Ekco RP364

[John10b]

I have attached part of the circuit, hope this allowed Mods.

[DonaldStott]

Thanks to Station X and Techman.

The replacement capacitors have been checked and seem ok and I am confident that they are connected back up correctly. I did have some problems with loose tracks and pads, especially around the ground plane, but I have done continuity checks where appropriate. In addition, I think that as the valve pin voltage measurements seem ok (?), albeit too high due to the rectifier DC output, I'm assuming that there are no further issues with the PCB. I note the comment about the mounting tags and the grounding circuit so I will go back and verify this as well as re-checking everything mentioned above.

As for some circuit leakage or leakage within the valves even, those checks will need to wait.

Meantime, I short-circuited the audio input leads at the amplifier and this resulted in an increase of the hum level!

[DonaldStott]

Originally Posted by John10b

I have attached part of the circuit, hope this allowed Mods.

Thanks John10b for clarifying the components and values in the power supply section.

[John10b]

The circuit I attached is that the same as yours Donald?
John

[Station X]

Originally Posted by DonaldStott

Meantime, I short-circuited the audio input leads at the amplifier and this resulted in an increase of the hum level!

How exactly did you short out the input to the amplifier and where? I hope you used a very short wire strap and not a couple of feet of wire.

[DonaldStott]

Originally Posted by Station X

How exactly did you short out the input to the amplifier and where? I hope you used a very short wire strap and not a couple of feet of wire.

Short length of wire (6 inches) with croc clips - on the PCB.

I assumed that I had to short the signal wire to its screen?

[julie_m]

You really need to make the short-circuit as near as possible to the input terminal on the PCB. I'd probably use a few cm. of insulated wire, single-strand if I had it; stripped at each end, tack-soldered to the screen connection and bent around so as to be easy to dab it onto the signal wire and off again. (Use something like a plastic chopstick with a V-groove filed in the end, if you want to protect your fingers from high voltages around the valves.) That way, you aren't introducing any new, unshielded connections at the input that might pick up hum. (Even if something is nominally earthed at one end, it doesn't take much resistance to become a hum source.)

If the hum diminishes when the input is shorted, that means the hum is getting in somewhere upstream of the amplifier input. If it persists, it's coming from somewhere inside the amplifier.

[vidjoman]

Are you sure the 'earth' point you shorted to is actually the earth. You said it was the screened cable but is it isolated via a capacitor? Or is there a break in the earth circuit somewhere. If the hum increased whilst shorted then it won't have been properly earthed.

I've just found the snip of the input circuit and to be certain of earthing the grid you need to short the slider (middle tag) of the tone control to earth.

[DonaldStott]

Apologies for my delay in responding but other matters were a priority.

Originally Posted by vidjoman

I've just found the snip of the input circuit and to be certain of earthing the grid you need to short the slider (middle tag) of the tone control to earth.

Tried this but the hum remains almost constant, very slight reduction, even with a short length of wire - 1cm?

I'm now struggling to understand two areas where I have issues to resolve: -

1. DC Output from new rectifier

If we go back to Post #80 where John10b calculated the expected current using the values shown in the circuit diagram with which I would concur and this gives us a baseline of 78mA. The DC output voltage from the new bridge rectifier is measuring 305V (246V AC from the mains transformer) whereas it should be 252V so I need to drop that by around 50V. Using my small stock of 10W power resistors of various values I connected up a 680 Ohm resistor and measured a voltage drop of around 54V across that resistor and using Ohm's Law this should give us a current of 79mA but at 4.28W - very hot! So why then if I measure the DC output after the resistor am I getting only 207V - probes connected to the DC IN + and - points on the PCB - and the DC output from the rectifier to the resistor is only 262V?

2. Tone Control

I've cleaned all the controls with De-Oxit Fader F5 to alleviate scratchiness and whereas the volume control and balance control seem ok there is something 'not quite right' (I think) with the tone control?

Stereo volume control is RV1A and RV1B and these should be 1M log pots - they are measuring 6.15 Ohms to 0.95MOhms and 4.2 Ohms to 0.92MOhms which seem ok to me?

Similarly the balance control (RV3) should be 50kOhms, it measures 10.75 Ohms to 44.6kOhms which again seems ok?

With the stereo tone control (RV2A and RV2B), both of which should be 1MOhm, they are measuring 2.63 Ohms to 0.74MOhms and 2.8 Ohms to 0.75MOhms. The maximum values are reached when the control is turned only about two thirds of the way round and the value of around 0.74 to 0.75MOhms remains constant between the two thirds point and maximum? So I have a feeling that the tone control may be compromised in some way but really unsure if that could be related to the hum and the rectifier output issues?


I'm really not that far away from making an appointment at our local recycling centre!

