On My Bench.

What people are working on at the moment
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Dave the bass
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#1 On My Bench.

Post by Dave the bass »

So as not to take over our-Ant's 'What are y'doing' thread I've created this thread which I'll update occasionally with whatever I'm playing/working on at the moment if anyones interested.

A chum's big 1980's Peavey power amp had a faulty TL074CN chip in it which was a bit of a pickle to fault-find, the replacement is working well but it seems their failure is rare. Nick suggested going a bit further into it seeing as I'd kept the faulty chip.

Using an experimenter board which I think I'm using correctly (I confess I've never used it since it was given to me in a bag of 'take-these-bits-or-they-re-going-in-the-bin' about a year ago from another chum having a clearout) I've found...

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The output from number 4 (or 'D' in the schematic) of the quad op amps in the package goes to about 14V all the time. Even with no components and just the OA powered with a DC supply across it number 4 op pin stays high I noticed.

Erks.
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Nick
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#2 Re: On My Bench.

Post by Nick »

That's good. I assume the other three outputs don't do that. If not then it's a reassurance that you found the cause and it's not so likely to happen again to the replacement.
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#3 Re: On My Bench.

Post by Mike H »

Nice one. :thumbright:
 
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#4 Re: On My Bench.

Post by simon »

SS dummy at the back of the class puts his hand up sheepishly...

So the one output is duff and the implication is then I think that it's unlikely to have been caused by external effects and it's "just one of those things" that the op amp has had an internal failure? And this is quite unusual? Is that the gist of it?
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#5 Re: On My Bench.

Post by Nick »

Well it's not conclusive but it's indicative of that. It could still be the amp broke the op amp.
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#6 Re: On My Bench.

Post by Mike H »

Certainly could have been caused by an external effect, for example if its o/p was connected to something it couldn't cope with. Even if only briefly.
:D

I think Dave is saying it's a permanent condition? If so, internal damage.

DIET: I'm repeating what Nick said (but more verbosely :lol: )
 
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#7 Re: On My Bench.

Post by simon »

Ah okay, I just wanted to be clear in my own mind. Nick said something about the cause and I wasn't sure if the finger was being pointed conclusively at the op amp.
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#8 Re: On My Bench.

Post by Nick »

If the op amp was ok out of the amp, it would be a indication that the problem went a bit deeper. As it is Dave's repair looks good to go.
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#9 Re: On My Bench.

Post by simon »

That makes sense
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#10 Re: On My Bench.

Post by Mike H »

Might have had beer spilt into it. That was sometimes the reason Dave Brooks (Danbury Transformers) was asked to make a replacment mains TX, but which had to look identical to the original, even to the extent of having the same scabby top cover put back on, because the amp was a valuable collector's item (Vox, Marshall etc.,). The old mains were a bit fragile anyway, the layers being insulated with waxed paper, so couldn't handle beer ingress as well. :lol:
 
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#11 Re: On My Bench.

Post by Dave the bass »

Good call, but I don't think any liquid has got into this amp, it was housed in a 19" Rack unit in use so not used as a beer table like a lot of stage combos can be.

Well, the amp is still working, I've soaked tested it for ages, power-cycled, swapped inputs, all the things you'd do to double check its OK, seems fine. I'm happy to hand it back to its owner.

I repaired a lighting controller for the same fella earlier in the year. It was for some on-stage LED Lumieres (sp?) that hang from a series of T-bars. That was a diode failure that burnt out a resistor and took out the L6598 controller chip. I found what I think was liquid ingress but but I'm not sure if it caused the fault on this occasion.
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On bench at the moment is his vintage (IMO) lovely big Trace Elliot pre and power amp. This has been interesting to work on, its been long-time stored in a damp leaky shed (perfect!) and theres a lot of corrosion on parts...
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This fuse had been wrapped with wire as it had blown at sometime, see more corrosion on the cans of the output transistors too?...
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The bearings on the fan are totally shot, it makes a complete racket, replacements are fairly expensive but a biker chum (also a retired Electronics Tech) came to the rescue with a good 2nd hand fan of the same spec, the original is so corroded the paint has lifted. Its done some serious hours of work, this amp was used a LOT...
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State of this cooling fan!
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The original reported fault, "It hisses a lot, like a rainstorm outside!" I was told is also accompanied by the 'Overload' LED by the input socket permanently illuminated which he didn't remember.
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Yesterday I did a "Hmm" and scratched my hairy chin before diving in. "Observation, observation, observation before you start" a wise TV engineer told me many years ago, he was brilliant to work with too.

