QED CD240 Amplifier Bridge Rectifier Issues

[Rol G]

 

[Rol G]

how do I test it? or should i simply replace it without testing it

 

[Galu]

The relay may not be faulty. See post #6 re the delay components R31 and C31 and the instructions from @BSST on how to test them.

R31 and C31 lie below the relay in your first photo.

Give us a close up of C31 as it's out of focus - is that a brown deposit I see on its top?

 

[Rol G]

pics attached There is NO brown deposit on top of C31 everything looks pretty clean, R 30 measures 271 ohm using the 600 ohm scale of a multimeter across R31 the meter stays on overload

 

[Rol G]

here are some more pics, what value is r31?

 

[Galu]

what value is r31?

R31 = 100 K. Use an appropriate scale on your ohmmeter to measure it.

Please confirm that C31 has 47 uF marked on it.

 

[Rol G]

yes c31 is 47mf, to test r31 do i have to disconnect one side?

 

[Galu]

Try with R31 in circuit first and report back your reading.

 

[BSST]

Also try swapping ohmmeter leads to see if you get same result.

 

[Rol G]

r31 measured in circuit with an old avo:
(ohms divided by 100)
in one position = 2000 ohm = centre of scale
reversing the probes = no reading

 

[Galu]

That reminds me. My late uncle's Avometer is languishing up in the loft!

You have read the scale correctly.

I'll let BSST comment on your findings as he appears to know what he's talking about!

 

[Rol G]

Did I mention that after keeping it on for a few weeks it now no longer comes back to life. it stays stone cold, just the pilot lamp works

 

[BSST]

When you power on, you should find about 33V at D1,C30. Look for rise in voltage at C31 and it should reach about +30V. If it doesn’t, you’re over the target. Easiest test is to tack in another resistor in parallel with R31 to see if that brings life. Otherwise, transistor or cap, etc.

 

[Rol G]

I tested r31 with the amp turned off. if the r31 piggyback idea does not trigger the relay, you thing changing c31 is the first component to replace. You also mentioned transistor Which one?

 

[Galu]

You also mentioned transistor Which one?

I'll answer that part: it's T3.

It's on your schematic and physically right next to the relay.

P.S. The voltages you are looking for are also shown on your schematic (post #25).

The way I read it, if the voltage across C31 does not reach +30 V then the capacitor needs to be replaced.

 

[BSST]

The transistor doesn't have a Ref Des, but it's the only transistor in the "Relay Circuit" page within the service manual. If a paralleled resistor doesn't fix the issue, I'd suggest removing the transistor. Then you'll be able to monitor the RC rise behavior without the transistor loading.

Edit: Just saw @Galu has ID'd T3. Thanks!

 

[Galu]

If a paralleled resistor doesn't fix the issue...

What value of resistor would you suggest to parallel R31?

 

[BSST]

If memory serves, R31 is 100K? I'd suggest 100k. The main goal is to confirm R31 is the culprit, then do a proper replacement.

 

[Galu]

If memory serves, R31 is 100K?

That's the value according to what you said earlier and to the colour code, but Rol G measured it as 2 K (in situ) on his Avometer (post #30).

 

[BSST]

2k seems way too small to make sense.

P.S. I just checked parts list and it shows 100k.