Phonoland

Hello,

The way I test amps is to put a signal into the inputs of the amp and check for output at the links (the links on the 13 pin jumper plug, you test at pins 1-3 and 5-9) The ouput here will be around 500 to 600 mv.
Basically you are testing the pre-amp, by using the dual beam scope you can compare the two channels against the schematic.
If you connect the the signal to the links you are then testing the output stages. Or you can test input to full output and again compare the two traces. These traces on the 125, 130, 250 watt amplifiers are a mirror image of each other because on these ranges of amplifiers the output is inverted to give optimum bass.

The way it was done on earlier amps such as the 602-02179 (or 2179) where the amp was in a normal format was to inverse the input at the cartridge.

If you are in the USA I suggest you send in your amp to Bruce Wentworth, here are his details:-

Bruce Wentworth
A&B Jukebox Repair
56 Old County Rd.
Plaistow NH 03865
Tel: 603-382-6272
http://www.abjukeboxrepair.com

Regards
Alan

Alan Hood
ami-man
UK
alan-hood@datex.co.uk

ami-man wrote:
With regards to the volume control, I would make up a test lead using a 10k log pot connected to a 5 pin straight molex plug connect the wiper to pin 3 (ground) and connect pins 1 & 3 together and connect to one end of the pot.

Alan Hood
ami-man
UK

Turns out both amplifiers are fine in all respects. I had someone bench test them. Still can not explain the missing channel and max. volume.

In the box, pulling the mute plug should allow for the amp to work, but this actually mutes the amp. Plugging the mute plug back in brings it to full volume, even when nothing is playing. Had no trouble on the bench test.

So now I'm trying to troubleshoot the volume control section. Alan, please verify for me the instructions you gave shown above. Should the wiper go to pin 3 (ground) and 1 & 5 go together, not 1 & 3? Could I do pins 1 and 5(?) separately to check each channel? (Selector switch in stereo, of course.)

-Glenn-

Glenn,
Very strange---
Are you having the mute problem with the mech in play or scan position ? Sounds as if it's in scan position, as that's exactly what it should do in that position--put the mech in play without a record on the TT (you will need to back-off the "no record trip" screw, or dis-connect the wire to it)
Yes 3 is gnd, 1 and 5 are the two channels--are you using the correct value pot ? Are you using the INTERNAL (rear outside of the cabinet) control--is the "jumper" set correctly ? Or are you using a "remote", plugged directly to the amp ? And yes--grounding one at a time will control that channel--
Ron Rich

Hello Glenn,

The dual control is wired as follows:-

Pin 1 linked to pin 2 and goes to one end of a 10K log pot
Pin 3 goes to both wipers of the two controls
Pin 5 goes to one end of the second 10K log Pot

Regards
Alan

Alan Hood
ami-man
UK

I ended up wiring the volume control directly to pins 1,3, and 5 just to get the box up and working. Sounds great!

Then you either have a short, or open, in the inter-cabinet wiring going from the VC plug on the amp, to the VC on the cabinet--
Ron Rich

Hi,

The issue with the cabinet wiring could be a open/sort or it could just be that the wires have been moved from the tags or that the jumper link is in the incorrect position.
This is why on first test on an amplifier I go direct to the amp with a volume control, you can then test with the cabinet wiring and sort it out.

Regards
Alan

Alan Hood
ami-man
UK