Avo EA113 temperature-related zero drift

[mole42uk]

Hello Phil,

RV4 does nothing observable. Looking at the chopper signal, I'm seeing a signal of approximately 600Hz but it isn't a square wave at all, it's more like a sawtooth but it is about 5v p-p.

Positive and negative supplies are good to the DC amplifier but although VT3 collector is 2v I measured VT8 emitter at 2.5v. VT8 base is at 3.1v so it's turned on while VT7 base is at -0.7v which voltage is not altered by RV4. I'm wondering about a c/e short on VT5.

Any guidance will be appreciated!

VT9 collector I get -2v, VT10 collector I get -2.5v and -3.6 on both bases.

[Phil G4SPZ]

Hi Richard. Yes, that does seem to be the logical conclusion. Are you able to measure any Vce across VT5? Certainly, given the values of the potential divider formed by R13 and R14, if VT5 were shorted then you'd get something like the collector voltage you are seeing.

The voltages around the multivibrator look a bit weird, though. I didn't record what the voltages were on my meter, nor did I look at the waveforms on a scope, but logic suggests that the collector of VT10 should be swinging between the positive and negative supply rails, hence the average DC voltage at its collector should be nearer zero than a negative value. There will be a sawtooth waveform at the bases, but the waveform at VT10's collector should be a square wave. I guess C32 may be causing the square wave to look distorted. It's impossible to get a probe onto the other side of C32, so maybe you could scope the waveform at VT10's collector?

You didn't mention whether the centre-zero switch does anything, but I very much doubt that it will. With VT8 turned hard on, the meter movement will be seeing a pretty heavy overload in the reverse direction. If you press the centre button, does it shoot up past FSD?

[mole42uk]

Some more tests last evening:

VT5 has near enough the same voltage on c & e and the base isn’t high enough to turn it on. I’m working on replacing that transistor but, of course, it is one of the harder ones to get at without dismantling!

VT8 has 3v on the base, so I think that would pull SB4 (tag 16) high enough to swing the meter hard over to the left.

One other thing is evidence of a spattering of component internals on the DC amp board but I cannot find where from!

[Phil G4SPZ]

Spattered components? Crikey! It's hard to think of any component going off in an explosive manner, other than a tantalum capacitor or maybe one of the supply rail decouplers? Perhaps some previous careless owner inserted the batteries the wrong way round?

Whatever, it does look like you're faced with a dismantling job.

Nice to meet you at the NVCF, by the way.

[mole42uk]

Yes, Phil, it was good to meet up at NVCF.

I'm puzzled about the origin of the spattering. It only appears on the exposed portion of the amp board, in other words adjacent to the battery housing. There is no corroborative evidence of battery leakage, I can't find a suspect tantalum (over and above the fact that all tantalums are suspect!!) or any other component that might have liberated it's essential elements with force. The main suspect in any test meter is the input circuit if it has been overloaded, but again, there's no evidence. The fusing is okay, the meter works perfectly well on the ac ranges and all the visible components connected to the input circuit look well and similar to the ones in my other EA113.

It may be a day or several before I get around to removing the amp board to change that transistor. I'll report back.