Troubleshooting Audio Amp

I'll get the probe I improvised re-jigged, but won't have time to do any more to the Uher til Sunday, unfortunately.

Yes, I'm sure you're right and it sounds like you have everything organized so well at your workshop. You do this kind of thing for a living? I'm just a hobbyist and not exactly outstanding at this if I'm honest about it. My workbench is an absolute disgrace. I just use up all horizontal space that's free in next to no time; always been the same.

Right, so the increased inertia helps to stabilize the speed? That makes sense. I assume those flywheels are so heavy for the same reason (tape deck flywheels, I mean).

Well that's what I'd been attempting to do when I failed to get the signal generator output into the Uher's audio input. As I mentioned, I suspected it was the cheap Chinese adaptor I'd been using. So I made up another one (also shown in the picture). Not very pretty and could have been constructed more thoughtfully, but will get the job done, no doubt. I also discovered there's *one* speed which the Uher and the F'graph have in common (19cm/s) which is a significant help. Not so important with test tones, but for music comparisons, invaluable.

I can do 630hz for sure, but why so specific a frequency? I also made another discovery about the possible cause of the varying speed. It turns out that if the machine isn't placed in one of the neutral positions on the speed selector dial when stored, then the rubber belt around the drive wheel will be under pressure from the pinion it butts up against, causing a flat spot. Bizarrely, there's NO mention of this in the owner's handbook! It does say that the machine should not be left unattended for long periods switched off with the 'play' key left depressed, but that's pretty much common sense. I only became aware of the importance of the neutral switch position from reading some guy's comment he'd left under a Youtube video of someone demonstrating this deck. Consequently, I'd been leaving the machine in this state when unused and I would put money that previous owners have done the same thing during their ownership too. So there may well be multiple depressions on that belt as a result. I'll check on Sunday when I next get the lid off. Hopefully if the material's reslient enough, it may recover in time.....

Just a minor point: It doesn't look as though there will be room for the capacitor inside the plug where it will be screened, so the capacitor needs to be at the tip of the probe with the resistor between it and the cable end. This is so that the resistor attenuates any stray signals picked up by the capacitor. If the capacitor were on the amplifier side of the resistor, anything it picked up from the mains or the proximity of your hand, acting like an aerial, would be fully amplified.

My workshop is a 6ft x 10ft wooden shed containing dreadful mixture of amateur, semi-professional and fully professional work in several different fields. The welding and blacksmithing can only take place outside in good weather and the Harrison L5 lathe is in a spare downstairs room in the house.

The workbench is a matted tangle of wire clippings and bits; I have to clear a space to bring in things I need to work on. Anything too big has to be dismantled on the kitchen table, then taken to the shed and worked on in sections - or the oscilloscope and soldering iron have to be taken to the kitchen. That means a very thorough clean-up in the kitchen before I can do any pastry-making.

The equipment comprises a large Solartron (valve) audio signal generator, a Marconi (valve) RF -VHF signal generator. The aforementioned measuring amplifier, a Philips twin-trace oscilloscope and several Farnell power supplies: 0-30v, 50-0-50v and 0-350v. I have a few resistance boxes and capacitance boxes for experimental work. For the bench mains supply there is a variac-controlled isolation transformer with 240v and 120v tappings. Other equipment is scattered around the house in odd corners and can be brought into the shed when it is needed.

One long wall is covered with nests of drawers containing resistors and capacitors, laid out on a grid pattern with preferred values aligned horizontally and the decades in vertical columns. Drums of wire are stored on broom handles hung in strops from the ceiling. The end of the shed nearest the doorhas a cabinet and a bench with the welding, metalworking and car repair tools. Small shelves along the tops of the walls house everything from transformers to household nails and spare rivets with a rivetting set for the Allen Scythe cutterbar!

A counter may not show up the problem because it averages the frequency over a period of time. A "wow-and-flutter meter" is the right thing to use but, being specialised, they are not very common. Ferrograph made a test set with all the necessary instruments built-in but they tend to be collectors' items and will probably need repair themselves before they can be used. Human ears work quite well on suitable recorded material.

The slower the flywheel, the heavier it needs to be - with the mass concentrated as near to the rim as possible. In the Uher, the capstan shaft has only a very lightweight driving wheel that rotates slowly, so the inertia of that is negligible. The speed,stabilising inertia is in a heavy brass flywheel on the cross-shaft, which rotates much faster and friction-drives the lightweight wheel through a rubber tyre on the latter. It is vital that this rubber tyre is in good condition, because any notches or lumps will tend to deflect the speed of the lightweight driving wheel (and the capstan) rather than being absorbed in the speed of the brass flywheel.

That is why disengaging the drive when the machine is being stored is very important.

It looks fine to me. Not all good working equipment needs to look swish and 'professional' - a lot of my fully-professional recording equipment is plugged together on site using ex-telephone switchboard jack strips with exposed wiring (so I can see at a glance what the connections are). They can always be discretely slid down the back of the table once the kit is working properly.

It is geometrically half way between 20c/s and 20Kc/s i.e. the middle of the audio band. [Try repeatedly dividing it by 2 or multiplying it by 2 on a pocket calculator, you will get to 20Kc/s or 20 c/s in the same number of operations]

It also has the second harmonic at 1260c/s and the third harmonic at

Tht is very important and will account for 'hiccupy' speed veriations. I seem to remember my Uhers needed to be put into the 'neutral' position to switch off the motor, so the user was unlikely to forget, as they were acutely aware of the need to conserve battery power on location. If yours has only been used on a mains adaptor, I can see how the misunderstanding could have occurred.

