable radios and larger attache case style portables employed valve or vacuum tubes required 90 volt dry batteries to run away from mains power.
Fortunately the Babette also came with its original battery eliminator – a mains powered card board cased box housed deep inside the bakelite cabinet – top right in the image above.
Unfortunately it was disconnected and the helpful instruction all in German. A pair of identical PP9 style battery clips presented two connection options. One labelled what I thought read “Netstell” Google translate told me was for network maintenance. This seemed too advanced, so I tried some variants and found Netzteil translates to power supply. The wrong choice would likely end in tears.
Powering up another surprise – almost immediately the radio crackled into life albeit with a background hum. Not the usual delay as valves warmed up – the transistor audio stage clearly takes no time, unlike modern DAB sets that need a few seconds to ‘boot’. But programmes were immediate too. The DF96 / DF97 valve circuitry appeared to work as fast as the transistors!
Sound quality seemed reasonable on medium wave talk radio but rather distorted on switching to FM most obvious with listening to music (UKW). So began my adventure and the reason for this post.
Background hum
Given the radio’s age and my recent experience with valve car radio – see Becker Europa Valve Car Radio Faults and Repair – I decided to check all the electrolytic capacitors before applying mains power. This also allowed me to confirm the battery eliminator’s 90v output from its top connections.
On the 6v supply, each 600uF capacitor, C95, C96 appeared to read over 1400uF with a Vloss of over 12% indicating a faulty electrolytic, although ESRs were below 0.1 ohms. As the battery eliminator used a series resistor between the pair of capacitors I had to disconnect each pair in order to see an accurate reading and confirm their value and ESR – a good lesson. As both capacitors looked in good condition, I suspect either the eliminator was newer than the radio or they had been replaced previously.
On the high voltage side again both capacitors looked okay measured independently I discovered C97’s ESR over 0.7 ohms a little too high.
Applying power via an isolated source I found the ripple voltages on the 6v supply was just 80mV while the 90v supply showed 0.35v. Neither seemed particularly high although there was 9v ptp across C97, the 90v 8uF reservoir capacitor.
I replaced C97 with a new 10uF 400v electrolytic that was still a little smaller than the original. This brought the ripples down to 3v and only 180mV on C98.
Checking the smoothing electrolytic capacitors in radio, C1, a 100uF, measured a high ESR over 0.2 ohms, so replaced it with a 150uF Philips 63v electrolytic. C90, a 500uF, appeared in spec but I decided to replace with a similar Philips 470uF. Note both a little smaller than originals – held in place by card.
Notice the hum frequency is 50Hz reflecting the single wave rectification Philips chose to use. Given the low current demands I guess it makes sense. But it also requires a higher reservoir capacitance.
Audible hum levels reduced significantly after the capacitor upgrade / replacements although some background hum remained that could be normal for this radio.
Audio amplifier repair – Distorted audio
Far from normal was the distortion I could hear. Using an oscilloscope and an RF signal generator, it’s fairly easy to discover why this radio suffers from poor audio quality. With a probe attached to the speaker, I found the positive waveform clipped, so instead of a nice smooth sine wave it looks more like a saw tooth hump!
This type of fault is typically due to the audio amplifier’s output stage which consists of a pair of OC72 transistors – one for the positive and negative waveform. Often one of the output transistors fail causing a harsh sounding distortion. Notice there is also hum present on the screen shot above which is due to my RF signal generator that needs a service too!
The problem is to discover what’s causing the distortion. It’s find it helpful to work through each amplifier stage for clues. On a radio of this vintage this can be challenging. As was common in radios of this age the chassis consists of a network of tag boards with components wired between. See below.
I hooked up an audio signal generator to the volume control to remove the RF section from diagnosis. The distortion was still present.
Following the signal path from the generator I discovered the amplifier’s first stage showed signs of the 2nd harmonic distortion, although the signal was very low. I suspected the 0.33uF input coupling capacitor – an ERD paper monster – and disconnected one side to test. It measured fine, but I replaced it with a much smaller polyester equivalent to gain access to another capacitor below.
I found a 10uF electrolytic which the component tester showed to be way off spec with ESR of over 9 ohms and a Vloss of 16%!
Replacing the electrolytic made no difference to the distortion as did the coupling capacitor.
So next I targeted the first transistor and replaced the OC71 with a silicon BC557 transistor resulting in a nice clean sine wave at its collector. Success? No, the output from the speaker still had the hump. And so did the driver, another OC71.
I decided to replace the 2nd OC71 with another BC557 to improve performance. Not a good idea. Silicon transistors have a higher Vbe bias voltage than germanium devices. While the first stage’s negative feedback accommodated the 0.5v difference, the second stage was not so forgiving. The circuit became unstable and oscillated at 72KHz, probably due to the higher current gain and lower self capacitance.
Checking the second stage’s OC71 in the component tester showed the transistor was functioning correctly. Something else must be to blame, so I restored the OC71 and looked to the output stage. Of course that’s where the fault must lie.The output pair are mounted on the opposite side of the module on long leads some hidden in-between the transformers. I’d been hoping to avoid removing them. I had no OC72 replacements in stock.
The audio amplifier is mounted as a separate module which I removed by unsoldering a pair of black wires and the centre screened input cable.
It’s much easier to work on the amplifier once removed as it can be powered directly via a 6v bench supply. And once free the negative earthed chassis can be ignored and the audio stage’s positive supply used as signal ground. However, fault finding is still challenging as each tag has multiple wrapped solder connections.
