Transistor selection for Leak Delta 70 amplifer repair help please...

[fergch]

Hi

I am attempting a repair on a Leak Delta 70 Audio Amplifier from I suppose the 1970's that does not work on one channel.

The amplifier has a modular construction, and swapping the left channel preamp and power amp boards in turn with the right results in only the right channel working on any combination so my assumption is that the power output transistors T10 and T11 for the right channel have failed.

Is this the most likely problem given the swapping of the daughter boards does not change the failed channel?

I have three problems to sort with someone's help please.

What transistor is the right and appropriate selection to replace the installed RCARCA39251 which I believe is a selected 2N3055 to cope with the 75v rails?

Will I need 4 replacements to install in order to maintain the symmetry on the circuit? (I am guessing I will need to replace all four power transistors).

Will I have to make any adjustments to the quiescent current on each channel after replacing the output transistors?

I realise these are rather basic questions, but just starting with a repair to the Leak Amp.

I have attached a circuit diagram I found on the web.

Help needed and gratefully received please.

[Gyro]

You probably took a bit of a risk swapping the power amplifier boards, failed output transistors could well fry the drivers. If you've swapped them and swapped them back without magic smoke then I think Output transistors are less likely to be the cause.

If you do need them then I found a UK site listing them at £4 each, err well one anyway!: https://www.silicon-ark.co.uk/transistors/bipolar-transistors/39251-silicon-npn-power-transistor-by-rca (I've never used them).

In the absence of magic smoke, I'd start looking for a connection problem on the dead channel, something that stays with the chassis. Are you able to trace the signal through the stages (scope)? Check voltages against the schematic etc.? It could be something as simple as a switch contact (input selector or speaker switch for instance).

First test, do the output transistors all warm up to the same temperature? If so they are probably ok, otherwise I'd get more suspicious. Note that the output transistor quiescent current (30mA, link above T10) is set on the by a pot on the power amplifier board - swapping them will probably cause them to run at the wrong values.

I don't think you'd need to swap all 4 transistors (if that is the problem) worst case, the two on the affected channel.

A methodical approach will win the day. 

[fergch]

Hi Gyro, thanks for your help...and for the link.

Well, clearly I am in the beginners class!

One thing I did before any of this was to give the switches a clean and exercise them well, and clean the pots with a drop or two of lighter fuel before any power was applied to the amp - it has been in storage for sometime.

The other odd thing was that I thought it was working on both channels, and I plugged an input into each selector (tuner 1 tuner2 disc 1 disc 2 etc), tried each pot for noise (treble base balance and vol), and pressed the push buttons for tests, tried speakers 1 and 2,  and I was getting un-distorted sound on both left and right. I left it running, not loud, came back and one dead channel! No smell or magic smoke though.

Then I thought of swapping left and right pre and power boards to see where the trouble was. I did not think of the consequences of the setting of the quiescent current on each channel! I now cannot be sure they are back in the right place - should have marked them of course. On inspection there do not seem to be problem components. I do not have a scope, only a meter...

I will have another look tomorrow, and see the temps of the output transistors, and report back. I will have a look at the voltages too.
Can you suggest some further logical steps please, my scatter gun approach has failed it seems, and I do not want to change stuff without good reason. I do not have much experience of diagnostic routines so thought this may be a good project to get started. Usually I have successfully built projects, and replaced obvious failed components in broken circuits, usually popped caps.

Thanks

And, if I get this Leak going, there is another Stereo 70 needing attention!!

[Gyro]

Don't worry, we're all beginners to some extent when suddenly faced with an unfamiliar bit of kit, that's where the methodical bit comes to the rescue.

Ok, let's see what we've got. You think it was working on both channels initially, one stopped, you swapped the boards and the working channel still work (same side).

That means it's something on the chassis (as you said) or something external.

- Have you tried the bad channel speaker on the good channel? Don't try the other way round (thinking worst case failure here!).

- The next step probably is the output transistor temperature test, as I said I'm still a little skeptical that it's the output transistors given that the driver board didn't blow when you swapped them.

- I notice from the schematic that there is a fuse feeding each channel, (above C6 on the PSU bit). If one of them is blown, then that's a big indicator - Don't just change it though, it probably blew for a reason. We can diagnose from there if that's the failure.

