Quad 405 1(12368-9)

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07-Feb-2020 04:06 AM

kffern

kffern

Posts: 16

Hi,

I have an untouched original amp and will be changing the caps.

I see C1 changed to 680uF from 68uF and R3 is still 22k in the Mk2 version. Can/Should I change to 680uF and what is the benefit?

Also, should I change any other of the original film caps even if they look OK. I have a 120pf mica I could  use for C8. The electrolytes are all leaking or bulging as expected.

Thanks,

kffern

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20-Feb-2020 05:28 AM

EJP

EJP

Posts: 1742

C1 didn't change at all. 0.68uF = 680nF.
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01-Mar-2020 08:16 AM

kffern

kffern

Posts: 16

Thanks for the reply.

I finished the amp with a pair of temporary used power caps and it sound pretty good. I still get 0.6mV DC on the output though. I first fitted C2 (100uf polarised electrolytic) as original and later reversed as per Dada recommendation. Doesn't seem to make a difference.

Once the Aussie dollar gets back up I plan to get your speaker protect. 

Regards,

kffern

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01-Mar-2020 10:20 AM

EJP

EJP

Posts: 1742

Anything under 10mV DC offset, or even a bit more, is insignificant.

EJP

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01-Mar-2020 11:08 AM

kffern

kffern

Posts: 16

Thanks, I should have typed 60 mV.

Regards,

Kffern

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01-Mar-2020 11:11 AM

kffern

kffern

Posts: 16

I can hear a slight hum from the speaker with my ear to the cone. Otherwise fairly quiet apart from a bit of noise on power on and off.

Regards, Kffern

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01-Mar-2020 03:03 PM

kffern

kffern

Posts: 16

I installed R6 and C4 for 1.5v sensitivity, changed the diodes and op amp to OPA604, 2K7 on D2 and 100nF on D1. R7 and R8 are 3K. Other than that all caps are to original spec.

2 x 1000uf caps on the power supply +/- 51.2 v.

One channel has 63mv and the other 64mv DC.

Hope someone has a suggestion.

Regards,

kffern

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02-Mar-2020 05:27 AM

EJP

EJP

Posts: 1742

If you change R6 and C4 you also have to change either C2 to 33uF or R4 to 6.8K.

60mV is certainly far too high.

EJP

Dada Electronics Australia

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04-Mar-2020 07:43 AM

kffern

kffern

Posts: 16

I had changed R4 to 6.8k as well. I will change C2 to 33uf. Is it necessary?

Any idea what could cause the high DC offset? I changed the PS caps to another old pair and the DC offset is the same. New ones should arrive next week.

I checked the other original 405 that I have and it has a very low DC offset. Pity I didn't check this one before I started.

Regards,

kffern


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04-Mar-2020 07:50 AM

EJP

EJP

Posts: 1742

As I wrote, either/or. Not both.

I think at this point I would wonder about C1.

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04-Mar-2020 09:03 AM

kffern

kffern

Posts: 16

C1 is a new Wima 0.68uF, 100V 10%,                                  Mouser: 505-MKS4.68/100/10P

Thanks,

kffern

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04-Mar-2020 12:53 PM

kffern

kffern

Posts: 16

This is the diode I used for D1 and D2, Mouser number: 78-BZX85B15, 15V, 1.3W, 2%, DO-41, Vishay

Could that be it?

Thanks

kffern

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05-Mar-2020 02:06 AM

EJP

EJP

Posts: 1742

Not if they are working, i.e. if the voltages at pins 4 and 7 are -15V and +15V respectively, within about 0.2V. If your DC offset is as low as 60mV the op-amp must be doing something. Otherwise it would be nearly 2V.

I think at this point I would swap out the op-amps for the ones from your other 405 and see if that fixes it.

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05-Mar-2020 03:35 AM

kffern

kffern

Posts: 16

I swapped the original opamp on one channel and got -4.0V dc on the speaker output! Tr8 appears to be getting hot. Only powered up for a few seconds though.

Checked the voltages at pins 4 and 7 and got -15.0 and +16.6V.

