Blown Resistor?

I already had that one from RM, Reinhardt. Plus I also have a hard copy service manual with a large scale schematic of another close variant of this model. Unfortunately, neither of these totally correspond to the 4000 I have here and the AGC section is *totally* different from either schematic. :(

The coating is cross-linked, so it chars rather than melting. If you ablate the resistive film with some huge overload, it just cracks off the coating as it goes.

Cheers

Phil Hobbs

Hi Phil, Not sure what you're doing on a repair group squandering that planet-sized brain of yours, but you are of course most welcome! I shall try to hunt down that 'suspicious' resistor tomorrow and post a close-up image of it, as it's one of those things where - for once - a picture really is worth a thousand words. Then everyone can chip in their 2c worth about what the cause might be. Stay tuned!

CD

Cursitor Doom schrieb:

Did you try to compare the AGC circuits of types 4200 var1/2 (stereo,

The AGC is a central part on the main PCB rather than on a separate board someone might have substituted at some point, so it seems like very much an outside chance your idea would bear fruit. Nevertheless, I'll give it a try..

Here are the photos of the suspicous resistor; let me know what you think. In addition to the blow holes, it's also slightly mis-shapen.

Doesn't look like a paint defect, nnless you've been poking at it with an exacto knife tip, after the fact.

Can't tell if it's smoke or just bad shadowing. Why not same exposure and lighting in all three images?

RL

Can you provide better photos of just the hole area (without shadows)

Hint, use a microscope

I've not been excavating! There was a bubble on the surface originally, which I scraped off with a fingernail to reveal that hole. I didn't dig any further.

It was very difficult to photograph, because the depth of field was close to zero and the resistor was only millimeters from the lens glass. I had to use a hand-held LED lamp for illumination, but full-on, frontal illumination wasn't possible due to the proximity of the subject to the lens, I'm afraid. The lighting varies because I was holding the lamp by hand each time.

Correct. There's *no* charring or smoke evidence at all here, Jeff. It's as clean as a whistle. There is also a much smaller hole a bit further around on the same level, but too small to snap with my setup.

Yes, it reads 1200 ohms as per its color coding, Jeff. However, if it

Not with my current set-up, I'm afraid. I was really pushing things to the limit for what I have as it is!

I have a stereocope and two microscopes, but they're not the type you can easily take photos through. I really must order something more suitable for this purpose, because the stereo I have is old-school analog a bit like yours.

Somewhat better but not good enough. I filled out the request form. It sent me an email with the download link. I downloaded the PDF, which only showed 1/2 of the Uher Report 4000 monitor schematic. Fortunately, the AGC circuit is shown. The PDF is also rotated +90 degrees which might explain the truncation. The schematic is clearer than the one I found.

The schematic request URL also includes a list of other people who are currently downloading the Uher monitor 4000 schematic. It shows about

I have about seven microscopes. Three of them work. The remaining four need repair. One can never had enough microscopes.

Trinocular head perhaps? Note the 3rd eyepiece for the camera:

You don't need a trinocular head. When I need to take photos on a binocular microscope, I remove one of the eyepieces, insert 0.5x lens, add a camera lens adapter, and take photos. Your binocular microscope should work:

For illumination, I use a cheap LED ring light:

Very nice. I had a BH a while ago; can't go wrong with Olympus. Currently I have a Russian Lomo (monocular) and a binocular Leitz-Wetzlar. The stereoscope's a Kyowa. I've got a Watson as well from the late 1950s; beautifully made in a hardwood case with the original bill of sale, but I don't use it as it's not very ergonomic compared with more modern designs. Shame really.

Yes, I really must get one of those next time I'm on Ebay.

Yeah, same here, Jeff. I *do* have a USB microscope *somewhere* as well, but haven't seen it for a few years!

A 'bubble' is a definite sign of a paint defect at time of mfr..

RL

Troubleshoot in the 'off' position and assume that AGC has nothing to do with your problem.

RL

I've never come across that before; just assumed it had blown through over-current. I was *convinced* I was really on to something. I suppose I could excavate the hole out and see how far down it goes down. I might do that tomorrow just out of idle curiosity....

No doubt good advice, but as I mentioned earlier somewhere, I've kind of lost the will to live with this problem so it's going off to a R-R specialist familiar with Uhers for a full service of the electronics and mechanics. It doesn't cost a fortune for these - unlike the Ferrograph I had serviced earlier this year.

Well, before I got around to that, my Ferrograph, which had just got back from a *very* expensive refurb, presented with the exact same problem: zero recording to tape despite plenty of audio input. Since the F'graph was still under warranty from the servicing people, I called them up and complained (politely - fortunately, as it happened). The chap on the phone said, "have you been playing old tapes on it?" I said, "Yes." He said, "Give the heads a good clean, then." I said, "They don't look dirty." He said, "Give 'em a good clean anyway and see if that fixes it." So I did (not having any faith it would help in the least, however). Nevertheless, full recording was instantly restored! So I thought I'd try the same thing with the Uher

- and despite the heads looking perfectly clean, that worked too! It's been nearly 50 years since I last used reel to reel tapes and it seems I'd forgotten a basic piece of regular maintenance and that was the cause of my problem. I'd spent *hours* going through all the circuitry looking for anomalous voltage readings and finding none, squirting test signals into the amp chain and not finding any red flags there, not to mention the bias oscillator and the AGC. Whilst all the time, the solution was a simple bit of housework. Back in the day, we didn't need to worry about oxide-shedding, but old tapes age the same as we do, it transpires; some worse than others. Ampex are the *worst* in this respect, I've learned. Don't play old Ampex. Don't buy NOS Ampex. Google "sticky shed" for more info and don't fall into the same rabbit hole I did!

I asked if you had cleaned the heads right at the beginning of this thread. The tineiest of gaps between the head and the tape can have quite drastic effects on reproduction and even worse ones on recording.

Another cause of those symptoms, which is sometimes discovered by complete beginners, is having the tape twisted so they are trying to record or play though the backing material. Some tapes that have 'bootlaced' can have an entire twisted section in the middle of an otherwise right-way-around tape. I have also seen spillages due to putting the spools on upside down, so they unwound under power instead of winding up.

The most confusing decks of all are those with non-standard tape paths and winding systems. One Truvox deck ran the tape from right to left and several early machines had anticlockwise takeup spools that wound the tape oxide-outwards.