Blown Resistor?

Mfring defect in the epoxy 'paint' finish.

RL

Unless something upstream that caused an over-current went open-circuit before the resistor could totally fail?

What model Uher tape deck? Low audio gain on record or playback. Stereo or mono? If it's stereo, then you have two channels available. I would guess(tm) that each channel has its own AGC. Therefore, if the AGC is misbehaving on only one channel, you can compare signals between the two channels.

Also, inspecting for mechanical component damage is a good way to start troubleshooting. For this author on YouTube does component level motherboard repair on Dell laptops. He usually starts with a thorough visual inspection. Looking for cracks in MLC (multi-layer ceramic) capacitors is very difficult without using a microscope. Try almost any of his videos:

Once your done looking for tiny volcanoes on resistors, maybe injecting an audio signal and tracing the signal with an oscilloscope might be useful. Unsoldering the 1200 ohm resistor may have fixed an intermittent. Try tack soldering a different 1200 ohm resistor and see if it magically fixes the low audio problem.

Jeff, it's a Uher Report Monitor 4000 so mono only. It would have been so nice to have another identical channel to compare against, but there ain't one, I'm afraid. I'm getting low/virtually no gain on record. Nothing wrong with the level of the source monitor's output to the internal speaker, though.

Good steer, Jeff; I like this kind of stuff and will check out his other uploads as well.

There was a bulge under the 'caldera' on this resistor, and a tiny secondary vent close by on the same level, so I don't think it's a failure of the ceramic coating. And Uher do things properly with good quality components; they're high quality recorders and the BBC used them in huge numbers for interview purposes. The main blow hole resembled John Hurt's chest in the original Alien film - under magnification, of course.

It didn't. The problem's still there. I have tried injecting a signal at the first audio input stage, but there's no particular point in the signal chain where the issue occurs; it's just low throughout the whole path, which I've not encountered before. A nice clean break at a specific point would have been so nice, but it's not to be with this one. :(

Have you cleaned the tape heads?

Yes, but not for that reason as the fault just suddenly manifested out of nowhere.

I recall a bad experience I had with the stereo version of this deck - the 4400 Report Monitor. I found a duff electro so whipped it out and got a replacement from my capacious parts bin. The bin was choc full of NOS caps that had been in store for years, so I thought I'd best re-form the replacement before fitting it. Problem was, when I'd done this, I forgot to discharge it before replacing the failed one. So I accidentally touched some other part of the PCB in the process and something sparked and I heard a crackle. Next thing I know, the f****ng thing bursts into flames! I kid you not, it actually caught fire! That was a rotten piece of carelessness on my part. A valuable lesson I'll never forget, but an expensive one. :(

If the source monitor is acting normally, I would guess(tm) that there's nothing wrong with the record audio amplification system. Something between the record audio output and the actual record heads is where I would start looking. Inject audio into the record audio input (aux or mic) and walk an oscilloscope up the signal path until you arrive at the record head. Also, check if the bias signal (about

Incidentally, I blundered across this article on fixing something similar but on a Uher 4400 (stereo). It turned out to be an intermittent switch contact:

I learned quite a bit about MLCC capacitor replacements from his videos. He describes in one of his videos how he got into the business. He bought what sounds like the entire parts and pieces inventory for older Dell and Alienware laptops from Dell.

Mic (with preamp) or aux (no preamp) input?

That would be too easy. If you can't find anything wrong along the signal path, try measuring the DC voltages on each device in the audio chain. Look for bias and power voltages that don't make any sense.

If you think it might be the input stage, inject a higher level audio signal into amplifier stages that are further along the amplifier chain.

Yup, already done, Jeff. And the bias osc on this machine runs at

There's no particular point in the amp chain where the signal disappears or even degrades; it's just way down all the way through, hence why I suspected something amiss with the AGC. That is *still* my main suspect, but I don't have a schematic for that section as it differs from everything else I've seen online and I'm one of those people who *need* a schematic. All the DC voltages in the amp chain were pretty much as per the readings given in the service guide - except for the stage prior to where the AGC samples the output level for comparison purposes (but even that's only down by 200mV (2.83V shown in the service book, but only 2.63V measured. Not really enough to get excited about. Likewise, the EBC voltage differences in the amp chain were all pretty much as expected - but not so with the devices in the AGC. I believe it's the AGC but I've gone as far as I can and am ready to farm this out!

Wow! That's the way to do it! :-D

Aux. There's no joy with mic level audio at the mic jack, either.

All solid advice, Jeff; but all already tried. I'm gonna send this mo-fo out for a special service and let the experts deal with it as there is probably quite a lot of re-alignment required too, the bastard.

I asked one of the AI's (probably CoPilot) and that's how it responded. If I asked a different AI, I'm fairly certain I'll get a different answer.

If you're injecting audio into the AUX input, and there's a preamp (mic amp) in front of the AUX input, the preamp output should deliver about the same audio level as you're applying from your audio generator. I would need a schematic or at least a block diagram to be certain. Try injecting audio into the Mic input and see if there's anything coming out of the preamp stage (which presumably goes to the AUX input.

Does the AGC respond to changes in audio input level? Probably not, but I want to be certain.

Try breaking the AGC loop so that it's running maximum audio level. If there is a diode peak detector, try lifting just one lead.

Things go about 100 times faster and better with a schematic. These days, schematics are deemed proprietary information and are hoarded by the manufacturers to discourage repairs and encourage landfill over use. Enough conspiracy theory for today. In the USA, it's a holiday.

Hmmm... looks like there are schematics available:

That's a very important and obvious clue. In the distant past, I would model the affected are using LTSpice. I'm not sure I could do that today with QSpice. These days, asking someone measure audio levels for you should be is easier:

Methinks you give up too easily. Maybe wait a few days, and come back to the problem with some fresh ideas.

Sorry, Jeff, but I've already spent all those wasted days on it and am all out of ideas. I'm not a repair tech; just a hack and I need an accurate schematic even to have a prayer at cracking an issue like this. Time for someone else more experienced with Uhers to take over methinks. Thanks for trying to help, though: you're a pal. :)

I used to repair them many years ago. if you live anywhere near Bath, drop in and I'll try to help you.

That's very good of you and I would - but unfortunately I left the UK many years ago. Thanks, anyway. :)

I'm sure I had the circuit diagrams for several of the Uher models - but i have no idea where they are now.

I seem to remember the 2-transistor mic pre-amps had a rather clever counter-intuitive volume control arrangement, which gave a wide range of possible volume input levels. The wiper was connected to the output of the second transistor, with one end of the track going to the followiong stage and the other end going to the emitter of the first transistor. As the volume was turned down it increased the negative feedback around the front end.

If you suspect the fault is in the AGC system, do you get an improvement if you switch the AGC off?

Cursitor Doom schrieb:

This one?

HTH

Reinhard

Nice. I couldn't find any schematics in the above download. The text is in German and English. I also looked in the "related manuals" section at:

Jeff Liebermann schrieb:

Sorry. Instead, give this a try:

HTreallyH

Reinhard

That's the machine, yes. But there's no schematics in this one.

Doesn't bring the audio back, I'm afraid. However, that doesn't exculpate the AGC, as they've embedded the on/oiff/fast/slow switch right in the middle of the stage.