| Electronics > Beginners |
| Transistor selection for Leak Delta 70 amplifer repair help please... |
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| 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 |
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