Ekco RP364

[Techman]

This should be a very easy fix.

My next test would be to tack an electrolytic capacitor of more than 16uf (anything up to 47uf) and rated at HT voltage (hopefully you've got new spares in stock) between a good ground and pin 7 screen grid of the valve in the main amplifier (or equivalent circuit run) and see what happens as regards to the hum. Obviously take care regarding correct polarity.

[julie_m]

The maths is proving a beast .....

In the meantime, since we clearly need less series resistance, try 470 ohms. It will be kicking out just under a watt for every hundred ohms.

[DonaldStott]

Quick update - bridge rectifier checked (out of circuit) using DMM in diode mode and all seems well with 0.5V voltage drop in one direction for all four diodes and OL the other direction.

Checked positive and negative pins to AC pins in both directions while checking between AC pins was OL in both directions. As expected meaurung across the negative and positive pins reads a double voltage drop of around 0.94V.

[DonaldStott]

Originally Posted by Techman

This should be a very easy fix.

My next test would be to tack an electrolytic capacitor of more than 16uf (anything up to 47uf) and rated at HT voltage (hopefully you've got new spares in stock) between a good ground and pin 7 screen grid of the valve in the main amplifier (or equivalent circuit run) and see what happens as regards to the hum. Obviously take care regarding correct polarity.

I tacked in a 47uF 450V electrolytic with the positive lead connected to Pin 7 Screen (g2) and the negative to the ground plane on the PCB but this made no difference whatsoever to the hum level. Connected negative lead to the chassis just in case but this also made no difference.

I'm just using a Decibel app on my iPhone which I know isn't that accurate but it's any relative change that's crucial here and not just my subjective assessment.

Tacked in some more 10W resistors in series with the DC output from the rectifier, 485 Ohms in total and got this: -

1. AC output from transformer, with rectifier connected to PCB via 485R resistor : 245.7V AC
2. DC output from rectifier, with it connected to PCB via 485R resistor : 269.7V
3. DC voltage across 680R resistor in this configuration : 42.25V
4. DC voltages at each of the pentode anodes in this configuration : V1B Pin 6 175.4V - V2B Pin 6 174.75V
5. DC voltages at each of the screen grids in this configuration : V1B Pin 7 186.7V - V2B Pin 7 186.6V
6. DC voltages across pentode cathode resistors in this configuration : V1B R12 13.90V - V2B R16 13.74V

[DonaldStott]

Quick Erratum - of course ‘680R resistor’ in Point 3 above should of course be ‘485R resistor’.

[Techman]

Originally Posted by DonaldStott

I tacked in a 47uF 450V electrolytic with the positive lead connected to Pin 7 Screen (g2) and the negative to the ground plane on the PCB but this made no difference whatsoever to the hum level. Connected negative lead to the chassis just in case but this also made no difference.

Well it was worth a try. I particularly suggested the pin 7 point of connection to rule out anything strange going on with open circuits elsewhere on that line and also I think you said that you had measured a significant ripple at that point. While you've got that capacitor to hand it'd be worth tacking it across both of the other smoothers in turn, just to see if it makes any difference.

Let's take a look at all the possible causes of mains hum, irrespective of, and after the volume control...

1) Bad HT smoothing caused by failed electrolytic smoothing capacitors, but you've replaced them all with new ones.

2)Part of the bridge rectifier not working - I would have thought that the reservoir and smoothing capacitors would have still coped with most of this, probably leaving just a small noticeable increase in audible hum from the speaker/s and perhaps a small drop in HT voltage.

3)An unacceptable extra load on the HT supply, dragging the voltage down and increasing the 'ripple' on the HT supply.

4)Leakage of AC from the valve heater supply getting into the audio stages, either via the circuitry or internal valve leakage.

So it has to be one of the above listed.

There's also the possibility of AC mains hum pickup from mains wiring wrongly routed across the circuit board or chassis near to the preamplifier stages, but after the volume control, or actually on the control itself. If the on/off switch is part of any of the controls then it's worth investigating that and also whether there's good continuity between all the metal back plates/covers of the controls and ground.

There's another very unlikely possibility of a bad batch of new electrolytic capacitors. There are fakes out there, but it's quite a few years since I last heard a story about fake electrolytic capacitors catching out a supplier. I guess you've no means of actually testing the ones you've got and I have to stress that I think it's very unlikely that you've bought bad capacitors, unless you used a very dubious supplier.

[DonaldStott]

Thanks Techman - lots of suggestions and things to try over the next few days, should keep me busy!

[DonaldStott]

I've spent the morning working through the helpful suggestions in Post #106 above from Techman and this has helped to narrow down, I think, the areas where the hum problem may originate. Points 1 and 2 can be discounted as I've double checked the smoothing caps and the diodes in the new rectifier - all ok. I've also played around with cable re-routing but this doesn't seem to affect the hum in any way?

