Apt Holman preamp restoration

[HalfApt]

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.

I'm not savvEE enough to analyze the individual parts of this circuit, but considering the functional blocks per the description, it's easier to understand how it works.

When the time delay expires, transistor Q17 & Q18 turn on providing a ground path for the relay coil.

Initially, max current flows through Q19 providing a "surge" to ensure positive engagement of the relay armature.

When C67 (1uF) fully charges, Q19 shuts off, so now a lesser current flows through R90 (820 ohm), just enough to keep the relay engaged.

So a voltage drop is expected at the mute switch, but it should happen relatively quickly, not slowly as you say. If my math is correct, the "surge" should last just a fraction of a second.

Is a drop to 17-18V reasonable? Seems a bit much to me, but I don't know, depends on the relay coil resistance, which isn't specified.

OTOH, if you get 24-25V at the mute switch (terminal 5 on schematic) when first turned on, that should have engaged the relay already, so don't now what to think about that.

D14 is a flyback/freewheel diode to snub back EMF spikes when the mute switch is opened. If it's shorted or somewhat reverse conductive, it can/will prevent the relay coil from energizing properly.

Since it's parallel to the relay coil, measuring voltage across it in-circuit is just measuring the drop across the relay coil.

The circuit will work without it, so might be a good test, but back EMF will be introduced in the +24V line. Don't know how bad that might be. The 24v unregulated supplies are only used for the relay & headphone amp.

Hopefully a knowledgeable/experienced EE might join in.

 

[Binkman]

@leesonic probably has it out the door. Probably been working like one of Santa's elves. Or.. sunning on some southern beach.

 

[dannyr]

It's also above my understanding much more so than yours. What's interesting is if I don't turn on the mute switch when I turn on the preamp it takes way less time to be able to trigger the relay with a switch toggle. I'm going to order the three transistors and swap em out next and see if it fixes anything.

 

[HalfApt]

It's also above my understanding much more so than yours. What's interesting is if I don't turn on the mute switch when I turn on the preamp it takes way less time to be able to trigger the relay with a switch toggle. I'm going to order the three transistors and swap em out next and see if it fixes anything.

Before you go random part-swapping, there are some simple tests that might narrow down the problem area by process of elimination. Requires noting more than a jumper wire.

If you can wait until tomorrow I can write up some steps & another graphic or 2.

 

[dannyr]

Of course I can. thanks for spending your time to help me try to solve this.

 

[HalfApt]

Of course I can. thanks for spending your time to help me try to solve this.

OK, here ya go, see the attached files.

Caveat: DO THESE TESTS AT YOUR OWN RISK - BE CAREFUL making jumper connections when the amp is powered on! Jumpering to the wrong part, or shorting to another part, can be damaging.

Smarter people than me might have better recommendations, but this is how I would proceed if the issue arose on my amp.

Be sure to read the READ_ME_FIRST file first

Hope it helps, good luck!

 

[Rybeam]

I have used an APT Holman for about 4 years now, I really like it. In fact I finally took the bottom off yesterday to get some cleaner into the mute switch. I discovered it was the cause of one channel dropping out, I hope it holds. I have considered sending it to AudioProz for a check-up / mod. Particularly after checking prices on similar quality pre-amps. Do the mods put it in league with the newer $1000 plus used pre's ?

 

[dannyr]

Aptman, awesome. thank you for going into such detail. I'll get to this soon when I have some daylight. I'll be careful and report back.

 

[HalfApt]

..... I finally took the bottom off yesterday to get some cleaner into the mute switch. I discovered it was the cause of one channel dropping out, I hope it holds .....

The mute switch itself CANNOT cause a SINGLE channel to drop out.

The mute switch is single-pole and only enables/disables the output relay. The relay is double-pole with separate contacts for left/right channels, so your problem is likely the relay. This is a known issue with older units.

See: http://audiokarma.org/forums/index.php?threads/apt-holman-pre-muting-relay.401961/

 

[HalfApt]

Aptman, awesome. thank you for going into such detail. I'll get to this soon when I have some daylight. I'll be careful and report back.

You're welcome. I hope it helps get it sorted out.

 

[dannyr]

Okay, the ground to mute switch pin 6 and the non ground side of R89 trip the relay right away. The resistor did and it also measures perfectly in circuit. Misread the testing instructions or I wouldn't have bothered to test R89's value. continuing...

Okay. "Test 4: check/verify Q18 function" function trips trips the relay as does "Test 5: check/verify Q17". Does this mean that I should replace Q18 next?

Thanks again. Believe it or not I had fun doing this test. To hear the relay snap again alone was kind of a treat.

 

[HalfApt]

Okay, the ground to mute switch pin 6 and the non ground side of R89 trip the relay right away. The resistor did and it also measures perfectly in circuit. Misread the testing instructions or I wouldn't have bothered to test R89's value. continuing...

Okay. "Test 4: check/verify Q18 function" function trips trips the relay as does "Test 5: check/verify Q17". Does this mean that I should replace Q18 next?

Thanks again. Believe it or not I had fun doing this test. To hear the relay snap again alone was kind of a treat.

What about "Test 3"? I presume it did NOT trip the relay, else no reason to proceed to Test 4?

If "Test 5: check/verify Q17" trips the relay, it means the problem lies in the delay circuit.

By way of explanation:

Q18 is just a switch that completes the ground path for the relay coil. But Q18 is turned "on" by Q17.

Q17 is also a switch, normally turned "on" by the delay circuit.

Jumpering across Q17 is the same as replacing it with a manual switch.

So the fact that jumping it DID trip the relay indicates that Q17 is NOT being turned "on" by the preceding delay circuit (or has gone bad).

Debugging this won't be as simple as jumpering.

