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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment. |
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27th Jan 2023, 4:11 am | #21 | |
Pentode
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Re: 1970's GEC power supply issue - help required
Quote:
By the voltage drop when connected to circuit, tells me there is a bleed somewhere. I think I may check R341 and 342 for integrity. Is it possible we are looking too far down the line I wonder ? Last edited by paul1962; 27th Jan 2023 at 4:31 am. |
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27th Jan 2023, 9:16 am | #22 |
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Re: 1970's GEC power supply issue - help required
It might be better to use the terms "off load" and "on load" to avoid confusion.
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27th Jan 2023, 2:58 pm | #23 |
Nonode
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Re: 1970's GEC power supply issue - help required
If the output emitter resistors are 1 Ohm it suggests that the quiescent current is 500mA.
This is Very high. If the output transistors are not already damaged they will be soon. I suggest you only turn on this amplifier for just enough time to take voltage measurements. If C331 and C332 are faulty/leaky some of the current may be passing through the loudspeakers. Try disconnecting the speakers and then quickly measure the voltage across R337 and R338. |
27th Jan 2023, 3:01 pm | #24 |
Nonode
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Re: 1970's GEC power supply issue - help required
If the output stage is passing too much current, the power supply will be overloaded and this can produce hum.
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27th Jan 2023, 3:08 pm | #25 |
Nonode
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Re: 1970's GEC power supply issue - help required
The potentiometer P309 in parallel with the thermistor TH301 sets he voltage bias that determines the quiescent current in the output transistors of that channel.
If the thermistor or potentiometer go open circuit or if their resistances go high, it will increase the bias and increase the quiescent current to an unhealthy level. This type of fault is quite common. |
27th Jan 2023, 4:23 pm | #26 | |
Pentode
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Re: 1970's GEC power supply issue - help required
Quote:
The voltage across R337 and 338 is 4.5v with speakers disconnected. I hope this helps. I will take passive readings of C331 and C332 to see if they are to spec, but I wonder what the odds are, where both channels are leaky/faulty to exactly the same degree that they show matched readings. |
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27th Jan 2023, 4:47 pm | #27 |
Pentode
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Re: 1970's GEC power supply issue - help required
I'm not sure if it helps at all, but here is a more inclusive circuit diagram from the service sheet showing the full power supply and main amplifier circuits.
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27th Jan 2023, 5:49 pm | #28 |
Heptode
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Re: 1970's GEC power supply issue - help required
Please do these tests and report back, ensuring the measurements are all accurate of course.
1. Unplug the speakers 2. Make sure the volume is at minimum during all tests. 3. Measure the DC voltage at the loudspeaker terminals on both channels, it should be Zero. If not, you may have leaky speaaker coupling capacitors C331 and C332. Do this after 30 seconds or so of running. 3. Measure the voltage across R337 and R339 4. Measure the voltage across R348 and R340. This indicates the quiescent current, hopefully, it should mention the values in the service manual or sheet.
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27th Jan 2023, 10:33 pm | #29 |
Dekatron
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Re: 1970's GEC power supply issue - help required
4.5V across R337/R338 would mean a current of 4.5Amps which is impossible so something is not right. What meter are you using? You are putting the meter probes on either end of the resistor you are measuring?
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27th Jan 2023, 11:39 pm | #30 |
Nonode
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Re: 1970's GEC power supply issue - help required
All electrolytic capacitors are leaky to some extent even if it is at the microamp level.
You may need to replace the speakers with resistors to get meaningful voltage measurements. Something in the range 100 - 1,000 Ohms would be suitable. Also measure the DC voltage at the negative end of C331 and C332. Red multimeter lead on ground (positive) and black lead on the capacitor. |
27th Jan 2023, 11:46 pm | #31 |
Nonode
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Re: 1970's GEC power supply issue - help required
Another method for finding faults in an amplifier is to measure the voltage across the base-emitter junction of the transistors.
A silicon transistor will generally have 0.6V across the junction and germanium transistors will have 0.2V. |
28th Jan 2023, 12:14 am | #32 | |
Pentode
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Re: 1970's GEC power supply issue - help required
Quote:
The voltages across R338 and 337 is definitely 4.5v. and as the output transistors are in danger of cooking, I don't feel it safe to keep reading them. If you think they will be ok, then I will continue to do so. |
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28th Jan 2023, 12:20 am | #33 | |
Pentode
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Re: 1970's GEC power supply issue - help required
Quote:
My mistake for putting the decimal point in the wrong place. the actual readings across the resistors should have been 0.45v .. oops Last edited by paul1962; 28th Jan 2023 at 12:30 am. |
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28th Jan 2023, 12:23 am | #34 |
Heptode
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Re: 1970's GEC power supply issue - help required
At this point we do need to know accurately, what the resistance values read of each of the four 'emitter' resistors. I think these may well have risen in value due to leakage in the output transistors. If they read high then the voltage that appears across each will be high too. You can still calculate the current through each resistor, but use it's new value, not the one on the diagram. As mentioned only switch on for a few seconds for each test.
