Apt Holman preamp restoration

[leesonic]

I'm working on an Apt Holman preamp for @srs735.

Here are some pictures before I took the covers off:







Lee.

 

[leesonic]

Here is the inside view, this is actually the bottom :



Filter board before :



And after. Wima film caps, Nichicon low leakage KLs, and KWs. This shows the difficulty of squeezing in film caps where there used to be electrolytics. The Wima caps usually have the shortest leads possible, but if you order the ones on the roll, the leads are longer and you can be more creative with the positioning.

 

[leesonic]

Some weird axial caps in the signal path here. These were replaced with KLs, but as these are radial caps, I had to reform the leads.



The two power supply regulators were "bolted" to the chassis without any heat transfer compound. I use the word bolted in quotes as both of the mounting screws were loose. They must have been getting pretty warm without adequate thermal coupling to the chassis.

Keen eyed observers might have noticed that I already changed the main filter caps to low impedance Nichicon PWs. The original caps almost fell out, the leads had corrosion on them from leaking electrolyte.

 

[leesonic]

Relay contacts are much easier to clean if you desolder the relay. I cleaned them with some 2000 grit sandpaper with DeOxit on it.



Here are the regulators, reinstalled with heat transfer grease. You can also see how I installed the radial caps in place of the axial.



Phono preamp with Wima films.

 

[leesonic]

At the moment, it works and sounds great. However, I'm having a problem with the phono preamp. There is significantly more hiss/noise in the left channel compared to the right. Since there are no op-amps in the phono pre, I'm wondering if there is a noisy transistor. I plan to change the op-amps, but want to get this noise problem fixed first.

Lee.

 

[Binkman]

Your usual fine workmanship lee! (Ya run out of Nad's? )

 

[62vauxhall]

FWIW, I have two of those, one of which had a hard life and needed more work than the other.

It had been worked on before - some new caps & relay but I wound up replacing all 41 electrolytics plus power switch & heap phone jack. All good until a random noise problem developed. The transistors were all OK so I sent it to AudioProz for service. Came back with two of the four IC's replaced and problem solved.

It did not occur to me to consider an IC as the culprit.

 

[srs735]

Your usual fine workmanship lee! (Ya run out of Nad's? )

Lee does stray occasionally. I have a Kenwood KA 7100 I bought from him that he restored. Right now I'm in the garage enjoying a Mitsubishi DA-R25 he restored paired with Bozak 302's. A pretty nice "garage system"

I'm looking forward to hearing what this Apt Holman is capable of sounding like. When he gets the phono stage figured out I wonder how this will compare with my
CJ-PV1?

 

[leesonic]

Your usual fine workmanship lee! (Ya run out of Nad's? )

Oh, I have plenty of NAD equipment. Four 3020s, three 3140s, two 1020s, three 4020s, two 2700, one 2600, one 2150, one 2200, and that's just from memory.

I don't only work on NAD equipment though. I have Pioneer, Sony, Sansui, Kenwood, SAE and Soundcraftsmen amps here to be worked on.

Lee.

 

[Binkman]

I had no doubt you worked on other models... Just being a little sarcastic in that your reputation for NAD is well known to the point that recently you won't give out the bom's anymore. (don't blame you.) Nads are hens teeth in my locale and I'd certainly consider sending one to you or pay for the damn bom! Hell you did all the redesign research to get the fav models right.

 

[HalfApt]

..... It did not occur to me to consider an IC as the culprit.

Yah, they can fail.

By sheer dumb luck, in 2000 I got a "broken" Apt-Holman preamp for $0 (long story).

Only one channel worked. The "broken" part turned out to be the volume control op-amp: one side had failed. Replaced it for 100% functionality.

I had always wanted one of these but couldn't afford it when it was new, so having one for less than $10 (parts + shipping) was like winning the lottery!

For anyone working on one, beware. There is at least one error in the PCB part reference silkscreen: left phono cap C11 should be C117. There may be others I haven't discovered yet.

Sean

 

[Klascala]

Nice work! I always LOVED that preamp's looks!

About to process a huge recap operation on a Yamaha CX-1 that has been unused in storage ever since it was built. The sound is rather thin and edgy and I guess the caps never had a chance to properly form. I'm following the great "Yamaha CX-1 thread" from our fellow member Bratwurst as a guideline for operations. Happy to see Nichicon KL and KW, and Wima MKS2, are your caps of choice as well I did buy some mundorf M-lytics as power caps tho!

