Ekco RP364

[John10b]

Could you measure the voltages again, from switch on, Valves cold, to when Valves warm up, ta.
John

[DonaldStott]

*ERRATUM*

References to C3 and C8 (0.1uF) in Posts #38 and #39 above should of course be C5 and C8 and these have been replaced.

There are a further two Hunts caps (C11 and C12: 0.0005uF) across the Tone controls (RV2A and RV2B) which were replaced with 470pF 1000V ceramics.



Apologies for the confusion.

[Station X]

How about taking some voltage readings?

[DonaldStott]

Originally Posted by Station X

How about taking some voltage readings?

Apologies for the long delay
But sometimes Life gets in the way
Of this Hobby

Voltage readings will be taken over the weekend …

[DonaldStott]

Apologies in advance for a fairly long Post but it’s possible to skip to the updated voltage readings if you want.

Having put the idler wheel challenge to one side I had moved on to dealing with the electronics and hopefully to address the loud hum. I decided to re-stuff the dual can capacitor with 2 x 47uF 450V electrolytics and replace any other components that were out of tolerance. Of course the instant my soldering iron touched the PCB, several tracks and pads just lifted off despite my best efforts! This was a particular problem with those components connected to the ground plane and, in my experience, typical of PCBs from this period.

As explained in my Post #35 above the hum actually got much worse following this work and the measured voltages, especially from the Westinghouse EC1 rectifier, were something I couldn’t understand? I assumed that I had a grounding problem as I heard a buzz when any metal part of the player was touched! So I went back over and re-checked all my work while reflowing/resoldering everything that had a ground plane connection. This included the main PCB along with all the connections on the tag strip associated with the tonearm/cartridge wiring.

And we have some progress!

My immediate observation was that the hum had decreased substantially and was now around 40-45dBA - still too high though. Some voltage measurements were taken and we currently have the following :-

Output from mains transformer : 243V AC
Output from rectifier : 238V DC
Positive side of C3 : 235V
Positive side of C2 : 193V
Positive side of C6 : 178V

V1 ECL82
Pin 6 : 183V
Pin 7 : 195V
Pin 9 : 87V

V2 ECL82
Pin 6 : 182V
Pin 7 : 194V
Pin 9 : 87V

These are readings I would have expected from a record player of this period i.e. reduced output from the Westinghouse EC1 rectifier and consequently a low HT1 and HT2 are also typical.

The hum is still too loud though and I’ve been investigating possible sources although I’ve ruled out any heater-cathode shorts. Other areas could be the very loosely twisted heater wires, dressing of mains cables and further grounding issues associated with the controls - on/off switch-volume, tone and balance:-



The tone control (RV2) is particularly stiff despite treatment with De-Oxit F5 and only rotates halfway round but in so doing the hum reduces slightly? I’m reluctant to tackle disassembly of this control (see picture) but there is clearly an issue here? Hopefully I won't need to source a replacement 1M stereo pot!

[David G4EBT]

Originally Posted by DonaldStott

I’m reluctant to tackle disassembly of this control (see picture) but there is clearly an issue here? Hopefully I won't need to source a replacement 1M stereo pot!

If needs must, a dual Alpha 1 Meg linear tone control potentiometer can be had from 'North of the Border'.

(North from my side of the border that is - not yours!):

£7.64 inc VAT plus P&P:

https://modulusamplification.com/dual-potentiometer---alpha-3208-p.asp?_=&variantid=5413

Mounting Hole Diameter:8mm
Shaft type: 6mm (1/4") shaft
Body Diameter: 24mm
Power Rating: 0.5w

MODULUS AMPLIFICATION,
Unit 14 Logie Way,
Logie Business Park,
Kirriemuir,
DD8 5PU.

[DonaldStott]

Thanks David but apologies for not including this additional piece of information earlier: -



I've looked at various suppliers but the shaft length is always an issue - needs to be about 3/4 inch from the split ring at the end of the brass threaded section i.e. the solid shaft section shown in the image below: -



Ideally it should also have a flat to accommodate the set (grub) screw.

[TonyDuell]

I assume the replacement potentiometers have a spindle that is too long.

