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Old 3rd Feb 2015, 4:54 pm   #1
mindburner
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Default RGD 1046 amplifer repair

hi all

I have just been given some old audio gear and one of the items was an RGD 1046 amp, with valves. I have been reading up on this amp and it seems to be held in high regard, especially the PX 4 valves.

I have used a meter to check all the components on the underside of the chassis.

The main smoothing caps all check out. One (immediately after the rectifier) looks like it has been opened at some time as the edge is rough, maybe 'stuffed' with a new cap. It check out though so in time I will check the type of cap inside.

The paper in oil capacitors all look ok with no leaks and check out on a capacitance meter. There is on (8MFD) that has a bulge at one end so this may need further investigation.

The resistors all in spec.

After a few hours of checking, I powered it up. The amp didn't blow a fuse and valves illuminated.

I am unsure how to inject an audio final into this amp at the moment, but I do have what seem to be an original speaker (with a red magnet)

After about 30 minutes, the UU7 rectifier valve was no longer warm. I think it must have died.

I have ordered a UU8 so hopefully this will sort out the problem.

As I am keen to avoid any more valve death, should I use some sort of fuse on the anodes of the rectifier? would this help?

I'm not sure how the original valve died but there is a possibility that I caused a short. There was a long jones plug lead coming from the amp, with red black, blue grey wires. I assumed this may have been some sort of speaker output, but it may have had mains voltage on it

Upon checking the lead it did seem to be some sort of interconnect with voltage out. I must get a circuit diagram to see if this is in any way connected to the UU7 rectifier

The out caps do not seem shorted, so is there anything else recommended to do before I try a new rectifier valve. Yikes.
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Old 3rd Feb 2015, 5:57 pm   #2
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Default Re: RGD 1046 amplifer repair

These things are worth a lot of money, so I would encourage you to have it restored professionally rather than doing it yourself, especially if you're a beginner.
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Old 3rd Feb 2015, 7:01 pm   #3
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Default Re: RGD 1046 amplifer repair

I agree.

You sound like you have the potential to be an very competant valve amp repairer, and you obviously know quite a bit about electronics and vintage components, but this is not the item to hone your skills on, as you could dramatically reduce its value.

Do you know about re-forming electrolytic caps, using lamp limiters and how to check coupling caps for leakage, for example?

Cheers,

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Old 5th Feb 2015, 4:37 pm   #4
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Default Re: RGD 1046 amplifer repair

Thanks for the replies. Yes I really don't want a bodge up on this amp and would much prefer not to go down the road of replacement of components before I know they are faulty to not.

As for the 0.1uf coupling capacitors. I have PIO quality caps which I could replace them with but I do not want to do anything that will alter the unit unless absolutely necessary.

I have done a capacitance test on them and they check out ok, but I have not conducted a leak test as yet. I shall do this right away.I really should have done this before as I am only starting to realise the quality and worth of this amp.

As for reforming, I that is where there is a gradual increase of voltage to reform the film inside the cap? Again I shall research this before any further work

I appreciate the advice
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Old 6th Feb 2015, 10:40 pm   #5
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Default Re: RGD 1046 amplifer repair

OK, I found some excellent info on this page
http://www.vintage-radio.com/projects/

I have made a lamp to put in series with the mains to warn of unexpected current draws.

I have tested all the stripboard components and found that all the coupling caps are leaky passing DC. I compared and contrasted this behaviour to a new 500v 0.1uf cap I had. Therefore a reforming attempt is a definite must

I was planning to remove the UU8 regulator use the existing chassis and take a 1n4007 with a 30k 2w resistor in series to create a DC feed but

I have also started work on building the capacitor reformer as I think this will be better and valuable tool for further work.

The resistors are not too bad all within 20% of tolerance. Some only just though.

Can one get NOS resistors in the same style? Putting new tiny metal film ones in would ruin the authenticity of the amp.

As I am learning, there is a skill in keeping a piece of equipment authentic and safe. It's easy to just swap stuff out but it's not always the right option.

anyway a picture of the stripboard with measured resistance etc
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Old 7th Feb 2015, 7:58 am   #6
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Default Re: RGD 1046 amplifer repair

NOS resistors can be sourced the problem is they are likely to have drifted in value the same as your resistors. Overall valve amplifiers are pretty forgiving in terms of resitors drifting in value. However, resistors in certain places like a cathode resistor or in the grid circuit need to be bang on. The other problem with old resistors is that they can be noisy.

Changing caps is pretty straight forward and can be done by a novice, but beware it is also easy to make a mistake and cause more problems or damage. I have done this myself as have others.

To be honest I wouldn't touch your amplifier and I have a reasonable understanding and a bit of experience having worked on several valve amps. They look simple, don't have too many components but the truth is there is a lot going on.

