Leak Delta 30 Amplifier - One Channel Down

[Wage Slave]

Yes! T5 now reads:


T5
Collector 37.5V (spec=35V)
Base 15.7V (spec=14V)
emitter 15.1V (spec=13.5V)

On the good board it reads

T5
Collector 37.6V (spec=35V)
Base 15.3V (spec=14V)
emitter 14.8V (spec=13.5V)

(C11, C17 and T1 are still out.)

[Alistair D]

Well done. The triumph of man over machine.

[Wage Slave]

More Transistor readings on the bad board:

T5
Collector 37.5V (spec=35V)
Base 15.7V (spec=14V)
emitter 15.1V (spec=13.5V)

T4
Collector 15.8V (spec=14V)
Base 2.1 (spec=2V)
emitter 1.5V (spec=1.5V)

T3
collector 33.2V (spec=31V)
Base 32.0 (spec=19.5V)
emitter 32.4V (spec=19V)

T2
collector 0.1V (spec=19.5V)
Base 0.7V (spec=4.0V)
emitter 0.2V (spec=3.5V)

T1
Not in circuit at present.

[Wage Slave]

Originally Posted by Alistair D

Well done. The triumph of man over machine.

Exactly. I have really enjoyed this a lot. Thanks so much!

[dave cox]

It's looking good !! - I'd be prepared to wager that it will work when the new 470k is fitted. Might be worth checking 'that' resistor on the working board as well since this seems somewhat like the classic failure of a high value resistor with high-ish voltage on it (although usually it needs quite a few more volts).

dave

[Wage Slave]

Originally Posted by dave cox

It's looking good !! - I'd be prepared to wager that it will work when the new 470k is fitted. Might be worth checking 'that' resistor on the working board as well since this seems somewhat like the classic failure of a high value resistor with high-ish voltage on it (although usually it needs quite a few more volts).

dave

Good thought. I've done that and am kicking myself. It looks very much like it has been replaced in the past on the working board. That was a pretty good clue in retrospect. You live and learn!

[Wage Slave]

So the resistor arrived today. Fitted it and replaced T1. Checked the transistor voltages and sure enough everything was identical to the good board. At this point I was supposed to wait a few more days for the caps to arrive and fit new ones for C11 and C17. Could I wait? Of course not. I went ahead and replaced the old ones, put the board back in and Bingo. Music, sweet music.

So, I am now enjoying glorious stereo, tone/balance control and switchable inputs. With the caps should be a 250milliamp fuse and a lilliput bulb for the mains indicator. And with that the repair will be complete hopefully. I'll post some pics later.

Thank you again to everyone and especially Station X, Alistair D and Dave Cox. Sorry to intrude on your hangover on Saturday Dave :smile:. You were all brilliant.

To anyone else out there who has a piece of vintage kit and wants to repair it, I would say go for it. By myself I would never have done it, but with the amount of expert help and patience you can get here, anybody should be able to do it.

The main thing is to learn to solder and desolder without damaging the components or tracks. It is easy to do damage - you need to develop a light touch. In my case I had built a fair number of projects involving flashing LEDs for Christmas decorations just for fun which developed the skills. I learned to solder/desolder and how to check tracks and joints very carefully looking for faults and how easy it is to muddle up the orientation of components of course.

Now I need a new project. There is not very much old kit knocking around here in Japan. It almost all got scrapped. What few bits and pieces survive get silly prices. They only have tiny lofts (if at all) and anyway access very difficult so little to nothing is kept.

I will likely have to get something back in Blighty and ship it out here. But what? A valve amp maybe?

[Wage Slave]

A few pictures.

[Station X]

Well done. It looks like in the end the problems came down to an open circuit 470K resistor and possibly a dry joint on a 1K resistor.

The problem was that as there's a lot of DC coupling in this amp, one fault will change the voltage readings on several stages.

[SteveCG]

Wage slave,

Glad to read that you've got the amp working again. However perhaps you can help me out with the pictures you have posted. I assume that the working amp card is the one on the furthest left hand side, with the one you have repaired next to it. On your enlarged picture, on the left hand card, which is somewhat blurred, I think I can see a typical 10% tolerance 470 K resistor of the sort I might expect to see in equipment of this age. However the resistor in the similar position on the next card does not seem to have the same colour code. Is this the resistor you have replaced? And if so then can you confirm what value it is please - or give the colour banding?

I like your Leak Amp & speakers set-up.

[Alistair D]

The resistor on the left is a standard 3 band resistor i.e the first 2 bands give the value and the 3rd is the multiplier.

