Anyone have experience with Collins 6P-1 Preamp repair?

[outoftune]

I have a pair of these in the shop for repair, they are racked with a modern PSU.

One channel has extremely high distortion, other channel lower gain than it should and noisy. I've tried swapping the tubes between the two channels but the issues seem to stay in the respective units.

I've been able to find a copy of the schematic here: https://bunkerofdoom.com/lit/collins_6p-1_BC_preamp/Collins_6P-1_Broadcast_Preamplifier.pdf

Aside from the schematic I haven't seen very much online. Just wondering if any of you have experience with these units and might know what is causing these issues? Any help would be appreciated.

 

[pucho812]

Well,
Take it from the top, check psu, checking with an oscope

 

[zamproject]

As pucho said !
Looking at schematic, topology, and part count, it should not be that difficult to find the issue ?

 

[rock soderstrom]

Yep, check PSU, all voltages and test all caps for leakage and capacitance. Any DC on the grids? Are the anode resistors still in tolerance?

Is R104 really a 100M (like mega) potentiometer?? I doubt it.

 

[zamproject]

Is R104 really a 100M (like mega) potentiometer?? I doubt it.

Yes that's probably a typo ?
Part list ask for fixed 0.10M ? 100k... not a pot
Also general description say fixed gain, pot is maybe an trim option, as with R105 divider you don't get a -volume- control from 0 to full.

 

[rock soderstrom]

Yes that's probably a typo ?
Part list ask for fixed 0.10M ? 100k... not a pot
Also general description say fixed gain, pot is maybe an trim option, as with R105 divider you don't get a -volume- control from 0 to full.

Yes, that is contradictory. I think it is a 100K pot to realize the "10VU" gain reduction as described in the manual. R105 defines the minimum gain.

Let's wait what @emrr Doug is saying.He knows this stuff.

 

[nielsk]

Check all the caps, these are often the problem.

 

[vitopower]

"M" ohms denotes "K" on the Collins 6q-1 schematic

 

[emrr]

Noise here is gonna be 1st stage plate or cathode resistor most likely. The oil caps are prob good, and much of the tone, unless they are really high ESR. The bathtub electrolytics can be peeled open like a sardine can and restuffed; actually the easiest mechanical solution. I show that in a post somewhere…

 

[rock soderstrom]

"M" ohms denotes "K" on the Collins 6q-1 schematic

Thanks for clarifying this!