If your going to go a head anyway, take lots of pictures from all angles so you can look to see where wires go, which component goes where and in what direction (polarity) Take your time, err on the side of caution and lastly keep a work log writting down what you did, measurements etc.

Good luck, Andy.
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Old 7th Feb 2015, 9:27 am   #7
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Default Re: RGD 1046 amplifer repair

I noticed that all your measured capacitors seem high in value.

Basic capacitance meters don't measure leakage, but they tend to read high when testing a capacitor with accurate capacitance, but leaky.

Leakage current is also voltage dependent, so to be sure a part is OK, it needs to be tested at a suitably high voltage. I use a hand-cranked 'Megger' insulation tester with switched voltage stress, so I can test at a voltage suited to the capacitor rating.

David
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Old 7th Feb 2015, 10:14 am   #8
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Default Re: RGD 1046 amplifer repair

Quote:
Originally Posted by Radio Wrangler View Post
... Leakage current is also voltage dependent, so to be sure a part is OK, it needs to be tested at a suitably high voltage. I use a hand-cranked 'Megger' insulation tester with switched voltage stress, so I can test at a voltage suited to the capacitor rating.
It can also be temperature dependent, so if they're mounted somewhere which gets warm then you should try to simulate that when you're testing them too. As far as voltage goes, the leakage can depend very nonlinearly on this. So, as David has said, a cap can appear fine at low voltage but leak badly at higher voltage. The critical word above is 'suitably'. Over the decades some capacitors seem to 'learn' what voltage they'll be operating at. My personal experience has been with nominal 450V electrolytics which are actually operated at 375V. If they've been unused for a few years then they will need reforming (as a general rule, by the way, other kinds of capacitors don't usefully reform). With patience the capacitors can be reformed up to 375V with a very high degree of success. But trying to push them beyond this - even to just 390V - raises the leakage current enormously, despite the printing on the outside of the can saying 450V. If the reformer persists and too much heat ends up being dissipated in what might be just a tiny patch of leaky film then the insulation can be irreparably destroyed there, and that's it. A capacitor which would have been fine at its operating voltage is now trashed. So take care not to overdo the volts. Restoring vintage kit isn't always about making every component perform as it would have done when it left the factory. It only needs to perform well enough to do its job.

Cheers,

GJ
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Old 7th Feb 2015, 3:27 pm   #9
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Default Re: RGD 1046 amplifer repair

The resistors are a bit of a mixed lot already- some older body tip spot types in with the later ring marked ones. For best performance they can be replaced with modern metal film types of higher power ratings (to get a similar sized part), but if authentic appearance matters it gets more difficult. For the tubular capacitors, they can be replaced with suitably rated polypropylene axial types; being smaller, these can be hidden inside the cleaned out cases of the originals if desired.

Although this amplifier is valuable (largely due the PX4 triodes in the output) it's not particularly complex as a design to work on- certainly not compared to some Philips efforts(!).
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Old 9th Feb 2015, 1:43 pm   #10
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Default Re: RGD 1046 amplifer repair

There's no point in using "period" passives; all carbon composition resistors are likely to have drifted in value by now, and all waxed paper capacitors will be electrically leaky. Modern, one-watt metal film resistors and polypropylene capacitors will be superior replacements in every aspect.
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Old 9th Feb 2015, 4:11 pm   #11
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Default Re: RGD 1046 amplifer repair

Thanks for the information. I have started opening some old 0.1uf caps that I have to see if I can insert a modern Vishay cap inside. They come apart quite well using a sharp blade rolled along the ridge band. The innards are pretty nasty and give off vapors, so gloves, mask, eye wear help.

I used some tylerman sleeving to insulate and the new cap leads go through the old holes nicely. Some epoxy and metal paint pen to finish off. They will hopefully look the part.

I have some 'allen' older style resistors believed to be from around 1990, so I can replace with these to keep the look. I only have a few values so others will be 2w metal.

It's a balance between authenticity and survival-ability. I want to actually use this amp so I would like to fit decent parts.

I still will try a reform of the electrolytics as I am building the reformer but I would still be worried these would fail sooner rather than later.
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Old 9th Feb 2015, 5:14 pm   #12
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Default Re: RGD 1046 amplifer repair

The attachment is of the amplifier installed in the RGD model 850G and the later production 1046G3.
The original 1046G amplifier did not have negative feedback, the simple top cut tone control consisted of a capacitor and variable resistor connected between the anode of the SP41 and ground. The 1046G3 has separate bass and treble controls.

DFWB.
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Old 9th Feb 2015, 5:32 pm   #13
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Default Re: RGD 1046 amplifer repair

The circuit of the earlier amplifier. This amplifier has the two smoothing chokes. Some 850G amplifiers also have the chokes or the not so good resistance capacity smoothing.
The amplifier in my 1046G 'gram will be converted to to use a pair of triode connected KT66 beam tetrodes instead of the scarce and expensive PX4 triodes and will be modified to 1046G3 specification.


