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Service Data Fidelity HF10 Stereophonic

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Doz
 Doz
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Anyone have a schematic? Just renovating this for my father, as his father bought it for him new in 1959. The original is long gone, but I managed to score one at retrotech.

 

This is me, sat next to it sometime in 1974.

IMG20230702162931 (1)

 

 
Posted : 08/12/2023 11:39 am
Nuvistor
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@doz I haven’t got one and have searched the internet, which you have probably done. Hopefully if none available the circuit should be straight forward for a 1959 design.

Frank

 
Posted : 08/12/2023 12:08 pm
Doz reacted
Doz
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It should be quite straight forward. It's got a selenium rectifier, which I'd like shot of before it stinks the place down, just wondered what the acceptable level of HT is, so I can calculate a value of series resistor to insert with my silicon diode. I'll start at 100 ohms and see where that gets me. 

It shouldn't  present many issues really ... on with the grease!

 
Posted : 08/12/2023 12:12 pm
slidertogrid
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I don't know if the Fidelity is any different. I use a 120R on the Bush SRP31 series and Dansettes. The Dansette contact cooled rectifier is usually OK. The Bush usually not! I have seen the amp still trying to work with 40V H.T on those! The HT isn't that critical so long as it's got some, I aim for 180 to 200V once the valve(s) have warmed up in the case of EL84 - ECL86. I either use a RS polo mint or one of those bolt down gold coloured resistors. Massive overkill I know but no chance of it going O.C!

Good luck with it, are you going to post a thread about the restoration ? A record player would be interesting...

Rich.

 
Posted : 09/12/2023 11:04 am
Cathovisor
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I know some engineers insert a zener diode in series with the resistor and the silicon diode to more accurately recreate the characteristics of the original rectifier: you may find a smaller resistor will then suffice. The zener replicates the forward voltage characteristic which in some cases can be tens of volts.

 
Posted : 09/12/2023 6:35 pm
slidertogrid
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One characteristic I have noticed particularly with single ended amplifiers EL84 or ECL82 is the residual no signal hum. Some amplifiers are virtually quiet at minimum volume others have a low level hum. This is with either an original good smoother/res or the smoother/ reservoir replaced with new modern capacitors. On an amp with low level hum increasing the smoothing has no effect, trying another valve has no effect. It's not really a problem more of an observation really. The cure is to convert to full wave rectification and then compensate for the higher HT.  I wonder if some amps were like it from new as only the mod will solve it. The ECL82 Amps seem more prone than the EL84 amps. 

 
Posted : 10/12/2023 12:15 am
slidertogrid
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Posted by: @cathovisor

I know some engineers insert a zener diode in series with the resistor and the silicon diode to more accurately recreate the characteristics of the original rectifier: you may find a smaller resistor will then suffice. The zener replicates the forward voltage characteristic which in some cases can be tens of volts.

@cathovisor  Hi Mike if I have understood this correctly does it create a small AC content to the unsmoothed DC, The half wave sinewave then not having a flat bottom? What is the advantage to this? I just wonder if it helps with the ripple current. 

 

 
Posted : 10/12/2023 1:46 pm
Cathovisor
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@slidertogrid I will ask people more knowledgeable than me to come up with the answer; what effect of a diode assembly starting to conduct at say, 10V rather than 0.6V will have on ripple current I'm not sure, but it does mean a reduction in heat in the series resistor which replicates the forward resistance of the old rectifier.

 
Posted : 10/12/2023 2:31 pm
slidertogrid
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@cathovisor Thanks Mike. I am going to have to tackle a family owned record player I have been asked to do at some point, it has a horrible little live chassis amp using the equally horrible UCL83 output valve, The layout and construction is truly terrible. Add to that it has been "worked on" in the past. I really am not looking forward to it. I like to keep things original but I am so tempted to bin this horrid amp and build something better with a mains transformer, a copy of a Dansette conquest but with just a tone pot is the way I am thinking... But if/when I tackle it it will be a subject for a new thread. First I need to sharpen the screwdriver and put the hammer on charge! 😀 

 
Posted : 10/12/2023 3:45 pm
Doz
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Posted by: @slidertogrid

I don't know if the Fidelity is any different. I use a 120R on the Bush SRP31 series and Dansettes. The Dansette contact cooled rectifier is usually OK. The Bush usually not! I have seen the amp still trying to work with 40V H.T on those! The HT isn't that critical so long as it's got some, I aim for 180 to 200V once the valve(s) have warmed up in the case of EL84 - ECL86. I either use a RS polo mint or one of those bolt down gold coloured resistors. Massive overkill I know but no chance of it going O.C!

Good luck with it, are you going to post a thread about the restoration ? A record player would be interesting...

Rich.

It turns out to be a contact cooled metal rectifier,, even after evicting all the leaky waxies and poor smoothing can, I only had 70V of HT... I popped a bridge in, and 100 ohms, and I've got about 220V, that'll do.

 

I'll post a video up when it's done

 

 

 
Posted : 10/12/2023 6:21 pm
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irob2345
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I saw a great answer to a post on the US site asking how to test those rectifiers. It went something like this:

1. Remove rectifier from the chassis.

2. Hold it over an empty trash bin.

3. Drop it.

4. If it makes a "clunk", it is bad. Leave it there.

 

 
Posted : 11/12/2023 7:48 am
Lloyd, slidertogrid and Doz reacted
Doz
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Posted by: @irob2345

I saw a great answer to a post on the US site asking how to test those rectifiers. It went something like this:

1. Remove rectifier from the chassis.

2. Hold it over an empty trash bin.

3. Drop it.

4. If it makes a "clunk", it is bad. Leave it there.

 

That gave me a much-needed laugh this morning 🤣 

 

 
Posted : 11/12/2023 7:57 am
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