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Old 31st Jul 2019, 4:31 pm   #21
ColinTheAmpMan1
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Default Re: Tektronix 7A26 problems

Dave,

Thanks, I am learning lots about this circuit from you.

I have had a bit more of a noodle about and swapping U1450 (ch1 second stage) and U2450 (ch2 second stage) makes no change.

I lifted a few ends of the R/C networks on pins 10 & 4 of U1350 and of U2350. The capacitors seemed fine, as did the 8k2 resistors paralleling them, but the supposed 8k2 resistors going to the -15 Volt bus were both 120R. I changed these for a couple of metal film 1% items, but while pins 10 & 4 on CH2 (U2350) are still 7.5 VDC, pins 10 & 4 on CH1 remain at 4.6/4.7 VDC. It also looks as though the 120R resistors might well have been original, too.

What is going on? I need to check a few more things that have been suggested, I think.

Colin.
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Old 8th Aug 2019, 10:20 am   #22
dave cox
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Default Re: Tektronix 7A26 problems

I wouldn't worry about the voltage on 10/4 too much, I don't think this is causal. In the later stages these are tied to ground so are likely not critical. Interesting that the R values are not as published though, is this the same on all your 7A26's ?

I think the 'most likely' culprit is R1347 drifting high ...

Do any of the resistors look a bit scorched / feel hot when running ? IIRC quite a lot are operating near the limit !

dc
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Old 11th Aug 2019, 6:23 pm   #23
ColinTheAmpMan1
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Default Re: Tektronix 7A26 problems

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Originally Posted by dave cox View Post
I wouldn't worry about the voltage on 10/4 too much, I don't think this is causal. In the later stages these are tied to ground so are likely not critical. Interesting that the R values are not as published though, is this the same on all your 7A26's ?
Now you mention it and I took a look, my other (working) 7A26 has 120R resistors instead of the documented 8k2. Mysterious...

Quote:
I think the 'most likely' culprit is R1347 drifting high ...

Do any of the resistors look a bit scorched / feel hot when running ? IIRC quite a lot are operating near the limit !
I think you have hit the nail on the head. R1347 in CH1 measures 364R while R2347 is 280R6. If R1347 has drifted high, how would this explain CH2 having a lowish gain, too? The value of 280R is not particularly common, especially at 0.5 W or higher. Using a finger-tip, both resistors (R1347 and R2347) are running warm, so I have ordered some 280R, 0.6 W, 1% Vishay resistors in the hope that the extra 100mW capability will improve the reliability. I will also mount them with a gap above the board, too. None of the resistors looks at all scorched, though.

Colin.
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Old 12th Aug 2019, 4:39 pm   #24
dave cox
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Default Re: Tektronix 7A26 problems



I don't think my amplifier has 8K2 either ...

dc
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Old 4th Sep 2019, 6:58 pm   #25
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Default Re: Tektronix 7A26 problems

Now I have my 7623A mainframe up and working again, I've been able to put this 7A26 into it and check it out. With both of the resistors replaced with ones which can cope with a little more current, the gain on both channels is pretty much equal and I can set the gain of both to the calibrated figure. There is a bit of extra gain available, but perhaps not quite as much as there is for my other 7A26. While I can live with things as they are, has anyone any idea what might be causing this?

Colin.
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Old 4th Sep 2019, 8:10 pm   #26
tony brady
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Default Re: Tektronix 7A26 problems

there is a slot in each variable cal knob - you can use a screw driver to adjust the level. perhaps it's this that is making the difference? you have to insert the screw driver into the slot then push in and adjust the level. the procedure is in the manual
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Old 5th Sep 2019, 11:43 am   #27
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Default Re: Tektronix 7A26 problems

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Originally Posted by tony brady View Post
there is a slot in each variable cal knob - you can use a screw driver to adjust the level. perhaps it's this that is making the difference? you have to insert the screw driver into the slot then push in and adjust the level. the procedure is in the manual
That is exactly how I have been adjusting the "cal" gain, Tony. I can live with the gain as it is, because I can calibrate both channels of the amplifier, but my other 7A26 has just a little more gain "headroom" and I suspect that something may be getting a little old and out-of-spec.

Colin.
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Old 5th Sep 2019, 3:55 pm   #28
dave cox
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Default Re: Tektronix 7A26 problems

It's very likely due to more of the same, out of spec resistors, as there is not much else to go wrong!

It maybe any of the same brand in either the 'emitter tails' or the collector loads of the 4 (IIRC) stages of differential amplification but problems *should* show up in the DC operating point as shown on the circuit diagrams.

I don't have a fully formed theory why the resistors you changed alter the gain but my own experience is that it does! I can see that it could mess up the available headroom on the output stage which might show up as a lack of gain. If that is the case then it should also show some non-linearity but I had already replaced the resistors so couldn't test that theory ...

dc
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Old 11th Sep 2019, 7:25 pm   #29
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Default Re: Tektronix 7A26 problems

I have just checked the 280R resistors that I have replaced and I measured 10.8V across both of them. If my arithmetic and knowledge of Ohm's Law is right, this equates to 38.6mA flowing and 0.42W in both cases. So, the Tek spec of 0.5W is a little close to the limit. I'm glad I put 0.6W replacements in there....Even now, they are quite warm to the touch, I measure about 50 degrees on each with a Fluke 62 Mini IR Thermometer.

Now to check the DC voltages and see if they are ok, or if some other resistors might have drifted, too.

Colin.
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