NAD 7020E. Transistor advice required.

[starone]

I've recently acquired a NAD 7020e receiver from the late 1980s, I believe. It powers on but doesn't do anything else so searching through the forums I came across a thread from 2008 that detailed the same problem as mine - no sound on any inputs or with headphones connected. After investigating further it appears that a disc capacitor (C227) had shorted, causing two transistors Q511, a BD139 and Q507, a 2SD699A to fail. I'm not skilled enough myself to have worked this out - it just so happens that the description in the previous thread was identical to the problem with mine so I checked these components out first. So now I want to fix it but am unsure with the 2SD699A transistor. The other components seem readily available but I can't find any reference to the said transistor, or an equivalent. Has anyone any experience with this receiver who can offer any advice.

Thanks
Martin

[paulsherwin]

The 7020e is basically a 3020e amp with NAD's digital tuner of the period bolted on. It dates from 1988.

Are you sure about those transistor types? I don't have a circuit to hand, but it's very unusual to have Japanese and European transistors factory fitted in the same design. Was the BD139 fitted during an earlier repair?

The 2SD699A seems to be available from a number of suppliers but I've no experience with it.

You would be unwise to swap out transistors simply because this fixed somebody else's fault. 'No output' is a pretty general fault. Have you actually tested these transistors, or taken circuit voltage readings?

[Bazz4CQJ]

The transistors specified in Post#1 do square with the NAD circuit diagram I have. Cannot comment on availability.

B

[paulsherwin]

Fair enough, it just seems odd.

[Refugee]

I have had an unrelated item fail to dead due to a corroded fuse holder.

[paulsherwin]

Martin, can you confirm that you have tested these transistors and confirmed that they are faulty?

[julie_m]

Some NAD amplifiers (there seemed to be a new 3020 variant every other week) had preamp outputs and power amp inputs, for connecting additional sound processing devices such as graphic equalisers without interfering with the tape monitor loop. These would normally be bridged with U-links. It's not as simple as that, is it?

Pictures of the socket panel would help, if in doubt .....

[starone]

Thanks for the replies. Firstly this amp doesn't have any links on the back so I'm sure it's an internal problem. The transistors look original and the types match the circuit diagram, which I must say I'm struggling with. I checked the voltages after the filter caps in the power supply and the +/- 27v rails were present. I took some readings at the output transistors but couldn't match the readings with the circuit diagram. What sort of voltages should be on these outputs? I've also taken a photo of the suspect capacitor which also shows the transistors in question. Both these transistors show a short between emitter and collector, however I've not removed then from the circuit to test yet. Also there's an image of the back panel (sorry the quality isn't the best).

Martin

[Radio Wrangler]

There looks to be a lightly toasted resistor to the North and it may be shadow but is the grotty paxolin PCB darkened somewhat in the middle? I'd expect dried capacitors in the area.

With a typical modern amp, I'd first check the + and - rail voltages, then I'd look at the DC level on the output. Dirty contacts in a speaker isolation relay if no audible output.

If you really have shorted output or driver devices, I'd expect the output stuck to one rail or the two rails shorted together.

David

[dave cox]

The 2SD699 isa whacking big darlington switch but it is only doing regulator duty so subbing something shouldn't be too much of a problem.

If you have +/- 27 volts then Q511 is working, as it is the series regulator for +27V. IF C227 fails with a short (and it does look like someone has taken a bite out of it) then it would short Q511 to ground killing it ...

dc

[starone]

Just got back to this after a few days away. The resistor, which should be 10 ohms is measuring 5 Meg so I guess it is fried. I've tried getting the +/- 27 volts but there's nothing so I removed the components and both transistors are shorted. I'm going to go ahead and replace these components, probably will have to order them from somewhere as Maplin doesn't stock the transistors. Cricklewoods do the BD139 but I can't find an online stockist of the 2SD699A, so will need a bit of advise as to what to use.

Martin

[paulsherwin]

The BD139 is quite a common transistor - there are lots on eBay now. The 2SD699 is much rarer and you will probably need to google for suppliers, but I would try to avoid substituting a different type if possible.

[Refugee]

In the photo in post#8 there is a ceramic capacitor above the two big electrolytic capacitors. It has a nasty looking mark on it as if some of the coating has burned off. It looks like C227.

[dave cox]

If you do find a 2SD699 for sale, then unless it looks like it's been de-soldered it will be a fake as, AFAIK, they've not be made for decades! I think a TIP142 / 147 might fit the bill, given the actual work the device will need to do (series regulator), but thats only a rough fag packet calc and the voltage rating is a little close ...

dc

EDIT:
That iffy looking ceramic cap is right across the +27 rail. It looks like a 50V type so 'should' have been OK ...

[dave cox]

I had another look at that regulator circuit and I think I see the possibility that the +27V rail might get momentarily whacked with the full rail voltage at switch on ( +48V ?) That’s scary close to the rating for the ceramic cap, C227, right in the firing line! Given how generous component manufactures are (not) a 100V part might be better.

dc

[dave cox]

Should it be a 2SD669A not 2SD699A ?

Looks like a T0220 or TO126 part (sans heatsink) on the PCB layout.

dc

[ms660]

2SD669A on the schematic, same on the 3020E schematic.

Lawrence.

[dave cox]

Just a blur on the only copies I've seen

[ms660]

Here's the specs for a 2SD669A:

http://www.ic72.com/pdf_file/2/564699.pdf

It mentions its compliment which is also shown on the NAD schematic.

Lawrence.

[starone]

Thanks to you all for taking the time to look over the schematic and offer solutions, it's very much appreciated.

I think, as has been said, the 2SD699A isn't going to be an option so I'll have a look at the suggested alternatives and try and get something ordered.

Thanks again.
Martin