Ekco RP364

[Edward Huggins]

Don't give up, the amp in this is well worth restoring. There was a time when Push-Pull ECL82 amplifiers were often the basis of a modest HiFi system. Depending on what type of replacement rectifier you fit, check to if it needs any form of surge resistor.

[Sideband]

That vile smell was a good indication in the days of TV repairs. On several occasions I would knock on the door of a customer and the smell when the door was opened...even next day had me reaching for a silicon diode and a 25 ohm surge resistor even before I'd taken the back off! Just replace it with a silicon bridge as suggested by David. You'll probably need a series resistor to reduce the voltage (suggest you start with something like 100 ohms) and then just change the value up or down to get the right output.

[DonaldStott]

Thanks vidjoman and John10b - it’s definitely the rectifier that has failed but there doesn’t appear to be any other damage or faults elsewhere. Will be interesting to hear what difference, if any, a replacement rectifier will make to the annoying hum?

Thanks David - interesting range of different smells from failed selenium rectifiers but unfortunately I seem to have had the ‘sewage’ variant!!! I'm still amazed by the small size and low cost of replacement rectifiers! Still need to work out how to remove the existing rectifier and construct something to hold the new one? I’m currently favouring the SIL type as it looks easier to fit within the available space.

[DonaldStott]

Originally Posted by Edward Huggins

Don't give up, the amp in this is well worth restoring. There was a time when Push-Pull ECL82 amplifiers were often the basis of a modest HiFi system. Depending on what type of replacement rectifier you fit, check to if it needs any form of surge resistor.

Thanks Edward - given my ‘stubborn genes’ I’ll probably persevere.

If required should the surge resistor be fitted before or after the rectifier?

[John10b]

Just checking if we have the same service manual, I have the EKCO manual do you have the same?
John

[DonaldStott]

Originally Posted by John10b

Just checking if we have the same service manual, I have the EKCO manual do you have the same?
John

I have the two-page ‘Ekco Service data’ from Paul Stenning’s DVD.

[John10b]

That is the same as mine, good.
John

[Sideband]

Originally Posted by DonaldStott

If required should the surge resistor be fitted before or after the rectifier?

For simplicity, I would fit it after the rectifier. You'll need two if you fit before the rectifier....one in each AC feed. You'll probably need about a 5 watt wirewound type

[DonaldStott]

Originally Posted by Sideband

For simplicity, I would fit it after the rectifier. You'll need two if you fit before the rectifier....one in each AC feed. You'll probably need about a 5 watt wirewound type

Thanks Sideband - very helpful.

I’m thinking of using a 5W variable resistor that could be adjusted to give me the correct DC voltages for HT1 and HT2 and then fit a permanent wirewound resistor with the nearest standard value.

[smiler411]

Originally Posted by DonaldStott

Thanks Edward - given my ‘stubborn genes’ I’ll probably persevere.

If required should the surge resistor be fitted before or after the rectifier?

It doesn't matter as they are in series anyway

[Techman]

Originally Posted by smiler411

It doesn't matter as they are in series anyway

Depends if it's a half wave or full wave rectifier, so it does matter. It's as Sideband says, after the rectifier for a single resistor.

Also, using a 5W variable resistor is a bad idea and it's likely to soon burn out and 5W isn't really enough for a fixed resistor in this position anyway, although you'd get away with it, it'll run hot - I'd recommend 7-10W.

Ideally you should have a small stock of power resistors of various values and appropriate wattage for doing this sort of thing.

[Edward Huggins]

A useful Thread as I'm about to start working on one of these. In the later 1950s, EAR produced a stand alone ECL82 PP amp rated at 8 watts. It sounded excellent through a 10" reflex speaker. Also at this time, the Radio Constructor magazine published a DIY design.

