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Old 23rd May 2021, 12:42 pm   #21
PJL
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Default Re: Fisher 400 output valves current draw

Very confusing as the component numbering is even different from the one I am using that is marked as 'serial numbers from 20001 to 29999'. What serial number is this one?

Lets find out why the screen is low first as it suggests something is taking too much current or the resistors in the chain are out of spec. Based on the schematic R105 (1.2K) is dropping around 129(50)V which implies a current of 107mA over twice the expected. I would have expected it to get VERY hot so it may be out of spec or the circuit may be different.

After that, it might be worth trying to get the LT 5.95(6.3) up a bit by reducing the resistor in the mains input.

Last edited by PJL; 23rd May 2021 at 12:54 pm.
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Old 23rd May 2021, 2:07 pm   #22
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Default Re: Fisher 400 output valves current draw

Version no. 48001 on.

Yes all very confusing...
Alan
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Old 23rd May 2021, 3:20 pm   #23
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Default Re: Fisher 400 output valves current draw

The serial number is 56828 V.

I am finding the actual layout difficult to follow. The voltage on the output from CR1 is 375v. I am assuming that R105 (the 1.2K one feeding the screens) is connected to the screen leads but the voltage on the screen lead side of it is 245V and on the other side is 227V. I cannot see where the connections of C75 get their power from as I cannot see a connection. All very confusing!

I am attaching some photographs in case they may be of help. Please excuse the soldering of the 10 ohm resistors, it was meant to be just temporary.
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Old 23rd May 2021, 3:58 pm   #24
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Default Re: Fisher 400 output valves current draw

Using the correct circuit diagram, that should be R41 (1.2K) and C52 (the multi section one)
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Old 23rd May 2021, 4:31 pm   #25
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Default Re: Fisher 400 output valves current draw

Quote:
Originally Posted by PaulR View Post
... I cannot see where the connections of C75 get their power from as I cannot see a connection. All very confusing! ...
It certainly is!
Is the problem finding the connection a physical one on the amplifier or on the schematic?

Apologies if you are aware of this, but the HT+ is a 'voltage doubler' circuit via CR2 and CR3 together with C56 and C57C. The capacitors need to be impeccable in this set up, loss of capacity looses you volts and any electrical leakage draws extra current and looses you more volts. What voltage do you get at the junction of the transformer tap and R43 (196V on the diagram)?

There is a lot of info on AudioKarma https://audiokarma.org/forums/index....er-400.689121/ which might help with locations.

Is it possible to have a picture of the mains transformer and a measurement of that extra resistor?
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Old 23rd May 2021, 5:23 pm   #26
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Default Re: Fisher 400 output valves current draw

It is the physical connections I am finding difficult to follow. I can see what looks like R41 leading to the screen connections and I can see CR2 connecting to the output transformers but I cannot see how CR2 connects to R41. What I am taking to be R41 has a lower voltage on the other side from the screen connection than the screen voltage itself.

The link is useful. I see that the multi can has been replaced. I have not done mine as it looked difficult to find a place for all those caps and there is no sign of leakage. There is absolutely no hum.

I am attaching more photos. The first is the underside of the multi cap. I am assuming that the brown red red horizontal resistor is R41. The orange lead goes to the screen connections.

The dropper resistor measures as 105 ohms.

The screws, or at least the washers, on the mains transformer do not look original but there are no other markings on it.
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Old 23rd May 2021, 6:05 pm   #27
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Default Re: Fisher 400 output valves current draw

Quote:
Originally Posted by PaulR View Post
It is the physical connections I am finding difficult to follow. I can see what looks like R41 leading to the screen connections and I can see CR2 connecting to the output transformers but I cannot see how CR2 connects to R41. What I am taking to be R41 has a lower voltage on the other side from the screen connection than the screen voltage itself.

The link is useful. I see that the multi can has been replaced. I have not done mine as it looked difficult to find a place for all those caps and there is no sign of leakage. There is absolutely no hum.

