Broken Neve 1272

[adc004]

Quick update...it looks like TR1 is bad. I removed it and tested with a multimeter, and it looks bad (E to C and C to E doesn't give me OL). I put the BC184C that used to be in TR2 into TR1 (right now, there's a BC550C in TR2).

When I connect the power, TR1 gives me 3.6V at the base (should be 0.9V), nothing at the emitter and about 15.5V at the collector. TR2 measurements are also wacky, and TR3 gets really, really hot (hopefully it didn't blow)!

The good news is, I did get my hands on another BA283 card to try, and everything works when I plug that in. So everything surrounding the card is ok.

Maybe for the next step, I'll try pulling out TR1 and replacing that with a BC550C (just in case the BC184C is messed up). My soldering looks ok, but I'll double check that there's not a short somehow.

The reading on Pin L on the oscilloscope looks good, so it seems like the problem is somewhere in the TR1, TR2, and TR3 area.
The first gain stage seems to be working. Anyone ever experience something like this?
Thanks!

 

[adc004]

The voltages on TR1 also make me think maybe a tantalum capacitor around TR1 has gone bad and is shorting...does that sound plausible? I'll check for that tomorrow.

 

[MagnetoSound]

Quite plausible. Does C1 read short? If you sent some +24 DC up into pin L that’s exactly what you might expect, since any reverse bias is catastrophic to a tant.

As you have the board out, it’s a good idea to replace C1 and the three electro’s anyway.

A short at TR1 would likely upset TR2 enough to make TR3 run hot. I would check all the transistors anyway, out of circuit.

Don’t mess with the bias pot, unless you already did.

 

[adc004]

I just did some quick measurements compared to the good BA283. It looks like C2 might be the culprit. On the good BA283, the voltage drop across C2 is ~3.3V, while on the bad one, it's about 100mV. But I don't know if the TR1 went bad again...I'll pull them out and check to verify they're all working. I didn't adjust the bias pot at all, so it should be where it was.

 

[adc004]

I replaced C1 with a 22uF electrolytic to see if it would help...now all the measurements around TR1 are ~0V, with the collector being ~100mV. I still have to pull the transistors out to check them (maybe TR1 is bad). Hopefully, the other tantalums are ok (C2 and C3)...I'll have to order those if they are bad.
Thanks!

 

[adc004]

Bit of an update...the collector of TR1 is measuring 24V (supposed to be ~3.6V). I thought that meant the 68k resistor was shorting somehow, but I took it out and measured it...it's fine (still measures 68k). TR2 was blown so I removed it. I decided to try measuring the collector of TR1 with TR2 removed (just in case that was shorting to 24V somehow), but that didn't change anything. It's still measuring 24V at the collector of TR1. The base and emitter are both showing ~0V.

But I just wanted to ask...does that sound right? Shouldn't there be a ~20V drop across the 68k resistor? I feel like TR2 blew (and caused TR3 to get really hot) because it was getting 24V DC at the base.

I also looked at the signal with the oscilloscope (1kHz sine wave). The signal going into pin L and into the base of TR1 is correct, but there is no signal at the emitter or collector.

The collectors of TR2 and TR3 are both showing about 24V, which I think they're supposed to.

 

[MagnetoSound]

Did you check C3 yet? If TR1 is good, a short across C3 would account for the equal voltages at B and E.

FYI, resistors generally fail open, or at high value. They almost never short. Tants always fail short. So when looking for shorts, tants are often the culprit. That said, I would expect C2 and C3 to be polystyrene caps. They can short too, although not always.

I suspect that if TR1, C3 and TR2 are sorted, you’ll probably be done.

 

[adc004]

You called it! C3 is shorted. I'll order one of those and see if that fixes things. It's a Suflex cap (I think that's an original).

I also found a little glob of solder connecting the base and emitter of the 2N3055. I'm sure that was throwing off all measurements and was probably the reason why it got so hot the other day. I hope it's not also blown, but it still measures ok. Frustrating!

 

[nielsk]

It is always good to inspect first for shoddy old work, there's a lot of it out there and can save serious time sorting that first! Never assume the correct components were installed as replacements, or that the damn thing was built correctly in the first place!
It is informative to check transistors, but often in these simple analog circuits if you see signal going in and not coming out and you have eliminated possible issues with surrounding components, just replace it and the ones in the same group. Unless it is something exotic and essential (the 2n3055 may be getting there...) it's time VS $ and if soldered they always suffer from removal / re-installing.

 

[adc004]

Good point...yeah, that would save some time. It was fun troubleshooting though...I learned a lot! The main thing that sucks is that every time I need something, I have to order it. But once I learned that the transformers were ok, it wasn't such a bad thing anymore.

 

[adc004]

Just wanted to give a quick update…it’s alive! Thanks to everyone here, my Neve 1272 is working again! Funny story, I ordered some 4700pf capacitors, which got lost in the mail. I ended up finding one at home (which is actually way too large) and put it in, along with replacing TR1 and TR2. Plugged it in, and everything is working and sounds great!

Thanks from the bottom of my heart to everyone here. I still have a little cleanup to do on it, but I’m very happy to have the preamp working again…and I definitely learned a ton…and it was fun, except of course for the first few days!