Output stages can be tricky to service, even for the experienced. A few tips I use: Use clip-on probe leads to connect to component legs where possible. With your meter probe securely on, then switch on, note the reading, the power off quickly. Have everything ready so as your meter reading is accurate, and your probe is not slipping off. A slipped probe can cause instant death to these circuits, it's happened to me a few times... Note all your readings clearly, then build up a picture of what's happening with the readings taken. Sometimes a few moments careful thought away from the equipment helps you realise what's going off! Regards SJM.
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28th Jan 2023, 12:55 am | #35 | |
Pentode
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Re: 1970's GEC power supply issue - help required
Quote:
I feel that we are looking at output stages and I get that is important and maybe i'm a bit old school, but my experience is mainly with things like Dansette's etc and the best way I can describe this hum is the same as when one of the multi smoothing caps (32uF/16uF/25uF) have gone bad, and this is about the same volume level. It is definitely generated from the power supply circuit I feel and identical to a Capacitor issue. Constant at any volume and not affected by adjustment of the volume control. There from switch on to switch off. C336 has been replaced due to possible high ESR, no change. C323 was basically knackered and been replaced (it used to pulse and replacing this made the hum constant, so at least something is better). C305 seems to be sound according to capacitor tester and waveform on the scope. The hum is affecting both channels of the amp exactly the same and as they don't use the same components, logic tells me that surely they can't be Siamese in their faults. If it was an amp issue, surely only one channel would be affected ? .. This is weird that both channels are showing exactly the same readings and same symptoms. Last edited by paul1962; 28th Jan 2023 at 1:03 am. |
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28th Jan 2023, 1:19 am | #36 | ||
Pentode
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Re: 1970's GEC power supply issue - help required
Quote:
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28th Jan 2023, 3:49 am | #37 |
Pentode
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Re: 1970's GEC power supply issue - help required
Ok, I found this within the service sheet.
It is the transistor chart and shows not only transistor numbers, but also voltage and current parameters. The voltages read are lower than these values and yet, the overheat of output transistors is still there. If the quiescent current figures are correct, then this shows some serious problems with current rather than an overvolt issue. I hope this information may help those who are helping me with this issue. It's one thing to see a schematic, but another to know what exact components are involved. I would just like to say, that I REALLY appreciate all your assistance so far and I bow to your helpful insights. Thank you all for taking the time to think about this with me. |
28th Jan 2023, 8:41 pm | #38 |
Nonode
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Re: 1970's GEC power supply issue - help required
To sum up the most important facts:-
Power supply gives out 24V but this drops to 17V when the amplifiers are connected. The manual states that the supply is 24V. This suggests either:- a There is a high resistance in the power supply (i.e. faulty rectifier) b The amplifiers are taking too much current from a healthy power supply. c Both of the above. 500mA seems much too high for the quiescent current which should be 65mA. I would be tempted to bypass the bias potentiometers with a small value fuse to convert the amplifiers to class B operation where there is no quiescent current. It should then work but with some distortion. |
30th Jan 2023, 3:07 am | #39 | |
Pentode
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Re: 1970's GEC power supply issue - help required
Quote:
I have taken a couple of days away from this issue, sorting out other things. I think we are probably looking at option B and I think although every cap in the circuits are showing as they should, both by cap tester and also component tester on my scope, I feel that it's time to hit this hard and replace C329 through to C332. I did run resistance checks on the bias pots, and they again, are at specification. I'm at a loss to know what else to do in regards this issue. I do agree with the current flow diagnosis and there is no way it should be approx 9 times manufacturers stated current. |
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30th Jan 2023, 3:57 am | #40 |
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Re: 1970's GEC power supply issue - help required
Have a look on the service sheet for any statement about what they measured their voltages with. Being GEC they're likely to have gone with their own 'Selectest' rather than AVO, but a 20,000 Ohms-per-volt instrument is likely. AVO 8 territory. I don't know the military ones myself, but I think they could well be like the earlier models and much lower ohms-per-volt.
There are areas in these circuits where this can produce significant differences in what you measure. David
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