Bearing in mind the problems with silicon transistors I decided to raid a working Ever Ready Sky Master which used similar germanium transistors. I discovered a pair of metal encapsulated OC72s, a later version of the glass cased pair fitted to the Babette. Testing their gain I found, to my dismay, they were far from a matched pair. One showed hFE of 53 while the other read 137!
As the output waveform was already showing asymmetrical distortion I decided there’s little point in replace with an unmatched pair that could exhibit similar issues. I unsoldered the base and emitters of the existing OC72s to check using the component tester. To my surprise they were in better shape than the Sky Leader’s.
As the output transistors were not to blame I had to admit defeat. A simple 4 transistor amplifier from the 1950s had me stumped. All voltages across the transistors were as expected. All transistor junctions measured correctly. Leakage currents were within specification but when driven with a small signal – no where near the maximum output power – the waveform distorted.
Perhaps one half of the output transformer winding had failed? It seemed easier to simply build a new audio amplifier than struggle with old tag boards and transformers.
A new hope
At first glance replacing the existing audio module seemed simple. But as I found, the Philips engineers were faced with a design challenge as the radio frequency valve stages and audio frequency transistor stage required different signal grounding. Most modern silicon transistor amplifiers have a negative signal ground or earth as they are predominately NPN types. But the Babette used germanium devices where PNP are most common as they are easier to manufacture so it has a positive signal ground reference.
So what you may ask? If you connect a negative ground amplifier to the Babette the radio’s power supply will interfere and modulate the audio signal. Even with large decoupling capacitors across the 6v supply that feeds the amplifier considerable distortion results when the input signal is not properly grounded. The very distortion I’me trying to remove!
The remedy involves a modification to the Babette’s detector circuit as shown below. Here the existing white wire that feeds the +6v supply from the band change switches is re-routed allowing the detector stage ground directly to the chassis.
TBA810AS Amplifier
As I had a low power IC audio amplifier to hand I used a prototyping board to construct a replacement module to fit inside the Babette’s chassis. There are simpler chips I could have picked but this is one I found on my travels some years ago that seemed appropriate to use.
Here’s a photo of the replacement amplifier fixed inside the Babette. Notice the +6v supply line (white wire) rerouted together with a new C69 4.7uF capacitor now with inverse polarity.
Powering up the new amplifier for a sound test was disappointing inside the radio. While the distortion had indeed reduced at low volume, turning the volume up resulted in nasty distortion and it still sounded quiet!
Connecting the oscilloscope across the Babette’s speaker showed me what the problem was and at this point I could have screamed in frustration. The new amplifier was clipping, both top and bottom, as it approached the 0-7v supply. A simple ohms check on the speaker confirmed my worst fears. It measured 240 ohms. I’ve never known a loud speaker with such high impedance. Even headphones may reach 32 ohms but 240 – what’s going on?
I looked again at the schematic that came with the radio. Was it a mistake? – the output transformer was not a transformer at all – it only had a centre tapped primary and the speaker was connected across the primary winding. What magic is this? What an idiot I felt. I’d assumed the circuit used a transformer and the speaker was 3 ohms like many valve radios and TVs used in the day.
My new amplifier was designed to drive low impedance speaker so it was hardly surprising it made more than a whisper driving a 240 ohm load. In desperation I rummaged around a collection of spare radios and found a 6 x 4 inch 3 ohm speaker that has been glued inside a Bang & Olufsen Beolit 600 – ( see Beolit 600 if you are interested in these radios). Note this is not an original Beolit SAAS speaker that’s larger and won’t fit in the Babette.
With the 3 ohm speaker installed finally I had a working Babette in fine voice and volume. Sound quality is much better and makes listening to music possible in comparison to the old germanium amplifier and speaker. If you were not too concerned with volume levels I guess the original speaker would be fine, it seemed to have a better bass response than the old one I used.
I will look for a better quality 4 ohm 6″ x 4″ speaker at some point and report back how it sounds.
Fault and remedy summary
- Hum (ripple) on supply lines:
- Check electrolytic capacitors in battery eliminator – found in spec with ESRs less than 0.1 ohms.
- Check electrolytic capacitors in radio – C1 100uF found with high ESR so replaced. C90 500uF had low capacitance so also replaced with 470uF. Note both smaller than originals – held in place by card.
- Distorted audio 1st stage (TR1):
- Check coupling capacitors C76 0.33uF (paper) replaced with polyester type; C77 10uF ESR ~10 ohms replaced with new radial type. Little improvement to 1st stage distortion.
- Disconnect and check TR1 OC71 – low Hfe, replaced with silicon BC557. 1st stage distortion much reduced but output still distorted.
- Distorted audio 2nd stage (TR2)
- Disconnect and check TR2 OC71- measures okay. Try replacing with BC557 but oscillation and instability results so revert. Distortion likely due to output stage feedback.
- Distorted audio output stage (TR3&4)
- Disconnect and check TR3&4 – Both measure okay but Hfe different. Fail to find matched pair.
- Modify detector circuit to negative signal ground and replace audio stage with new negative earth amplifier – no distortion on low volume but clipping at higher levels.
- Clipped audio at higher volume – replace existing 240 ohm speaker with 3 ohm – success, distortion and volume good.
Useful links
Here are the links mentioned in this post
- Scan of Babette LD472ABT Schematic Diagram –
- Radio museum – Babette LD472ABT – https://www.radiomuseum.org/r/philips_babette_ld472abt.html
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