Anyone want to give some troubleshooting guidance on 700C preamp?

[herbd2]

Hello,
I have a 700C preamp which I recently aquired which has no sound out of the right channel.
I'm wondering if someone would be interested in "hand holding" me through troubleshooting the possible issue. I've done recapping before so I know how to solder and know the basics of using a meter, but not sure how to establish a troubleshooting path using the schematic. The only thing I have done so far (other than confirm it is definitely the preamp issue and definitely right side output) is check for +/- 45V on the power board and I am reading that per the checkpoints on the schematic.... Questions and/or help would be welcomed. Thank you.
Relays kick in and sound seems clear from left channel. Same on both A and B inputs.

 

[Alobar]

Same on both A and B inputs.

Have you tried the headphone jack? I'm not an expert, or even a novice but I do have one of these in my daily use so take an interest..

 

[herbd2]

Have you tried the headphone jack? I'm not an expert, or even a novice but I do have one of these in my daily use so take an interest..

I believe the headphone jack is tied to the outputs...but no I haven't tried it.

 

[mbz]

As a first step dowload the service manual.
Power off/unplug the unit, open it up and do visual inspections for any bulging caps (receeded plastic sleeve) and check for dry
solder joints (google for pix) and charred resistors. Problem is most likely caused by poor electrical contact either switches or solder
joints. Once this is eliminated will look at component failure. Will try and work out some measurements to help track down the issue.

Need to deoxit/faderlube switches and pots and check for dry solder joints

 

[gslikker]

Hi Herbd2, work backwards from the output. So the low filter board is basically where the output comes from. Familiarize yourself with the schematic just going back from output to this filter board, the volume control attenuator, and back from that. Having headphones does help, as touching a boards inputs with a measurement probe will probably give some spike which is amplified and audible.
The headphone amplifier is connected to the outputs, as Alobar states.

An idea is also check all dc voltages on all boards on different places (they should be the same at left and right cannel locations everywhere, isn't it?) Do not hurry but take the time to familiarize yourself and check everything you see against the location in te schematics.

For pictures of the boards without being covered with cables and some more information on what was wrong with mine, see my page on the unit here. http://members.quicknet.nl/gerard.slikker/kenwood-700c.htm

The areas of concern in this amp are deteriorated parts, because it is old.

Unfortunately, I have not much time for "handholding" ...

 

[herbd2]

Hello and thanks guys. I think I got lucky! I did what you said and started at the outputs working backwards. When I got to the low filter board, I found 2 loose solder connections on one of the film caps. I resoldered both legs, put back together and now I have both channels. So I will consider this a serious case of good fortune as I was planning on spending most of the day working on this. Thanks again!

-Herb

 

[gslikker]

ok enjoy the amp, it is worth it!
You have to be careful that all the toggle switch covers feel secure. If loose, they really can fall out of the openings and get lost. So if loose, do not hesitate and make sure glue them back. I bought mine missing such a cover