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Crown D150A - Puzzled. Fixed itself, then distortion came back. DC offset.

Johnny_Law

Johnny_Law

o.O
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**Edit 7/17/21: Skip to Post #51**

**Edit 12/27/20: Now seeing +800mV on Ch.2 output via DBT; Ch.1 is OK**

All original transistors and diodes, new electrolytic capacitors.

I'm seeing some negative DC offset on one channel, and my outputs from the op amp do not match the schematic. Various other voltage test points don't match the references in the schematic either.

-1.7VDC at Channel 2 output [varies, usually around -2V, but has gone up to -4V at times, seemingly randomly] EDIT: Now, on powerup, it initially is - 300mV but falls to around -900mV where it stabilizes.

0.0VDC at Channel 1 output

45V power supply good; IC supply 10V also good (see below)

Measurements of IC1 (739 dual op amp) - all V(DC)

Pin 1: -0.3 [Ch.1 WRONG - should be 1.75]
Pin 2: 9.2
Pin 3: 6.2
Pin 4: 6.2
Pin 5: 0.0
Pin 6: 0.0
Pin 7: -9.9 [GOOD - power supply]
Pin 8: -0.1
Pin 9: 0.0
Pin 10: 6.2
Pin 11: 6.3
Pin 12: 8.9
Pin 13: -0.3 [Ch.2 WRONG - should be 1.75] EDIT: Now +30mV
Pin 14: 9.9 [GOOD - power supply]

I've removed and tested the following transistors in my cheap transistor tester, results OK:

Q200, Q201, Q204, Q205, Q206, Q212

I've tested the following diodes as OK:

D207 (out of circuit), D208 (in circuit)

I've tested the following resistors as OK in circuit:

R216, R224, R237

Service Manual available here:

https://www.crownaudio.com/en/site_elements/d-150a-service-manual-part-1
https://www.crownaudio.com/en/site_elements/d-150a-service-manual-part-2

Would really love any suggestions as to where to go next. Do we think the op amp is bad? I tested a second one from another D150A I have on deck for repair, no change. Thanks.
 
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cademan said:
What was the initial problem to begin with?
Click to expand...

Reported "one channel sounds funny."

Passed dim bulb test.

Am now seeing negative DC voltage in Channel 2 and -0.3V outputs from the op amp on both channels as outlined above, am puzzled.
 
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Well, back at the bench to re-measure, turned it on again and now the channel 2 output shows +0.2V DC. I changed nothing. What would cause it to fluctuate I wonder. Power supply voltage is still right on target.

Using the bias pot, I got the DC offset down to 0.05V in the bad channel.

Listening to music, it produces sound but the sound is clearly distorted especially the high frequencies. Yep, that matches the reported symptoms.

Edit: Now the DC offset is perfect, and music sounds great through that channel. The amp has fixed itself, apparently. Now I am even more puzzled.
 
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Lesson learned. Geez I wish it was as simple at that more often. Thanks!

Also lesson learned that the reference voltages in the manual aren't necessarily correct ...
 
It is important that the amplifier is well warmed up, and that the input level controls are sprayed with fader lube and cleaned before trying to adjust the 2 per channel DC offset trimmer controls, as outlined on page 10 of the Crown D 150/A service manual part B that you provided following the instructions 7.8 "Electrical Checkout & Adjustment Procedures..
Marking the settings/position of the trimmer pots (R105/205 & R103/203), then spraying them with a little fader lube and then rotating the from one end to the other several times & then returning them to their marked positions, same cleaning as should be done with the input level controls..

Follow the DC offset adjustment adjustment procedures with no input signal connected, speakers, or dummy load connected part A & then B, and barring any other old age issues, the amp`s performance and stability should be fine.

Check C4 & C5 for swelling, and R10 for proper value as their hard working & commonly stressed from time, and are in the important voltage doubling circuit.

I have a hand full of D 150/A & DC 300/A/II amps, and was a Crown authorized service center in the early nineties.

Good luck.

Ciao
 
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Thank you! Very thankful that you posted here, feel very lucky to have you helping out being a Crown service center, thank you.

C4 and C5 are new, R10 measures OK. Pots & trimmers all sprayed, lubed. Yesterday, I let it sit on the bench power on no inputs / trimmers ccw for a few hours and was able to get it to sit and stay under 1mV. Felt good about that.