The resistor on the right is a 4 band, the first 3 bands give the value and the 4th is the multiplier. The bands on this one will be yellow violet black orange (470x10 to the power 3).

Both the resistors are the same it is just the numbering that is different.

Do a Google search on 4 band resistor.

Al

[Wage Slave]

Just as Alistair said, the coding has changed. Sorry about the blur I didn't see it on the camera screen and it was all put back by he time I came to uploading. The mauve body might mean half a watt, I am not sure. I bought it locally if that makes a difference.

The caps came today so I changed C11 and C17 on both boards as per Alistair's recommendation. Is it my imagination or does it sound slightly better and does sound arrive at the speakers slightly faster on power up? Also one channel was slightly delayed before whereas now it doesn't seem to be.

What sounds really good on this combo is speech which is great for my Radio 5 habit and for listening exercises when teaching English. This is a bugbear of mine. It is really difficult to start catching what people are saying and yet every classroom I see has nothing more than some pathetic little CD player for up to 40 kids. At one school they bought the cheapest one they could find especially for me. When I rejected it in favour of lugging my own around they thought it was because their one was pink! The thought that sound quality could be important never even crossed their minds.

It's not just language either. They use the same stupid cheap crappy things for music and other subjects. I really think people have lost a certain ability to recognise good sound. They will tolerate any old rubbish these days. It's not just HiFi. Listen to an old radio or TV - They all sound pretty good to very good. A lot of modern ones by contrast sound dreadful.

[Wage Slave]

Drat. I forgot. One more pic:

[SteveCG]

Thanks for the fresh photo, yes I can now see clearly the colour coding of your new resistor. I am familiar with the 4 band colour code - its just that my collection of resistors does not have many of that type!

I noted that the printed circuit mount type of resistor was used on the board (for example R20 and R27) as well as the wire ended type. It's been my experience that these pc mount types are less reliable than their wire ended relations. I don't know whether others have had similar experiences.

P.S. I wonder why the Japanese go in for poor quality sound when many things they make are carefully designed and executed? Is it because they like things small - and few good loudspeakers are small?

[Wage Slave]

There is another problem with this amp that I cannot figure out at all. It concerns the tuner 1 input and doesn't seem to affect the tuner 2 input.

If I plug a source into it and power up at first the right channel is very muffled and distorted plus there is loads of hiss. The left channel is fine. After a few minutes it gets a lot better and after 10/15 minutes it is almost perfect although some hiss remains.

As I said, tuner 2 seems fine. Disc 2 shows exactly the same problem. I doubt if it is related but Disc 1 hums loudly even with nothing connected to it.

A source plugged into the tape playback input works perfectly.

The circuit diagram is here. I have studied it for a while but can't see what tuner 1 has that tuner 2 doesn't that could cause this. I looked to see if the white wires have been snipped to convert it to a mic input and I dont' think they have if I am looking at the right white wires. Anyway, if it had been converted then it would be absurdly sensitive wouldn't it?

['LIVEWIRE?']

Tuner 1/Mic input goes straight to a 560 ohm resistor, whilst Tuner 2 couples via a potential divider, including the 'Hi/Low' switch, though I admit I can't see anything which could cause your problem, except possibly problems with the switching(high resistance contacts?)or maybe in the input stage of the RH Channel

[Wage Slave]

Yep, thanks. I've been puzzling over exactly that. Perhaps you are right and it is something to do with the switch contacts. I did try wiggling it a bit to see if anything changed and it didn't. Perhaps another blast of contact cleaner is in order.

Odd the way it improves dramatically after a few minutes.

[Alistair D]

I would expect Disk 1 to hum with nothing connected to it. This input is fully floating so thi input socket and the unscreened parts of thi cable will act as an aerial. Some manufacturers would have soldered a resistor of 1-2Mohms across the back of the socket to tie the input down. The simple fix here is to obtain a couple of phono plugs and solder a bit of wire from the inner to the outer connection of each plug. That if you ever need to use that input just remove the plugs.

Measure the contact resistances both the Hi/Lo switch and sw1/1.

There could be a problem with C11 but the symptoms dont make sense.

Al

[Wage Slave]

Will look at those switch contacts. It seems the most likely cause. I changed C11 as part of the recent repair so it should be good. I can live with this problem as a) It does improve and b) tuner 2 appears to be fine. I will test tuner 2 again tomorrow after it has had a rest overnight to confirm that is the case.

Thanks for the tip re silencing disc1. I'll do that just because it sounds horrible.

[Michael Maurice]

Which speakers are you using? They look like 2020's or 2030's. Fine Loudspeakers. I bought a pair of 2030's new from Peter Jones (part of the John Lewis Group) in 1974.