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Repairing Hi-End Preamplifier
Repairing Hi-End Preamplifier
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Old 2nd April 2018, 11:49 AM   #1
jahsavage is offline jahsavage  United Kingdom
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Default Repairing Hi-End Preamplifier

Hi All

I have a preamplifier that died on me while reconnecting a cable from its power supply. Its a Hi End commercial product and about 12 years old so I have no circuit diagrams. I photographed each side of the circuit board and went about drawing it myself. I was hoping to find a match on line to aid debugging it but this is a strange one. Anyway I discovered all the diodes/zener diodes from the red input supply line to pins 1 and 9 are all short circuited, they have the codes SX15, TJ20, QR35, TJ20 but I'm unable to match them against anything so I'm unsure if they are zener or straight diodes, can anyone identify them or shed some light on the circuit operation so I might be able to take a guess?
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Old 2nd April 2018, 12:31 PM   #2
petertub is offline petertub  Sweden
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What brand is it, and what does the maker say about repair or schematics ?
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Old 2nd April 2018, 01:03 PM   #3
jahsavage is offline jahsavage  United Kingdom
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So I've sent two emails so far, one to a technical address and one to general info but after 2 weeks I have yet to receive a reply. I don't wish to embarrass the manufacturer and hope to eventually get some information but you would hope for better service from a company that sales products that costs tens of thousand of pounds.

I do understand they probably want me to go through their dealer but the costs do not make sense for amplifier of this age. I will fire off an email to them but since I was not the original customer I'm not expecting much, but I also don't have much to loose!

I'm approaching retirement and wish to enjoy my equipment for many years into the future I need to be able to repair it myself so this is a good trial for what I can expect to run in to. Fortunately I'm not approaching this subject from cold since I have a degree in electronics and in my early 20s built many audio components, I admit I'm rusty but keen to re-learn and spend my retirement building tube amps and enjoying the equipment I've already purchase.
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Old 2nd April 2018, 01:35 PM   #4
Mooly is offline Mooly  United Kingdom
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Repairing Hi-End Preamplifier
I don't think naming the manufacturer embarrasses anyone tbh. If you hope to fix this yourself then you need as much info as possible. And so do we if we are to advise
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Old 2nd April 2018, 02:45 PM   #5
rayma is offline rayma  United States
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Quote:
Originally Posted by jahsavage View Post
I discovered all the diodes/zener diodes from the red input supply line to pins 1 and 9
are all short circuited, they have the codes SX15, TJ20, QR35, TJ20
I'd say those are all current regulator diodes. The plate is loaded with a CCS diode (two in parallel).
The power supply regulator is a series CCS diode (two in parallel), and a shunt Zener (three in series).

In a pinch you can use appropriate resistors instead.

Last edited by rayma; 2nd April 2018 at 03:09 PM.
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Old 2nd April 2018, 02:47 PM   #6
vinylkid58 is online now vinylkid58  Canada
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Quote:
Originally Posted by jahsavage View Post
So I've sent two emails so far, one to a technical address and one to general info but after 2 weeks I have yet to receive a reply. I don't wish to embarrass the manufacturer and hope to eventually get some information but you would hope for better service from a company that sales products that costs tens of thousand of pounds.
If a manufacturer is still in business, and won't reach out to a customer that needs help in a timely manner, then they should be called out publicly.

jeff
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Old 2nd April 2018, 03:02 PM   #7
jahsavage is offline jahsavage  United Kingdom
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I'll give the local dealer and manufacturer a little bit longer before naming names and burning that potential bridge.

On technical level I expect to see a first pass filter after rectification but on this amp we have a 20uf and a pair of diodes/zeners? So are we using their forward impedance as part of the filter calculation or does this confirm they must be zeners?
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Old 2nd April 2018, 03:07 PM   #8
rayma is offline rayma  United States
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Quote:
Originally Posted by jahsavage View Post
I expect to see a first pass filter after rectification but on this amp we have a 20uf and a pair of diodes/zeners? So are we using their forward impedance as part of the filter calculation or does this confirm they must be zeners?
See post #5.
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Old 2nd April 2018, 03:31 PM   #9
Alllensoncanon is online now Alllensoncanon  United States
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Repairing Hi-End Preamplifier
As Ray Ma's post you have two Zener and two Current Regulating Diode. Something like these

https://www.mouser.com/datasheet/2/362/crd-1407.pdf

These device have a 100V limit. My guess is the Zener is there to drop the voltage, so these CRDs won't blow up on power up.

I would take a meter and measure the other channel (which I suppose have not also blow up) Cathode Current and the B+ between the two set of diode. From that you can order the appropriate replacement.

Last edited by Alllensoncanon; 2nd April 2018 at 03:34 PM.
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Old 2nd April 2018, 03:39 PM   #10
rayma is offline rayma  United States
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Quote:
Originally Posted by Alllensoncanon View Post
My guess is the Zener is there to drop the voltage, so these CRDs won't blow up on power up.
The rightmost pair of series CCS, together with the shunt Zeners, form a regulated power supply.
The leftmost pair of CCS are the plate load. The two values are different. The plate load CCS is
smaller in value than the Zener CCS.
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