BUT this morning, turned on and waited an hour - Channel 2 was back up to 200mV with Ch.2 pot full CCW ... could not zero out with trimmer R205. Channel 1 is fine, at <1mV.

With music, I'm hearing some distortion to the highest highs (cymbals, chimes, etc) on Channel 2, it comes and goes.

Have you ever seen pre-drivers or drivers develop intermittent leaks into the signal path causing intermittent signal distortion? I wish I had an oscilloscope to investigate, but I only have a DMM and my ears.
 
Johnny_Law said:
Thank you! Very thankful that you posted here, feel very lucky to have you helping out being a Crown service center, thank you.

C4 and C5 are new, R10 measures OK. Pots & trimmers all sprayed, lubed. Yesterday, I let it sit on the bench power on no inputs / trimmers ccw for a few hours and was able to get it to sit and stay under 1mV. Felt good about that.

BUT this morning, turned on and waited an hour - Channel 2 was back up to 200mV with Ch.2 pot full CCW ... could not zero out with trimmer R205. Channel 1 is fine, at <1mV.

With music, I'm hearing some distortion to the highest highs (cymbals, chimes, etc) on Channel 2, it comes and goes.

Have you ever seen pre-drivers or drivers develop intermittent leaks into the signal path causing intermittent signal distortion? I wish I had an oscilloscope to investigate, but I only have a DMM and my ears.
Click to expand...


Does this amp have the stereo/mono bridging switch on the back of the cage, if so make sure that it has been sprayed and cycled a few times with the amp off remembering to return to the stereo position, and look over all solder connections on the board, and touch up/reflow if any are questionable..

What is the history of this amp, if you have any knowledge(like was it driven hard (PA use for yrs. ?), or has been in storage, or unused for a lengthy period of time ?

The D 150 service manual pdfs you posted has the most excellent trouble shooting methods/guidelines, if you would take the time to absorb it`s info...

You have one channel operating correctly, so you have a living part for part schematic to compare to the acting up one..

You should really have at least a scope and signal generator, as well as a good DVM, to work on these type amplifiers, or any audio equipment for that matter..
If you have 200mv of DC offset it`s no surprise that you`re hearing distortion, as that channel is DC biasing your speaker, and no speaker/s should be connected to any DC amplifier until all issues have been resolved, or the amp could be damaged/stressed more, and or freak out eat your test speakers, if not fused properly.
 
Thank you @Bill Ferris !

The amp was likely driven hard at a recording studio, where I bought it (in Healdsburg CA). I bought it knowing it would need repair, the owner of the studio had a tiny little workbench in the basement where it looks like he kept things going. But he dug this out of a pile, and it had a piece of tape on it with the note "One Channel Sounds Funny" ... that's all I know about the past life / condition.

I've been poking around measuring and trying things, but still have around ~300mV at the output of Channel 2. Channel 2 will play music OK but you can hear slight distortion especially in the treble. Channel 1 is fine.

Since last week ...

-Stereo/Mono switch deoxited, no change
-Followed the +300mV backwards from the output transistors and drivers.

Q210E: 0.3V
Q210B: 0.6V

Q209B: 1.2V (1.0V on good channel)

Q207B: 1.8V (1.6V on good channel) / this is also Q202C

Q202E: 45V (same on good channel)
Q202B: 45V (same on good channel)

D203 tests good out of circuit

Q203B: 0.4V (0.13V on good channel)
Q203E: 0.3V (0.0V on good channel)
Q203C: 1.73V (1.4V on good channel)
Q203 tests good out of circuit

I've taken other measurements as well but those seem like a good place to start.

Would love any thoughts. I thought it seemed like Q202 may have an issue, but it measured good, and I swapped with Q102 and no change.
 
petehall347 said:
i hope you meant dc offset pot
Click to expand...

Yes, I misspoke. R203 (at min offset), R205 (at max offset) offset controls per p.7-10, Section 7.8 Electric Checkout and Adjustment Procedures, Adjustment #1 "Quiescent DC offset".

I also think I mis-reported the value, it was damn close to 0 at 0.5mV.

That said, it was shortlived and went back up to ~300mV where it's stayed since.