I am attaching more photos. The first is the underside of the multi cap. I am assuming that the brown red red horizontal resistor is R41. The orange lead goes to the screen connections. ...
The screen resistor R41 is the large green or brown job (see the last picture in post #5 of the AudioKarma link). The resistor you are referring to is R40.
The caps do not need to be physically leaky to be electrically leaky. This receiver runs very hot as normal, I would be planning replacements regardless.

Quote:
Originally Posted by PaulR View Post
The dropper resistor measures as 105 ohms.

The screws, or at least the washers, on the mains transformer do not look original but there are no other markings on it.
That is the most appalling thing! If that is in the mains side of the transformer it is a real safety hazard. Exposed terminals and no grommets where the wires pass through the chassis! Also it is gently cooking the capacitor block behind it, as if there wasn't enough heat in there already.

And just to note the valves in the picture have clearly had a long and possibly hard life, the gettering is fading to brown.
Alan

PS, that does look like a proper Fisher mains transformer and looking at pictures on the net so is the mounting hardware. But is it a 240 volt item, I do not know...
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Old 23rd May 2021, 6:25 pm   #28
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Default Re: Fisher 400 output valves current draw

The dropper resistor is attached to that aluminium sheet sheet with a Spire Nut-clip that's been re-purposed.

The aluminium sheet is the heat shroud which should be around the valves, but someone has bent it outwards away from them to make space for that dropper resistor.

Someone has really beggered about with this unit.

If the heater volts are too low, then presumably the primary folts are too low as a quick estimate you said 5.9v instead of 6.3, so then measure the volts on the primary as it is, then multiply by 6.3/5.9 to get the proper primary voltage it should have. It's about 7% out. The resistor could be a bodge to use a wrong transformer like a 220v one.

Those washers on the transformer do look wrong. Americans are not generally sloppy about hardware and I'd have expected those UNC nuts with captive crinkle washers.

David
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Old 23rd May 2021, 7:03 pm   #29
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Default Re: Fisher 400 output valves current draw

Thank you Alan, the connections make sense now. R41 measures 9K.

Yes that resistor is on the mains side of the transformer.

The voltage on the primary is 205v multiplying by 6.3/5.9 gives 219v which bears out your hypothesis David.

So where do we go now? Are there 240 to 220v transformers available at a reasonable price? I have found 230 to 220 but I cannot find the correct one. I realise that there are other things to sort out but ,maybe we should get the power supply sorted first. I take the point about the valves but if I can more or less balance them on each channel at least to start with I think I would leave it at that for the time being.
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Old 23rd May 2021, 7:15 pm   #30
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Default Re: Fisher 400 output valves current draw

The transformer issue, you have to get rid of that resistor it is a safety problem.
Easiest way is to have an external bucking transformer, see here https://sound-au.com/articles/buck-xfmr.htm The figure 4 method is preferred.
Problem is until you get the current flow correct you may have to juggle the final bucking transformer voltage choice.

I wonder why R41 is such a high reading, has it been replaced or just tired? Fix that first, it may well improve the screen volts as advised. Problem then is that it will draw more current and may result in lower HT volts, if the rest of the 'system' is tired.
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Old 23rd May 2021, 7:21 pm   #31
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Default Re: Fisher 400 output valves current draw

I like it when a hypothesis comes together.... 220v is an American sort of primary. That resistor bodge wouldn't be there if there were the right primary tappings available. Running it on 220v from an autotransformer or bucking transformer is asking for future mistakes and accidents.

The proper way out would be to see if Ed or Mike would re-wind it for you with a proper 240v primary.

But that depends on how much value you place on the set.

David
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Old 23rd May 2021, 7:56 pm   #32
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Default Re: Fisher 400 output valves current draw

I have just spoken to my son about it as it is his. He knows of somewhere that would supply a 240 to 220 self contained transformer but I agree that the more elegant solution would be to get the existing one altered.

I will make enquiries about having it rewound. In the meantime I might as well continue with it as it is, being careful of that dropper. To make matters worse it doesn't have a case so the connections on the dropper would have been very easy to touch.
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Old 23rd May 2021, 8:16 pm   #33
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Default Re: Fisher 400 output valves current draw

So next step is to measure R41 which should be the big orange one and should measure 1.2K but must be much higher based on your voltage measurements. You should disconnect one end to get an accurate figure.

