Restomod Dynaco Stereo 120 — high DC offset

[TXGardener]

I am rebuilding a Dynaco Stereo 120, with UpdateMyDynaco power supply and replacement caps, and the eBay TIP-mod amplifier boards, and I am seeing two issues that I could use some help with. 1) High DC offset (~200mV), and 2) the #4 and #14 eyelets are reading 71.3V instead of 36 and 37V, respectively. I measure 34.09V across R C7 and 34.12V across L C7. Just about every piece is new, and Q5 and Q6 are 2N3055s. Is there an obvious place to start troubleshooting the DC offset? Is a ~200mV DC offset even a problem on a TIP-modded Stereo 120? Thanks in advance.

 

[TPettenati]

1) Cap coupled amps usually don't have an adjustment for DC offset, and you may need to connect an 8 ohm dummy load (or speaker you don't care about) to get a realistic number.

2) Which board, left or right? Just noticed that the test points are different on the schematics, right versus left. Also what is on #7/#8 on both boards? It should be approx. 1/2 of the 72 volt supply.

Use your Test Points picture and write down every voltage for the left and right boards. Maybe it will make more sense.

 

[TXGardener]

Thanks for responding, @TPettenati. Seems like maybe I jumped the gun a little. The instructions did say that measurements should be taken with the inputs shorted and an 8 ohm load on the outputs (speakers). Well, I followed the instructions, and here are the measurements. Looks like the DC offset is actually quite low (3mV on eyelet 11), so I hooked it up to my A-25s and it sounds pretty sweet.

I am still concerned about point 2 on the left and point 14 on the right running about twice as high as they should be, but everything else seems right on the money. ¯\(ツ)/¯

Spec (V)	Left PC-14 (V)	Right PC-14 (V)
01. 72/0	71.3	0
02. 37/36	71.3	34.31
03. <0.5	0	0
04. 37	71.3	71.3
05. 72	71.3	71.3
06. 36	35.03	35
07. 36	34.4	34.32
08. 36	34.4	34.32
09. 71	70.6	70.6
10. 71	70.6	70.6
11. 0	0.003	0.003
12. 0	0	0
13. 0	0	0
14. 36/37	34.35	71.3
15. 0/72	0.001	71.3

 

[TPettenati]

#4 on both boards is wrong. Looks as if the path from #4 through Q4 is open. The path is #4 thru R28 to Q4-E to Q4-C thru R14 to ground. Check to see if Q4 is the correct transistor and installed correctly, or possible blown.

 

[TXGardener]

Thanks. I’ll pull one of them and test it, but it seems strange that both Q4s would be bad.

 

[TXGardener]

I checked and rechecked Q4, and there were both installed properly. Then I looked at the wire connection points again, and that's where I discovered my issue. It turns out that the wires at #4 on both amps were not fully soldered to the back of the PCBs. So, I slathered both sides of the connection point with solder to make sure that the wires were 100% making contact, and...#4 is now reading 36.3 on both channels! Thanks for taking the time to help me. I appreciate it.