APT Holman preamp manual, schematic?

sdstarnes

Member
2007-12-20 1:31 pm
Hey all,

Newbie here. Glad to have found this forum as I am puting a "vintage" system together and will be most likely needing some help.

Just purchased an APT Holman preamp, and Adcom 535 amp off of Craig's List. Does anyone have access to schematics and/or owner's manual for the APT Holman pre amp? Can they be bought new? Any "tweaks" for the pre amp?

Thanks!
 

sealevel

Member
2008-01-04 6:19 pm
Not so much a tweak, but from what I've seen on the net, the mute
circuit is the problematic area of this pre-amp. Thats why I know
where the schemo is, as mine is suffering from this issue. Haven't had
the time to trace thru and fix it yet. Otherwise and outstanding pre-amp.

sealevel
 

unclejed613

Member
2006-12-28 12:19 am
sealevel, if the mute circuit is causing a channel (or both) to drop out, here's the factory fix (i worked as the customer service tech at APT for a few years):

1)remove the cover from the relay
2) put a piece of brown paper bag between the contacts of the relay
3) hold the contacts closed and use the brown paper to burnish the contacts.

4) reassemble the relay
 

sealevel

Member
2008-01-04 6:19 pm
Thanks!

I'll step thru the relay contact cleaning sequence.
When it initially began, cycling the mute switch a few
times would correct the no/min output, but as time
(9 months) went on, it no longer responded to the
cycling. Had to slide a crown ic-150 into its place.


sealevel
 

sdstarnes

Member
2007-12-20 1:31 pm
Noise while turning system power off.

I have noticed that when I shut down or power on, amp, preamp, etc, there is a very noticeable buzzing noise that comes through the speaker. When I repeat the process with the mute button pressed in there is no noise. Also, is there any way to replace the APT preamp power cable with a 3 conductor type? This one sees to be somewhat worn.

Thanks!
 

rogerbent

Member
2006-08-13 3:30 am
Apt Preamp Mute Relay Will Not Pull Up

If you are having trouble with the operation of the mute relay in the Apt/Holman Preamp, it is quite likely the c67 (1 mf, 50 v) and/or the C66 (10 mf, 16 v) capacitors. These form a 4 to 6 second timing circuit and if the caps have dried out, there is nothing to charge and the relay depends on a pulse of energy to activate the relay. Once the relay circuit was repaired, she sounds great!
 

unclejed613

Member
2006-12-28 12:19 am
there's also another way to fix the relay contacts that seems to work well as long as you don't over-do it. remove the relay, and connect a 9V battery or a power supply limited to no more than about 200ma across the open contacts. use another supply to energize the relay about 5 times. do this for each set of contacts. this generally provides just enough current to remove the oxide layer, but not enough to burn them.
 
Hello,
I'm resurrecting this very old topic because I've problems with an Holman preamp.
My late brother had one sitting with a lot of HIFI gear. As I liked the look of it, I asked my sister in law if I could have it. And got it.
It is running quite fine but is in an unknown state. When I got it, the source switch button was loose (but I could not remove it !). I opened it up and it seems that it has not been tampered with (at list this is not showing) and as my brother never told me about it I do not know how and when he got it.
Now, the preamplifier is fine except on Phono 1 where the right channel is more powerful than the left one. If I put the turntable on the second phono input, everything is fine. So as the phono preamp is common to the two inputs, I "think" it is not the culprit. That leaves me with the switching circuitry, jacks and the like.
How could I check the source switch and repair it if faulty ?
Thanks a lot for your help and advice !
 

seanc

Member
2009-03-18 12:50 am
... I was quite sure it was a switch problem.
Before you single out the input select switch:

1) Phono-1 has 2 additional switches that come before: load impedance & load capacitance.

If either of these switches (or the switched part values) is faulty, you'll have a channel imbalance to some degree.

Try setting impedance to 100k & capacitance to 50p. If imbalance is gone, one of these is to blame.

2) There are channel balance trim pots for both channels. The trim pot for Phono-1 may be too far out of adjustment, or dirty/faulty.

BTW, do you know what kind of connector the additional MC preamp use ? And what size is the board ?
The MC plug-in card uses one of these connectors.

This one provides a vertical orientation: Molex 09-48-3105

This one provides a horizontal orientation: Molex 09-48-3106

I've never seen an actual Apt card or schematic (or know anyone who has one) so can't comment on size, but am 99% certain they were oriented horizontally (parallel to main board), on top of the RIAA stage circuitry.

From numerous measurements taken inside the unit, I estimate the original board would be limited to about 75 x 80mm.

Because I would like to test a Leach MC preamp on this piece of kit.
How would you configure a Leach circuit inside the Apt? The typical Leach is battery powered, you'd be hard pressed to find room on an internal board for batteries, and changing them would be a PITA.

I've built 2 Leach variants in the past. Both work well, but are unsuitable for the Apt interior, so I am building a custom AD797 MC card instead. I have the board in-hand, and hope to start assembling it this weekend.
 
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So, as I said when I was working in software maintenance, a bug is never where you watch for it.
I planned to use my old Philips generator, feed it to the two phono 1 input and look at the output of the preamp with my old Tektronic 2235. The oscilloscope starts fine but the traces are restraining themselves to dots in a matter of minutes. The generator does not produce any signal.... What a day. I should use them more often.
So I opened up the preamp, checked visually everything (the relay had suffered a soldering iron hole in it's cover ! ) but everything was fine. I used contact cleaner to clean all switches, potentiometers and adjustable pots. And closed up the device.
Then I had an idea : I checked the cables at the phono: Right side gives me 498,2 Ohm and 443 mH with the ATVM95E fitted. But the left channel gives anything from a pure resistance to a 325pF capacitance.... So it seems I've an intermittent on the turntable cabling...
What a day !
It seems that the preamplifier is fine.
Thanks for your help !