DFWB.
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Old 10th Feb 2015, 10:46 am   #14
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Default Re: RGD 1046 amplifer repair

Hello,

Just found this thread.

Hope this post is not too far off topic.

I remember buying one of these from Friend’s Father back in the mid 70’s for 50p. I have to say – in guilty way – I stripped off all the wiring and tag panels etc. Also discarding the PX4’s for a pair of pentode connected EL34’s to build a guitar amplifier!

In my defence this gear was worthless (well 50p) back then so, I suppose it was fair game.

Oh well, one had to learn how work on this gear and this was one of the unlucky victims of my early experiments!

Regards
Terry.
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Old 10th Feb 2015, 4:45 pm   #15
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Default Re: RGD 1046 amplifer repair

Quote:
Originally Posted by FERNSEH View Post
The circuit of the earlier amplifier.
hi that schematic is much appreciated.

Quote:
Originally Posted by Valvepower View Post
In my defence this gear was worthless (well 50p) back then so, I suppose it was fair game.
Hey 50p was bug money back then

It's easier to find out the value of stuff now days I guess with the old web sphere an all. I remember playing with an old variable capacitor as a kid. Pretending it was part of a ray gun. My brother always had old radios and oscilloscopes around.

I was just lucky with this amp. It's strange as I was repairing a pair of Quad 303 mono block amps and I was doing some research. I kept reading how good the Quad's where but solid state just didn't compare to the best valve amps. So I was looking for a valve amp for a few weeks. I nearly bought a Chinese copy to play around with. Then I managed to get the 1046.

I just need to get another now. Might be easier said than done
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Old 10th Feb 2015, 11:39 pm   #16
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Default Re: RGD 1046 amplifer repair

Quote:
Originally Posted by mindburner View Post
I just need to get another now. Might be easier said than done
It's an easy enough amplifier to replicate.
The RGD radiograms are at long last being preserved. Along with the excellent amplifier, the radio unit is also a very good performer.

DFWB.
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Old 11th Feb 2015, 8:44 am   #17
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Default Re: RGD 1046 amplifer repair

There may be some helpful info in this article for restoring old PA amps:
dalmura.com.au/projects/Renovating%20PA%20amps.pdf
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Old 11th Feb 2015, 9:40 am   #18
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Default Re: RGD 1046 amplifer repair

Quote:
Originally Posted by ajs_derby View Post
... all carbon composition resistors are likely to have drifted in value by now, and all waxed paper capacitors will be electrically leaky. Modern, one-watt metal film resistors and polypropylene capacitors will be superior replacements in every aspect.
True, and if all that matters is performance then stripping and replacing everything is the way to go. But, to be honest, if all that matters is performance then selling this and buying a good 1980's transistor amp is the way to go. Do remember that making the amp less original will lower its value. The sorts of people who are prepared to pay serious money for these are often the sorts of people who want them not to have been 'got at'.

Cheers,

GJ
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Old 13th Feb 2015, 10:08 pm   #19
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Thumbs up Re: RGD 1046 amplifer repair

Quote:
Originally Posted by trobbins View Post
There may be some helpful info in this article for restoring old PA amps:
dalmura.com.au/projects/Renovating%20PA%20amps.pdf
That's an excellent resource. Thanks for the heads up. I have so much to learn but already I think I have 'got' some of it.

Quote:
True, and if all that matters is performance then stripping and replacing everything is the way to go. But, to be honest, if all that matters is performance then selling this and buying a good 1980's transistor amp is the way to go. Do remember that making the amp less original will lower its value. The sorts of people who are prepared to pay serious money for these are often the sorts of people who want them not to have been 'got at'.
I'm trying to do a bit of both at the moment. I took a break as I had a very poorly Quad 303 amp that really was in a bad way. It was a matter of cleaning, swapping components and replacing rusty connectors.

With the 1046 I have hollowed out the old 0.1uf caps and put in decent replacements. I used JB weld and the job came out pretty well. I used new resistors but I have all the original and other metalcap capacitors in case i do want to sell. I can then revert back.

I'm not even sure if I want to paint the transformer housings. I quite like the look of the paint flaking off. And it certainly is original.

On a separate note. I have successfully built the capacitor reformer and it works great. Well apart from the V meter. I though I was going crazy until I realised that the meter was an AC one, the seller had sent by mistake. I just need an enclosure..

I have got one of the 16uF's down to 12mA from over 50v, so might be promising

A few pics.. one of the caps after being opened, don't look too bad me thinks. And one of the reformer before boxing and operation.

Again I appreciate all the advice give so far
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Old 13th Feb 2015, 10:20 pm   #20
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Default Re: RGD 1046 amplifer repair

Impressive work
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