Roberts Classic 927 radio repair

[HwAoRrDk]

I have a radio that I'm attempting to repair, a Roberts Classic 927 (model no. R9927). Apparently, while it was on (I wasn't there at the time), something went 'fizz/crackle' and it just stopped working.

Suspecting that something like an electrolytic capacitor had popped, I opened it up, but couldn't see anything that looked bad. This radio can be powered either from mains AC or from batteries (4x 1.5V = 6V), so suspecting that perhaps it may be something with the mains power supply, I tried hooking up the battery terminals to my bench PSU and gave it 6V. It works!

So, something is wrong with the mains power supply. I began to trace the circuitry, but I am puzzled as to why it is built like it is, and how it failed.

As far as I could determine, the mains power circuit is as follows:



The mains AC comes in via a 2-pin figure-8 connector and goes straight to a small transformer. The transformer has 3 wires out: middle one goes to circuit ground, the other two both each go to a series rectification diode (with parallel ceramic cap), which are then commoned together and smoothed by an electrolytic and small ceramic cap. This DC power then goes to a switch integrated into the AC socket, which appears to be used to switch out the battery power when AC is plugged in.

I am confused as to why they are using the transformer like this. Can anyone explain?

From what I can determine, the transformer is at fault. When powered, there is zero voltage on the secondary side. If I measure the resistance of the windings, on the secondary I get 3.5 ohms between the 'ground' tap and each other one. The primary side appears to be open-circuit. The transformer has a label on it stating has a thermal fuse, but there is no sign of overheating whatsoever. Are thermal fuses typically on the primary side? If that has broken, that would explain the open-circuit measurement.

Don't know whether I should bother looking for a replacement transformer, or just bodge a DC barrel jack into the thing and rewire it to run off of that.

[TimFox]

This transformer connection is a perfectly normal "full-wave center-tap" circuit.  It was ubiquitous in the vacuum-tube era, where a dual-diode rectifier (e.g., 5Y3GT) was connected in this way, in order to reduce the number of diodes required.  When solid-state rectifier diodes became cheap, the "full-wave bridge" circuit without center-tap and using four diodes became more popular.
For either polarity of the input AC waveform, only one of the two diodes conducts.  Of course, with a capacitor-input filter, the capacitor charges up to almost the peak voltage of the transformer output, so the diodes conduct for a small fraction of the half-cycle, when the input voltage has risen above the roughly-constant voltage on the capacitor.

[BrokenYugo]

Yeah, DC jack or new transformer, something this small isn't worth trying to repair/rewind unless you want the experience. Sounds like you've poked at it already, so I assume this isn't an easy fix like a cracked solder joint on the board.

[Audiorepair]

The transformer primary should read just a few ohms, so clearly it it toast.

Probably best bet is, as you say, an external DC supply.


If you remove the transformer, or otherwise cut the secondary traces, so you no longer have a few ohms across the output, you could then wire in your DC socket across the Anodes of D1 and D2.

This will keep in all the smoothing capacitors, and it won't matter what polarity the external supply is, or how you wired it.


There will be circuitry downstream of your diagram to account for discrepancies between battery voltage and transformer voltage, but it would be sensible to use a maximum of 9v supply, preferably a regulated one, they are not expensive.

[Audiorepair]

Oh, wait a minute.

My proposal of connecting the DC supply direct to the diodes only works with a full wave rectifier circuit, this is half wave.



All part of the learning process, eh.

[TimFox]

No, it's a full-wave rectifier, just not a bridge.
But you are right that you can't connect the DC supply across the anodes.
If the diodes are OK, he can connect the external supply across C3 or C4.

[HwAoRrDk]

Ahh, I see what you mean about being full-wave rectification. Yes, that makes sense, thanks.

This radio's not that old - maybe 15 or so years old, definitely made in the 21st century - so it's not like there would've been a lack of full bridge rectifier parts. Maybe they were just re-using a centre-tap transformer they were already using elsewhere.

I had a look at transformers on Digi-Key, Farnell, Mouser, RS, etc. Couldn't find anything that small (approx. 35x35mm, excluding mounting tabs) with 6V out.

I think I'll just modify it to have a DC barrel jack. I have a spare 6V wall-wart power adapter. Yes, perhaps across the 100uF cap, maybe with a diode in series with the positive for polarity protection.

[Audiorepair]

6v in front of a diode will give a supply voltage of around 5.4v, due to the loss of the 0.6v across the diode.

Better IMHO to aim for a 9v regulated supply which is guaranteed to exceed the 6v battery expectation, but not enough voltage to blow stuff up downstream.

[Audiorepair]

It has in the past been common practice to put a reversed biassed diode across the DC IN jack, so the wrong polarity shorts out the external power supply rather than destroys the unit attached.
Under sustained wrong polarity conditions, the diode melts to a short, protecting the equipment, and who cares about the power supply.

It is rare to find an inline diode to protect against wrong polarity.