All along I've suspected some form of grounding issue as I noticed a very, very slight reduction in the hum when I touch any of the control spindles, the chassis or any of the metal parts in the auto-changer - safety first of course. So I'm focusing down on something related to Points 3 and 4 in the Post?

I had previously discounted any form of heater-cathode leakage as both valves were tested out of circuit with my DMM with no continuity between the heater Pins (4 and 5) and the cathode Pins (2 and 8) or between any other pins? I don't have a valve tester to go beyond this stage but I'm assuming that both valves are ok?

What I did discover, however, is that when I probe the valve pin sockets on the PCB (valves removed) I do indeed get some continuity! This is between valve pin sockets 2 and 4, 2 and 5, 4 and 8, 5 and 8 - I'm assuming that this is not to be expected and may be indicative of some form of short in the valve base, but I'm clutching at straws now??

My logic is to remove the valve bases from the PCB to ascertain if the continuity issues between what will be the valve pin socket pads persists?

[Cobaltblue]

Originally Posted by DonaldStott

I've spent the morning working through the helpful suggestions in Post #106 above from Techman and this has helped to narrow down, I think, the areas where the hum problem may originate. Points 1 and 2 can be discounted as I've double checked the smoothing caps and the diodes in the new rectifier - all ok. I've also played around with cable re-routing but this doesn't seem to affect the hum in any way?



What I did discover, however, is that when I probe the valve pin sockets on the PCB (valves removed) I do indeed get some continuity! This is between valve pin sockets 2 and 4, 2 and 5, 4 and 8, 5 and 8 - I'm assuming that this is not to be expected and may be indicative of some form of short in the valve base, but I'm clutching at straws now??

My logic is to remove the valve bases from the PCB to ascertain if the continuity issues between what will be the valve pin socket pads persists?

The circuit diagram shows that the heater winding is connected to the HT common so you will get readings between the heater pins and the valve cathodes via the associated cathode resistors.

Cheers

Mike T

[DonaldStott]

Originally Posted by Cobaltblue

The circuit diagram shows that the heater winding is connected to the HT common so you will get readings between the heater pins and the valve cathodes via the associated cathode resistors.

Cheers

Mike T

Thanks Mike, as they say, every day is a School day!

I’d better stop clutching at straws …

[DonaldStott]

Last throw of the dice before the Festive/New Year break- I'm going to replace the 47uF 450V electrolytics with new ones just in case there is an issue that my test equipment can't see e.g. the ESR of both is around 1.3 Ohms which seems ok to me?

Too many options available from trusted suppliers - I'm looking to acquire caps branded by e.g. Panasonic, Nichicon, Wurth, Rubycon or RS Pro - any recommendations?

Finally (for now) what should I be looking for in relation to Ripple Current?

[DonaldStott]

There we go, safely stowed under the desk until New Year:-



If, however, you have any further suggestions in relation to the HT voltage or hum problems then don’t hesitate!

Seasons greetings to all Forum Members.

[Edward Huggins]

At 112 posts, I do think you should get a second opinion. As amplifiers go, this is quite a simple and proven circuit design. When working as it should, it will have a low hum level, only more noticeable at above 75% of the volume control travel.

[julie_m]

I used to be able to do equivalent circuits, honestly ..... The transformer is a voltage source with a series and a parallel resistance representing its losses, and we've got 3 sets of conditions, so applying Ohm's Law and Kirchof's Laws that gives 3 equations, with 3 unknowns. And the Devil's own job to tease out the like terms and gather them together. I used to know tricks for doing this stuff. Scary how quickly you can forget things .....

Anyway, enough of the existential crisis, 470 ohms is still too much, by voltage measurements; try 270. When there's 16V on the cathodes, everything else ought to be right by circuit behaviour.

[John10b]

Donald enjoy your festive break.
John

[DonaldStott]

Originally Posted by julie_m

Anyway, enough of the existential crisis, 470 ohms is still too much, by voltage measurements; try 270. When there's 16V on the cathodes, everything else ought to be right by circuit behaviour.

Thanks Julie but I’ll leave Kirchof's Laws to experts like yourself!

I did try various values for the 10W resistors and noticed that I was getting close to the required HT1 and HT2 values with a 240 Ohm resistor - any further testing and measuring will need to wait until the New Year.

[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

[Techman]

Originally Posted by DonaldStott

All along I've suspected some form of grounding issue as I noticed a very, very slight reduction in the hum when I touch any of the control spindles, the chassis or any of the metal parts in the auto-changer - safety first of course.

Just a thought - you're not running this player via an isolation transformer by any chance?

[DonaldStott]

[QUOTE=Techman;1734943]

Originally Posted by DonaldStott

Just a thought - you're not running this player via an isolation transformer by any chance?

Simple answer - no.

[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.