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Old 3rd March 2014, 07:53 PM   #21
sbrads is offline sbrads  United Kingdom
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I think you may have to force your amp to stay on in as safe a manner as possible to get more sensible readings.

If you can disconnect the bridge +/- outputs, 2 resistors in series with the DC rails would make it unlikely you would blow anything up. It would be even better if you could put them in the +-47v lines after the reservoir caps c140 c141 but I expect you would have to cut tracks to do that. Try two 100R but if they smoke you'll need higher wattage ones or lower values but then there's more chance of blow ups. Even 22R would offer a decent amount of protection.

You could force the mains relay RY102 permanently on by shorting Q122 collector/emitter (or C145).

I think CB101 unplugged is OK.

Last edited by sbrads; 3rd March 2014 at 07:56 PM.
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Old 3rd March 2014, 07:59 PM   #22
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Hi,
He can remove the Q130A/C that it is output the high voltage to the speaker and that will allow the amplifier to stay on and will not cause any problem.
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Old 3rd March 2014, 08:05 PM   #23
sbrads is offline sbrads  United Kingdom
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Quote:
Originally Posted by tauro0221 View Post
Hi,
He can remove the Q130A/C that it is output the high voltage to the speaker and that will allow the amplifier to stay on and will not cause any problem.
Could be so. I would still like to see resistors in the supply while there's a lot of probing going on - it's so easy to slip and blow transistors up.
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Old 4th March 2014, 02:23 AM   #24
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Hey guys,

Thanks for you replies!

I have remeasured from Q110/Q112/Q109/Q111 back and have found the voltages on the attached diagram. To me it seems that there is the expected voltage drops across each of the transistors so they're not SC, however you guys would know better! Note this is still with the bridge connected, I just wanted to clarify exactly what you mean first before disconnecting it!

Should I desolder the bridge and then place 2 100R in series between the -47v and 47v connectors, effectively joining the + and - rails? Then, short out the relay to stay open? What would this achieve? Wouldn't there be no power to the circuit then or only what is going through D108? Or have I misunderstood?

Cheers,
Felix
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Old 4th March 2014, 07:05 PM   #25
sbrads is offline sbrads  United Kingdom
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You've misunderstood, I meant break the connection at +47v and insert one resistor across that break so that all power to the amp has to go through that resistor, ditto with the -ve side, purely to limit maximum current availability. It would be best done after the reservoir caps if possible, if not then at the bridge. Not essential if you're careful, but something that can save a lot of heartache.

Your latest voltage measurements seem to show that the output stages are OK as the bias isn't far off what you may expect, about 2v across all 4 b/e junctions. Q105 doesn't look like it's conducting due to insufficient b/e voltage. I think the 37v at the output is due to something being off at the input end, so a lot more reading round there could help. Specifically, all the Q101 Q103 voltages. I presume there's about 15v at the left end of R109, that's common to both input stages and could shift the output voltage if it's way out.

Last edited by sbrads; 4th March 2014 at 07:12 PM.
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Old 4th March 2014, 09:26 PM   #26
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Hi,
Can you check Q105 to see it is not shorted from base to emitter? If it is shorted from base to emitter it will feed negative voltage back.
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Old 4th March 2014, 09:38 PM   #27
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Hi,
I think the voltage from base to emitter are OKAY. Check the collector of Q105. It should read -1 volt at the collector. If not then Q105 it is not conducting.
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Old 6th March 2014, 07:10 AM   #28
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Hey guys,

That makes so much more sense sbrads, I don't know why I thought that ha.

Anyway, I've gone and measured the input stages to see what you guys think. If nothing comes of this then i'll look at chucking in those resistors and permanently powering it to get some better measurements. I've been trying to get the best readings possible, however strangely after turning it on and off enough times it slowly drops from 31.2v down to around 28 then magically comes back up again....strange?

Once it starts dropping I've been measuring across points as well which tells me if they are the same, so by doing this I think they should be more accurate


I tested Q105 and Q106 and they aren't shorted.

My conclusions from this (every component is reflected on the R channel also):
1. The voltages on bases of Q101/Q103 mean they aren't turning on. This then means that there is no voltage drop across R107 which means Q105 won't turn on either.
2. I have 0v drop between the TP101 rail and the base of the Q101, which initially made me think that C103 was S/C, but it isn't. (or is this to be expected?)

Don't know if there is anything in this either but I noticed if I turn on (and it shuts off) 3 times in a row quickly then RY101 clicks on then turns off with everything else.

Thanks!
Felix
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Old 6th March 2014, 02:45 PM   #29
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Hi,
This is the way I see it. Transistors Q105 and Q107 need to conduct to balance the bases of transistors Q111 and Q109. This will bias the bases to -1.1 as shown in the schematic. If not then it can go + 37 or -37 depending of the problem. Q105 need to be conducting to the amplifier to work. Check the collector voltage of Q105 and it should be negative. I know you check it for short but it can be open too collector to emitter.
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Old 6th March 2014, 07:27 PM   #30
sbrads is offline sbrads  United Kingdom
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2 amps doing the same thing though, it's weird, so back to basics. All these type of amps have overall A.C. gain determined by 2 resistors ratio (R117, R115) and their DC isolation is provided by one capacitor (C109), so that the DC gain can be unity, set by 100% DC feedback through one resistor (R117). There is another low value resistor R153 (just off your diagram) that connects the output to R117 that I'm ignoring.

This DC feedback means that the output will be at 0v (desired!) if the input (base of Q101) is at 0v, more or less. Most circuits will have this base pulled down to 0v via a medium value (10k or so) resistor to make sure it's near 0v. Now, during your fault finding you have CB101 unplugged, so an external ground on the E pin won't be there. I'm therefore intrigued how you have been measuring 0v at the bottom of R101 (THE important place for there to be 0v) as I can't see where the 0v gets there from looking at the circuit. If that can float up, the output will go up with it.

What with the schematic errors, I could easily be way off the mark here LOL.
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