 

[dannyr]

Hi, when I originally got this preamp years ago it worked okay. I decided to recap it and also clean the relay due to liking it so much. After all the work the relay would take a bit to engage but only after toggling the mute button on and off. It would never trip on it's own anymore. I replaced the relay with a new one thinking that me cleaning the old one did it in but it didn't help. Lately, it takes a good while longer before I can get it to engage using the same mute switch trick. It seems if i don't use the preamp for it a bit it gets worse. I can't remember if it then gets better or not with time. I got into the habit of having the mute engaged before I turned everything on but the increasing time until I can play music is annoying me. Are there other parts or sections that I should look into? Thanks.

 

[HalfApt]

Hi, when I originally got this preamp years ago it worked okay. I decided to recap it and also clean the relay due to liking it so much. After all the work the relay would take a bit to engage but only after toggling the mute button on and off. It would never trip on it's own anymore. I replaced the relay with a new one thinking that me cleaning the old one did it in but it didn't help. Lately, it takes a good while longer before I can get it to engage using the same mute switch trick. It seems if i don't use the preamp for it a bit it gets worse. I can't remember if it then gets better or not with time. I got into the habit of having the mute engaged before I turned everything on but the increasing time until I can play music is annoying me. Are there other parts or sections that I should look into? Thanks.

The mute/relay circuit is independent of the audio circuitry, runs only off the power supply, so possibly bad solder joints, component(s) failed or incorrect value are about all that can go wrong.

Did you "recap" the power supply and/or mute circuit? Did you replace any other components in the power supply/mute circuit?

If so, did you replace w/same value as originals found on PCB, or did you go by schematic values? There are some discrepancies.

 

[dannyr]

Hi HalfApt. Thanks for taking the time to reply. All I did was change all the electrolytic capacitors in the unit with the same values and a relay with a direct replacement from a link that I got on here. I think that I read the values off of the unit's boards but it's been years. There are only two capacitors in the mute circuit right? I remember one or both being 10uf but one could have been 47uf.

 

[HalfApt]

Hi HalfApt. Thanks for taking the time to reply. All I did was change all the electrolytic capacitors in the unit with the same values and a relay with a direct replacement from a link that I got on here. I think that I read the values off of the unit's boards but it's been years. There are only two capacitors in the mute circuit right? I remember one or both being 10uf but one could have been 47uf.

Mute circuit has 3 electrolytics: 2 x 10uF & 1 x 1uF

10uF for power supply filter, 10uF for delay timing, and 1uF for relay activation.

If you got the 1uF & a 10 mixed up, it won't work right. Voltage ratings are important too.

As you say it worked before recap, but not after, these should be your most likely culprits.

Attached files may help. Red markups are my corrections to PCB & schematic errors.

 

[dannyr]

Thank you for taking the time to chop out those pages and post them for me. I really appreciate it. You cleared up a lot. I remember swapping the two 10's for the second time before or while changing the relay but I may have left the 1uf or even put a 10 in it's place. I'll have to open the unit to see. I hope that I have a 1uf somewhere. I'll let you know what happens.

 

[dannyr]

I went into the preamp tonight. It took me awhile to remember how to disassemble it all. Anyway, all of the three capacitors are the correct value/voltage, 2x10uf 35v and 1x1uf 50v, and in the right orientation. I reflowed/cleaned all the solder joints in the section but nothing changed. What's interesting is then I measured the output voltage of the mute switch going to the relay that it starts at 25volts and slowly crawls down. once it's under a certain voltage flicking the mute switch works which may be somewhere between 17 and 18 volts. It continues to slowly go down but I didn't have it on long enough to figure out where it stabilized. What would be your next move if you were me? Replacing all all the transistors and diodes?

Update: I removed D14 and it tested fine out of the circuit.

 

[dannyr]

okay. when the mute switch is engaged before the unit is powered on, the input voltage of the mute switch is 2.5ish volts and the output is 25. I measured D14 circuit and it was .4volts both ways. It measures like a shorted one but it's connected to the relay so I don't know if this is normal or not.

 

[restorer-john]

I like the 'bespoke' PCB spacer... the stick on grey rubber foot!

It's quite an unusual but attractive unit. I haven't seen one before. Thanks for posting.