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Apt Holman preamp problem
Apt Holman preamp problem
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Old 16th October 2019, 11:20 PM   #11
rayma is online now rayma  United States
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Make sure they really do have different amplitudes, by connecting the same signal
to both scope inputs. The two displayed sines should then be the same size.
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Old 17th October 2019, 10:37 AM   #12
georgesgiralt is offline georgesgiralt  France
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So the input of the phono preamp is the same for both channels. See the picture.
Rayma, the scope and probes are "perfect" for both channel. It is the phono preamp which is at fault. I can ear it so it is quite important. ;-)
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File Type: jpg IMG_20191017_103940.jpg (716.5 KB, 151 views)
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Old 17th October 2019, 11:33 AM   #13
sgrossklass is offline sgrossklass  Germany
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Could you get the signal out of the tape loop so as to determine approximate gain at 1 kHz? It would be interesting to know whether the "loud" or the "quiet" channel is the faulty one.

Have you already tried feeding in the signal after the input selector switch's phono-related section, just to rule that out? Repeat at R6/106 to rule out C12/112 as well. There's also a phono balance related section on the input selector, bad contact there could reduce right channel gain as well... but you said it's the left channel that is weaker.

At the end of the day I would not be too surprised if it turned out to be a bad C13, C17 or perhaps C12... regular polar electrolytics that have been kept at the mV level for 4 decades are, in all likelihood, toast or at least in need of some serious reforming. I would only trust dedicated low leakage types like the orange Elnas from back in the day to make it, and those tended to be used in sub-10 F sizes, not like anything in question.

It can't hurt to check solder joints on all the components in the feedback network... 7 passives per side, that should still be manageable.

Last edited by sgrossklass; 17th October 2019 at 11:38 AM.
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Old 17th October 2019, 01:29 PM   #14
georgesgiralt is offline georgesgiralt  France
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Hello,
Thanks for your help. I removed C13 (easiest for me) and tested it. It is toast. So I ordered all the phono preamp electrolytic and the power supply caps (I have nothing in stock right now). I change them and keep you posted.
Have a nice day !
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Old 18th October 2019, 05:09 AM   #15
georgesgiralt is offline georgesgiralt  France
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Hello !
Yesterday, I got an Email stating that RAYMA has answered on this thread.
The post did not show on the site ???? Lost in cyberspace ?
Here it is :

---Quote (Originally by georgesgiralt)---
So the input of the phono preamp is the same for both channels. See the picture.
Rayma, the scope and probes are "perfect" for both channel. It is the phono preamp which is at fault. I can ear it so it is quite important.
---End Quote---
Very good. Now we must determine if it is a simple gain difference , or an RIAA frequency difference.
***************


I did some more tests to answer this :
I varied the frequency of the signal generator from 50 Hz to 16 kHz. Alas, nothing significant change. See the pictures. First is 50 Hz, 100Hz, 400 Hz, 800 Hz, 4kHz, 10 kHz and 16 kHz. I omit the 1kHz because it was that frequency I previously used. I have corrected the gain of the scope to have the trace as wide as possible. Thanks for your help.
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File Type: jpg IMG_20191017_193702.jpg (828.6 KB, 120 views)
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File Type: jpg IMG_20191017_193614.jpg (798.4 KB, 95 views)
File Type: jpg IMG_20191017_193939.jpg (735.0 KB, 13 views)
File Type: jpg IMG_20191017_194005.jpg (708.8 KB, 13 views)
File Type: jpg IMG_20191017_194027.jpg (819.4 KB, 15 views)
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Old 18th October 2019, 02:42 PM   #16
rayma is online now rayma  United States
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Quote:
Originally Posted by georgesgiralt View Post
I got an Email stating that RAYMA has answered on this thread.
The post did not show on the site ???? Lost in cyberspace ?
I deleted that post after seeing that you found bad capacitors, which could be the problem. Good luck.
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Old 21st October 2019, 06:08 PM   #17
georgesgiralt is offline georgesgiralt  France
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Hello ! The worst capacitor is C17. It has leaked and shows around 30 pF capacity. By comparison, C13 is quite new ;-)
I've replaced both and the preamp is PERFECT. (my phone did not register the image I took of the scope... Grrr)
In order to keep things symmetric, I exchanged C113 and C117 which were not bad but not pristine either...
I've refrained to change other capacitors, even if I've them.
I'll keep this for next time ...
I'm listening to it now. "La forza del destino", by Mr Verdi an RCA red Label...
Happy to have recovered this fine piece of gear !

I'll thank you ALL for your help ! I'm very bad at troubleshooting, so your help was INVALUABLE !
Thank you !
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Old 21st October 2019, 06:16 PM   #18
stratus46 is offline stratus46  United States
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Quote:
Originally Posted by georgesgiralt View Post
Hello,
Thanks for your help. I removed C13 (easiest for me) and tested it. It is toast. So I ordered all the phono preamp electrolytic and the power supply caps (I have nothing in stock right now). I change them and keep you posted.
Have a nice day !
I think if you had used a square wave rather than sine you would have seen a larger difference between the channels. Remember squares will give insight 1/10 to 10 times the base frequency which can be very useful for comparing response. 200Hz to 1 KHz will tell you lots.

G
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Old 21st October 2019, 07:35 PM   #19
georgesgiralt is offline georgesgiralt  France
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Well I doubt it. I would not have found the link between the low output and C13....
When I say I'm not good at troubleshooting ....
You will have found the flaw and the problem, not me. This is why I asked for help. And help I got !
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Old 24th October 2019, 09:14 AM   #20
georgesgiralt is offline georgesgiralt  France
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Hello,
I've another question about this preamplifier.
Given the 10 F capacitor failed badly (leaked totally) and that there are a bunch of them inside, will it be wise to replace all of them now or wait for them to fail in the "if it is not broken, don't fix it". And the 100F are also in the line for replacement...
Thanks for your help !
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