KT88 construction problem

[A.N. Beal]

Dear John,
1. The channel which was playing up on dummy load was the left. It now works OK and sounds OK. The right channel worked OK on dummy load and all quoted voltages and resistances measure correctly but there is no sound.
2. It is the integrated amplifier.
3. At full volume the left channel has low level his and hum. The right channel is (as far as I can hear) completely silent.
Regards,
Alasdair

[Richard]

Hello again,

It sounds like a problem (open circuit) with the signal path of the R channel.

It could be at any point along the way so use your meter on continuity and ohms ranges to home in on it.

1. First check to see if all the R channel inputs are faulty or just the CD one.
2. Start with a meter probe in the input RCA socket (make sure it contacts the centre pin) and the other probe on chassis earth probe. The amp will need to be on and the input being probed will need to be selected. You should get a reading around the value of the vol pot (50K or 100K). Compare to the good L channel to check your method and expected result.
3. Continue checking along the signal path until you find no continuity on the bad R channel and confirm against the good L channel.
4. For checks along the path after the relay pcb the amp can be switched off (it only needs to be on to operate the relays during checks which include them).

[John Caswell]

Alasdair,
Because there is apparently dead silence I would also suspect a short circuit on the signal path.
The problem being that as it is an integrated amplifier when powered off all the inputs are s/c as there is no power to the relays.
You will need to disconnect the signal feed from the relay board to U1 grid to check continuity, s/c, o/c etc.

John

[A.N. Beal]

Dear Friends,
I tried swapping the L and R signal lead connections to the input tag board but the fault stayed on the right channel, so the fault must be 'downstream' of this connection. However I have checked input and driver tag boards and connections to valves etc., so far without success.
Any suggestions on particular connections to check - or is there any possibility that it is something in the output circuit and, if so, can you suggest any particular points or voltages to check?
I shall have another go at it tomorrow night but any suggestions for particular points or voltages or resistances to check would be welcome.
Regards,
Alasdair

[Richard]

Have a read through John and my last posts again and think about how you are trying to home in on whether the fault is an open or a short circuit, and where along the signal path of input RCA to output speaker posts it is.

I suggested it may be an open circuit as that is the most usual due to a poor joint. John suggested a short circuit and of course he is right as, if the relays aren't being opened, it will cause a short circuit of the signal path. The most likely cause of the relays not opening will be an open circuit, probably a poor joint.

Where to start, find out if it is the relays or not;

do as I suggest in point 2) with the amp switched on and the input you are probing selected. Probe the L and R channels RCA to chassis ground. Tell us what readings you get - it will be 50K or 100K on the good channel and it may be the same on the bad channel or it may be open circuit or short circuit.

Check first that you know what your meter will show if the probes are shorted together or left apart. Tell us the results. This will show if the relays are opening or staying closed, and if the problem is with the relays or further along.

[bob orbell]

Hi Alasdair, I must agree with Richard and John, you must now start from the inputs onward, it is no good jumping in half way, by doing so it will only confuse you, we have all had problems and will continue to have problems, but you must proceed in a logical namer. Best of luck. BOB

[A.N. Beal]

Dear Friends,
You will be relieved to know that the problem is now solved, all is well and it is sounding good.
I hadn't thought about the possibility of an open circuit on the relay wiring shorting them - but that wasn't it. I checked resistances on all the phono sockets with the power on and they were correct on both channels. I also checked them through the potentiometer to the input tag board and on to valve U1 pin 8 and all was well.
I then tried my best to follow possible signal paths through the circuit and again found no problems. I then checked checked key points to earth - no problems.
Running out of ideas, I resorted to connecting the multimeter across adjacent tagboard tags and comparing left channel with right and .. hey presto on the driver tag board 14 and 15 were shorted together on the right channel, shorting out resistor R10, but not on the left. The stray end of a link wire was the villain and quickly fixed .. but I made the mistake of not testing on dummy load before connecting speakers
When I switched on, I was rewarded by a loud buzz and howl - and switched off in a hurry. It seems that the right channel had the same reverse feedback polarity problem as previously afflicted the left but it had been masked by the short circuit. Opened up again, reversed the connections to U2 pins 1 and 9, tested on dummy load (I'm learning!), all well, connected up and all's well at last, with lovely music.
Thanks for all your help and sage advice,
Alasdair

[John Caswell]

Hi Alasdair,
Well now that is nice to hear
I am glad that you have solved the problem(s) although I am a little puzzled by the fact that you say both channels were incorrectly wired. However if you wired one side wrong then copied it to the other that answers the problem. On a similar point a lot of builders have often forgotten the wire that connects the DC heaters to the HT standoff potential divider R25/26 - this caused me a lot of grief initially!!
It is always nice when we hear that a problem has been solved, as it can be extremely difficult to diagnose long distance (blind and deaf so to speak) and sometimes it is just experience that tells us what to suggest or try.
Well done!

John

[bob orbell]

Yes, well done Alasdair, is this your first kit build? and is the amp. up to your expectations? Also do listen to it for a few months before thinking of upgrading it, I assume that it is the standard kit and not the XL. If you need more info. or help, please just ask. BOB

[Richard]

Yes well done, a bit of a fight for that success eh So with hindsight either the transformer windings were colour coded incorrectly, the schematic for the 5687 is wrong, or pins 1 and 9 were transposed in building. Worth us bearing in mind