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Old 28th Jul 2007, 1:57 pm   #21
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Default Re: Rigonda Stereo - Reviving the beast

I'm sure you're right in saying each board is an amplifier. I would expect the o/p valve to be nearest the o/p transformer.

You could select a likely looking candidate as the grid coupling capacitor, connect a test lead to one end and then with the meter on ohms scale probe round the valveholders looking for a very low resistance. Repeat for the other end of the capacitor. It should be connected from anode of one valve to the grid of the other.
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Old 28th Jul 2007, 11:18 pm   #22
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Default Re: Rigonda Stereo - Reviving the beast

Thanks for the tip, Graham. Never thought of that one, but it seems like the easiest option here I think. Have discovered that a laser pointer can be an aid here in seeing which terminal is actually which when jumping from one side of the board to the other.

Have been out at my parents house today, so don't have access to much of my equipment. I did bring the chassis with me though, and have subjected it to a thourgh clean (vacuum cleaner, paintbrush followed by liberal application of contact cleaner to the switching system - being careful to keep it off the back of the tuning scale). Pleased to report that it looks a good deal healthier, and that the switches all work now. Though SW1 still sticks a little bit. Can work on that more later. I'm just glad that the cleaning didn't unearth any more obviously cooked components.

Unfortunately re-stringing the pointer for AM has been less successful. I've figured out how it came off now, when I was putting the chassis back into the case last night it must have caught on the upper edge of the case apeture (there's very little clearance) and just jumped out of the very shallow grove in one of the pulleys. I am now discovering that this is an absolute pig of a thing to re-string! The dial cord in this case is actually steel wire, and as such is very prone to "sproinging" into any shape and in any direction OTHER than where you want it to go. It's like trying to re-string a radio using a slinky! If anyone's done this before, I'd be most grateful, because so far it's driving me insane!

I did manage to pick up the resistors I needed today, so will get them changed tomorrow. Any repairs to the amplifiers such as this I'm doing in pairs, as I figure that it's best to keep things balanced.

Hopefully things will progress more tomorrow.
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Old 29th Jul 2007, 9:32 am   #23
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Default Re: Rigonda Stereo - Reviving the beast

Some Blu-Tack comes in handy!! Holds one thing in place while you get it over another bit!

Cheers,

Steve P.
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Old 29th Jul 2007, 12:58 pm   #24
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Default Re: Rigonda Stereo - Reviving the beast

Steve, you're a genius. Never thought of that!

Will give it a shot when I'm back in the flat later and get an update here to let you know how I get on.

Cheers.
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Old 30th Jul 2007, 8:54 am   #25
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Default Re: Rigonda Stereo - Reviving the beast

I'd echo Steve's advice about Blu-Tack. Also, some of those little triangular document clips (Staples and the like) and a pair of artery forceps (but watch out for damaging cord)
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Old 31st Jul 2007, 9:38 am   #26
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Default Re: Rigonda Stereo - Reviving the beast

Zelandeth

Why are you changing the grid coupling capacitors when you've already determined that there is only 0.08V on g1?

TimR
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Old 31st Jul 2007, 11:52 am   #27
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Default Re: Rigonda Stereo - Reviving the beast

Because they look to have been overloaded somewhat!

Anyway.... Another tip... Put a light behind the board. You'll then be able to see the tracks and the parts.

Cheers,

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Old 31st Jul 2007, 1:21 pm   #28
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Default Re: Rigonda Stereo - Reviving the beast

Hi all,

Surely if there is 0.08v+ on the grids then the caps need changing? Shouldn't you have at least a few volts negative with respect to cathode?

Or.... is the 0.08v+ measured between grid and chassis?

Anyhow, looks like a fab set, I would like something like this one day.
Cheers
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Old 31st Jul 2007, 3:52 pm   #29
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Default Re: Rigonda Stereo - Reviving the beast

Hi Lee

The output stages are single ended class A...ie autobias. The control grids are returned to earth via the grid leaks. The cathodes are positive WRT the control grids. The bias is developed across the cathode resistor. Therefore if the grid is at chassis potential and the cathode is at a positive potential WRT to chassis the control grid is therefore negative WRT the cathode. The cathode is generally at about 10V so 0.08V on the grid is going to make no difference at all.

My point is, if you are going to make a measurement to diagnose a fault or check a suspect component and you find the component to be not defective, then why change the component.

