Hello enthusiasts,
I read this great forum since 12 years ago, but today I decided to create an user and ask you for help.
I bought an working Grundig FineArts V12 amplifier and I started to give it a new "life" by replacing old electrolytic caps inside. Before I started the work, I measured the power supply voltages and they seamed to be quite right compared to the schematics. The little exception that I had it, is that main power supply is designed to work at 230V ac, but in my area I have constant 240V. I suppose that the manufacturer has considered a +/-10% tolerance for the main supply voltage, so maximum will be 253V.
Anyway, when I started to make the measurements, I observed that on right channel, on output, I had a dc offset of -150mV when tone defeat is On and -80mV when tone defeat if Off. On the other channel the dc offset is ok (+26mV).
I started to replace all elec. caps, I remeasured the dc offset on the right channel, nothing changed. I decided to replaced all the transistors (incl. final trans.), all the diodes and all the resistors with a metal film, same dc offset at the output. All the replaced components were new and bought from a trusted supplier.
I started to dig up even more, to measure transistors voltage between left and right channel and saw that on T405 the collector voltage is 2V higher that T402 collector (on the schematic the voltage difference should be 0.7V). On the good channel the difference is ok. I mention that transistors in the current mirror were replaced with new ones.
Another thing to mention is that T406 (also T306) are quite hot (I measured about 50 degrees C or 122F) and when I start to cool down, dc offset lowers a bit.
At this moment I am stuck and don't know what to do in order to reduce or correct the output of dc offset. For this reason I am asking for your help.
Please find attached the schematic of the mentioned amplifier.
Kind regards,
Chris.
I read this great forum since 12 years ago, but today I decided to create an user and ask you for help.
I bought an working Grundig FineArts V12 amplifier and I started to give it a new "life" by replacing old electrolytic caps inside. Before I started the work, I measured the power supply voltages and they seamed to be quite right compared to the schematics. The little exception that I had it, is that main power supply is designed to work at 230V ac, but in my area I have constant 240V. I suppose that the manufacturer has considered a +/-10% tolerance for the main supply voltage, so maximum will be 253V.
Anyway, when I started to make the measurements, I observed that on right channel, on output, I had a dc offset of -150mV when tone defeat is On and -80mV when tone defeat if Off. On the other channel the dc offset is ok (+26mV).
I started to replace all elec. caps, I remeasured the dc offset on the right channel, nothing changed. I decided to replaced all the transistors (incl. final trans.), all the diodes and all the resistors with a metal film, same dc offset at the output. All the replaced components were new and bought from a trusted supplier.
I started to dig up even more, to measure transistors voltage between left and right channel and saw that on T405 the collector voltage is 2V higher that T402 collector (on the schematic the voltage difference should be 0.7V). On the good channel the difference is ok. I mention that transistors in the current mirror were replaced with new ones.
Another thing to mention is that T406 (also T306) are quite hot (I measured about 50 degrees C or 122F) and when I start to cool down, dc offset lowers a bit.
At this moment I am stuck and don't know what to do in order to reduce or correct the output of dc offset. For this reason I am asking for your help.
Please find attached the schematic of the mentioned amplifier.
Kind regards,
Chris.
A couple of thoughts...
Given that the power amp sections are AC coupled at their inputs (C302 and C402) it should not be possible for anything before that (such as tone defeat) to change the DC offset.
Given that you say it does would have me checking for any incorrect voltage polarity across those input caps and also looking if there was any DC present before those caps.
If that was OK then a scope check for high frequency instability in the front end is needed... because those caps should make the power amp and its offset 100% immune to DC shifts before them.
Also... and to late now... replacing all the transistors is not recommended. Unless all are exactly the same as the originals (and that means of that era and not generic new production devices) then you can run into all kinds of subtle issues.
At the very least the bias settings will need readjustment due to the different devices.
Given that the power amp sections are AC coupled at their inputs (C302 and C402) it should not be possible for anything before that (such as tone defeat) to change the DC offset.
Given that you say it does would have me checking for any incorrect voltage polarity across those input caps and also looking if there was any DC present before those caps.
If that was OK then a scope check for high frequency instability in the front end is needed... because those caps should make the power amp and its offset 100% immune to DC shifts before them.
Also... and to late now... replacing all the transistors is not recommended. Unless all are exactly the same as the originals (and that means of that era and not generic new production devices) then you can run into all kinds of subtle issues.
At the very least the bias settings will need readjustment due to the different devices.
Thank you Molly for your response.
Regarding Tone Defeat, this kind of amplier uses equalizer (and tone defeat) in the feedback loop of the amplifier. The connection is between terminals of C307 (or C407) with the XL, XR, YL and YR wires. The eq looks passive for me and it looks it doesn't have any DC supply.
To be honest, I didn't checked for DC before C302 and C402, but I will do that. 🙂
About scope check, do you suggest to do it with input signal or without?
The transistor replacement was done only for the affected channel and I was doing it in order to see if anything will change, but the issue stays the same. I was able to find on farn*** the same kind of transistors expect the T406 (BC880), for which I've found as replacement the ZTX705. In this case, I mounted the transistor and when I saw that nothing changes, I remount the BC880.
Regarding Tone Defeat, this kind of amplier uses equalizer (and tone defeat) in the feedback loop of the amplifier. The connection is between terminals of C307 (or C407) with the XL, XR, YL and YR wires. The eq looks passive for me and it looks it doesn't have any DC supply.
To be honest, I didn't checked for DC before C302 and C402, but I will do that. 🙂
About scope check, do you suggest to do it with input signal or without?
The transistor replacement was done only for the affected channel and I was doing it in order to see if anything will change, but the issue stays the same. I was able to find on farn*** the same kind of transistors expect the T406 (BC880), for which I've found as replacement the ZTX705. In this case, I mounted the transistor and when I saw that nothing changes, I remount the BC880.
Clean from contamination, rinse with PCB alcohol (possible atmospheric contamination). Clear pots and switches.
Input differential pair, electrolytics at the input, in the NFB circuit, bootstrap.
Input differential pair, electrolytics at the input, in the NFB circuit, bootstrap.
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My apologies, so it does...
So the switching between defeat and not seems to add a 68k (R451) as a DC path across the fixed network of the amp (R306 and R307). When in 'Defeat Off' the tone network seems AC coupled via C451.
All of that suggests that it would be normal to see a shift in DC offset as the switch is changed because this will alter the bias current flowing in T305. T305 and T302 should always see similar currents for minimum offset, although not in itself the whole story because DC balance also depends on such things as the tail current of the diff pair.
All I can suggest is you try and compromise and get the offset closer to zero by initially trying different pair groups for the diff pair. First try just swapping the two devices around.
Scope check would be with no signal just to confirm there was no HF instability present.
You could also try very subtle tweaking of the 560 ohm to alter the tail current a little. Don't go to low or high though as the output would suddenly latch to one of the rails.