🙂 The static voltage of this rtvc amplifier is 280mv. I'd like to know how much its static current should be adjusted for the best state. I hope to have a pleasant audition effect,，thank you,
That doesn't seem very likely. Vce=.280 v is deep saturation, and saturated transistors do not operate class A.
You appear to have 4 TO5 transistors. Probably one is RIAA gain, one is tone control plus output. Per channel. Rather a simple circuit. Not likely to be ideal. The PAT4 from the same era used 4 transistors per side, not two. I don't see any plastic film caps, which would be necessary to set RIAA curve and tone control contours accurately. Maybe those grey cylinders I can't read are film caps.
Ordinarily the collector of the amplification transistors in the middle of the power supply voltage would be ideal. Current somewhere between 10 ma & 200 ma. You appear to have 1000 ohm and 3300 ohm resistors, which doesn't give a lot of choice of collector current. In 1971 (yellow cap date code 7145) low noise transistors were expensive, and unlikely to be used in a unit with all electrolytic capacitors for RIAA & tone controls. There was a lot of skin flakes & hair floating around fab rooms in 1971, which creates noisy transistors. Whereas fab rooms these days are squeaky clean with no occupants except maintenance men in tyvek suits. Run of mill modern transistors like ON 2n5401 2n5551 MPSA06 MPSA56 are very quiet without the noise specification.
Nice front panel case & controls. Any 5532 IC circuit off ebay would probably do a better job of amplification. Turning single supply into split for op amps would be a matter of a virtual ground from resistor divider or op amp. You could trash that huge e-cap before it shorts & burns that nice transformer. Electrolytic caps are rubber sealed time fuses, doomed to die sometime. I don't see a mains fuse, which could protect the transformer. Another cost cutter feature that really should be corrected.
You appear to have 4 TO5 transistors. Probably one is RIAA gain, one is tone control plus output. Per channel. Rather a simple circuit. Not likely to be ideal. The PAT4 from the same era used 4 transistors per side, not two. I don't see any plastic film caps, which would be necessary to set RIAA curve and tone control contours accurately. Maybe those grey cylinders I can't read are film caps.
Ordinarily the collector of the amplification transistors in the middle of the power supply voltage would be ideal. Current somewhere between 10 ma & 200 ma. You appear to have 1000 ohm and 3300 ohm resistors, which doesn't give a lot of choice of collector current. In 1971 (yellow cap date code 7145) low noise transistors were expensive, and unlikely to be used in a unit with all electrolytic capacitors for RIAA & tone controls. There was a lot of skin flakes & hair floating around fab rooms in 1971, which creates noisy transistors. Whereas fab rooms these days are squeaky clean with no occupants except maintenance men in tyvek suits. Run of mill modern transistors like ON 2n5401 2n5551 MPSA06 MPSA56 are very quiet without the noise specification.
Nice front panel case & controls. Any 5532 IC circuit off ebay would probably do a better job of amplification. Turning single supply into split for op amps would be a matter of a virtual ground from resistor divider or op amp. You could trash that huge e-cap before it shorts & burns that nice transformer. Electrolytic caps are rubber sealed time fuses, doomed to die sometime. I don't see a mains fuse, which could protect the transformer. Another cost cutter feature that really should be corrected.
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There's at least four small plastic transistors behind the selector switch, and four TO-220 transistors on the right in the aluminum fins. It could be a full integrated amp in the 8WPC range.
But I do not understand the question.
EDIT: I've found ten small plastic transistors. 18 total, 9 per channel, is not too few for a 1972 set.
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I see eight.
Impossible. You need two for each, at least.
I see 14 film caps, and 8 polystyrenes.
But I don't understand the question either. What is 'static voltage', and where was it measured?
Might be lost in translation.
I would assume how to adjust =
1) DC offset on speaker output.
2) Output Transistor Idle/bias current
would need to see schematic.
might be single supply , capacitor coupled amplifier
maybe only diode thermal track
bias / idle current is likely preset very low
and intentional
and the trimmers are for DC offset
only guess.
I would assume how to adjust =
1) DC offset on speaker output.
2) Output Transistor Idle/bias current
would need to see schematic.
might be single supply , capacitor coupled amplifier
maybe only diode thermal track
bias / idle current is likely preset very low
and intentional
and the trimmers are for DC offset
only guess.
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This is a British made amplifier (by Radio and TeleVision Components Ltd) with an output of 20/30 watts per channel. It was known as the Viscount IV It was sometimes advertised as 20 watts and sometimes 30 - with uprated output stage perhaps. It was also available as a kit. There doesn't appear to be any schematic available.
It is single rail. Presets nearest the output stage will set quiescent current. With no circuit details available I wouldn't go more than around 15ma per channel. The other presets will be midpoint voltage. Either set with a scope for symmetrical clipping (probably not with a load if you want the amp to survive) or set to one half supply voltage as measured on the appropriate side of the speaker coupling cap,
If it is capacitor coupled there can't be a DC offset, and the trimmers can't therefore be for that.
This thread isn't making much sense.
Makes perfect sense
Many Single supply amplifier have trimmer to adjust output
to 1/2 the supply voltage.
and as mentioned dc offset visible on scope
for symmetrical clipping
yes obviously after the coupling cap no DC
bias is likely kept low/ preset non adjustable on purpose
around 6 to 15 ma since only appears diode thermal tracking
Many Single supply amplifier have trimmer to adjust output
to 1/2 the supply voltage.
and as mentioned dc offset visible on scope
for symmetrical clipping
yes obviously after the coupling cap no DC
bias is likely kept low/ preset non adjustable on purpose
around 6 to 15 ma since only appears diode thermal tracking
... and before it considerably more than the 280mV the OP specified.
