Rowe R-90 amplifier troubleshooting

The right channel is weak in comparison to the left, lower volume and significantly less bass. I swapped woofers and checked all the speakers/wiring, no change. Also reversed the phono cartridge leads, no change.

I'm just in the beginning stages of troubleshooting this, but I've discovered something I think is odd and wanted to see if it's actually normal. When I turn the balance control all the way to one side or the other, I still have sound coming out of both sides. It does show a preference to the channel it's turned to, but there is still significant output to the opposite channel. I would expect that if I turn the control all the way to the left channel, I would only hear sound from the left channel and vice-versa. I have checked the stereo-mono switch for proper position and have checked it with a meter to make sure it was working properly.

Is this normal behaviour for this amp, or does this indicate where my weak channel problem could be, or is this another issue altogether?

Thanks!
Nelson

Maybe I was too wordy?

If I turn the balance control all the way to the left or right channel, should I still hear sound from the opposite channel?

Thanks!
Nelson

Nelson,
I can not remember, on these amps---On some amps, you will hear it, on others no. Ron Rich

Thank You Ron.
I managed to track down one of those elusive shop manuals and it arrived today! Maybe I can find what I'm looking for in there somewhere.

In the meantime, I'd be very appreciative if anyone out there with a mid-late 80's Rowe box could just bop the balance control to one side or the other and tell me how it behaves.

Thanks all,
Nelson

In some (stereo) amplifiers, the balance control only "balances" the mids and highs of the music, leaving the bass alone.
This maximizes the (non directional) bass at all times, while shifting the upper frequencies.
The schematic would show this to be true if a capacitor is used as a "grounding" point for the balance control, or designed into the operational amp cicuit.

Nelson, on these later units Rowe stopped requiring the cart to be wired out of phase. Instead they used an op-amp set up for unity gain to flip the polarity and (IIRC) located just before the input to the P.A. driver stages.

This allowed them to keep the mono bass and prevent acoustic feedback, but the new design also allowed the preamp signal to be tapped for a conventional external amplifier without the need to have a switch or relay to flip one speaker load's polarity when switching from juke to other sources. Using a mono P.A. setup was now possible. It also allowed an aux input to use the juke amp and speakers since the inversion took place after the input point.

It is possible that the inverter IC or it's circuit has a problem.

As for the balance control, from what I recall, because of the need for the bass speakers to be across the channels they kept the partial attenuation characteristic. Rowe balance controls were never full range. I doubt this area has anything to do with your problem.

The best way to deal with this sort of problem is to take the amp and o-t package out, put them on your workbench and use a test input and look for gain differences. I'd begin at the point where the preamp/agc/vc board couples to the driver board. That way you can learn which end of the circuit has the problem.

I haven't had one of these machines since late 1994, so I may be missing something specific to these models.

Rob/NYC

Hi Nelson,
What I do, with this type Rowe amp, is first check the output transistors--then check the bias setting. Once both are "good", I swap output boards, to determine if the problem is in one of them, or the pre-amp board.
This gives me a "starting point", at least--
Ron Rich

Thanks for all of the input! I've been away from electronics troubleshooting for over 15 years so I'm a bit rusty.

Ron - your idea of swapping the driver boards was the fastest way to eliminate or incriminate that aspect so I went for that first. Unfortunately that didn't make any difference but at least I know where the problem isn't.

Matt - Rob indicated the same thing in his response, and from what I can determine, this is indeed by design.

Rob - A lot of great info! I'm a bit limited in how deep I can go without more sophisticated equipment, but I think you've put me on the right trail. The audio actually does sound much like one channel is out of phase. It looks to me that the inverter circuit can be easily bypassed by simply redirecting the preamp output for that channel directly into the driver board. My thinking is that I could gain some more insight into what area isn't working properly. Have you ever tried bypassing the inverter, or would this be a waste of time and/or possibly damage something else?
I have no plans to connect anything other than the turntable to this, so I'm not above going old school and throwing the cartridge and speaker out of phase on that channel to get it sounding right.

Thanks!
Nelson

Hi Nelson,
Rowe uses that large (13 A-MP pin) jumper to "invert" signal--on some applications, they jump it differently so it's NOT using the inverter--see the schematic, and you should be able to figure out how it's done--
Ron Rich

Hello Nelson,

The standard jumper plug if your pre amplifier is a 6-07925-04 is as follows:-

pin 1 to 3
pin 2 to 7
pin 5 to 9
pin 6 to 8

if you want the outputs in phase rather than one side inverted remove the bue link (2 to 7) and link pin 2 to 3.

Regards
Alan

Alan Hood
ami-man
UK