You changed 2 x 10uF caps in that circuit. One supplies filtered DC to run the circuit, the other provides the time delay. It's possible one of these is the culprit: might be a bad cap, or bad solder joint, or however unlikely damaged one of the other components around them.

Before swapping parts, check voltages. The manual description says "When .... exceeds the 0.6v turn-on threshold of Q17."

So check for voltage at the indicated spot: have the meter probes connected then turn the amp on. Wait for the normal 4-6 second delay period to see if the voltage gets to 0.6v or more.

If it DOES, and relay doesn't trip, then Q17 is faulty.

If it doesn't, check supply voltage at the indicated spot. I'm not sure what it should be, since they don't specify transformer secondary voltage, but I'm thinking it should be around 24 v.

If that checks good, R&R those caps again, double-check solder joints, etc.

 

[leesonic]

@leesonic probably has it out the door. Probably been working like one of Santa's elves. Or.. sunning on some southern beach.

Neither. I was packing up stuff for a move that didn't happen. I'm still actively looking for a house, so do I carry on packing stuff up, or take a break from packing and get back to restoring?

Lee.

 

[dannyr]

Right. test 3 didn't trigger the relay. I believe that I swapped the two 10uf's twice but I may have damaged a trace going to the delay circuit. I'll poke around and let you know the voltages that I measure. Thanks once again.

 

[sqlsavior]

Nice preamp in nice condition. I bought an 'untested' one that had one channel out - no surprise, really - in the photo the balance knob was all the way to one side. Sent it to Audioproz for a repair and a recap. Loved the sound. Then one channel of the phono preamp went out, and Audioproz said the original transistors were NLA, and used a substitute. When it came back I no longer liked the phono section, so gave it to my brother who doesn't play records. I regard them as excellent-sounding, but problematic, based on my experience.

 

[dannyr]

Halfapt, I did the tests that you were so kind to lay out for me and I'm getting around .5volts. the voltages rise during the first 4-5 seconds and just stop around .5v and very slowly rise after that. The diode gives 23.5dc. Thanks again for all the help.

I looked and couldn't find any bad joints etc. I'll dig out some 10uf caps and see if they help next.

Update. I changed the two 10uf caps and really cleaned between the mute circuit's traces. The good is now the test point above R88 is now .55volts. I can get the replay to trigger by pressing the mute switch on and off quickly instead of waiting forever which is how the unit was after I originally recapped it.

Would lowering R88 maybe work? Going into it is 1.15 maybe 1.2 volts.

 

[Binkman]

Neither. I was packing up stuff for a move that didn't happen. I'm still actively looking for a house, so do I carry on packing stuff up, or take a break from packing and get back to restoring(quote)

seasons greetings.. moving this time a year is a bitch.

 

[Rybeam]

Apt, can a repair shop bypass the mute relay ?
I never use it, old habit is to use volume control for such things.

Thanks for your info.

 

[HalfApt]

Halfapt, I did the tests that you were so kind to lay out for me and I'm getting around .5volts. the voltages rise during the first 4-5 seconds and just stop around .5v and very slowly rise after that. The diode gives 23.5dc. Thanks again for all the help.

I looked and couldn't find any bad joints etc. I'll dig out some 10uf caps and see if they help next.

Update. I changed the two 10uf caps and really cleaned between the mute circuit's traces. The good is now the test point above R88 is now .55volts. I can get the replay to trigger by pressing the mute switch on and off quickly instead of waiting forever which is how the unit was after I originally recapped it.

Would lowering R88 maybe work? Going into it is 1.15 maybe 1.2 volts.

@23.5v after diode D12 sounds good. 0.5-0.55v at Q17 is too low (IMO), but the delay circuit with all the inter-related resistors, caps & diode is where my knowledge is incomplete, so can't say what part(s) may be causing it.

I don't see how R88 can be the issue, as it doesn't come into play until AFTER Q17 has turned on Q18.

I put the circuit into LTSpice to see if I could understand how it behaves. I'm none too good with simulations, and this circuit is kinda bizarre, so not sure if all the parameters are right, but it seems to jive with what should be happening in reality (my Apt triggers right at 3 seconds). Sim results show this:

1) Voltage at R85 (220k, end opposite diode D12) should rise to @17v at 8-9 seconds (hits 15v at @3.5 seconds).

2) Voltage at base of Q17 rises to 0.75-0.78v after @3-4 seconds

3) Current thru relay coil starts at 24 mA (surge), and settles to 15mA @3.3 seconds.

Some possibilities if parts are out-of-spec. Unfortunately, I don't think you can test/measure any of these in-circuit, would have to remove them to check:

If R86 (2.2 Meg) gets too high, delay time increases (2.4 Meg makes it 8+ seconds) and voltage at Q17 doesn't settle down as cleanly.

If R87 (3.9 Meg) gets too low, delay time increases (3.5 Meg makes it 8+ seconds) and voltage at Q17 doesn't settle down as cleanly.

R89 (1 Meg) doesn't affect delay time, but the smaller it is, the higher the voltage ripple at Q17.

Diode D13 - this is only used & needed during power down (rapidly discharges C66) but if it's leaking current it might be problematic.

That leaves only the capacitors. If you've replaced these with new, known good caps, multiple times, I don't know what else to try. Need someone else a lot smarter than me.

What brand/series of caps are you using?

 

[HalfApt]

Apt, can a repair shop bypass the mute relay ?
I never use it, old habit is to use volume control for such things..

Sure.

You could remove the relay and put 2 wire jumpers in place, enabling the outputs 100% of the time.

A simpler solution is to remove R88, replace Q18 with a jumper. The mute switch would still work w/out any delays. eg: engage the mute, power on, wait 2-3 seconds, disengage mute; and when done listening, engage mute, power off.