[jmelson]

Note there is also an output coupling capacitor not on the board.  It would be very easy to check that,

Jon

[Gyro]

Yes, agreed.  I should have mentioned that explicitly (It was the thought behind the 'Don't try the good channel speaker on the bad channel'). As the amplifier has a single rail supply, the cap failing short would take out the speaker voice coil (it would also make a loud 'crack' which would hopefully have been noticed). If it has failed dry then I'd expect at least a minimal level of output from the speaker.

There are quite a lot of connections and switch contacts between the output transistors and the speaker that still need to be treated with suspicion (and continuity tests) before condemning the output transistors.

@fergch: Something that I just realized and really should have mentioned before!... The collector (case) of the upper output transistor (T10) on the schematic will be up at amplifier DC supply voltage (75V in the case of the Leak 70). That it enough to give you a nasty 'sting' or worse if you're holding other metalwork at the same time. If feeling for temperature, make sure you are not touching anything else (left hand in pocket) and not in bare feet. The case of T11 will be sitting at about 36V so is safer to touch. Sorry about that! 

[CJay]

House marked 2N3055s I seem to remember, at least they worked pretty well with 2N3055 transistors so the standard warning, make certain you buy from a reputable place, there are a lot of fakes out there.

On a DC coupled amp, one of that era particularly, you have to be methodical and careful as the fault can and will cascade back, destroying everything back to and including the mains transformer if you're unlucky so no more board swapping (as Gyro has pointed out) and no fuse swapping until you know what components have failed.

On something that old you need to pay attention to the switches and wiring, don't assume, check, with a meter, that you have continuity where you should have and verify voltages are where they should be on the edge connectors.

*edit* Don't expect miracles either, they weren't that great even when they were 'good'

[fergch]

Thanks for your all help and tips...

Seems I got off lightly: I have not been shocked, and still no smoke form heated components, more to luck rather than my judgment, which is way off...

I have been tracing the routes through the chassis side, no longer swapping boards, and I think a few scabby joints need attention.

There is not much space between the boards, and I am hoping to get some voltage readings when the light is good and I can probe safely, (- a little scared by the 75v waiting for the unwary finger!)

The fuses are intact in fact, something I checked before opening the cover. Inside, the three fuses are also intact for continuity.

The output transistors feel the same temp but I checked with the amp unplugged, thought it would be safer for my inexperienced fingers dangling among the components. I am now expecting to find something on the chassis as you  have explained.

Although not expecting miracles, apart from a successful repair without injury, I thought the Leak was in its day a respected amp.
Are they not worth the effort to fix, or is it better to find an alternative. In which case, have you any suggestions please??

[Gyro]

That sounds like hopeful progress (sorry about the late warning!) as long as all 4 output transistors are running at the same temperature (and as long as that's warm and not cold), in combination with intact fuses,  then I think your problems are hopefully fairly minor - it's a pretty good indication that they're all alive and reasonably correctly biased. You can put your credit card away for now. I think a lot of the voltage testing might be redundant for now, at least for the power amp stage (don't feel too scared about the 75V, it's enough to give you a nasty nip, but if you keep your hands dry and keep one in your pocket, it's pretty unlikely you'd come to any harm. The biggest risk is accidentally shorting something to chassis through careless probing).

Scabby joints are a start, but yes continuity checks on the chassis (first) are now the first order of the day. Work backwards from the speaker terminals (actually, check negative as well as positive, it might just as easily have lost its ground connection). You ought to be able to trace DC continuity from the positive speaker terminal back to the negative connection on the output coupling cap (C34), compare both channels as a guide. From C34 +ve you should then have continuity back to R52/R53 on the on the power amp board. As jmelson suggested, C34 could be faulty, check to see if it's shorted. Also confirm that you've checked the faulty channel speaker on the good channel (?).

The tape monitor switch contacts are another favorite. If you can feed a signal in at the tape input, then you will bypass all of the Preamp PCB and input switching, further narrowing things down (feels just like old times  ). The only thing in circuit in front of the Power amp PCB then is the volume control. Again, you can check continuity (compare channels).

Hopefully this will yield some results. Fingers crossed.

