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Old 6th March 2013, 10:48 AM   #31
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OK, you have discovered something more that's useful to know about this design. The 21,5 and 23V voltages are about
what I was expecting but they aren't critical because the transformer input stage and capacitor output coupling mean
that the signal is isolated from the rest of the amplifier. Did you check 12V at the bases of Q6,7?
If that is within 0.5V, I think we can assume it's functional and I would set bias to midrange at 50mV in both channels.

I'm guessing, but I assume the hum in the bad channel was worse at a higher bias like 150 mV? If that is so, it means
the hum is still there and something else like the position of wiring, a cap, even a film type, may have deteriorated.
You can look at the hum waveform and trace it with your 'scope set at high sensitivity to find it's strongest location
and identify whether it's principally 60 Hz or 120 Hz which will tell you the origin - mains or PSU and this can help locate it.
After all, if it's radiated from mains wiring or the transformer, there's no point searching the circuitry for component faults.

Good work, anyway
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Old 6th March 2013, 03:24 PM   #32
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Hi Ian. Did not get to the 12V check, will do that tonight.

The hum was yes worse when the bias was higher. It's not so bad and I was thinking if it was in both channels I might let it go Once the volume's up to about 1 or 1.5, i.e. as soon as you start hearing the source, you don't hear it any more.

I spent five or ten minutes last night poking around at wiring, tapping things, bringing a ground to all the ground connections with a jumper, etc., couldn't find anything.

I was thinking the same, to try tracing the hum with the scope. I'm gonna guess it's 120Hz; what would that imply?

I have a couple more electrolytics on the tone board to replace this weekend but I'm guessing they won't be the problem. The fact that it has no response to the volume or any other controls seems to me to indicate it's being picked up after the tone control board and volume pot, at least, no?

Perhaps foolish also, I haven't tried putting the cover back on to see if maybe it's external and diminished by the cover...
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Old 6th March 2013, 09:23 PM   #33
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Noticed last night that I misread two cap values, C10 and C11, on either side of the FM Stereo lamp. I currently have 33uF instead of 100uF. Seems to be working fine, worth ordering new capse & changing?
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Old 6th March 2013, 09:54 PM   #34
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Quote:
Originally Posted by mmmalmberg View Post
I was thinking the same, to try tracing the hum with the scope. I'm gonna guess it's 120Hz; what would that imply?..........

The fact that it has no response to the volume or any other controls seems to me to indicate it's being picked up after the tone control board and volume pot, at least, no? ...
If it's 120 Hz hum, which you can tell because it has a rough edginess to the tone that is is heard in several higher harmonics, then it is almost guaranteed to be generated by the rectifier and it will most likely be in the power rail. That hum will be fed to every circuit and the only way to remove it is to increase the size of the large electros and adding more and smaller caps through the circuits, known sometimes as bypassing or decoupling caps. This is well and good but takes time and money to find the most effective points to add the parts and sometimes, RC filtering is necessary to have any effect on low level circuits.

Nevertheless, the hum is also radiated by power leads, which should always be routed away from sensitive wiring and circuits such as preamps and the input stage of the power amp. Typically, in a single rail amplifier like this one, the power supply and ground return should be twisted together and routed away from the boards and signal connections. Twisting a feed and return wire pair helps cancel some of the noise field around the wires and that means less noise/hum.

If it is a deep, sonorous 60Hz, that's likely electromagnetically induced from the transformer or mains wiring which, if you think about it, is part easy and part impossible to deal with unless the circuits are somewhat immune to it, as balanced supplies can be. Still, we have to make the best of what is there, warts and all. I don't think it would have been a noticeably noisy receiver originally, but certainly, buyer expectations in the 1960's were a lot less for hum and noise than they are now.

A lot of guys are peeved when they buy an old amp. based on it's reputation of good sound quality, only to find that it hums, thumps and has objectionable noise when controls are operated. Sometimes, those "endearing qualities" were there in the original amp and nobody thought twice about it but now, noise levels are well under -100dB wrt full output and this is dead quiet. Playing even good vinyl now would seem quite noisy at only -75 dB.
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Last edited by Ian Finch; 6th March 2013 at 09:57 PM.
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Old 6th March 2013, 10:04 PM   #35
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This amp sounds pretty great now overall. The hum's only in one channel which makes me think it's not in the design of the amp and was probably not that way new. It would however be quite livable as is. Plus it's not my amp and I'm not being paid

I do of course have new p.s. caps in there along with everything else, not lots larger than stock, I went from 1500uF to 1800uF times 2 caps.