The DC offset is still adjustable via R203 ... but at lowest is still ~50mV. Adjusting R205 will change the offset value slightly but not much. Maybe this is a clue.
 
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Here are my measurements thus far.

Circles around components means they've been removed from the circuit and verified good, then put back in.

Parens ( ) indicate the value on the good channel for comparison.

I hope you can download it and zoom in.

I suspected a few things that have strange values but they tested fine ...

R211 is supposed to have 0mV on the far size of Q201E. R211 tested OK. C217 tested OK (not circled yet).
Q204B is supposed to be -200mV, but is +55mV. Q204 tests good but Q204E shows the +300mV output voltage and Q204C is 200mV too high at -400mV instead of -640mV at the D208 anode.

All drivers and outputs have correct BE bias, but B is just 300mV too high ... so Q207/Q209/Q210 emitters are 300mV too high. I suspect this implies the voltage originates upstream of Q207B, and in fact I measure Q202C at +1.85V compared to the reference +1.45V. However Q202 measures good and in fact I swapped with Q102, no change.

I don't know where else positive DC could enter the circuit aside from Q201 or Q200 but both measure fine, and R211's voltage at Q201E matches the other channel anyway.

Note that IC1 Pin 13 is odd. The schematic shows +1.75V, but I've measured it at -90mV which would seem wrong. [This value edited from what's on paper below.] However, the good channel measures -230mA as do both channels of my other D150A which is now working great. So I'm not sure about that, if it's really supposed to be +1.75V or not. I can't see how it would correctly bias Q201, PNP, if it were positive per the schematic. Either way, -90mV seems wrong, and a change from post #1 above.

PXL_20201122_213221154.jpg
 

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Some more measurement shows that I'm actually only seeing 130mV across R216.

When viewed in light of the -60mV at Q201B (or, Pin 13 of the IC), I think implies that the IC is not biasing Q201 correctly.

The fact that I'm seeing 290mV on the other side of R211 maybe supports the hypothesis that no (or not enough) current is flowing through Q201.

So let's see if an IC swap fixes it ... or not.
 
Swapping in a good IC did not fix this. Same issues remain.

Will continue to look at what might be wrong, without any obvious culprits.
 
Thinking about this, if the output of the opamp is too low on Channel 2, then perhaps it is seeing a voltage differential between the inverting and noninverting inputs of Channel 2.

Sure enough, pin 8 (inverting input) is showing around +7mV now while pin 9 (noninverting input) is showing 0mV.

Note that my previous measurement of pin 8 in the first post was -0.1V (-100mV), while I was seeing -4V at the output.

So it seems like there is a correlation between pin 8 voltage and the DC offset of the output. Checking the numbers confirms as much:

First Post: -100mV inverting input to -4000mV DC offset = gain of 40
Now: 7mV inverting input to ~275mV DC offset = gain of ~39

That's close enough to suspect a direct correlation, and that this is the real issue.

I will now investigate what might be causing the pin 8 inverting input to (A) not be 0mV and (B) vary over time from -100mV before to +7mV now.
 
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Turned my attention back to R200, the 100K pot. Cleaned it again, lo and behold after extensive deoxit and back-and-forth turning, at its most negative position the DC offset crossed into the negative. However, it was way into negative. A tiny breath clockwise launched the DC offset back up into the ~100mV range. I continued to clean it, used fader lube as well. Now the DC offset seems to be hovering around 60mV without much adjustment possible. I'm letting it sit for a little while and will check again.

Needless to say it seems like R200 may be the cause of the whole mess. I may swap the pot from R100 into the R200 position to see if that fixes it.
 
I swapped R100 into R200 and woohoo! DC offset adjusted back to 0.

BUT ... I played music and it was still distorting. Only with the input pot at around halfway and up ... quiet volumes it sounded fine.

BUT THEN ... something popped and now I'm seeing ~+43V on both channels.

Ugh.
 
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Pulling parts to test them ... two 1N4004s bit the dust. Each broke in half as I removed them. Must have been bad when in circuit? Ugh, if only I knew if they were already bad before I pulled them! Schrodinger's diodes?

I'm fresh out of spare 1N4004s so this'll have to wait until the next Mouser order (I'll order new 100k trimmers as well as a replacement for a bad L01 transistor and anything else I find out needs replacing).

PXL_20201128_220424134.jpg
 
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