The Leak 70 wasn't a bad amp in its day. It might not sound sparkly by today's standards but could still yield pleasant sounds.

After that things

[CJay]

Although not expecting miracles, apart from a successful repair without injury, I thought the Leak was in its day a respected amp.
Are they not worth the effort to fix, or is it better to find an alternative. In which case, have you any suggestions please??

They were definitely one of the better ones, but like a lot of stuff, people's opinions of them have been magnified by some rather strong prescription rose tinted glasses, put up against a modern amp and its deficiencies would become clear but never discount the value of the experience of repairing it, plus if you intend to listen to a source/material that's contemporary with it then it'll probably be rather pleasing.

It's worth the effort to repair, it's a fun project to revive a classic like that.

[fergch]

Well just to let everyone know, the amp is working, and I have all my fingers!!

Thanks for all the help and suggestions, fortunately I did not end up frying anything in my ignorance, and lessons have been learnt!

I think the problem lay in the scabby joints, I re-soldered (leaded proper old school solder) and gave the push button switches another clean with some contact cleaner and both channels have come to life.

Again, thanks for the help, this forum is definitely the place to be...

Now I may tackle the Stereo 70....
Watch this space, I may well need some more help....

[Gyro]

Well done, hope it sounds nice! 

[CJay]

Nicely done, I'm pleased it's working and being enjoyed, despite not holding any attraction for me I do like that the old stuff is being used, onward for the Stereo 70 now.

[IanMacdonald]

Important points for solid state power amp repair:

Any fault at all in a power stage can cause an excessive current draw and cascade damage. Therefore after changing anything,  always test first with a current limited supply.  This can be a dim bulb tester, a suitably rated resistor in the supply line(s), or a regulated PSU. Any of these will do, just don't hit it with full power and hope.

The supply electrolytics can stay charged for a long time. Use a resistor to discharge them, otherwise you risk damaging components.

Never be tempted to 'jockey' parts. Including, speakers. If you transfer the fault to the other channel, you blow the other channel. (It was rare to see an amp come in with one channel blown,  it was either none or both. Reason, the owner invariably swapped the speakers and so transferred the shorted wiring over, doubling the repair cost.)     

On DC-coupled amps with no output cap, NEVER connect a valuable speaker to a suspect amplifier. Always test first for DC on the output terminals. Or, use a fused dummy load resistor of suitable rating. (On capacitor-coupled amps there will seem to be DC on the output if there is no load, but this is just because the cap has not been able to charge.) 

After any output stage component replacement, it may be necessary to set the standing current. If too low there will be distortion on quiet passages, if too high the amp will run hot. See the manual for the exact procedure, though they are all basically similar. With a DVM having a good millivolt range, the easiest approach is to probe the voltage across the emitter resistors.

Be suspicious of special diode packs used to set the bias point. These were notorious for going o/c. If they do, the standing current goes straight to max and the output stage toasts. I used to replace these as a matter of course. If an amp has blown and there were no signs of abuse such as shorted wiring, then these are the key suspect.

Never ground any of the speaker terminals. This applies especially to scope probes. Connect the shield to the amp case, NOT the speaker -ve.  Some designs, especially bridge types, have voltages on both sides of the speaker.

If doing any amount of amplifier work a pair of test loads is useful. Each consists of a beefy 4R or 8R wirewound resistor as appropriate (with fuse or thermal cutout, ideally) plus a higher value series resistor feeding a speaker socket. 100R is generally suitable. Thus, you can monitor the output of the amp up to full power without it being excessively loud, and without risking your monitor speaker.

The old fashioned slider rheostats as once used in labs or lighting dimmers are ideal if you can find a pair of around 10R, and can handle anything up to a big PA amp. The tap being used to provide a variable monitor volume!

Incidentally there is no harm in testing an amp to the full rated power (just starting to clip) with a music signal into a dummy load, for say up to 15min. If it can't pass this test then it will not last anyway, so you might as well find out now rather than have a disgruntled customer. Don't prolong sinewave tests though.

HTH.

[fergch]

Ian

Thanks for these tips. Clearly I have a lot to learn, and so this information is very useful. I will bear it in mind as I tackle the Stereo 70.

I think I got off lightly with the Delta 70!!