I'll figure out the frequency and poke around with the scope and see what I can find. If I can't nail it by the time the last order of little caps arrives this week I'll probably just button it up and return it; I'm sure it sounds 100% better than it did.

-Mark
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Old 6th March 2013, 10:06 PM   #36
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If it were in the rectifier could I just put a new better diode bridge in there? But I don't get how noise from the rectifier would be isolated to one channel...
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Old 6th March 2013, 11:52 PM   #37
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It seems I've posted over your #35 and it's a related matter.
You may have realised that the FM pilot lamp is connected direct to the AC secondary supply! Q404 switches the bulb to ground to give it power according to whether or not a stereo pilot tone is present but it's quite funny that 60Hz is right in there at the detector. I would refit 100uF caps there actually - the more smoothing the better, regardless of whether it sounds OK at the moment.

A rectifier is a rectifier unfortunately and you won't hear any difference in performance using various ultrafast recovery SMPS diodes which are currently in vogue with DIY and high end designs. The point is that they all have to switch the AC on alternate half cycles to keep the caps charged but each switching causes a necessary 120 Hz sawtooth ripple on the DC as the electrolytics are alternately charged. Filtering that ripple is not viable considering the size and cost of the chokes and caps needed to completely smooth the DC, The alternative of regulation is also not the panacea it might seem.

All basic transformer PSUs are like this and they are quite satisfactorily dealt with by adequate filter caps, electronic circuits that are immune to noisy rails (high PSRR) and keeping the power wiring separated and away from trouble spots as discussed.

If only one channel hums, it has a trouble spot that the other channel doesn't - that's all it boils down to. The obvious candidates are wiring and parts location (too close to noise source or poor circuit/wiring layout) and faulty components and connections such as filter or bypass caps, various solder joints and any connectors, switches etc. Any of these can be quite different in either channel, so the problem is not so much one of the channels being differently designed, but differently wired up, positioned or faulty.

The big deal with audio hum is grounding of the different sub-circuit boards and this always needs patient checking if wiring has been disturbed or corrosion has set in and caused poor contacts etc.

The first thing I would do is remove the signal input wiring for both channels right at at the power amplifier board. Then let logical tracing find the culprit, if the hum is still worse in one channel. Once you only have that simple handful of parts and the wiring bundles in the amplifiers to consider, things should be much easier to deal with, particularly since you don't have DC coupling so ground loops will be less of a problem.
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Old 7th March 2013, 12:23 AM   #38
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That sounds like a good idea, if it's still humming with the power amp inputs disconnected then we know it's in the power amp. If it's not then we know it's either in the preamp or in the wiring coming from the volume pots to the preamp, yes?
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Old 7th March 2013, 12:30 AM   #39
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Quote:
Originally Posted by Ian Finch View Post
You may have realised that the FM pilot lamp is connected direct to the AC secondary supply! QUOTE]
You know I only saw it connected to AC and thought "Wow I really don't understand how that tells whether it's stereo or not!", but didn't figure out the transistor or any other functionality...Still probably a bit over my head but I'll take a look at it
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Old 7th March 2013, 05:20 AM   #40
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Well I didn't quite get to everything tonight. First I figured I'd get bigger caps on the stereo lamp circuit; didn't have the 100 uF's specified so put 220's on, can always change them out again but didn't notice any difference either way.

Powered up and checked some voltages. 1.1 & 1.2VDC on Q's 6 & 7, schematic calls for 1.2V.

Rechecked bias, for some reason one channel had shifted from where I believe I left it, so I went back and forth between them and got both at 50mV.

Pulled the input to the power amp from the side that hums (right). It actually seemed to get louder with that input disconnected. Intuitively I tried momentarily grounding the base of the input transistor, Q12, which eliminated the noise.

I didn't pull the input to the left power amp. I poked around with the scope and found the loudest hum at I the base of one of the output transistors, I think Q1, while it was low at the other output transistor. I saw it at a lower level on the other side of the input transformer, at C24, 4uF, only on the + side of the cap. Can't say I came away with any understanding of what's going on

The boards and the p-p chassis wiring use lots of riveted ground connections, but visually they look very clean, and I poked around at a bunch of them the other night with a ground jumper and didn't find anything. I've jostled and tugged wires all over the place, not coming up with much...frustrating. I guess I could try resoldering every ground connection next.
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