[dazzlevision]

I now have a complete spare chassis from a Marconiphone 4238 on the bench (this, and the Ferguson 3238, were the last two models to be released that used the DC43 deck and valve electronics). The 4238 chassis is dated late 1968 (according to the date codes on some electrolytic capacitors and the ITT Selenium bridge rectifier) and must be one of the last ever production run.

I’ve been careful to use the same reel of EMI “brown” (1960s) tape on both decks, in order to get an accurate comparison between the two.

The coloured dots and lines on the rear of the R/P head are the same as on the Ultra 6212 head.

After powering up, I measured the output of the 55kHz HF bias oscillator stage (ECC82) at the point which feeds bias to the record/play head via a parallel pair of a fixed and a variable capacitor (to set the correct R/P head bias current).

The level was considerably lower (140Vpk-pk) than that measured on the Ultra 6212 chassis, so I tried a replacement ECC82, which didn’t work at all. After replacing the original valve, it still wouldn’t work! This was due to a manufacturing defect.
Pin 2 of the B9A valveholder was much shorter than it should be and only just got whetted in the solder bath at the factory. Stressing the valveholder during valve removal broke this rather tenuous connection. A new valveholder restored oscillation and with a new Mullard ECC82, the bias voltage was much higher (240Vpk-pk).

I then inserted a 1% 100 Ohm resistor in the earthy side of the R/P head’s track 1 wiring, in order to check the bias current, which was 23mV rms (the service manual states the acceptable range is 20–30mV).

A frequency response test recording was made at 3.75 ips, with a 1kHz reference, plus 10k, 12k and 14k tones. 14kHz is the upper limit of the claimed +/-3dB response at this speed.
The (normalised about 1kHz) results were: 0, -4, -3 and (at 14kHz), there was an audible tone (in addition to a diminished 14kHz sinewave on the ‘scope) when played back, which seems to indicate some sort of intermodulation effect during the recording process. I had the same effect on the Ultra 6212. This result seems acceptable as far as the response check is concerned.

I then measured the values of several (carbon composition) fixed resistors in the treble boost circuitry used in record mode. They were all well above the upper tolerance limits. I replaced all of them, as they may well have had an adverse effect on the response.
The new 3.75 ips results were: 0. -6, -6 and the same audible tone at 14kHz.

I also made some test recordings at 7.5 ips (both before and after resistor changes), at 1, 10, 14 and 18kHz (18kHz being the upper +/- 3dB limit at this speed).
The “before” results were: 0, +1, +8, +8.
The “after” results were: 0, -3, +3, +3.

So, a bit of a mixed bag. The results at 3.75 ips are out of spec, but those for 7.5 ips are in spec (and no audible playback tone at the 7.5 ips top test frequency either, unlike at 3.75 ips).

The specifications listed in service manuals for these DC43 tape deck models state a “frequency range”, with no +/- dB limits given. So, with some visible head wear on the sets I’ve been working on and the ageing of certain passive electronic components and the valves, the most recent results that I’ve obtained are acceptable.

I’ll put my Ultra 6212 back on the bench and fit a known good ECC82 bias oscillator valve, to see what effect that might have on erasure and frequency response performance (in case the one fitted is out of spec).

I’d be interested to hear from anyone who can explain the mechanism that causes the audible range playback tone at a recorded spot frequency of 14kHz at 3.75 ips.