C77 is key: pre-amp smoother.
C52/53 are smoothers for the AFC. Irrelevant to hum.
C63 is the record deck input smoother only.
C55 is the smoother for the FM radio section ONLY
C87 is the output coupling cap from the preamp to the power amps..again irrelavant
ah Not a million mile away if needs be from Tony or I..
sounds sort of 'thetford' or a bit east..
Nah 95% confidence in C77.
snip
OK, I'll see if I have a spare for that available without getting the others in first.
Will report what happens.
"sounds sort of 'thetford' or a bit east.." The latter, for sure. Think A143. But I'll try to do it myself first, with your help!
OK. Are we sitting comfortably?
My conclusion of the identification of the real C77 was based on the fact that only one other cap. of the 300uF value was the one on the long PCB which I had not done anything to, partly because it is well encased and clearly a bitch to get at. I had no access to its underside to look at pcb connections. So, that was my target for replacement. It was on the correct board, the one at bottom left of the schematic. So I removed the front of the Goodmans, giving me access to two tiny nuts whose removal allowed the pushbutton rack, and the attached pcb, to be precariously swung up as far as various wires would allow.
There was some collateral damage,
Now, with a small inspection mirror underneath the pcb, I was just able to get my soldering iron to the spots where the C77 wires came through, and desoldered them. On installation of the new, smaller cap., though, the section of pcb strip, which is at an end of a section and so not supported on both sides, appeared to come loose from the pcb, although still attached to the strip as though by a hinge. The old cap., having its wires exactly matching the locations of the holes, sat right down on the pcb, whereas the new small one is much narrower, and so sits there with its legs splayed out, and above the pcb surface, and thus is missing the stability of being hard against the pcb. I believe I have fixed it, and the cap. is now firm in its location. Re-assembled the unit as far as is necessary to test, and.. v v v It still hums.
I think I'll put in a low bid for that Technics unit tomorrow morning.
I found a Service Manual online, which has the component layouts I was wishing for earlier. C77 is the one that I changed today, I am happy to confirm. Looks as though I have some more reading to do.
if the pcb has a cracked earth track so the capacitor is not actually going to earth...
Yes, indeed. I was wondering whether to temporarily attach a wire to the exposed leg and ground it. I might even be able to do that without removing the pcb.
What is the correct description of the damaged resistor, ie 'wirewound'?, and how do I find it on the RS website? I have come up with 199-6081, and 666-1673 as possibilities, but am not sure if they are correct, nor what Wattage to look for. Pretty low, I would think, as it's a signal input.
Thanks.
I have a horrid suspicion that you have in fact replaced a C62, and damaged R60 which is a 470k ohm resistor, not wirewound at all, but metal film with a spiral cut into it..
C77 should be in location area B2 if that means anything on the board
Right.. Just to re-cap;)..
I think we need to check a basic assumption. IIRC you said that this still hums if you turn the Volume control right down and whilst doing that keeping the volume control right down turn the balance control one way and then the other.
Does that make any difference when to do that to the Hum on Either channel as when you have the balance control right down at least one amp output stage has its input shorted to earth, and that isolates all the other pre-amp low level circuitry before it.
In such a case the only caps which are significant are C88 and C90 and possible C92 and C93. Thats about all of them that are in the power amp output stage apart from the output coupler C98 which if open or reduced capacity would give no or little output and if short would man the mid rail volts would be either way high or low.
Check the volts on C88 what are they and check the volts on C98 both sides..
Also can you check the volts on C90.. And then report back please..
I know that I have damaged R60, I am trying to find a replacement on the RS site. I am not sure of its description, hence my request for the correct one.
I have confirmed that I have in fact replaced C77, from the layout on the Part 2 of the Manual that I found (and had to pay for!). I have yet to try it with a second wire to ground from the dubious connection wire. (+) to earth, yes?
Reply, part 1.
Correct. No front-panel potentiometers, nor selector buttons, have any effect on the hum, in any way.
Have not tried shorting anything, only disconnected one output amp. input wire. Hum on that channel disappeared, confirming that it is generated somewhere upstream, I thought. Is it ok to short out inputs to earth? Sounds dubious to me, unless, as you say, that's what happens when the Balance control is used. I'll take a look at that. Remember, I'm not an electronics engineer!
Will do. Checking C90 and C92 will require some dexterity to get the board(s) situated so that I can measure the volts on the board side while it's 'On', but I will get there somehow. Output, the last time I put a signal in, sounded adequate! Both C98s were replaced 18 months ago, when I replaced the exploded C55. Checking their voltages will require more dexterity.
This may be a day or two before I report back.
remembre there is a bit of preamp AFTER the volume/balance controls Tony.
TR15/16. Smoothed by C77
I agree that using the balance to see if the hum is pre or post that is worth doing.
The Balance control makes no difference to the hum level.
ok, that leaves as far as I can see only both C90s or the R113s or some kind of hum loop
Both C90s have already been replaced, as have the C92s. I have not replaced any resistors. I know I need to replace R60, when I can find the correct item on the RS site, but that's not part of this problem.
How does the fact that breaking the signal connection before C89 resulted in a loss of the hum on that channel fit in to this? Quoting myself: " There was a marked reduction in the hum from the disconnected channel. If I wasn't listening for it, I probably wouldn't have worried about it. "
I will still do the tests requested.
Let's see what we have here: All ref. to chassis, (+) probe. (-) probe to test point. DMM set to volts DC.
Cap. (-) (+) C98(1) 25.8 0 C98(2) 26.3 0 C90(R) 19.1 .2 C92(R) 49.2 -.02 C93(R) 20.4 -.02 C88(1) 53.9 0
(1) and (2) mean one side or the other, I don't know without working it out from drawings!
Sometimes soldering on test wires is easier...
See my post of 18:12!
It sold for £30.
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