170DC right side distortion at higher levels + left side heat issue.

[jd3rdcoast]

Have had this amp for a few years. Couple issues want to address:

1) Recently noticed right channel audio is distorting at higher output levels. Have swapped sides and follows right output plus remains when bypass 3250B preamp. Popped the cover and checked DC offset. Was 12mV R and -57 mV L. Set both without problem. Cleaned output pots w/D5.
Problem remains I see the R amp board has a partial recap.
Should I just recap the whole board and take it from there?

2) While doing above noticed the L channel is running much hotter than R side but sounds better. Primarily the two heatsinked transistors in middle of board are very hot to touch. Outputs are not hot. This board looks original. Anyone have any thought on the uneven heat issues?

 

[wlhd1610]

I don't think a recap will solve your issue.
Also since one channel is fine it won't be the single dual main filter cap .
I would check the bias to see if there is an issue there.
You say the output transistors are not hot but the two middle ones are ,are those the driver transistors on the actual board?

Bob

 

[jd3rdcoast]

You say the output transistors are not hot but the two middle ones are ,are those the driver transistors on the actual board?

Yes, Q712, Q714 and Q711, Q713 on 2 heatsinks on the P700 board. I have attached the P700 schematic for reference. Both heatsinks get quite hot on the left P700 board but not on the right. This is sitting with power on, gains at minimum and no inputs/speakers connected to unit.

 

[wlhd1610]

I'm at work but I believe you can check the bias voltages between J 707 and J711 for the one pair and J 710 and J 714 for the other pair of outputs.Compare those voltages to the same J points on the other amp module to see if they differ.
The 170DC does not have bias trim pots.
Report back!

Make sure you power down when removing /applying your test leads.
No sparks-no arcs!

Bob

 

[jd3rdcoast]

check the bias voltages between J 707 and J711 for the one pair and J 710 and J 714 for the other pair of outputs.Compare those voltages to the same J points on the other amp module to see if they differ.

Here's what I got for the suggested bias measurements:
Left: J707/J711 = 36.0mVDC & J710/J714 = 38.6mVDC
Right: 0.0mVDC & 0.0mVDC respectively.

I went ahead and measured each J lug on this connector and got the following:
J# R L
J713 -51.5 -51.2
J716 -51.5 -51.1
J705 51.5 51.0
J708 51.5 51.2
J706 11.6mV 0.6V
J709 15.0mV 0.63V
J712 17.0mV -0.57V
J715 20.0mV -0.57V
J707 21.0mV 45.0mV
J710 15.0mV 52.5mV
J714 17.5mV -3.0mV
J711 16.5mV 5.0mV

Discrepancies seen at R(J706-J715) not at 0.6V per schematic which is emitter side of Q713/Q714. These started about 45mV on power up then slowly ramped down and settled at these values after several minutes as temp stabalized.
Also, only L (J714/J711) at very close to 0V.
I had read a comment in a thread that these 170DC's are known to run hot and not to be concerned. Do you think the hot left side is normal?
Could this affect the Right side issue of signal distortion at elevated volumes?

Thanks for your feedback;
JD

 

[wlhd1610]

Well something is wrong on right side , you don't have any bias voltage/current at all.
That would definitely give you distortion issues.
Check the .33 ohm resistors .
Compare voltages to left amp module.
I would think that 15mv bias per pair on left side would be correct but calculation would have to be done.
It's almost midnight here in the Catskill Mts.

Bob

 

[jd3rdcoast]

Well something is wrong on right side , you don't have any bias voltage/current at all.
That would definitely give you distortion issues.
Check the .33 ohm resistors .
Compare voltages to left amp module.
I would think that 15mv bias per pair on left side would be correct but calculation would have to be done.
It's almost midnight here in the Catskill Mts.

Bob

Hi Bob, Sorry took a while to get back to you. The Wife had a birthday this weekend. It wouldn't have went well if spent all weekend on my bench.
Well, I checked the voltages across the 0.33ohm resistors at the junction pins as follows:
R753/R754 (J10/J14) Left Amp (good): 29mV and 0.7ohms Right Amp: 0.0 mV and 0.7ohms
R755/R756 (J07/J11) Left Amp (good): 28.7mV and 0.7ohms; Right Amp: 0.0 mV and 0.7ohms

I also measured voltages at the EBC's of Q712/Q7/13/Q711/Q714 with the following:

Obviously issues there on the right side. I must admit I just dont know where to go next other than start pulling components and testing individually. No burnt resistors and all the ones I checked ohm'd out good. Will need to lift some diode legs to get good test. Remember, unit functions OK but with distortion at higher volumes. Stumped.

 

[USMC Spike]

Hi JD,

I'm following along as I also have a 170DC amp. What did you find out after
this last post? When you were stumped? Was it the diode failure?
Or the transistors in the channel starting to go bad?
Or Something else?

Thanks,

USMC Spike

 

[jd3rdcoast]

Hi JD,

I'm following along as I also have a 170DC amp. What did you find out after
this last post? When you were stumped? Was it the diode failure?
Or the transistors in the channel starting to go bad?
Or Something else?

Thanks,

USMC Spike

USMC Spike and I have com’d on PM. I will update this thread this next week. Suffice to say I did have a flaky transistor but also a self induced issue since resolved. This is a slow process for me as I am overly cautious with my baby.
Also have been under the weather and other commitments. Waiting on those KSC1845’s to get restocked as well to finish complete overhaul.

Working on upgrading the main filter caps with some KG Gold Tune 15K’s but will have to make some slight mods to fit. Will be following Catrafter example.

Stay tuned.