1176 LN rev d. woes

[bobtheninja]

Hi,

I've taken it upon me to fix the 1176 LN rev d that lives at the studio I work in. I'm not the worlds greatest repairman, but I have built compressors and preamps and had success, and I really enjoy the process.

The symptoms are:

I can only get it to reduce gain by 5 or so dB on the 4:1 setting. I think this has to do with the pre-amplification stage of the compressor, that I can't amplify the signal high enough above the threshold to increase the gain reduction beyond 5 dB.

Also - the positive rail on the power supply is giving only giving me 27.68 VDC when measured on R80. My negative rail is -9.99 VDC when measured at CR9.

I've also measured voltages at each transistor (except for the FETs). See below for the results.

Component	C	E	B	Voltage
R80				27.68
CR9				-9.99
Q1
Q2	1,66	0,50	0,977	VDC
Q3	10,43	1,09	1,65	VDC
Q4	3,64	0,37	0,92	VDC
Q5	21,53	2,46	3,04	VDC
Q6	25,04	1,85	2,47	VDC
Q7	13,66	3,52	4,05	VDC
Q8	27,43	13,06	13,66	VDC
Q9	14,94	2,80	3,35	VDC
Q10	27,41	14,32	14,95	VDC
Q11
Q12	14,06	-1,60	-2,49	VDC
Q13	13,30	-1,51	-0,94	VDC
Q14	27,41	9,88	10,43	VDC

Let me know if you see anywhere I should be taking a closer look. And also feel free to hurl insults at me for being stupid and overlooking something - I know that I'm not very experienced at this, but I'm eager to learn.

Thanks in advance

 

[Rob Flinn]

Maybe compare your voltage readings to the attached diagram. If they stack up then I would be using a scope to trace the signal & side chain.

 

[bobtheninja]

Thank you for your reply, Rob. Much appreciated. I pulled the lightbulbs in trying to get the positive rail voltage up, and now I'm at 29,96 VDC at R80. I'm generally pretty close to the voltages in the diagram you attached (wow, that is very useful!!), most of the time a tiny bit low, except for at Q12 and Q13 where the voltages were coming in a little hot. I could get them down to the diagram by adjusting the trimmer R75.

I did some more troubleshooting and pulled out Q1 and inserted a 0dBu signal to the line input. When having both the output and input fully clockwise I measured 15 VAC at the output, which equals roughly 26 dB amplification. If I'm not mistaken this should be around 45 dB, so maybe there is a gainstage that isn't working as it should. I have an old scope so I will try to trace the signal through the circuit and see if I can figure out something through that some time this week. I'm guessing the problem has to do with the amplifier circuit not providing enough gain to get the signal high enough above the threshold to provide more compression than the 4-5 dBs I'm getting.

Thanks again! It's very useful just to document the process as well. Helps my thinking. Let me know if you have any leads.