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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment. |
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26th Jan 2020, 12:55 am | #1 |
Octode
Join Date: Feb 2010
Location: Leicester, UK.
Posts: 1,431
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Ampro projector amplifier
I picked up one of these Ampro projectors recently and tonight looked at the amplifier. I'm just considering how far to go regarding servicing. At the moment it works, but it is all original. The smoothing caps are hunts (2x double 16uF cans) and are still doing there job perfectly. All of the capacitors are paper in oil, I think most other electros are TCC Picopack types and all other capacitors are Paper in oil types, again look to be tcc or hunts.
The amplifiers DC is provided by two SenTerCel finned rectifiers in series and they too are working well with no fault. I just wonder what other peoples opinions would be on how far to service this? I was thinking of changing at least the coupling capacitors and then maybe just see how things go. I'm not too sure of the reliability of the rectifiers, they work now but I'm not confident that they will be reliable. Same goes for the electrolytics and oil caps, the last thing I want is a bang and having to clean oil from all inside the amp. It would be nice to keep it fairly original as it works very well but I'm sure with it being 60+years old maybe it is due for a service. |
26th Jan 2020, 1:49 pm | #2 |
Dekatron
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 5,997
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Re: Ampro projector amplifier
I would replace all the paper in oil caps as the temperatures inside these units are formidable. The only paper in oil that seem to survive are the large military grade cans, so I would treat these the same as wax types.
My older Premier 10 uses a mains and driver transformer but yours uses audio coupling capacitors to the output transformers. I replaced all capacitors despite them having been replaced before. |
26th Jan 2020, 5:22 pm | #3 |
Octode
Join Date: May 2009
Location: Stevenage, Herts. UK.
Posts: 1,515
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Re: Ampro projector amplifier
Which model is it?
Have you measured the HT? The selenium rectifiers can go 'soft', I partially bypassed the ones in my 'New Educational' with a diode and series resistor to restore the HT. . |
26th Jan 2020, 11:14 pm | #4 |
Octode
Join Date: Feb 2010
Location: Leicester, UK.
Posts: 1,431
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Re: Ampro projector amplifier
I think I will just change them then. I did think it would be wise as I wouldn't really trust any caps that age but they are working well. The amplifier and in fact the rest of the projector is in almost pristine condition, it was nice to open up the amp and find it in pretty much the same condition it would have been in some 50+ years ago.
I will also replace the rectifiers, I think mine may also be a New educational model, it has no actual model name on it but the amplifier is certainly the same. I do think that the rectifiers have gone soft as there isn't a good amount of volume, from memory I was getting around 170v DC at 117v input when I powered it externally, which definitely sounds low. Do you recall what diode and resistor value you used? I imagine the 1N4007 will do to replace the rectifiers. |
27th Jan 2020, 12:06 am | #5 |
Octode
Join Date: May 2009
Location: Stevenage, Herts. UK.
Posts: 1,515
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Re: Ampro projector amplifier
Ok, if it runs off 117v it can't be an educational model as these were designed to run directly off 240v, although it was possible to run just the lamp off an external 110v transformer if maximum brightness was required.
It sounds like you have a 'Stylist' what output valves does it have? Stylist should be 50C5's (at least in the US version), Educational should be 35L6's. I left my selenium rectifiers in place and functioning for authenticity, but gave them a helping hand with a 1N4007 and a resistor chosen to give correct HT voltage. Last edited by wd40addict; 27th Jan 2020 at 12:21 am. |
27th Jan 2020, 12:46 am | #6 |
Octode
Join Date: Feb 2010
Location: Leicester, UK.
Posts: 1,431
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Re: Ampro projector amplifier
Sorry should have made myself more clear. It is a 240 volt model, I removed the amplifier and ran it at 120v for a while before refitting it, when I did I checked over the caps and took some voltages just to make sure everything was more or less working, that is the only time I measured the HT. I will do so again tomorrow and post some voltages.
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27th Jan 2020, 9:19 pm | #7 |
Octode
Join Date: Feb 2010
Location: Leicester, UK.
Posts: 1,431
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Re: Ampro projector amplifier
Took some readings tonight and the HT is certainly down. The amp gets 225v AC from the projector as it is fed via a dropper resistor on the voltage selector, measurements are as follows
DC o/p @ rectifier 188v DC Anode voltage of 35L6GT 104.3v DC (should be 215v) I'll change the rectifier along with a few caps and hopefully that will improve things. |