Hello all. I've got this old reel to reel -mostly- working, however I'm getting a severe amount of distortion at the midrange and lower frequencies on the input side, which feeds through to any recordings I make with the machine. The only way to get a reasonably flat response is to turn the input knob to a very low value, to the point that it's barely audible on the tape. Playback of pre-recorded tape from another machine sounds fine.

Only modifications that have been done is replacement of T6 "AC187" which was working intermittently. I adjusted DC bias of the output pair as they were sitting at over 20mv, stable at 6mv now. All other components including caps are original.

I checked the voltages around T10 "BC108A".
Voltages seem a bit high - Collector 12.96v, Base 0.44v, Emitter 0.65v
Spec is 10.8v, Base 0.1v, Emitter 0.78v

Also have no movement of the input level VU meter but that's the least of my worries at the moment.

You guys have been great help to me before, if anyone has any advice I'd be very grateful. I've attached the schematic.

Regards
D

 

Welcome to AAC.

Classic signal tracing methodology would have you inject a signal into the signal path and measure the distortion at each step to see where the signal becomes distorted. Have you done this or something like it?

It will at least narrow down the locus of the trouble.

 

Hello all. I've got this old reel to reel -mostly- working, however I'm getting a severe amount of distortion at the midrange and lower frequencies on the input side, which feeds through to any recordings I make with the machine. The only way to get a reasonably flat response is to turn the input knob to a very low value, to the point that it's barely audible on the tape. Playback of pre-recorded tape from another machine sounds fine.

Only modifications that have been done is replacement of T6 "AC187" which was working intermittently. I adjusted DC bias of the output pair as they were sitting at over 20mv, stable at 6mv now. All other components including caps are original.

I checked the voltages around T10 "BC108A".
Voltages seem a bit high - Collector 12.96v, Base 0.44v, Emitter 0.65v
Spec is 10.8v, Base 0.1v, Emitter 0.78v

Also have no movement of the input level VU meter but that's the least of my worries at the moment.

You guys have been great help to me before, if anyone has any advice I'd be very grateful. I've attached the schematic.

Regards
D

T10 is the bias oscillator circuit, R441/2 are the recording volume controls, there is a section in the service manual for recording settings.

If you have a scope, then look at the output from T10 to see if it's oscillating .