[Edward Huggins]

Don't give up on this just yet. Why not pop it round to one of our Members in the Clyde area to measure and signal trace?

[John10b]

Hi Donald
Just to be clear:
1. output from New Rectifier in original circuit = 305V.
2. Output from New Rectifier with additional resistor in series = 207Volts.
If this is the case it doesn't make sense, so you need to check your connections.
John

[julie_m]

A tone control circuit is unlikely to be affecting the power supply voltage, because it would normally have only AC coupling to the amplifier inputs. And if the voltage is being pulled low, that suggests excessive current through one or both of the output valves; which realistically could only be influenced by something DC-coupled to them.

I think we need to take some voltage measurements under a few different conditions, as follows:

1. AC output from transformer, with rectifier not connected to anything else
2. DC output from rectifier, with it not connected to anything else
3. AC output from transformer, with rectifier connected straight to PCB
4. DC output from rectifier, with it connected straight to PCB
5. DC voltages at each of the pentode anodes in this configuration
6. DC voltages at each of the screen grids in this configuration
7. DC voltages across each of the pentode cathode resistors in this configuration
8. AC output from transformer, with rectifier connected to PCB via 680R resistor
9. DC output from rectifier, with it connected to PCB via 680R resistor
10. DC voltage across 680R resistor in this configuration
11. DC voltages at each of the pentode anodes in this configuration
12. DC voltages at each of the screen grids in this configuration
13. DC voltages across pentode cathode resistors in this configuration.

Apologies if it looks a lot! But the actual measurement probably will be quicker than the description was. Numbers 3 onwards should be done with the valves warmed up and full anode current flowing.

If something is drawing excessive current, the answer will be found somewhere in those readings. And of course you might spot something amiss in the process.

[DonaldStott]

Originally Posted by John10b

Hi Donald
Just to be clear:
1. output from New Rectifier in original circuit = 305V.
2. Output from New Rectifier with additional resistor in series = 207Volts.
If this is the case it doesn't make sense, so you need to check your connections.
John

Doesn't make sense to me ether which is why I'm asking the questions.

Originally Posted by julie_m

I think we need to take some voltage measurements under a few different conditions ...

If something is drawing excessive current, the answer will be found somewhere in those readings. And of course you might spot something amiss in the process.

Many thanks Julie for taking the time to detail these steps - this is exactly the methodological approach I was looking for as I was unsure of how to isolate different sections, AC and DC. Will do my best to take the required measurements over the next few days.


NOTE: With the record player connected to my isolation transformer I'm seeing a ripple voltage on my oscilloscope of around 60mV on Pin 7 (g2) of V1 (ECL82).

[Station X]

Originally Posted by DonaldStott

NOTE: With the record player connected to my isolation transformer I'm seeing a ripple voltage on my oscilloscope of around 60mV on Pin 7 (g2) of V1 (ECL82).

In that case check the smoothing capacitors.

[julie_m]

Originally Posted by DonaldStott

Many thanks Julie for taking the time to detail these steps - this is exactly the methodological approach I was looking for as I was unsure of how to isolate different sections, AC and DC.

Thanks for being so understanding -- especially as it was on my suggestion to measure the ripple on G2 in the first place, when you turned it on and the rectifier let the smoke out!

[DonaldStott]

Originally Posted by julie_m

Thanks for being so understanding -- especially as it was on my suggestion to measure the ripple on G2 in the first place, when you turned it on and the rectifier let the smoke out!

Measuring the ripple on g2 and the rectifier failing was just an unfortunate coincidence - it was clearly on its way out and I just hastened its demise.

[John10b]

Hi Donald
I've reread all your post and as I understand it you have replaced most if not all capacitors, including the two coupling capacitors C5,8.
Considering all you have done I can only suggest you follow the advice given to you by Julie. I'm sure you will find the problem, I will be very interested to know how you get on.
John

[DonaldStott]

Originally Posted by julie_m

I think we need to take some voltage measurements under a few different conditions ...

Here are my findings - not sure how to interpret these measurements?

1. AC output from transformer, with rectifier not connected to anything else : 272.5V AC
2. DC output from rectifier, with it not connected to anything else : 233.6V
3. AC output from transformer, with rectifier connected straight to PCB : 240.3V AC
4. DC output from rectifier, with it connected straight to PCB : 304.5V
5. DC voltages at each of the pentode anodes in this configuration : V1B Pin 6 230.7V - V2B Pin 6 230.4V
6. DC voltages at each of the screen grids in this configuration : V1B Pin 7 247.8V - V2B Pin 7 247.4V
7. DC voltages across each of the pentode cathode resistors in this configuration : V1B R12 18.75V - V2B R16 19.07V
8. AC output from transformer, with rectifier connected to PCB via 680R resistor : 257.7V AC
9. DC output from rectifier, with it connected to PCB via 680R resistor : 274.7V
10. DC voltage across 680R resistor in this configuration : 57.5V
11. DC voltages at each of the pentode anodes in this configuration : V1B Pin 6 165.9V - V2B Pin 6 165.9V
12. DC voltages at each of the screen grids in this configuration : V1B Pin 7 177.5V - V2B Pin 7 177.3V
13. DC voltages across pentode cathode resistors in this configuration : V1B R12 13.58V - V2B R16 13.29V

NOTE: The Ekco Service Data has a misprint where the Pentode of V2 is labelled 'V2A' when of coure it should be 'V2B'.