That indicates rather a DC coupled amplifier with a fault, rather than an AC coupled amplifier. If indeed it is a DC offset at the output, which he OP still hasn't clarified.
Hi, this amplifier actually has a good sound and good thrust. I measured that the collector is 50V, and measured the last two fine adjustments. The DC voltage is 240mv. The figure is attached for you to see, but I don't know whether the bias current is tested in the same way? Directly connect the DC current gear of the multimeter. I hope to have a good job and bring it back to the original state. Thank you,
The DC voltage measured in the first two fine adjustments is 10V, which is the same on both sides. Attach the figure for everyone to see,
You need to understand what you are adjusting and how you measure it. If you get this wrong you will damage the amp. What you show isn't relevant.
You must first find the correct points to measure to before adjusting anything... if I had the amp in front of me I could do it in seconds but describing it is more difficult.
To set the bias current requires you to either locate a suitable resistor in the outputs stage (a so called 'emitter resistor' and calculate the current by using ohms law and the voltage across this known resistance. Failing that you need to either insert a resistor in the supply to the output stage or insert an ammeter in series with the supply to the output stage... both of which require experience.
To set the midpoint voltage is easy. Measure the total supply voltage and divide the reading by two. Now set the correct trimmer to give that calculated voltage on the amplifier output before those two speaker coupling caps.
You must first find the correct points to measure to before adjusting anything... if I had the amp in front of me I could do it in seconds but describing it is more difficult.
To set the bias current requires you to either locate a suitable resistor in the outputs stage (a so called 'emitter resistor' and calculate the current by using ohms law and the voltage across this known resistance. Failing that you need to either insert a resistor in the supply to the output stage or insert an ammeter in series with the supply to the output stage... both of which require experience.
To set the midpoint voltage is easy. Measure the total supply voltage and divide the reading by two. Now set the correct trimmer to give that calculated voltage on the amplifier output before those two speaker coupling caps.
There are some odd things in there. The TO5 driver transistor pairs each have a 3-wire lead connection, circled and located between them. In the upper amp, they appear to connect to a device fitted to the heatsink. It's likely a transistor or a couple of silicon diodes in a typical bias control circuit for the 1960s-70s but what of the lower amplifier? No connecting leads or temperature reference device for that channel?
Seeing as the electrolytics have been replaced at some more recent time, I'm thinking that what we're looking at is a repair/refurb gone wrong. Perhaps this needs an expert eye and more clear close-up pics of all the PCB and heatsink mounted parts, both channels, top and bottom side. Otherwise, unless a schematic can be found or at least partly deduced from the components, an unknown oldie like this is going to lead us on a merry chase from uncertainty to disaster, given that many components will be anyone's guess as to what to replace them with, should something go wrong.
Seeing as the electrolytics have been replaced at some more recent time, I'm thinking that what we're looking at is a repair/refurb gone wrong. Perhaps this needs an expert eye and more clear close-up pics of all the PCB and heatsink mounted parts, both channels, top and bottom side. Otherwise, unless a schematic can be found or at least partly deduced from the components, an unknown oldie like this is going to lead us on a merry chase from uncertainty to disaster, given that many components will be anyone's guess as to what to replace them with, should something go wrong.
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Hi, mooly, you're right. There was a leak70 before. I just put an ammeter in series on the collector power supply and adjusted the bias. It's a pity that I can't find its schematic diagram and can't do anything about it. Thank you for your great knowledge and your help. I hope to have a happy working day,
Hello, Molly, Hello, I finally debugged the quiescent current of this too old machine. I disassembled the back of the machine, disconnected the collector, connected the ammeter in series, and then began to debug slowly. After the machine was turned on for half an hour, it was heated, and then slowly adjusted. I adjusted it at 18ma on each side. As shown in Figure 1, the collector supplied 50V, disconnected any one, connected the multimeter in series, and turned on the current of 50mA. Figure 2 and figure 3, Then go adjust the positive pole of the multimeter and connect it. Then disconnect the other group and start to adjust it slowly. It's a very pleasant work. Thank you for your guidance,
If it is capacitor output, and you look without a speaker, you typically WILL have some DC. With no load, the turn-on transient takes about forever to come to zero. Hang 100 Ohms on it and wait a while. Does it go away slow?
For more fun, many designs "leaked" DC through NFB path or even Vas stage load. If there is 100 Ohms of "no-load load" (as other parts or dummy load), and you have 280mV, then when you attach 10 Ohms of speaker it will be 28mV, utterly zero for our purposes.
And I would not worry about 0.28V in a speaker suitable for a 20 Watt amp. That is 0.02 Watts at 'zero frequency', and no matter how bad the speaker maker cheats, that's not going to burn or stretch or significantly decenter a "20 Watt" speaker.
Thanks for those pics - great hi-res images there. It seems that the wiring to the lower amplifier bias transistor was tacked on the PCB copper traces below, from the outset as a modification, instead of topside like the visible leads for the other channel. There probably are a couple of transistors mounted against the heatsink somewhere in that small clamp device which looks to be empty but they could even be Ferranti/Diodes Inc. E-line type which are tiny and near flat, both sides.
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