Packard Bell DPA-150 Amp Channel Mismatch

[Flanklim]

Hi all - I recently purchased a Packard Bell RPC-67 and am restoring the unit. I recently replaced all the electrolytics, but when powering the unit on, I get almost no low end out of the right channel. I took the power amp out and measured some voltages while powered, and noticed some channel mismatch at the output. When applying a 100Hz signal on the input, I get ~1V lower voltage on the right channel.

One mistake I made was wiring in the filter capacitors in reverse, and tripping the on-board circuit breaker, before realizing my mistake. Can this be what's causing the missing low end? No, right?

I am using the schematic found on this thread: https://audiokarma.org/forums/index.php?threads/packard-bell-dpa-150-console-amp-schematic.876884/
but it is blurry and I can't quite make out the expected voltages written there.

Does anyone have a tip for me or dealt with something similar before?

A clearer schematic would help as well, if anyone has access to Sam's Photofacts set 907 folder 7, and wouldn't mind sharing.

 

[stillspin'n]

Do you have a way to check the signal level at the input to the driver transformer? That would be the blue wire from the driver transformer on both channels. If the signal is the same at that point, them it's in the output section after the transformer. If not the problem is in the input or driver transistor section. Should be able to compare the difference in voltages between the channels. Check resistors in the weak channel for ones that have gone up in value.

 

[Flanklim]

Do you have a way to check the signal level at the input to the driver transformer? That would be the blue wire from the driver transformer on both channels. If the signal is the same at that point, them it's in the output section after the transformer. If not the problem is in the input or driver transistor section. Should be able to compare the difference in voltages between the channels. Check resistors in the weak channel for ones that have gone up in value.

Hey, thanks for the help.

I do have a way to check the signal there. I’ll give that a try this week. If I recall, I may have checked that with a DMM, and the voltage was about the same. I’ll double check that though.

I also didn’t think about the resistor values changing, I’ll give that a look as well.

Thanks again!

 

[TomSc]

Wow - transformer phase splitters in a SS amp. That's gotta be pretty rare. I agree with stillspin'n on checking voltages and resistors. It could just be a broken wire or bad solder joint where the wires from the transformers are connected. I remember a loss of bass when a balanced cable went bad during my live sound days. It was usually a broken connection in the XLR plug and accompanied by a drop in output. The transformers in this Packard Bell are essentially converting a single-ended signal to balanced signal feeding the push-pull output stages.

 

[gadget73]

early stuff often did this. I find it pretty wild, its basically a tube amp design from the mid 1930s.


agree with checking resistors though, if something in the feedback network in particular is off it will affect gain side to side.

 

[Flanklim]

early stuff often did this. I find it pretty wild, its basically a tube amp design from the mid 1930s.


agree with checking resistors though, if something in the feedback network in particular is off it will affect gain side to side.

Wow - transformer phase splitters in a SS amp. That's gotta be pretty rare. I agree with stillspin'n on checking voltages and resistors. It could just be a broken wire or bad solder joint where the wires from the transformers are connected. I remember a loss of bass when a balanced cable went bad during my live sound days. It was usually a broken connection in the XLR plug and accompanied by a drop in output. The transformers in this Packard Bell are essentially converting a single-ended signal to balanced signal feeding the push-pull output stages.

Thank you both for the tips.

I’ll have time to check in these in the next couple days, and report back if I find anything.

On both your notes on the push-pull, I also noticed that the drivers in the right side are wired in “reverse” of the left drivers. Is this to flip the phase to match side to side?

 

[gadget73]

looks like its wired the same, just the schematic is drawn mirror imaged. Not uncommon, basically its just to make it look nice.

 

[Flanklim]

looks like its wired the same, just the schematic is drawn mirror imaged. Not uncommon, basically its just to make it look nice.

Oh sorry, I meant on the physical drivers outside the amp. Here’s the left. I believe the right side was wired where each driver +/- were swapped compared to the left.

 

[gadget73]

ah, yeah thats a phase reverse for whatever reason. Maybe there is an inversion somewhere else in the unit only on one side and this is how they sorted it. Also possible they did it to give a wider stereo effect. Reversed phase does give that, but it usually creates a big hole in the middle and depending on the particulars it often sucks the bass out. Or maybe because of the common box it needs to be flipped to sound correct.

can always try reversing it and see what it does to the sound. Won't hurt the amp, but you'll know fairly quick if its worse or not.

 

[Flanklim]

Hey all, update on this.

I replaced all resistors in the left and right channels of the input driver section, and now the amp measures the expected voltages on each channel. When testing the output signal with a signal generator, I get great response out of the right side now.

Now, the right channel sounds perfect when back in the console, but the left sounds pretty bad, especially at higher volume. It's pretty tinny and louder than the right channel. My guess is that this mismatch is coming from the preamp, or potentially the potentiometers (?).

Any tips on how to proceed? Thanks!

 

[gadget73]

flip the cables at the input of the power amp to confirm it moves side to side. If it does, time to pull out the preamp and see whats going on.

If it does not move, switch the speaker connections left to right.

 

[Flanklim]

Another update.

I realized the left side tweeter was busted (measured inf ohms) and needed to be replaced. After replacing this tweeter, I noticed the right side now sounded terrible compared to the left. I cleaned the potentiometers and it still sounded bad. After using an RCA cable plugged into the power amp, I found out that there was another source of channel mismatch coming from the preamp. I'm going to take this out soon to start replacing the out-of-spec components there.

Thanks all for the help on this. Cheers!