Leak Delta 30 output chokes

[PaulR]

I have just bought a fairly untouched Leak Delta 30. An initial look over revealed that the main smoothing cap isn't leaky but has much reduced capacitance. I have also noted that the inductors following the output caps look as though they have had a hard life and are somewhat burned. I haven't checked for leakage on the caps themselves yet.

Can anyone please advise what the power rating of replacement inductors should be?

Thank you

[valves4amps]

Hi I have no idea of what the specification of the original was. However, if we assume the output impedance is 8 ohm then power = current squared x impedance. Now for continuous use at 30 watts current now equals around 15 amps. So you could wind some 15 amp wire around a100 ohm resistor about 10 turns would suffice. It is there to avoid spikes in the feedback from damping the speaker along with a zobel network.

Dave

[Herald1360]

If IsqrdZ = 30 and Z = 8, how can I = 15A? About 1.9A seems more likely.

Oh and it's load impedance, the output impedance of the amp will be much less.....

FWIW, the damage to the conductors is likely to be cosmetic rather than functional unless they're OC or have shorted turns.

[PaulR]

Although called a Delta 30 it is only 15W per channel. 15A along the speaker leads does seem a lot. I haven't powered it up yet in view of the condition of the main smoothing cap. Also the quiescent current trimmers are right at the end of their travel so I have decided to replace all the other caps as well as the trimmers themselves before I apply power. Having tested the output caps they are very leaky at only 10v.

It was mainly for cosmetic reasons I was going to replace the chokes as they aren't actually open circuit.

[valves4amps]

Sorry chaps, brain freeze. You are all correct, terms wrong (load impedance) and factor of 10 out approx. for current. Some days I think too much instead of writing it out first on paper. These senior moments are getting too frequent.

Dave

[Diabolical Artificer]

"I have decided to replace all the other caps as well as the trimmers themselves before I apply power." Worth putting in 10 turn presets, the one turn type are usually fiddly to set. If they're at the end of the travel have any resistors in the Vbe circuit gone off spec?

Andy.

[Radio Wrangler]

With multi-turn presets, there is no indication where you are in the range. For one used to set quiescent current, make extra sure it is at the low current end before applying first power. Many bias trimmers have a range from 0 to 'Oh dear!' amps.

David

[PaulR]

Thanks all. I was thinking of getting it working safely via a lamp limiter then looking at the various voltages. I was wondering about the diodes in the bias circuit. I know this sounds feeble but I couldn't actually find them when I had a look yesterday! Hopefully when I look again they will jump out at me!!

[thermionic]

Originally Posted by PaulR

....... I was wondering about the diodes in the bias circuit. I know this sounds feeble but I couldn't actually find them when I had a look yesterday! Hopefully when I look again they will jump out at me!!

The two diodes are mounted on the heatsink which holds the output transistors. If they are faulty, standard 1N4148 diodes will be fine.

Very strange that the the small chokes are overheated, I’ve not seen that on these amps before. I would carefully check the output transistors with a meter with the driver pcbs removed before applying power.

SimonT

[PaulR]

I am doing some wallpapering this morning and looking at the amp whilst the paper is soaking! I realised that the diodes are mounted between pins C and J of the circuit board - or at least were. The leads to the heat sink have been cut at the heat sink end and the diodes are missing. I will investigate further as to how it could work like that. More papering to do in the meantime!

[PaulR]

The diodes have been replace by a single diode between the end of the pot originally connected to Pin C and the earthy end of R54. I am not sure how it worked at all like that.

Does anyone have a photograph of how the diodes were originally mounted on the heatsink?

Did I say untouched? At least all the work has been done to a good standard unlike the Armstrong I did last, which looked as though the soldering had been done with a blow lamp.

[PaulR]

Apologies. The diodes were there. They were tucked behind the tag strip, and the wire end I assumed to be the end of one of the leads was actually the junction between the two diodes. I had assumed that they would be mounted on a tag strip not just tucked away like that. It was presumably done to keep them at the same temperature of the power transistors.

It doesn't explain the addition of the additional diodes though.

[PaulR]

An update on this.

I replaced the main smoothing cap, the output caps and all the caps on one of the main amp boards together with the trimmer pot. I also removed the added connection from the trimmer. All the resistors were within a reasonable amount of the correct value. The channel I have done works well and I got the required quiescent current well within the range of the trimmer which was actually a 25 turn one as I couldn't get anything else to fit which was in stock. It sounds good through headphones without the slightest hint of hum so I will now go on to do the other channel, hopefully with the same success.

[Radio Wrangler]

Well, even if the second channel doesn't go so easily, you now have a good working one to compare against.

David

[PaulR]

True. Being able to remove and swap the boards is very useful. I am trying to stop myself from looking for a Delta 70 now. I really don't need any more amps and cannot bring myself to sell them once repaired.

[Sideband]

Originally Posted by PaulR

I am trying to stop myself from looking for a Delta 70 now.

I have a Delta 70 in constant use. It only needed some recapping and the quiescent current adjusting. Faultless for the last five years. Using at this moment with a Leak Troughline Stereo tuner. You won't be disappointed!

[PaulR]

Tempter!! I am ashamed to say that I have put a continuing search on ebay for one.

[thermionic]

From a performance point of view, the Stereo 70, or Delta 70 has better harmonic distortion figures than the Stereo 30 / Delta 30, according to Stephen Spicers book ‘ Firsts in High Fidelity’, which I can highly recommend if you’re a Leak freak!

[PaulR]

Interesting, I will have a look for a copy. This is the first Leak amp I have owned and I bought it because it looked an interesting project. I have only heard one channel so far as a couple of transistors have blown on the other power amp board and I am waiting for replacements.

If anyone is setting the quiescent current from scratch with a new variable resistor a good starting point is around 60 ohms or a bit less I have discovered by the way.

[PaulR]

Whilst waiting for the replacement transistors I thought I would look at the output ones. They must have been replaced at some point and the replacements are R2008B made in Britain with a code of 78 25. Does anyone know anything about these? I can't find much in relation to audio amps. Presumably the code suggests they were made in 1978.