Both V1 and V2 are ECL82 valves.

[julie_m]

Originally Posted by DonaldStott

1. AC output from transformer, with rectifier not connected to anything else : 272.5V AC
2. DC output from rectifier, with it not connected to anything else : 233.6V

This doesn't look quite right. The unsmoothed DC from a bridge rectifier should be about 2*sqrt(2)/pi times the RMS AC voltage supplied to it. That's about 0.9; so knowing that 270 is about 3*90 and .8 * 90 is about 80, we should be looking at 240-ish coming out of the bridge rectifier; my trusty calculator says 245.3.

But maybe it's just the winding resistance. Check the connections to the bridge rectifier, and that all diodes in it are good?

Originally Posted by DonaldStott

3. AC output from transformer, with rectifier connected straight to PCB : 240.3V AC
4. DC output from rectifier, with it connected straight to PCB : 304.5V

That's looking a bit better. With a smoothing capacitor, we would be expecting sqrt(2) times the voltage input with no current being drawn; that would be 339.8V with 240.3V input. 10% down under load doesn't sound like much cause for concern.

Originally Posted by DonaldStott

5. DC voltages at each of the pentode anodes in this configuration : V1B Pin 6 230.7V - V2B Pin 6 230.4V
6. DC voltages at each of the screen grids in this configuration : V1B Pin 7 247.8V - V2B Pin 7 247.4V
7. DC voltages across each of the pentode cathode resistors in this configuration : V1B R12 18.75V - V2B R16 19.07V

Well, those voltages are high as we'd expect, but there's nothing to complain about with respect to the channel matching!

Originally Posted by DonaldStott

8. AC output from transformer, with rectifier connected to PCB via 680R resistor : 257.7V AC
9. DC output from rectifier, with it connected to PCB via 680R resistor : 274.7V
10. DC voltage across 680R resistor in this configuration : 57.5V

The voltage from the transformer isn't dropping by as much here, which we'd sort of expect with the lighter loading due to the 680 ohm resistor in series with the amplifier. But I would have expected the DC output from the rectifier to have been higher with the extra resistance in series with the amplifier.

Originally Posted by DonaldStott

11. DC voltages at each of the pentode anodes in this configuration : V1B Pin 6 165.9V - V2B Pin 6 165.9V
12. DC voltages at each of the screen grids in this configuration : V1B Pin 7 177.5V - V2B Pin 7 177.3V
13. DC voltages across pentode cathode resistors in this configuration : V1B R12 13.58V - V2B R16 13.29V

OK, time to scribble some numbers on the back of a fag packet and hold up a wet finger into the wind. It's a single-ended Class A amplifier, which is as near to a resistive load as "damn it" is to swearing. Without the 680 ohm resistor, we get a total current through both the pentodes of (18.75 / 330) + (19.07 / 330) = (18.75 + 19.07) / 330 = 37.82 / 330 = 0.1146A = 114.6mA. There isn't going to be much current flowing through the triodes with 220K anode resistors, call it 116mA through the whole amplifier; and at 304.5V, it's looking like a 304.5 / 0.116 = 2625 ohm resistance.

With the 680 ohm resistor, we get a total current through the pentodes of (13.58 / 330) + (13.29 / 330) = (13.58 + 13.29) / 330 = 26.87 / 330 = 0.0814A = 81.4mA. Call it 83 with the triodes, and we have 274.7 volts from the rectifier minus 57.5 across the resistor = 217V. That looks like 217 / 0.083 = 2614.5 ohms.

The service data says we're looking for 16V across 330 ohms, twice; which is 32 / 330 = 0.0969A = 96.9mA; call it 98 with the triodes, and 252V at the rectifier output looks like 2571.4 ohms. So it doesn't look like too bad an approximation.

I'll be back when I'm done crunching the numbers .....

[DonaldStott]

Thanks Julie for taking the time and effort to Post such a detailed response, some of which is beyond my technical knowledge and expertise.

I note that there doesn’t seem to be much of an issue with respect to channel matching which instinctively points, in my opinion (!), to components in the power supply - mains transformer, bridge rectifier, smoothing capacitors and a few resistors?

I will of course await the outcome of your number crunching …