Steve P

How can the component be overloaded! If the working voltage has been exceeded then it may have been damaged. However, as the HT is pretty well fixed to within a reasonable degree by the domestic mains supply and the component is designed to operate with pretty well the full HT voltage across it then I fail to see how it can be overloaded. It would be extremely unlikely that its dissipation will have been exceeded as it is a small value so it would have had to pass a lot of heavy duty RF to exceed its maximum dissipation.

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Old 31st Jul 2007, 3:57 pm   #30
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Default Re: Rigonda Stereo - Reviving the beast

Sorry - Wrong choice of word I suppose. I meant to say that it looked to me like the ends were bursting out slightly. And with new EL84's being a bit pricey, it might be a good idea to change them both to be on the safe side.

Cheers,

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Old 31st Jul 2007, 5:21 pm   #31
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Default Re: Rigonda Stereo - Reviving the beast

Quote:
Originally Posted by Hunts smoothing bomb View Post
Hi all,

Surely if there is 0.08v+ on the grids then the caps need changing?
[...]
There are a few other possibilities, Lee; the EL84 itself could be passing grid current, due to gas or grid emission, or the valve base or print could be leaky.
0.08v across something like (probably) 1 meg is not a lot of current!

Quote:
Originally Posted by flyingtech55 View Post
Hi Lee
[...]
The cathode is generally at about 10V so 0.08V on the grid is going to make no difference at all.
Absolutely.

Quote:
My point is, if you are going to make a measurement to diagnose a fault or check a suspect component and you find the component to be not defective, then why change the component?
Agreed, unless it looks physically unwell - Russian caps of this sort are usually fine.
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Old 31st Jul 2007, 6:11 pm   #32
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Default Re: Rigonda Stereo - Reviving the beast

Those caps were replaced because while they were identical to many others in the set, they were the only ones where the rubber bungs in the ends were bulging out, and sure enough when removed one of them turned out to be leaking slightly. As I had suitable spares handy, seemed sensible to change them.

The 0.08V there was relative to chassis. Measuring relative to k (pin 3), I get -4.2V and -3.8V. Cathode being positive wrt the control grid (and around 5V positive relative to the chassis).

I did try the lamp behind the boards trick, unfortunately they're just a bit too thick and opaque to actually be able to see the tracks through. You can see that there's a track THERE, but not quite where it goes.
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Old 31st Jul 2007, 7:50 pm   #33
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Default Re: Rigonda Stereo - Reviving the beast

Quote:
Originally Posted by Zelandeth View Post
I did try the lamp behind the boards trick, unfortunately they're just a bit too thick and opaque to actually be able to see the tracks through. You can see that there's a track THERE, but not quite where it goes.
Another trick that I use is to scan the print side of the PCB using a scanner (or a digital photo might do at a pinch).

Then use your software to make a mirror-image of the scan, enhance it so that the tracks and board are as contrasting as possible, and print it.

Now you have an "x-ray view" of the tracks, which you can refer to while you're looking at the component side of the PCB

N.
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Old 31st Jul 2007, 11:00 pm   #34
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Default Re: Rigonda Stereo - Reviving the beast

I might have to try that X-ray trick actually. Got a decent camera and a tripod, so should be able to get the angles right.

Haven't had a chance to do much on this set this evening (was de-dusting the other five, decided when I had to wash my hands for the fourth time in twenty minutes that it was time to clean them!), but did have a poke around earlier.

There's definitely something amiss with the right hand amplifier board (looking at it from the rear of the chassis) or its feed. It's not doing a lot, and it's doing it in quite a muffled manner. My gaze is immidiately falling on the stereo seperation control, which when first touched was causing all kinds of wierd behaviour from the other channel. It strikes me that this should act like a balance control - but all it seems to do is turn the volume up and down on the good channel. I was hoping it might be something simple like a duff pot there, but they both vary smoothly between .5 ohms and a meg. So much for that theory!

Upon further investigation, both channels seem to work okay when they feel like it...but they seem to be mutially exclusive so far! With the stereo control out, one channel works...in, and everything seems to switch to the other side. Hmmm...I think the switch has to be regarded with suspicion there.

I have noted that the EL84's seem to run really rather hotter than any of the other valves in the set - but not being familiar with them, I don't know if this is normal behaviour. There's also a very slight blue glow inside the shielding in the valve, I know some valves do that, this normal for an EL84?

If someone could tell me how the "Stereo separation" control on the back is MEANT to work, I'd be most grateful. I'm also at a loss to explain the function of the key on the front marked "Band" which presently appears to act like a graphic equaliser preset for music...which is all that it might actually be!

