[Phil G4SPZ]

Thank you for re-opening this old thread, Bill.

There’s been a more recent repair episode involving this particular instrument, and the fault turned out to be due to a single component, so I thought I’d add it to my previous report.

Over time, I noticed that on DC voltage ranges the meter started drifting from zero, and this was specifically temperature-related. On cold days, the pointer would drift down below zero, but on warm days it would drift up well above zero. On DC, there is no external ‘set zero’ control, and so I was forced to use the meter in ‘centre zero’ mode. The meter remained accurate, but this problem bugged me.

It took me the best part of a year to get around to fixing it, much of that time spent studying the circuit, reading up on chopper-stabilized DC amplifiers, and trying to fathom out just how the DC amplifier worked. I appealed for advice on the Forum, and fortunately both Peter Munro and Ron Bryan respectively came up with some missing pages from the manual and some suggestions.

I initially started out looking for heat-sensitive components, but quickly found that even the radiant heat from my fingers in the vicinity of VT5 or VT6 was enough to upset the zeroing. Placing a finger on either transistor sent the pointer racing off above or below the scale end stops, so it seemed that the chopper stabilizer wasn’t working. I calculated from the circuit values that the astable multivibrator that drives the chopper circuit should be running at around 600Hz, and with the aid of a simple signal tracer I found that it was indeed running as it should.

DC voltage checks showed that there was a negative potential of a few millivolts at the emitter of the chopper demodulator VT4, whereas with no input this should have been zero. Ron had pointed out that bipolar transistors used in this type of circuit are often connected in inverted mode (i.e. emitter acting as collector and vice-versa) as this achieves a very low Vce(sat) figure in the microvolt range when the demodulator transistor is turned hard on by the square-wave drive from the multivibrator. I also reasoned that, with zero input, there should be no AC at all coming out of the second voltage amplifier VT3, but sure enough, using the signal tracer there was a pronounced 600Hz buzz at its collector. Not only that, but the same buzz could be heard across the 64uF electrolytic emitter bypass capacitor C2...

Bingo! An emitter bypass capacitor should behave as a virtual short-circuit at the lowest frequency of operation. I temporarily lashed-in a replacement capacitor across C2. The buzz immediately vanished, and the pointer returned to zero. I snipped out C2 and replaced it with one of the same value, 47uF (although the Avo components list states 64uF) and the drift vanished. When I measured the old capacitor, it read just 270pF! After careful adjustment of RV4 and RV1 the meter was perfectly stable, and well within the +/-1% accuracy quoted in the manual. The other big improvement is in the pointer’s speed of operation; no longer is it sluggish, and it now behaves like any other electronic multimeter.

I took the opportunity to replace R81 and R48 with the new 68 Megohm resistor kindly sent to me by Alistair D exactly a year ago, in series with a 390k resistor selected from my own stock to bring the combined value as close as possible to the required total of 68.38 Megohms. This restored the 1,000 volt range to full accuracy.

Needless to say, I am absolutely delighted to have been able to restore this desirable meter to full working order by the identification and replacement of a single faulty component.

Having spent so long studying the operation of the DC amplifier in order to understand its method of operation, I can’t resist sharing it with you for the benefit of anyone else who has the misfortune to have to repair one of these meters. Feel free to skip this bit if you’re losing the will to live, but prior to this little project I knew nothing about chopper-stabilised DC amplifiers!

The DC amplifier circuit occupies part of the ‘amplifier board’ which sits behind the meter movement and in front of the range board, making access very difficult. The DC amplifier itself is a chopper-stabilised analogue operational current amplifier with a basic gain of 37, i.e. 1uA input gives FSD on the 37uA movement. The gain is set by the feedback resistor R10 in conjunction with the series multiplier resistors on the range board, resistors R8 and R9 being switched in to reduce the amplifier gain on the 300V and 1,000V ranges.

VT1 is a shunt chopper modulator which is driven by a square wave from the astable multivibrator VT9 and VT10, running at around 600Hz. On the positive-going multivibrator pulses arriving at VT1’s gate via C32 and D3, VT1 turns hard on and shunts the input voltage to zero. On negative-going multivibrator pulses, the input is coupled via C2 to the base of VT2. VT2 therefore sees a succession of positive pulses proportional to the magnitude of the input voltage. VT2 and VT3 form a conventional AC amplifier, with base bias and DC negative feedback via R4. C2 bypasses VT3’s emitter resistor R7, which is returned to the -2.8 volt rail.

An amplified version of the input pulse train appears at VT3’s emitter, and is AC-coupled by C3 to the chopper demodulator transistor VT4. VT4, connected in ‘inverted mode’, shorts the AC signal leaving C3 to ground in synchronism with VT1, resulting in an amplified positive-going train of pulses at its emitter, which is filtered and smoothed (integrated) by R11 and C5. The resulting DC potential across C5 is applied to the base of VT6 which, in conjunction with VT5, form a long-tailed pair or differential amplifier. VT5’s base bias is fixed, and the current through tail resistance R14 is substantially constant. RV4 provides the means of setting the zero (with no input) by adjusting the ratio of current flowing through VT5 and VT6.

With no DC input, the voltage at the junction of C3/VT4 emitter is zero (or just a few microvolts above zero, due to the clamping action of VT4) and hence the voltage at VT6's base is also zero with respect to the 0 volt rail. However, VT6 is conducting, due to its emitter being returned to the -2.8 volt rail via part of RV4 and R14. VT5 is also conducting, to a degree determined by the position of the 'set zero' pot RV4. VT7 and VT8 also conduct, to the extent that the net potential difference across the meter, i.e. between the junction of D4 cathode/R17/R16 and RV1/R20, is zero. Note that the circuit diagram contains an error here; “SB4” should read “SB6”, and “SB3” should read “SB5”. These refer to switch contacts shown on the instrument main wiring diagram, and this error caused me a good deal of head-scratching at first! Essentially, SB6 goes to the movement’s negative terminal, and SB5 goes to its positive terminal. Resistors R17, RV1 and R20 form an adjustable shunt across the meter movement, and hence RV1 is the main ‘set calibration’ control for the whole instrument. Once set, with 30mV applied on the 30mV range as described in the manual, the calibration should be correct across all ranges.

With a positive-going DC input present, VT6 conducts harder and the current through VT5 falls, causing VT7 to turn on harder. The current flowing through emitter-follower VT8 falls and the potential at its emitter and at the cathode of D4 moves more negative, causing current to flow via R20 through the meter and moving the pointer clockwise. The same mechanism applies with a negative-going DC input. The useful centre-zero function is provided by R1 and R2; a fixed bias is applied via the centre-zero switch which effectively makes the movement sit at mid-scale. The instrument then responds to input voltages of either polarity.

There is a feedback mechanism whereby any drift in the output under no input conditions sends an opposing potential back to the input, via the gain-setting resistors R8, R9 and R10, which act to correct the drift. The output of the DC amplifier is a current drive of up to 37.5uA to the meter movement. If drift occurs anywhere in the DC-coupled stages VT5 to VT8, a proportion of this drift is applied via R10/R9/R8 back in anti-phase to the input. As the entire amplifier is inverting, this negative feedback acts to stabilize the DC operating conditions of the amplifier and render an external ‘set zero’ control unnecessary.

Capacitor C2 had gone open-circuit, allowing AC feedback, causing the DC conditions to float about and permitting the multivibrator pulses to appear as an output at VT3’s collector. This manifested itself as the original symptom of temperature related drift. The thumbnails show the circuit diagram and the offending faulty capacitor.

Phil