According to the service sheet it should have 40V across it. The wattage dissipated by the resistor is W=V*V/R which works out as 1.33W but I would play safe and use a wire wound 3W or even 7W resistor.

The problem with the Fisher is the correct 7868 valves are pretty much unattainable so don't worry about the mains transformer until it is all working correctly.

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Old 23rd May 2021, 8:21 pm   #34
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Default Re: Fisher 400 output valves current draw

R41 measures 9k. I hadn't realised that wirewound resistors would alter so much. I have just ordered a 3w replacement. I have also asked Ed if he can give me an idea of the cost for rewinding the mains transformer.

Progress, hopefully. The help given on this forum is invaluable.
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Old 23rd May 2021, 8:51 pm   #35
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Default Re: Fisher 400 output valves current draw

Wirewound resistors fail open circuit but solid carbon resistors do change value with time or if they are mistreated.

To complete a full restoration you should check all the resistors and replace ones out of specification particularly as this is a stereo amplifier and you want both channels to match! Again, I would hold on till you have decided on the replacements for the 7868's as the costs will start to spiral.

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Old 23rd May 2021, 9:23 pm   #36
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Default Re: Fisher 400 output valves current draw

Thank you. I have had a reply from Ed regarding the transformer and will pass it all on to my son as he will be paying.

Any thoughts on these Russian replacements? https://valvetubeguitaramps.com/prod...-brimar-el506/
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Old 23rd May 2021, 10:28 pm   #37
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Default Re: Fisher 400 output valves current draw

Personally, I would go for the bucking transformer solution, it requires a fairly small standard low voltage transformer that is wired out of phase to the mains so reducing the input voltage to the transformer. The Fisher amps are quite collectable and buyers are likely to want it to be original.

Can't comment on the valves but they need to be a very similar specification as the amplifier has a fixed bias that is not adjustable and cannot be made more -ve.

Lets see what the situation is with the replacement resistor.
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Old 24th May 2021, 7:05 am   #38
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Default Re: Fisher 400 output valves current draw

No adjustable bias, that's a bit cheap. One could swop out R35 for a 1k pot or better still swop out R34 & R35 for a pot & two resistors. I have some nice Bourns 1k 3W linear presets if you wanted to do that Paul.

It does look very untidy, a bit of tidying up wouldn't go amiss, nothing drastic just straightening things out a bit to help layout ID.

Andy.
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Old 24th May 2021, 9:22 am   #39
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Default Re: Fisher 400 output valves current draw

Quite a lot to think about!

I am more inclined towards having the mains transformer rewound. The existing one is neither one thing nor another. If it was 110V I would leave it alone and suggest that my son gets a step down transformer. I realise that they have 220V in the USA for higher power equipment but this set is marked 110V so the existing transformer is non-standard. If it had a 240V one at least it would be standard for this part of the world and could be used as it is. I think he bought it from abroad but when I asked my son where it came from he could not remember. The replacement Fisher output transformer came from Canada but I am pretty sure that the set came from somewhere else.

Regarding the bias supply, I agree that it would be good to make this adjustable and I may well take Andy up on his kind offer. There isn't a lot of space in the relevant area. I restuffed C60 but I could leave the new caps "bare" to create more space for a pot. I wondered about replacing R138 with a pot as there is more space but that looks pretty hefty so I am not sure whether a suitable pot would be available. I did replace it with a 22 ohm one in an attempt to increase the bias a bit when I replaced the caps and the bridge rectifier.

I agree it is untidy, particularly around the output valve bases but I just tacked those 10 ohm resistors in as an initial test of the valve current draw and things took off from there!
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Old 24th May 2021, 9:53 am   #40
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Default Re: Fisher 400 output valves current draw

Paul R,
Just to mention the original R41 is a 7 Watt resistor.

The Electro Harmonix 7868 are about the only new made versions available, little choice unless you can afford NOS USA tubes.

Until you get the negative bias voltage up, there is little point making it adjustable. Did you replace the bridge CR1 earlier?
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