So far however one thing has become clear. Receiving FM, even through the somewhat questionable tatty little speakers I'm using at the moment, this is going to be one cracking sounding set when it's done.

As you can see below, the chassis responded well to a good clean!
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Last edited by Zelandeth; 31st Jul 2007 at 11:08 pm.
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Old 1st Aug 2007, 9:23 am   #35
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Default Re: Rigonda Stereo - Reviving the beast

Zel
Going back to tracing the coupling cap - I just thought that there might be a grid stopper (1k-ish resistor) connected directly to the grid pin.

EL84s (and most other audio output valves) do get quite hot - enough to burn your fingers quite quickly!
Don't worry too much about the blue glow - they sometimes did this slightly from new.

The chassis is looking good.
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Old 1st Aug 2007, 9:31 am   #36
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Default Re: Rigonda Stereo - Reviving the beast

Hello Zel

You can expect the EL84s to run hotter than the other valves; in single ended output stages they run at more or less their rated anode dissipation all of the time. The blue glow is not uncommon with the EL84 but can sometimes be a sign of distress. The cathode to chassis voltage you have measured seem surprisingly low for a correctly biased EL84; you can expect to see around 10 to 12V across the typical 270 ohm cathode resistor. There may be a problem with these resistors, the electrolytics across them or the screen grid supply to the EL84s.

Hope this helps.


John
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Old 2nd Aug 2007, 8:36 pm   #37
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Default Re: Rigonda Stereo - Reviving the beast

Thanks for the tips guys, I've been quite busy, so haven't had a chance to do much with this (or any of my other) radio(s) since my last post.

I did just FINALLY manage to re-string that confounded AM dial-cord on about the four hundredth attempt. Really did test my patience that one, and didn't do my fingers any good whatsoever. Think I lost several years worth of sanity doing that!

So...back to checking the feed to the screen greed supplies, and the cathode feed resistors...
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Old 4th Aug 2007, 10:09 pm   #38
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Default Re: Rigonda Stereo - Reviving the beast

Present project is building up what I'm referring to as a "layout schematic" by tracing out the circuit and marking out the circuit as found on a photo of the board from above. This is being done in Paint Shop Pro on a multi-layer image. Separate layer for each thing. Thus far I've got all components, and most of the off-board connections done, as well as the smoothing network. Probably halfway through the first actual amplifier board. Once I've got the circuit itself down, I should be able to cross-reference with the schematic I have to mark which component is actually what. THEN I'll feel confident enough to really start hunting for faults. The plus side of doing this is that it's letting me get to know the circuit a lot better.

The main visible problem I've got is still this wierd mutually exclusive stereo thing that's going on. I can have one channel or the other, but not both.

I've noted that when I'm poking around the board, if I touch the terminals (or even bring a probe near to) the connections to the channel that isn't working on the balance control, there's a distinct hum appearing at that speaker - but there's no effect whatsoever on the working channel. Grounding fault somewhere?
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Old 8th Aug 2007, 4:04 pm   #39
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Default Re: Rigonda Stereo - Reviving the beast

Hi Zel,

Can't help with your stereo output problem, but you may be interested to learn that the 'BAND' switch on your Rigonda is likely to control the BANDwidth of the AM radio (MW/LW/SW). Some of the more upmarket Rigondas featured well specified radios which included this function. The switch will basically switch between wide (full audio frequency response) & narrow (limited audio frequency response but improved selectivity, improved immunity to adjacent channel interference). It's a nice feature to have. Unlikely to be operational on FM/VHF. The Rigonda Bolshoi had a variable control!

Regards,

Seth.
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Old 10th Aug 2007, 6:54 pm   #40
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Default Re: Rigonda Stereo - Reviving the beast

Seth, That would seem to tie in with the behaviour actually. Will do some experimentation to see if I can confirm that shortly.

Due to a requirement to clear the flat up a bit, I've put this project temporarily on hold, hopefully will get back to it next week.

Glad to report that I did manage to get the chassis back into the cabinet without knocking the AM dial cord off this time (the fact that I stuck it in place with masking tape first may have played a part...).

Following a good clean, the set's certainly looking a lot better! I'll be taking the gold panel on the front and repainting it this weekend, along with the bronze wedge shaped bits at the ends of the tuning scale (think I still have some bronze paint left from my Renault 25 Monaco I had a few years back...). This is about the electrical work, not cosmetic though, so I'm not going into details.

Electrically, I'm no further on...stereo's still behaving odd, but I can't see any obvious causes. Nothing getting unduly hot or appearing otherwise stressed. More investigation required I think.
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