Aha! big dry joint on one of the smoothing cap rails, see where my pointy thing is pointing?...
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Cleaned up and remade the joints on that rail. Powered up, Overload LED on but the amp works fine although there is intermittent noise breakthrough. Probing around with an insulated pointy thing I found an area of the PCB where theres 3 small transistors and three trimmer pots which I believe are used to set the overload threshold at 3 points through the pre-amp section. 'Believe' as I cant get any service info from Trace Elliot on an amp this old, however the did kindly send me a late 80's schematic (hats-off to TE) that used a similar arrangement. Tapping this area reveals hiss/hash increases and decreases in the background, ooo, getting closer perhaps.

I cleaned the pots in case they were 'noisy', no change, then I started gently squirting freezer spray around the 3 small transistors, bingo, noises drop or trigger! Aha, one of the transistors is breaking down maybe? More very gentle freezing to narrow down the fault to one transistor. Theres a lot of solder flux around this area too from the manufacture in ~1981/2.

Then nothing, no Overload light on, no hisses and pops, just a perfectly working amp! Now this is where it gets weird.

I couldn't induce the fault again, heating (small heat gun with a nozzle) and cooling (can of freezer with a tube) the suspect transistor doesn't do a thing. As a precaution I removed all 3 transistors, checked their fwd volt drops using the diode test function on my ancient DVM and cleaned the corrosion off their poor little legs then replaced them all in the same positions but not before removing all the flux with IPA, there was a LOT of it.

Spotless now...
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And a day later the amp remains perfect. I know its a bit far-fetched but I wonder if me freezing the transistor/s has altered the chemistry of the large amount of flux around that area on the PCB and that was what was slightly conductive maybe and tripping the Overload sense circuitry? Or maybe the transistor was noisy/breaking down and the heat/cooling cycles has temporarily jolted it back to life? I dunno TBH.

Odd, but interesting.
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Ray P
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#12 Re: On My Bench.

Post by Ray P »

Dave the bass wrote: Sat Dec 16, 2023 9:18 pm Odd, but interesting.
Self portrait, Mr B?
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#13 Re: On My Bench.

Post by Nick »

I just repaired a l07d turntable. The problem there was a dual op amp that had developed a dry joint on one of its pins. Its been there for 40 years working fine, but the legs of the 8 pin DIL had become corroded and I suspect the corrosion had worked its way down between the solder and the leg. Seemed to fix it, but leaving it sitting in damp cold workshop problem came back until it was running for a bit and had warmed up. I think the general crud that was on the PCB was absorbing moisture (it was a high impedance area in the circuit).

Just saying that to sympathize with Dave, as things can just go wrong with time and use. Though I never believe they can, and am sure there is a hidden problem I haven't found, but maybe it just is.
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#14 Re: On My Bench.

Post by Dave the bass »

Nice fault finding Nick, cap-doffed.

The damp conditions this amp has been stored in has taken its toll.

There's 3 x CA3140E socket mounted 8 pin DIL ICs that I've wondered about removing and cleaning pins and re-seating. I don't know what to do because of the amount of corrosion TBH! Change both the ICs and the sockets might be the best route although they're working fine and gently tapping with a probe doesn't induce any nasties audibly presently.
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There's a nasty corroded electrolytic cap too I've noticed, I'm tempted to change this...
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I've touched up solder joints that have appeared powdery and found some are just flaking apart and have done my best to remake them, none of them seem to have affected the amp's functionality but I couldn't let them go in the state they were. It was only around that one area around the trimmers and transistors that the random noises stopped after heating/cooling cycles.

I'd like to see this amp in use again, the owner would too of course but the storage conditions have taken their toll and not helped.

Ray P wrote: Sat Dec 16, 2023 9:49 pm
Dave the bass wrote: Sat Dec 16, 2023 9:18 pm Odd, but interesting.
Self portrait, Mr B?
Ha! Yes, I'm happy that way :)
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#15 Re: On My Bench.

Post by Mike H »

Fab!
And a day later the amp remains perfect. I know its a bit far-fetched but I wonder if me freezing the transistor/s has altered the chemistry of the large amount of flux around that area on the PCB and that was what was slightly conductive maybe and tripping the Overload sense circuitry? Or maybe the transistor was noisy/breaking down and the heat/cooling cycles has temporarily jolted it back to life? I dunno TBH.
I think the general crud that was on the PCB was absorbing moisture (it was a high impedance area in the circuit).
I was starting to think that, the flux deposit was holding moisture.

Years ago I built something that hissed - my temporary solution was to tape thin paper over the tweeters. Seriously. :shock: Didn't stop it but made it less annoying.

Turned out, it was shunt-Zener regulated, and there wasn't enough overhead of Voltage, so at least one of the Zeners was on the threshold and so generating white noise. Which it then bunged into the supply rail. Took me a while to work out what was going on tho.

I seem to remember having a bit of an issue even getting Zeners at that time, for some reason it was difficult, as in, just can't get 'em, so I often ended up with using large metal ones with threaded studs, that I could get. And these ones might I think have been 'bargain bundle' too, so the values may have been suspect.
 
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