[John10b]

I think Donald iwas intending to adjust the variable to get right voltage, according to circuit data, and then fit an appropriate size fixed resistor.
John

[DonaldStott]

Originally Posted by Techman

Ideally you should have a small stock of power resistors of various values and appropriate wattage for doing this sort of thing.

I do.

Originally Posted by John10b

I think Donald was intending to adjust the variable to get right voltage, according to circuit data, and then fit an appropriate size fixed resistor.
John

You are correct John, this is my plan - just need to find an appropriate variable resistor to fit on a temporary basis.

[DonaldStott]

Faulty Westnghouse EC1 rectifier removed and new rectifier installed.

New voltage readings much as I expected due to the change from selenium to silicon: -

Output from mains transformer : 240V AC
Output from rectifier : 305V DC
Positive side of C3 : 305V
Positive side of C2 : 251V
Positive side of C6 : 230V

Current : 61mA

So we are about 50V over across all components in the power supply section - using Ohm's Law I should be able to calculate the value of the required (10W) resistor?

[John10b]

Donald how did the rest of the circuit behave after fitting new rec?
I did a calculation on original circuit values and the current to be 78 mA, from that I get a figure for additional resistor to be 679 ohms to bring down the voltage to 252Volts.
Please correct if you think I'm wrong.
John

[DonaldStott]

Originally Posted by John10b

Donald how did the rest of the circuit behave after fitting new rec?

Voltage readings below after fitting new rectifier (Service data shown in RED):-

V1 : ECL2

Pin 2 : 19 16
Pin 6 : 238 199
Pin 7 : 252 215
Pin 8 : 1.71 1.5
Pin 9 : 117 90


V2 : ECL2

Pin 2 : 18.7 16
Pin 6 : 232 199
Pin 7 : 248 215
Pin 8 : 1.82 1.5
Pin 9 : 115 90

All measurements too high as expected with 305V DC output from the new rectifier.

Originally Posted by John10b

I did a calculation on original circuit values and the current to be 78 mA, from that I get a figure for additional resistor to be 679 ohms to bring down the voltage to 252Volts.
Please correct if you think I'm wrong.
John

I've checked the Service Data and can't see a figure for the expected current - I'm very interested in how you came up with the your figures?


NOTE : Fitting the new rectifier had made absolutely no difference to the hum level which still persists, so no progress on that front!

[Station X]

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. You can connect resistors in series or parallel if need be.

Hum is generally down to one of two reasons. Faulty smoothing capacitors, or mains pick up into the leads leading from the cartridge to the amplifier.

I guess the smoothers have been changed, but are they connected the right way round?

If you short-circuit the audio input leads at the amplifier and the hum disappears it's down to stray pickup.

[Techman]

Originally Posted by DonaldStott

I'm very interested in how you came up with the your figures?

NOTE : Fitting the new rectifier had made absolutely no difference to the hum level which still persists, so no progress on that front!

I also wonder where those figures came from. I can perhaps make a guess and you're wrong and you also got your decimal point in the wrong place, but I'm only guessing Perhaps John knows something I/we don't - happy to be put right!

As said, much better to have a selection of resistors to experiment with - and you already said that you 'do' have them several posts back. Also good to see that you're going to use 10 watt types - the less heat dissipation the better!

Without checking back, I think you said that the hum was independent of the volume control setting, so likely to be ineffective smoothing. I'm guessing that one of your replacement capacitors isn't connected for some reason - either that or they're faulty. Perhaps the mounting tags on the original can form part of the grounding circuit and something hasn't been connected back up correctly? Unlikely, but possibly some circuit leakage or leakage within the valve even.

[John10b]

It was a bit late in the day for calculations, but this is my observations and calculations.
I have the EKCO circuit, which shows the following: note original circuit.
252VDC out of Rec
R4 470 ohms connected in series to Rec output.
215 V out of other side of R4.
37V across R4.

I = V/R = 37/470 = 78mA...............1

I repeated above with the news Rec fitted and output at 305Volt

Let me know what you think.
John