Thanks for your patience.

kffern

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05-Mar-2020 04:09 AM

kffern

kffern

Posts: 16

I have double checked R8(3.0k)/D2 with 2.7k decoupling and R7(3.0K)/D1. They measure slightly less in circuit but are identical to the original that I bought for the other 405.

Regards,

kffern

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05-Mar-2020 04:55 AM

EJP

EJP

Posts: 1742

You could only get -4VDC if there was a fault or the opamp wasn't working. Can you try another one?

16.6V at pin 7 is a bit too high, but it isn't critical, unless it has a ripple component, or unless it indicates that the zener is O/C.

I think you have some fault around the op-amp and the DC offset chain. Double check it all on the track side for dry joints, tiny solder bridges, etc.

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05-Mar-2020 06:03 AM

kffern

kffern

Posts: 16

I checked the tracks. seem OK. Put in one of the opamps from the spare 405. Maybe higher than 4V on the speaker outputs but smoke emanated  from the area above D2,R8, +Ve PS section of the board.

Having the same problem from both boards makes me think its one of the components I have put in.

Regards,

kffern

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18-Mar-2020 04:55 AM

kffern

kffern

Posts: 16

I figured out the problem! I measured 10ohm between the signal earth (DIN plug) and the chassis earth (rectifier screw). Even though as far as I can see, I have rewired as per the diagram on the instructions. I ran an extra earth wire from the signal earth on the DIN plug and that problem is solved. 0.1mv on both outputs. I still have a bit of hum on the left channel (the board on the opposite side to the caps) which I presume is interference on the coax cable.

My next issue is that R30 and R31 get very hot. I measure 75 degC on my touchless thermometer. Both my amps have different resistors for R30 and 31 (both 560ohms I presume) with ceramic rings on the legs, yet the parts list has the same parts number.

Thanks,

kffern

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18-Mar-2020 05:19 AM

EJP

EJP

Posts: 1742

You're supposed to measure 10R between signal ground and chassis. It is a ground-lift resistor, an anti-hum measure.
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18-Mar-2020 05:45 AM

kffern

kffern

Posts: 16

Thats strange. I measure 0ohm on the untouched (by me) amp.

I disconnected the extra ground wire and measure 100mV DC offset. Does that tell us something?

R30 on both amps measure 580ohm. It is grey in colour and slightly smaller than the red-brown R31.

Regards,

kffern

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19-Mar-2020 02:31 AM

EJP

EJP

Posts: 1742

You will measure zero if you fit a link as you did above, or if you fit non-insulated input connectors such as certain RCA sockets without plastic washers both sides.

R30/31 hot means overcurrent, which probably means one of Tr7/9 or Tr8/10 is faulty.

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19-Mar-2020 03:23 AM

kffern

kffern

Posts: 16

Thanks. Both amps only have original DIN with shorted grounds. Could it be the new speaker binding posts I installed? They are from Altronics P9210/P9212. They have insulation on both sides of the chassis. I filed another notch on the side of the hole and they seem to fit well.

The DIN-phono cable came with the amps and I made another with 4 core Belden. Have tried with both cables.

Regards, kffern


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19-Mar-2020 04:21 AM

EJP

EJP

Posts: 1742

The grounds at the DIN socket should not go to chassis, only to the shielding that runs to the corner pin of each PCB. From there it goes to chassis ground via the 10R ground lift resistor.

The bottom speaker posts are connected to ground at the star earth. It doesn't matter if they touch the chassis as well.

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19-Mar-2020 06:13 AM

kffern

kffern

Posts: 16

Is that R2?

Looking at the second amp it looks like it has been worked on. One Op-amp has a socket and one doesn't with a different looking op-amp. Can't see the marking.

Looks like I was sold a couple of sick pups.

Regards,

kffern

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20-Mar-2020 02:25 AM

EJP

EJP

Posts: 1742

Yes it is R2.

EJP

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20-Mar-2020 08:16 AM

kffern

kffern

Posts: 16

Yup! R2 is reading 1 to 1.4ohm on both boards of the second amp. They don't appear burnt and look like the original resistors.

Thanks for your help.

kffern

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