In which case, carefully measure the length you want from the pot body, clamp the excess spindle in the bench vice and cut it off with a (junior) hacksaw. You can also easily file a flat on the spindle.

[DonaldStott]

Originally Posted by TonyDuell

I assume the replacement potentiometers have a spindle that is too long.

Quite the contrary, the only replacement potentiometers I can find have shafts that are too short.

[DonaldStott]

Thanks for all the help and ongoing support but I think that discussion of potentiometer replacements is a bit premature when I haven’t yet identified the source of the offending hum.

Given the voltage readings I’ve Posted I’m not sure if a replacement rectifier is required? I also need to look at the heater wiring and dressing of the mains cables - should these also be shielded? Has anyone experimented with an artificial centre tap for the heater wiring or have I been watching too many YouTube videos?

[julie_m]

If it's not a daft question, what are the screen grid voltages like? There's been another thread here where excessive ripple in G2 turned out to be the cause of hum in a Rogers Cadet using ECL86s.

[John10b]

I presume the voltages are as per circuit, unfortunately I have been unable to download the circuit, which I understand shows the correct voltages, can someone indicate the voltages shown on the circuit, thanks.
John

[DonaldStott]

Originally Posted by julie_m

If it's not a daft question, what are the screen grid voltages like? There's been another thread here where excessive ripple in G2 turned out to be the cause of hum in a Rogers Cadet using ECL86s.

The screen grid (g2) is Pin 7 of the ECL82 - the voltages are listed in my Post above and both are close to my measured HT1.

According to the Service Data HT1 should be 215V while HT2 should be 212V.

[DonaldStott]

Originally Posted by John10b

I presume the voltages are as per circuit …

All measured voltages are low when compared with the Service Data starting with the rectifier output of 238V - should be 252V.

[John10b]

Thank you.
John

[DonaldStott]

One step forward, ten steps back!

I connected up my oscilloscope with the record player on my isolation transformer all ready to measure the ripple voltage on Pin 7, the screen grid (g2) of the ECL82s.

All seemed well at first then I heard that 'fizzing' noise we all know and I associate with failing components - no smoke but of course this was accompanied by a really horrible smell coming from somewhere! Put the record player on the lamp limiter and this immediately glowed very brightly indicating (to me) that there was a short somewhere!

Even though it was on the lamp limiter the rectifier was only measuring around 11V DC output and had clearly failed - what do you think?

So progress with this Ekco RP364 is going in the wrong direction and may be put to one side meantime or even abandoned altogether!

And now the really horrible smell has permeated the whole house!!!

[John10b]

That's unfortunate, but I'm sure it can be sorted, it's a decent amp, and the circuit, which I just received, is clear with much information.
Carry out some cold checks on the rectifier, the capacitors etc, I hope the mains transformer is ok!
John

[vidjoman]

The smell indicates that the rectifier has given up some magic smoke. Possibly was failing some time ago but working enough to get you looking for faults elsewhere.

[John10b]

Without going off topic, that smell would bring back memories of my TV days, you could walk into a house and immediately tell what the problem was.
John

[David G4EBT]

Originally Posted by vidjoman

The smell indicates that the rectifier has given up some magic smoke. Possibly was failing some time ago but working enough to get you looking for faults elsewhere.

Yes - it's a contact cooled selenium bridge rectifier and when they fail, they smell like rotten eggs, rotten cabbage or sewage.

It's failure ought not to have caused any damage, except perhaps to domestic harmony!

The DC current drawn by the amplifier is well below 1Amp, so a 2A silicon bridge rectifier such as this one would be generously rated:

B250R Bridge Rectifier Diode 2A 600V

https://www.switchelectronics.co.uk/products/b250r-bridge-rectifier-diode-2a-600v

Alternatively, if a 'flat format' SIL (Single In Line) rectifier would more conveniently fit, one like this:

https://cricklewoodelectronics.com/GBP208-Bridge-Rectifier-S...0sDgccpsuaoYpqX_Iqq_rFyNrq-rL2HQez8

If the DC output is appreciably greater than the desired 252V, you might need a series resistor to drop the voltage.

Hope that helps a bit.