I have a Precision Fidelity C7A Revised that I acquired around 1981. I was looking at it, and the associated schematics, with the thought of replacing the almost 45 year old electrolytics.
Based on what I see on the Web, it is most likely a C7 that was modified around 1981 to C7A Revised specs (the schematic indicates that the Revised circuit was introduced with serial number 81221 - presumably 81 = 1981. Mine is 79xxx. The channels are side by side, and run front-to-back, and it has a C7A Revised faceplate and volume knobs. I don't know if it was a factory retrofit, or modified by someone else.
Based on schematics available online, it definitely has the Revised circuit. The LM723 regulator is gone, having been replaced with stacked zener diodes, the additional electrolytics shown in the schematics are there, etc.
On anomaly: my unit doesn't have C102, a 25 uF 450 v electrolytic that bypasses the two zeners. I certainly didn't miss it for 40+ years.
Can anyone fill in any information about this? Is the unit actually missing a (somewhat non-essential) capacitor, or is the schematic incorrect?
Also, any knowledge of whether during the early 1980s, Precision Fidelity performed any retrofits of the Revised circuit to original C7s?
Thanks.
Based on what I see on the Web, it is most likely a C7 that was modified around 1981 to C7A Revised specs (the schematic indicates that the Revised circuit was introduced with serial number 81221 - presumably 81 = 1981. Mine is 79xxx. The channels are side by side, and run front-to-back, and it has a C7A Revised faceplate and volume knobs. I don't know if it was a factory retrofit, or modified by someone else.
Based on schematics available online, it definitely has the Revised circuit. The LM723 regulator is gone, having been replaced with stacked zener diodes, the additional electrolytics shown in the schematics are there, etc.
On anomaly: my unit doesn't have C102, a 25 uF 450 v electrolytic that bypasses the two zeners. I certainly didn't miss it for 40+ years.
Can anyone fill in any information about this? Is the unit actually missing a (somewhat non-essential) capacitor, or is the schematic incorrect?
Also, any knowledge of whether during the early 1980s, Precision Fidelity performed any retrofits of the Revised circuit to original C7s?
Thanks.
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I've attached the schematics that came with the preamp, but I've seen the same ones all over the Web. I also read somewhere on-line that the available schematics could have inaccuracies.
I fished out the paperwork that came with the unit: It is "C7A Preamplifier Owners Manual", a cut sheet "C-7A Revised Preliminary Literature", the schematics (including binder marks on the edges) and the original warranty card, dated 11/18/81.
Based on the apparent 1979 serial number, the included preliminary Revised literature, the "first generation" (front-to-back) PCB, the Revised faceplate and the original purchase date of 1981, I suspect that this was a factory update of a C7A. But who knows at this point.
But back to the missing C102 - it is a very quiet unit, even without the bypass capacitor. But I'm curious: did someone forget to install it, or is the schematic misleading...
I fished out the paperwork that came with the unit: It is "C7A Preamplifier Owners Manual", a cut sheet "C-7A Revised Preliminary Literature", the schematics (including binder marks on the edges) and the original warranty card, dated 11/18/81.
Based on the apparent 1979 serial number, the included preliminary Revised literature, the "first generation" (front-to-back) PCB, the Revised faceplate and the original purchase date of 1981, I suspect that this was a factory update of a C7A. But who knows at this point.
But back to the missing C102 - it is a very quiet unit, even without the bypass capacitor. But I'm curious: did someone forget to install it, or is the schematic misleading...
On the C7/C7A/C7A revised power supply schematic it says that after serial number #81211 the revision A circuit is used. They include a table of components to add and delete. I assume deleting means remove or don't fit.
Currently I would expect R50, Q1, R51, C103, Q2, C19C, C9 to be present. What is in place of R52 either a link or a 10R half Watt resistor I don't know, most likely a resistor.
C101 if it is present should not be connected to the Emitter of Q2 but it can be connected the other side of R52 if its a 10R resistor.
All other components including U1 LM723 should be absent that is apart from CR10 and CR11, added together CR10 and CR11 have a Zener voltage of 320 Volts, they are usually a little higher and will probably provide the 325 Volt supply (or close enough).
Q1 and Q2 are followers they just follow the Zener voltage.
As to whether C102 should be fitted - it was probably there to slow down the rise time of the +325V supply line. The time constant of C102 and R50 is about half a second. But C102 could possibly also stress Q50 when the power turns off if it turns off quickly (fault condition).
So probably C102 was first suggested to slow down the +325 Volt supply rise time, then somebody realised it was not needed or it caused problems and was left out.
Have a look at your serial number if its well after #81211.
C102 is not there to improve noise it will not do that.
If the amp has been working for years without C102 then leave it out.
As regards replacing Electrolytics C19A, C19B and C19C would be the important ones so it might be a good idea to replace them they can be 33uF 630 VDC or 47uF 630 VDC.
There seems to be a second fragment of circuit on the Right Channel schematic suggesting that the +325V supply line goes through a 1K resistor to a fourth smoothing capacitor. If this is present replace it with 33uF 630 VDC or 47uF 630 VDC.
C20A, C20 and C21 are all there to smooth the low voltage DC heater supply. They are very high values the makers did not want any mains hum.
Whether you replace them is debatable. If hum is not a problem and CR5, CR6, CR7 and CR8 are not unduly hot also C20A, C20 and C21 are cool, seems there is little reason to replace them.
I don't see any other electrolytics but maybe there are?
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I agree with all of the above.
As I noted, I don't consider C102 essential (and per your comment, possibly detrimental - thanks) and didn't miss it for 40+ years. But as a point of curiosity, I wasn't sure about the accuracy of the schematic.
