Seduction bass performance question.

[Paul Joppa]

This problem has come up before. The two most likely causes are an error in the RIAA circuit, or a failure of one of the bias LEDs. The resistance and voltage checks will most likely provide enough information to diagnose the problem.

 

[Grainger49]

I sent the schematic, resistance and voltage checks to him.  I'm sure he will post when he gets the data together.

 

[garrardfisher]

Terminal      Voltage
26,36          0,0
27,37          69.4, 71.9
29,39          68.7, 70.9
30,40          70, 73.6
34,44          134.6
32,42          73.5, 69.9

I don't see any major problems there.

All 4 LED's light up....is that not a guarantee that they are good?

 

[Grainger49]

If the LEDs light, they are good.  They will have 1.5V across them when lighted. 

I'm not looking at the expected voltages now but if you are within 15% either way, you are good.  I expect something in the RIAA circuit.  The Capacitors are the major part of it as are the 66.5k, 9.64k and 1M Ohm resistors.

Verify those values.

 

[garrardfisher]

Resistors are good.  66.4K & 66.5K    .997Meg & .999Meg

I'd sure like to avoid de-soldering 6 caps in order to measure their capacitance.  (If necessary, I will)
Besides that, it seems logical that this would be a problem that involves a component that is common to both channels because neither has sufficient bass.  If it were only one channel, I would suspect an RIAA circuit capacitor and be able to focus on that channel.

I verified continuity between the ground side of the 10n, 1.2n & 33n and ground plate.  All good. 

 

[garrardfisher]

BTW, all voltages are within 5%.  The B+ is 9% low. ( should be 150, I measured 136.4)

 

[saildoctor]

Hi Brian - is the Seduction stock or does it have the C4S upgrade?  What kind of tubes are you using?  Are the output caps the stock 'Orange Drops'?  You are dead certain that the each cap is where it's supposed to be right?  I remember I nearly goofed swapping some (the 33nF and 0.1uF I think) when I build mine because the ones I got were so similar looking.

 

[Paul Joppa]

Yes, the tube voltages look good, so it is probably not a bad tube.

The bass is controlled mainly by the 66.5K resistor, the 9.64K resistor, and the 0.033uF capacitor. A bad solder joint in this region would kill the bass. So:

Check resistance between T37 and T39, should be 66.5K. T37 to T40 should be 75K. Now get some clip leads if you don't have them (cheap ar Radio Shack, very handy!) and shore out the capacitor leads of the 0.033uF. Then measure resistance T37 to ground; it should still be 75K.

No good? then maybe the 0.1uF is not connected. Resistance from the lead that goes to T36, to ground, should be 1.0Meg.

Notice I am asking for cap leads, not terminals - so the solder joint between them is tested.

 

[garrardfisher]

Hi Brian - is the Seduction stock or does it have the C4S upgrade?  What kind of tubes are you using?  Are the output caps the stock 'Orange Drops'?  You are dead certain that the each cap is where it's supposed to be right?

Stock, no C4S upgrade (yet).  6922EH tubes.  Yes, orange drops.  Yes it is easy to mix these up, but I am dead certain about the caps and their locations (double checked with a cap number code chart).  Thanks for the suggestions, I really appreciate it.

I tested the resistance per Mr. Joppa's advice.

T37 and T39 = 66.5K 
T37 to T40 = 75.9K

T27 and T29 = 66.4K
T27 and T30 = 75.7K  (I'll have to buy aligator clips tomorrow, I don't have any)

That is if I have the probe on the bare metal of the terminal.  I can't understand why my DMM sometimes doesn't register when I put the probe on the solder.  Solder is conductive right?  Typically when I can touch the lead where it pokes out of the solder, then I get a reading....weird.

Maybe that is how my .1 coupling cap might be disconnected.  Really odd that both channels would be disconnected, even though soldered in place.
I will double check my 0.1uF connection and all connections in the RIAA circuit tomorrow.  The terminal eyelet is filled with solder, but maybe I didn't wrap the lead around the terminal well.

Please stand by....

 

[garrardfisher]

Continuity is good on the .1uF caps.  I probed the leads of the 220ohm and 66.5Kohm resistors and the .1uF lead.  So the solder joints are OK.

I also confirmed the continuity of the 10n, 1.2n & 33n caps the same way.  Probed the leads to test the solder joints. 

 

[garrardfisher]

Check resistance between T37 and T39, should be 66.5K. T37 to T40 should be 75K. Now get some clip leads if you don't have them (cheap ar Radio Shack, very handy!) and shore out the capacitor leads of the 0.033uF. Then measure resistance T37 to ground; it should still be 75K.

No good? then maybe the 0.1uF is not connected. Resistance from the lead that goes to T36, to ground, should be 1.0Meg.

Well, in the previous post, I reported good resistance values  for T37 & T39.  T37 to T40 was 75K too.

I got the clip lead made and shorted the .033uF cap.  It took a long time for the VOM to stop, but it landed on 60KOhms for T27 & T37 to ground.

What does this imply?

 

[Paul Joppa]

Read second paragraph.

 

[Grainger49]

Reheat the solder joints on all of these components. 

Paul is saying to measure from component leads so that the solder joint is verified.