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APT Holman preamp manual, schematic?
APT Holman preamp manual, schematic?
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Old 9th October 2019, 10:06 AM   #21
georgesgiralt is offline georgesgiralt  France
diyAudio Member
 
Join Date: Jul 2008
Hello
Here am I again with my problem. Still present ...
What I've done so far :
Turntable side
Checked wires, cables and carts. Every resistance of cables and cart are the same on right and left channel. On the 3 carts I own. So far, so good.
Holman preamp side
Cleaned all switches and pots. Cleaned all adjustable pots. Device seems to be untouched, electrolytic caps are original (as far as I can judge, from the photos on Internet).
The left channel is still lower in output than the right channel even using a mono record (I own a couple). More importantly, the default is present on Phono 1 and on Phono 2 input !
So any advice and troubleshooting tips are very welcome !
Thanks a lot in advance !
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Old 9th October 2019, 02:18 PM   #22
seanc is offline seanc  United States
diyAudio Member
 
Join Date: Mar 2009
Quote:
Originally Posted by georgesgiralt View Post
Hello
Here am I again with my problem. Still present ...
What I've done so far :
Turntable side
Checked wires, cables and carts. Every resistance of cables and cart are the same on right and left channel. On the 3 carts I own. So far, so good.
Holman preamp side
Cleaned all switches and pots. Cleaned all adjustable pots. Device seems to be untouched, electrolytic caps are original (as far as I can judge, from the photos on Internet).
The left channel is still lower in output than the right channel even using a mono record (I own a couple). More importantly, the default is present on Phono 1 and on Phono 2 input !
So any advice and troubleshooting tips are very welcome !
Thanks a lot in advance !
Take a systematic approach:

1) Skip the turntable, feed it a high level source: CD, tuner, etc

If problem gone, problem is in phono stage. If problem persists ...

2) disable "Infrasonic Filter" switch on back of unit (button out).

If problem gone, problem is in infrasonic filter circuit. If problem persists ...

3) Flip both "R>L" and "L>R" switches up.

If channel imbalance changes to other channel, problem is in Tape Monitor or Ext Processor switches. If no change in channel imbalance ...

4) flip "Tone Defeat" switch up.

If problem gone, problem is in the tone control stages

If problem persists, it could be in the Volume , Mode or Balance control stages, or the final output buffers/relay, and further testing needed.
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Old 9th October 2019, 04:18 PM   #23
georgesgiralt is offline georgesgiralt  France
diyAudio Member
 
Join Date: Jul 2008
So I tried to put a CD with 1kHz mono signal in. Both channels are OK on whatever high input I use.
So IMHO, the problem lies in the phono input circuitry.
I'll try to put a signal generator at the phono input and check the infrasonic switch and/or the left level at the output of the phono preamp.
Thanks for your help !
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Old 9th October 2019, 04:37 PM   #24
seanc is offline seanc  United States
diyAudio Member
 
Join Date: Mar 2009
Quote:
Originally Posted by georgesgiralt View Post
So I tried to put a CD with 1kHz mono signal in. Both channels are OK on whatever high input I use.
So IMHO, the problem lies in the phono input circuitry.
I'll try to put a signal generator at the phono input and check the infrasonic switch and/or the left level at the output of the phono preamp.
Thanks for your help !
The infrasonic switch comes AFTER all the inputs, so if CD was OK, the infrasonic filter is not the cause.

Does your turntable/cart work OK with other preamps?

How bad is the imbalance? You haven't said if it's small or large.

If small, it's possible that the previous owner may have adjusted the phono balance trimpots to match his cartridge. Have you tried adjusting these? (try to remember their initial setting if it doesn't help).
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Old 9th October 2019, 05:42 PM   #25
georgesgiralt is offline georgesgiralt  France
diyAudio Member
 
Join Date: Jul 2008
Hello,
First, the turntable played good on a Luxman L-2 integrated amp. I bet I unsoldered one wire when I switched to the Holman. Now this is repaired. Both cables and carts are OK. Switching input cables right/left make no change. The left channel is still the lowest.
Next, I tried various High level sources : Tuner, set top box, CD player and all high level inputs (tape untested) are OK.
The Phono input left channel loudness is very low compared to the right channel. On both phono channels. (it is sufficiently lower to be easily heard on every record played. be it stereo or mono).
I've cleaned ALL switches using KF Contact F2 product, and all pots and adjustable.
The balance adjustable resistor in the phono preamp where set at the middle. I cleaned them, and set them back to middle. I tried to fiddle with them to correct the imbalance but to no avail.
I've repaired the BF Philips gen and made the Tektronic scope back in use by removing and plugging back all cards. It runs fine now.
If tme permits, I'll try to set up a measuring jig and troubleshoot it tomorrow...
All idea you may have won't be overlooked ! Thanks for your response.
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