The deletions/additions you describe through analysis of the schematics are correct. I previously verified them by comparing the schematics and the actual unit. (The 47 uF caps shown in the boxed section of the schematic are next to V2 and V3.)
The serial number on mine is well below the start of "official" Revised production. But it does have the Revised version faceplate/knobs, and the Revised circuit (on an early, modified, PCB - I became aware that different PCB versions existed when the Web became a thing and photos became available) [except, as noted, C102].
Yes, 630 V parts are a good idea.
Finally, I plead guilty to conflating two topics in the original post:
(1) C102 vs the schematic;
(2) The provenance of this particular unit. The serial number, considered in light of the purchase date, and the Preliminary Revised cut sheet that came with the unit, seem to indicate a possible factory update of a C7A to the final version in order to sell it as a Revised. Having spoken with the first owner many years ago, I don't think he personally modified it, or had it modified by a third party.
As I noted, I don't consider C102 essential (and per your comment, possibly detrimental - thanks) and didn't miss it for 40+ years. But as a point of curiosity, I wasn't sure about the accuracy of the schematic.
The deletions/additions you describe through analysis of the schematics are correct. I previously verified them by comparing the schematics and the actual unit. (The 47 uF caps shown in the boxed section of the schematic are next to V2 and V3.)
The serial number on mine is well below the start of "official" Revised production. But it does have the Revised version faceplate/knobs, and the Revised circuit (on an early, modified, PCB - I became aware that different PCB versions existed when the Web became a thing and photos became available) [except, as noted, C102].
Yes, 630 V parts are a good idea.
Finally, I plead guilty to conflating two topics in the original post:
(1) C102 vs the schematic;
(2) The provenance of this particular unit. The serial number, considered in light of the purchase date, and the Preliminary Revised cut sheet that came with the unit, seem to indicate a possible factory update of a C7A to the final version in order to sell it as a Revised. Having spoken with the first owner many years ago, I don't think he personally modified it, or had it modified by a third party.
Thanks for clarifying comments. I think the schematic should look more like this:
R52, C101 and C19C should be present as drawn.
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And the Phono Preamp circuit, with the additional resistor and capacitor:
Modifications in the days of paper circuit diagrams were always a problem and sometimes added reluctantly. Even then (as happens a lot these days) modifications become the release of a new model or a so called "facelift" model and are part and parcel of increasing sales. The mods seen here are typical of what happened in the past (before standards like ISO9000 etc) and to me look perfectly genuine and done in good faith.
I'm glad to see this circuit. I have been wondering about my current work where I plan to use a maximum of two Triodes per channel, so to see a preamp that uses six Triodes per channel to equalise and amplify a phono input to line voltage is very encouraging! (it took me a while to realise that nothing is done to Tuner, Aux and Tape In).
That above said, this entire C7A design has a very cautious approach, the way the Heater supply is rectified and smoothed is an example of extreme caution.
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Modifications in the days of paper circuit diagrams were always a problem and sometimes added reluctantly. Even then (as happens a lot these days) modifications become the release of a new model or a so called "facelift" model and are part and parcel of increasing sales. The mods seen here are typical of what happened in the past (before standards like ISO9000 etc) and to me look perfectly genuine and done in good faith.
I'm glad to see this circuit. I have been wondering about my current work where I plan to use a maximum of two Triodes per channel, so to see a preamp that uses six Triodes per channel to equalise and amplify a phono input to line voltage is very encouraging! (it took me a while to realise that nothing is done to Tuner, Aux and Tape In).
That above said, this entire C7A design has a very cautious approach, the way the Heater supply is rectified and smoothed is an example of extreme caution.
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Precision Fidelity C7A tubes: 4 x 12AX7 (ECC83) and 2 x 12AU7 (ECC82).
The heaters for each tube are wired in series so the heater voltage is 12.6 Volts. So C20A, C20 and C21 would need to be at least 16VDC (as they are now).
A photo of Precision Fidelity C7 I think - from another thread:
The PCB is very nicely and carefully laid out with Ground Plane everywhere to avoid noise pickup etc.
The LM723 as was well never intended by TI to be used for such a high voltage - a bad idea from the start, here is TI's idea for using the LM723 for high voltage:
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The heaters for each tube are wired in series so the heater voltage is 12.6 Volts. So C20A, C20 and C21 would need to be at least 16VDC (as they are now).
A photo of Precision Fidelity C7 I think - from another thread:
The PCB is very nicely and carefully laid out with Ground Plane everywhere to avoid noise pickup etc.
The LM723 as was well never intended by TI to be used for such a high voltage - a bad idea from the start, here is TI's idea for using the LM723 for high voltage:
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I had this pre more than 25 years ago: the power supply gave a lot of trouble with the 240V UK supply. I took it to Bill Beard to see if he could repair it. [He tested it whilst I was there
and I remember him being amazed by the almost limitless headroom]
What I do remember is that many of the schematics available on the web - back about 15 years ago - were incorrect and were causing people problems.
My suggestion is that if you need to alter the factory transformer voltage have a new transformer made to suit your local mains... and totally rebuild the PS.
You will then have a decent basis on which to update the signal circuits of what was a really nice preamp.
and I remember him being amazed by the almost limitless headroom]
What I do remember is that many of the schematics available on the web - back about 15 years ago - were incorrect and were causing people problems.
My suggestion is that if you need to alter the factory transformer voltage have a new transformer made to suit your local mains... and totally rebuild the PS.
You will then have a decent basis on which to update the signal circuits of what was a really nice preamp.