Decca diadem 1965

Decca Diadem after repair & varnish to the lid.

The Diadem was quite ahead of it's time for Decca, most UK radio manufacturers were still building radios in big square boxes or naive designed wooden cabinets. This model aimed at the G-Plan type homes of the mid-60's. It's about shoe box size and when lid is closed all controls are hidden or not collecting dust, the only clue it's a radio comes from the speaker grills at each end, only one speaker is fitted as stereo reception hadn't hit the market quite yet.Most in-door radios of the day were mains powered, this uses 9 volt gained by fitting 6 x 1.5 volt large torch type batteries, providing plenty of current for the 2 OC81 output transistors, but also conserving power when using AM reception the FM transistors were powered off.
On the rear it has inputs for a record player and an output for a tape recorder with a headphone or extension speaker plug. Telescopic aerial is hidden on the rear looking like a bolt head, but pulls out for FM reception and a socket was added for a dipole aerial. A tome wheel and push button sensitivity switch is found at the front. A large oval speaker provides good quality sound. The speaker grille on the right is a dummy but does allow rear left speaker sound modulation to escape.

The repair
Before I switch on a new arrival I always give it a once-over internally, it gives you an idea of the kind of life the unit has lived. Two 1/2 turn clip screws hold the bottom cover in place to reveal what looks like a modified inside (worrying). A hole had been made in the rear and a mains cable had been fitted, but the most worrying part was the mains voltage  had been wired to the battery plug that fitted normally in the 6 x U2 battery compartment. Obviously this was a huge mistake and after removing the metal tuning dial I found it covered in spark residue. When it had been switched-on the the past part of the copper printed circuit had been vaporized all over the metal plate in a huge bang.

The AM oscillator transistor was located to the right of the osc coil, I fitted BF450 on print side for ease

After replacing a short length of print with a length of wire I found, not surprisingly, a 5 ohm short across the battery rail, this turned out the be a faulty OC81 output transistor, that I replaced. No noise apart from a faint hiss was coming from the loudspeaker, I used an audio signal generator to test the sound output part of the circuit, starting at the volume control and all seemed well. Next I found a circuit on-line for this model, I've included it  here below. The first 2 AF116 transistors are for the FM signal and transistor 3 also AF116 provides oscillations and RF amp for the AM signals. This turned out to be faulty, even though it read ok on a test meter (about 850 ohm from base to emitter and collector) and o/c reversed wire checked. I fitted a BF450 that has about a 6k ohm forward resistance, but works fine with no circuit changes, although the pin connections are totally different, check them on-line..

Decca diadem circuit diagram

Before any work was carried-out, I cleaned all the inside out with a car vacuum cleaner and a small paint brush, a spider ran over my hand escaping the speaker chassis, lol Mother panel is held by 2 screws at top (hard to get at, (remove a section of print when re-fitting to make it  easier to get at screws) and 2 overlong bolts at the bottom of the printed circuit steel frame. The white tuning dial rear was cleaned and 4 LED's were fitted to improve viability of the display.  I wired them in parallel via a 50 ohm resistor to the 2nd 1.5 v battery giving the LED's 3 a volt supply. The positive side of the circuit I fitted to the cold side of the on/off switch.

Cleaned tuning dial rear where I fitted 4 x LED's taking care they were hidden behind the open display

 Overall I found this to be a well made piece of kit and a pleasure to repair, although when I purchased it, it was quite expensive considering all the damage, I removed scratches from the top by removing original varnish with paint stripper and then applying 4 coats of quick drying mat varnish. I think the Diadem is quite rare as no on-line sites give it a mention, until now!

Lovely clear and large tuning dials, originally without a dial lite 

Ooh I almost forgot, when checking for low sound on FM, I found the 5k ohm skeleton preset pot open circuit, this was found in the audio  decoder circuit by the 2 crystal detector diodes.

ebay photo, showing  some small scratches - now removed along with mains lead?