Orpheus Radio Repairs


"On the Bench"


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11th February 2004.

I'll start by saying that this set isn't mine, but has come to me for repair. The set is a standard 3 band superhet of the mid 1930's, using a 4 valve plus rectifier and tuning indicator arrangement. The valve line-up is FC4, VP4B, TDD4, AC/PEN, IW4-350 rectifier and TV4(A?) tuning indicator. It is an AC only set, and has a HT energised speaker, which doubles as a smoothing choke. There is provision for a gramophone input, and a tone control is fitted to the back of the chassis. There is also a switch on the rear of the chassis marked AVC and QAVC, which I think is a noise blanker, and works by modifying the AVC (AGC) response.

The two pictures show the chassis after an initial clean. Empty four pin socket on the top of the chassis is for the loudspeaker.

The first problem wwith this set is no service data, so an empirical aproach has been adopted!

Initial tests on the receiver were:

Check the speaker and ouput transformer for continuity

Check the mains switch and transformer for continuity - ther is also a filter choke in the mains input, which was checked.

All the valves were removed from the set, and the mains applied - at least the dial lamps work! A few quick checks with the meter confirmed heater power for the rectifier (4V), and heater supply for the rest of the set (4V centre tapped to earth). The HT winding gives about 360V from each side open circuit (centre tap to earth), but importantly, both side read the same. After a few minutes running, the transformer temprature was checked, the cool running being a good indication that there are no shorted turns.

Next, the rectifier valve was re-fited, and the set powered up. The HT at the rectifier cathode reached 460V, so the set was quickly turned off - the HT smoothing capacitor is only rated at 450V!

After replacing the ouput valve (AC/PEN), and plugging in the speaker, the set was powered up again. HT voltage at the rectifier cathode was now 350V, and at the anode of the output stage only 190V, indicating that alot of current was being drawn by the ouput stage. The trouble was traced to a leaky AF coupling capacitor on the grid of the ouput valve. Replacing this returned the voltage to about 250V.

Replacing the detector valve (TDD4) produced a second working stage, and a good deal of gain from the "Gram" input, which is switched in by the wavechange switch.

Next, insert the IF valve (VP4B), feeding a signal to its grid (top cap), via a 0.1uF capacitor, indicates an IF of 130KHz, but not a great deal of gain, so I'll need to investigate a bit more!

24th March 2004.

I've just come back to this set after a couple of "rush job" DAC90's. As there was little or no IF gain, a little more investigation is needed. Measuring the voltages around the VP4, they compare well with the valve manufacturers recommended operating conditions (remember, no service data for this set!), so I'll move on, and do the same with the FC4 frequency changer, which reveals......

No volts on the Screen grid! A bit more investigation shows that there are two resistors in series feeding the screen (10K and 51K), with a de-coupling capacitor from the mid point to ground (one end of the pair goes to HT, the other to the screen grid). Replacement of the 10K resistor has returned some life to the set.

The audio is a little distorted, and the set only seems to pick up strong stations on medium wave and short wave (nothing on long wave), so I think the next step is IF alignment, and then another good look at the wavechange switch and the LW coils.

I've also straightened a couple of bits of metalwork, and re-routed a few wires to stop the tuning pointer from fouling various bits as it goes round the dial.

26th March 2004.

After a quick run through the IF alignment (centre frequency 130KHz), a lot of the distortion has gone, and altough the set is quite sensitive (easily picks out a 1uV signal on long and medium wave, and 6uV on shortwave), the audio is definatly not what it should be - there is still distortion, and not a great deal of volume. It must be time to re-visit the AF stages.

A good poke around under the chassis reveals two electrolytics which by-pass the cathode resistors of the AF stages, they are both 25uF 25V units, which is OK for the TDD4 (3V cathode bias), but not so clever for the AC/PEN (20V bias - a bit close for comfort. They have both been replaced by 33uF 50V units - the rule for the voltage is to run the cap at 1.5 x the peak voltage, but never with less than 10% of its rated voltage across it - the one for the TDD4 only just about makes it, maybe I should have used a 16V rated component here (if I'd had one in the spares box!).

Measuring the old caps revealed that the capacitance of the TDD4 by-pass unit had fallen from 25uF to 0.1uF, which would account for the lack of AF gain. The capacitor from the AC/PEN was still in specification. Neither showed any leakage.

The low IF does lead to image problems on Short wave, and most stations can be tuned in at two points on the dial!

I've also removed the voltage selector plug on the rear chassis apron from the circuit, as it had two exposed sockets connected to the mains supply and very easy for small fingers to find. The selector is still in place, but has been disconnected, and the transformer taps are now hard wired for 240V.

Underside of Chassis.

2nd April 2004.

I replaced the set in it's cabinet - and it's stopped working! A little investigation showed that the resistor feeding HT to the frequency changer had come away from the HT busbar. the wire joint had not been wrapped before soldering. Re-making the joint properly, and re-soldering cured the problem.

After the set had gone back into the cabinet again, it was tested well. The sound qaulity is not as good as it should be except at very low volume levels, with distortion and rattles form the speaker.

5th April 2004.

A substitute speaker (a permannent magnet 8 ohm unit) was wired in in place of the original (field left connected, as it is the smoothing choke), and the audio sounded fantastic! Looking closely at the speaker, it is bolted together, so it was carefully dismantled, which showed rust on the pole pieces. The pole pieces and the air gap were cleaned well, and the speaker reassembled - use paper shims to help centre the voice coil. re-testing the set gives lovely clear sound, and a lot of volume! The speaker has a "Radiospares" replacement field coil - I've never seen one before!

All in all, this has been an interesting job, especially as there is no service data for this set! It's now down to the sets owner to re-finish the cabinet.

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Copyright J.Beacon & M.Wroe-Parker 2003