A fine but important point there! We often think 'stuff in series' can go in any order, but so often there are exceptions due to unexpected effects which must be allowed for. I do have some more suitable sub-min 10n caps which would be much easier to shield. I'll use those on the Mk II version. :-)

Same here, except with me it's DiY spiced tea. :)

Well, it's damn nice to know I'm not the only one who works in something approaching complete chaos. I was beginning to think I was unique in that respect! I had a theory that having a workshop like a minefield where if you fell or tripped, you'd impale yourself on something would force you to take great care and in doing so, hopefully ward off dementia, but since I got diagnosed with age-related osteoporosis, I've been forced to tidy up the more obvious and lethal hazards at least.

Yes, as you say, such a meter is a bit exotic and beyond my needs for a one off assessment. However, I do have counters with gate times which can be closed down very tightly. I'd be surprised if they couldn't resolve these variations. However, this is not my usual area at all so I expect to discover otherwise; we shall see.

Makes perfect sense. It's an astonishing omission to make from the owner's handbook for a mechanism that had been in production for very many years and must have been a known issue by the time the documentation was reprinted/revised.

Ah, right. I suspected there had to be a good reason behind that choice. Thanks for the clarification/explanation.

There's no give away 'feel' to it like when you release the 'play' key you can sense the tension coming off. That doesn't happen with that knob at all. Now I have to ask: how and where did you learn all this stuff? You're clearly an expert in this field whether you acknowledge it or not and there are vanishingly few women who go down such a technical pathway, certainly not in the 'old days' at any rate! Did you experience discrimination on account of it? Must have been bloody difficult in an all-male environment.

Make sure it has a high enough voltage rating if you ever want to fault-find valve equipment.

The speed selector is a very counter-intuitive thing to operste, it doesn't make sense until you look inside and see what they are trying to do with it and how it controls the positioning of the cross-shaft. Another thing that isn't obvious is that it has a locking mechanism (at least in some models): if the knob is turned through 90 degrees, the speed selector can't be moved. There is a line marked on the face of the knob but it isn't at all obvious what it means. I assume this is to prevent the motor being accidentally turned on by catching on the reporter's sleeve when using it on the shoulder strap.

I'm very old, so I have had a long time to learn it.

It's more complicated than that: I spent most of my life trying to act like a man and only recently realised I had been a transwoman all along. This meant that I didn't experience the same obstacles a woman would have met but, instead, I was constantly bullied for not being 'manly' enough. Luckily my two parallel careers were in electronics and biology, biology has always been open to women - and to men with a more feminine outlook on life. Electronics was very mysogenistic to start with but is becoming more woman-friendly now. My local radio club has three husband-and-wife pairs of licenced radio amateurs. Most of the other clubs in this area have at least one open transwoman and some have a few others still "in the closet" - those statistics would be a good research project for someone. This is a far cry from the early 1950s, when Jean Walton had to change her name and persona to John Walton before Decca would treat her/his electronics and mechanical design abilities seriously.

I first worked in marine radio design but, when I left that, my main day job became biology. I continued to have electronics as my hobby and worked as a sub-contractor to the (UK) National Sound Archive in my evenings and weekends.

Because I was running the sound recording job on a shoestring, I could only afford to equip the 'studio' (a bench across the bay window in the front room) with equipment that had been sold at scrap prices - but my work had to meet full professional specification. This meant I had to learn, very quickly, how to rebuild knackered bits of kit back to factory specification and calibrate them to the satisfaction of my N.S.A. boss, who was extremely helpful but a stickler for accuracy.

He was a BBC-trained studio manager and recording engineer from the days when discs were the main medium and he had built his own recording studio from scratch (with a home-made recording lathe). He acted as my mentor and most of what I now know in the audio field can be traced back to his teaching.

Nowadays the first question most people ask, when presented with a problem, is "what can I buy that will do the job for me". I am still of the old school that asks "What have I got that can be adapted to do the job? ...and if I haven't got anything, can I make something?"

Well, as a right-of-center Libertarian I fully support your choice. That may surprise you if all you know of people like myself is what you hear about us on the BBC! The sad fact is that classical Liberalism is dead. I am still one at heart, but can no longer describe myself as such any more because the meaning of the word has changed out of all recognition and the 'neo-Liberals' are about as illiberal is one can get. They claim they're this that and the other, but if you dig into it, the form of freedom they advocate is the one

Being transgender isn't a choice - the only choice I have is what I do about it. Do I go around feeling permanently ill at ease with myself and hating what I have to do in order to fulfil society's expectations or do I say "Sod it! That's their problem if they can't cope with me being me"? I don't set out to create situations where people are upset

- but I won't let them impose their prejudices on me. That is one of the benefits of growing old (mandatory) and cantankerous (optional).

The BBC has an agenda on many topics; having heard their coverage of things I happen to know about, I no longer trust anything they say. At best a lot of their journalism is sloppy and they just repeat propaganda because its easier than searching out the truth - at worst they knowingly repeat blatant lies that happen to suit their preconceived agenda.

Back to tape recorders then....

Or vintage electronics in general! Can't go wrong with that. :)