Q14 shows up at the top of p11, associated with the muting switch.
I'll need to find a way to read mV. Maybe I can do that with my scope?
Didn't quite make it to that point, hopefully tonight, thanks!
My scope's dual trace so I should be able to do that, perhaps after I get the hum sorted out. Scope is a Philips, found a manual for it and the rotation adjustment is internal. Just what I need, another gizmo to fix, I have three receivers going at once right nowI had a think about the FM stereo and since it is derived passively with a diode bridge type detector, there is little to question about the signal balance there, if the stereo indicator is coming on appropriately when signal level is good and the bridge diodes are working correctly. Use your scope to look closely and compare levels at both left and right outputs from the Multiplex Decoder board. Deselect the connection to the preamp when doing this, so there is no load interfering with the measurement. If you have a dual channel or dual beam 'scope, it's a good opportunity to try that out. Without knowing the brand, 'scopes usually have a "trace rotate" or beam rotate control either on the front panel or internally. Watch yourself around high voltages, though - EHT in larger CRT scopes can punch holes in you.
I'll use an insulated tool when I do that, rubber boots and tie one hand behind my backCould I also swap the suspect transistor with another in the circuit, or would I risk damaging anything in doing so?
Thanks, this is a DC switch and at times, all terminals will be almost shorted. As long as hiss between stations
is cut back when the panel mute switch is on, it's fine. Any NPN such as BC547,8,9 would likely work there.
Wot, no DMM? Even a $10 one should go down to mV and $25 gets a deluxe model with transistor checker too!
You can use your scope but it will need a little calibration, preferably with a square wave of known accuracy.
I'm always wary of testing DC with a scope because of the unknown ground potential, since scopes have
a grounded earth on the probe input connection. I don't think you'll have a problem though, as long as the
receiver operates OK when the chassis is grounded. Old US sets may not be.
IIRC, there is a cal. terminal on Philips scopes and this is a 0.5V square wave on my TEK and also a cheapo
Taiwanese brand I have. Obviously, you can only calibrate the 1V range with a fixed output but the range
switch should be sufficiently accurate to cover other ranges. Just don't try to measure with the variable
control on the range switch or "Mag" engaged as these are not what you want for accuracy.
OK, good luck!
Oh, yeah well, I probably have five analog meters and three digitals, almost all of them have something else wrong... Mostly I fry them trying to measure current somehow. But yeah I have at least one dmm that will work if I get a battery for it - guess it's time to get that together
I may just leave that for another day, after I've got the trace leveled and I don't have so many balls in the air... Getting some batteries sounds more attractive right now
Thanks....Transistors are easy to test using a multimeter so do not swap. The hum related problems mostly do not happen due to a faulty transistor.
Gajanan Phadte
Gajanan Phadte
OK I did a quick once over on the sockets with deoxit, carefully sliding the transistors in and out a bit. There's a lot of them and I didn't want to cause any new problems... Won't turn it back on 'til tomorrow so it has plenty of time to dry.
The schematic shows 40-60 mV bias. I found one of my analog meters has a .6V range and I used that, was able to set each side to a very steady 50 mV. This was accompanied by a reading of .5V, vbe I guess that is, rather than the .65V shown. To whatever degree my vintage radio shack vu meter is accurate.
The DC voltage at R81 wrtg was 15V, and at R82 12V. FWIW...
The schematic shows 40-60 mV bias. I found one of my analog meters has a .6V range and I used that, was able to set each side to a very steady 50 mV. This was accompanied by a reading of .5V, vbe I guess that is, rather than the .65V shown. To whatever degree my vintage radio shack vu meter is accurate.
The DC voltage at R81 wrtg was 15V, and at R82 12V. FWIW...
This seems quite wrong. It means the output is biased unequally between the rail of 48V and ground.
12V to ground means there is 36V to the positive rail when it should be closer to half the 48V positive rail.
Because of the circuit design, there will be some DC offset from about 24V at the output (before the electrolytic).
I think I misunderstood the operation of BAL ADJ, so vary that to see if the voltage also varies. I'm sure it will
to some extent but I think we should first check that the voltage is 12V at the bases of Q6,7 as is indicated on
the schematic. I think that there is actually no separate bias adjustment on the amplifier and 40-60mA just
indicates the normal expected range with the parts fitted. Don't swap transistors here as some may have
been selected to get best operation.
You say your voltmeter is a VU meter? Only use a straight DC range measurement and when you need accuracy,
DMMs are the only way to fly. They may not be strictly necessary but allowances have to be made for mechanical
meters unless they are high quality and calibrated.
You can read DC on an analog AC (VU) meter but it wont be accurate at low voltages because of the necessary
diode rectifier losses. Then again, you may mean something else by your description of what you are using.
Using analog multimeter. Sorry 'bout that.
Am not swapping any transistors, tonight I'll poke around some more. The voltage across those resistors did vary quite directly with the BAL ADJ pots and hit .5V somewhere in the middle of their ranges.
I'll also double/triple check that I'm measuring at the correct point on each side.
Thanks
-Mark
Am not swapping any transistors, tonight I'll poke around some more. The voltage across those resistors did vary quite directly with the BAL ADJ pots and hit .5V somewhere in the middle of their ranges.
I'll also double/triple check that I'm measuring at the correct point on each side.
Thanks
-Mark
Last edited:
OK SO... deox'd all the transistor sockets last night and gave them a day to dry out.
Tonight re-checked these BAL ADJ voltages. I tried using the BAL ADJ pots to adjust the voltages to ground from these resistors. The closest I got was to get one at 21.5V and one at about 23V, each pot at one end of their range. From there, checking back to the voltage across the resistors, one was at about 20mV, and one was at at about 150mV.
I set them back to 60mV each across the resistors. The voltages to ground were still about the same. In other words the adjustments seemed to have a much greater effect across the resistors than relative to ground. I also noticed that as I brought the voltage down on the side that was temporarily high, the slight hum I've been chasing got lower. I think changing that voltage may have just been affecting the output level.
Tonight re-checked these BAL ADJ voltages. I tried using the BAL ADJ pots to adjust the voltages to ground from these resistors. The closest I got was to get one at 21.5V and one at about 23V, each pot at one end of their range. From there, checking back to the voltage across the resistors, one was at about 20mV, and one was at at about 150mV.
I set them back to 60mV each across the resistors. The voltages to ground were still about the same. In other words the adjustments seemed to have a much greater effect across the resistors than relative to ground. I also noticed that as I brought the voltage down on the side that was temporarily high, the slight hum I've been chasing got lower. I think changing that voltage may have just been affecting the output level.
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
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
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...
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...
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.
Last edited:
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
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
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.
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.
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
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.
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.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.