Ekco RP364

[julie_m]

Originally Posted by John10b

Perhaps we should have listened to station X post #78, when he found trial and error was the best way to get right value. Any way it's been interesting.
John

Once you've found a value that's too low and a value that's too high, pick one somewhere in between and if in doubt, closer to the one that's out by less. It's almost quicker to do than to explain, especially if you've rigged up some crocodile clips for quick connection and disconnection.

[DonaldStott]

Originally Posted by Techman

OK, at least that's another thing ruled out. Isolating transformers have been responsible for all sorts of strange problems, going right back to my student days in the radio lab at college...I'm thinking particularly of a BRC2000 that wouldn't run due to the AC waveform distortion out of the lab's isolated supply.

Thanks Techman - although I have an isolation transformer which is used when connecting any equipment up to my oscilloscope I’ve never examined the AC waveform for distortion? Another job and maybe another Thread for the New Year so let’s not get side tracked.

[DonaldStott]

As Station X (Graham) mentioned way back in Post #78 'Experience has shown me that the easiest way to determine the value of the dropper resistor is by experimentation. Just start with whatever value of wire wound resistor you have in the 100R to 500R region and then increase or decrease the value as required.'

Various values for the dropper resistor were tried over the Holiday period and I've settled on 270R 10W which, of course, is the value that julie_m recommended.

So we now have these measurements with Service Sheet values shown in RED: -

• AC output from transformer, with rectifier connected to PCB via 270R resistor : 241.7V AC
  
• DC output from rectifier, with it connected to PCB via 270R resistor : 253.3V (252V)
  
• DC voltage across 270R resistor in this configuration : 26.4V (94mA 2.38W)
  
• DC voltages at each of the pentode anodes in this configuration : V1B Pin 6 194.0V (199V) - V2B Pin 6 193.0V (199V)
  
• DC voltages at each of the screen grids in this configuration : V1B Pin 7 207.0V (215V) - V2B Pin 7 206.5V (215V)
  
• DC voltages across pentode cathode resistors in this configuration : V1B R12 15.3V (16V) - V2B R16 15.4V (16V)

Slightly low across the board for the pentode anodes, screens and cathode resistors but not too far from the Service Sheet values?

[julie_m]

That looks a bit under, but close enough. Give it a listening test! If there's no audible distortion, call the job a good 'un.

Long version: When you design an amplifier, you pick an operating point in the middle of a section of the transfer curve (anode mA versus negative grid V; remember, the cathode is already at some positive voltage, so the grid is going to look negative with respect to it. And the capacitor across the cathode resistor tries to keep the DC voltage bias as constant as possible, despite the varying cathode current) which is as close as possible to a straight line. The input voltage can be thought of as a vertical line coming up from a point on the X-axis until it strikes the transfer curve, bounces off it and then travels horizontally to the Y-axis, where it represents the output current. With a dead straight line, you would obviously get a perfectly faithful reproduction of the input signal, but in real life there is nearly always some distortion unless you spend enough money ..... and sometimes you actually want a bit of distortion, say, to make what should be an exponentially-decaying sine wave from a guitar string sound a bit more interesting .....

So the real question is actually two questions: Are we moving the operating point far enough to wind up on a rougher section of the transfer curve, and is it going to make enough of a difference to be heard? My gut is saying probably not, but let your own two ears be the final judge.

[DonaldStott]

Many thanks juilie_m - both helpful and informative.

I’ve been listening to the record player and while my ageing cloth ears found the audio quality to be acceptable (new stylus, new cartridge and new tone arm wiring) it’s difficult to be certain through the annoying loud hum!

Just out of interest and my ongoing education in such areas is there any way to determine where the operating point is on the transfer curve for this amplifier? Probably beyond my technical understanding but I’d like to learn more.

Now that the power supply voltages seem ok (?) the next step is go back once again to try and identify the source of the hum - new low ESR smoothing caps are first on my list.

[Edward Huggins]

In theory the hum on this very simple 2 stage stereo amplifier should be an easy fix and clearly has not yet been traced after 125 Posts. Understanding "transfer curves" may be interesting, but is unlikely to help you. Why not get a Member near to you to trace through the circuit? If there were to be any charge, it would be modest.

[julie_m]

The first thing to do would be to determine whether the hum is coming from within the amplifier, or being picked up somewhere in the wiring from the cartridge to the control panel; the next steps needed will be a bit different in each case. If the amplifier is generating hum, that could be a smoothing problem; whereas if it's just blamelessly amplifying hum coming into it from outside, that's a different problem.

So what we are going to do is, partition the circuit into two at the amplifier input, by short-circuiting the triode grid to 0V. Then there will always be 0V at the amplifier input, meaning it has literally nothing to amplify; and then if there is any hum on the output, it must be coming from within the amplifier. And since this machine happens to be stereo, we can leave the other channel unshorted as a control for the experiment, for quicker comparison.

Make a temporary link from one of the triode grids to the nearest ground point, using as little wire as possible. That will ensure no signal at all is entering that channel of the amplifier. Leave the other channel alone, for comparison, switch on and allow to warm up. If both channels still sound the same, this means the hum is being generated inside the amplifier; if the hum has gone away on the channel where you grounded the input but is still there on the unaltered channel, this means the hum is being picked up on the wiring to the amplifier.

Just in case there is some sort of bizarre, asymmetric situation going on here, with different faults in each channel, try it also with both channels shorted to ground, and finally with the short removed from the first channel. I'm not expecting different behaviour in each channel, so I apologise in advance for putting you through what ought to resemble an exercise in futility; but it just might save a bit of later.

[Station X]

Originally Posted by Edward Huggins

In theory the hum on this very simple 2 stage stereo amplifier should be an easy fix and clearly has not yet been traced after 125 Posts. Understanding "transfer curves" may be interesting, but is unlikely to help you.

Hear! Hear!

[DonaldStott]

Understood - Moderators please close this Thread.

[julie_m]

Nooooo! You can't give up now, when it's this close to working!

If both channels still hum even while shorted, the problem is affecting both channels; so it's probably something common to both channels, which can only be the power supply. If grounding a triode grid removes the hum from its channel, that means the hum is entering somewhere between the cartridge and the final connection to the amplifier board. Does the hum on an unshorted channel change if you move any of the wires around? Post pics, in case anyone spots anything .....

[John10b]

Donald after all your hard work in trying to solve the problems with your RP it seems a pity to give up now, but of course I can understand your frustration.
I have been rereading your posts and one capacitor that has not been replaced is C2, in post #39 you say it's be " double checked ". My suggestion would be to do a quick simple check and put one across C2, no need to solder. See if this reduces your hum.i should add make sure you discharge the capacitor after the test, you don't want to get a " belt".
John

[vidjoman]

You’ve done a lot of work on this project. I recall somewhere that screened cables were said to be faulty. Did you replace them exactly as the original, which may have only been earthed at one end, but you’ve connected the screen to an earth at both ends. If this is the case then you may have caused an earth loop and introduced hum in the screened cable. I’ve seen this several times in the past where someone thought that it was wrongly manufactured and earthed both ends and actually made it worse.