Assistance/advice sought re problems encountered in upgrading Stereomour

[rusty-jade]

After much procrastination, I decided to upgrade my Stereomour.  I replaced the 1.2K and 360 Ohm resistors on the PCB with Triad CX-7 chokes.  I also replaced the 100uf power caps with Solen 33uf 630V film caps and also used Mundorf 47uf, 250V caps as bypass. A picture of my last attempt is attached.  There were some difficulties encountered because of lack of space and I had to use 23awg hookup wires as a trace of the circuit and some of the circlets on the PCB had come off when desoldering and removing the stock parts the first time. After finishing the soldering of the upgrade, I checked the resistance and voltages per the Stereomour manual and everything was normal.  However, there was a slight buzz ( like a fly) audible only when music has stopped.  Suspecting it was a ground solder problem, I resoldered some of the parts w.r.t. the upgraded parts. But still, the buzz remained.  Over the next several days, I resoldered several times, plus some wires connected to the PCB as they had snapped, but all in vain as the buzz still remained.  Lokking back now,d I should have left things alone as the upgraded sound was excellent aside from the buzz.  Unfortunately in my last attempt, besides the buzz, there was a popping noise.  I switched all the tubes but the pop remained. Then I measured the resistance( all normal) and voltages and here is the anomaly:

- on T14, only 76 volts.  I suspect this may be the filter cap, but the strange thing is that I have a meter that measures capacitors and the cap connected to T14 measures exactly.1uf when not powered on, same as the other channel.

- on the Active Load Board, OB measures only 76 volts ( instead of 230), AND
- the Kreg on the "b" side measures 0

Since the low voltage is the same onT14 and OB, is there some connection? I am not sure how to proceed. Advice/suggestions from anyone will be greatly appreciated.

Brad

[Paul Joppa]

For the low measured voltage, the immediate problem is that the driver cathode (tube pin 8 and kreg on the B side) is grounded, whereas is should be at 2.5 volts as set by the 431 regulator chip. Once the cathode is biased up at 2.5v, the plate voltage (OB is in fact connected to T14 and to the plate of the driver, tube pin 6) should rise to the desired 230v. Note this 230v is more variable than most; if it's between 180 and 280 you will be good to go.

The question, however, is "why?" It could just be a wiring error - for example, if tube socket pin 8 was bent to where it touched pin 9 (which is grounded). Or it could be the 431 chip is dead. I don't see a way to kill that chip, since it was working before, so the shorted pin 8 is my best guess until more information is available.

The buzz is harder to guess. It may be just the wires to the filter chokes, which appear to run very near the sensitive grid  terminals of the driver. Looking at the picture, I'm pretty sure you didn't want to hear that! I can't see how those chokes are mounted, but if they and their wires could be moved away from the driver temporarily that might answer the question. It does not look like the wires are long enough to re-route them away from the driver tube socket, so if that is the problem then you'll have to do something more creative ...
but it's always best to be sure you know what the problem is before implementing solutions!

[rusty-jade]

Paul,

Thank you so much for your assistance.  I lifted the Active Load PC Board as much as I could w/o snapping the wires and looked at the driver tube pins 8 and 9.  They are not touching and in fact, I had no occasion to lift the active PC board and touch anything underneath during the upgrade; so it may be the 431 chip.  If so, is the guilty one on the "b" side of the board? I think I will order a new 431 chip tomorrow. Additionally, I noticed that the MJE5731A transistors were bent quite a bit during my installation of the upgrade. Could that also be a cause of the problem?

Regarding the buzz, the wires of both chokes are not close to the grid terminals of the driver, but on the outer side of the white bypass capacitors.  In fact they are closer to the power tube sockets than the driver tube. I have attached another picture to indicate this.  If you can think of anything else, please let me know. . Thanks again for your help.

Brad

[Jim R.]

If the buzz doesn't go away with the repairs to the regulator chip, try subbing the resistor back in for one choke and leave the other ps filter choke in place and listen to see if the buzz on the side you put the resistor back in is not buzzing.

I can't see the pics so not sure how the ps filter chokes are mounted but they could either be pickig up stray radiaton from the power transformer, or they may be radiating into the audio circuit.

-- Jim

[rusty-jade]

Thanks Jim.
My priority now is to fix the popping sound and the low voltages noted in my post above.  So I will try first by replacing the 431 chip.  In looking at the schematic diagram, I notice that one leg of the 431 is connected to ground and another to pin # 8 of the driver.  So if the chip is defective, its replacement may eliminate both problems I had described. Will let you all know.

Brad

[Paul Joppa]

I hope it works, please post in either case. We don't actually know it's dead, it may still be shorted somewhere, but it's cheap to find out. If it is indeed dead, I'd sure like to figure out how that happened so we can prevent it happening again! So keep your eyes open while fussing with it...

[rusty-jade]

This is an update to the issue I had described previously.  First, I wish to thank Eileen who had to send the LM431 not once but twice as the first one got lost somewhere in Washington state by the USPS during the holiday season.  I received the second one several days back and installed it onto the active load Board. The low voltages I described previously were back to normal but I had several other problems, like popping sounds, etc. which I managed to resolve.  However, the slight white noise I described previously is still audible on both channels - it is not loud or volume depend and can only be heard when no music is playing.  At this time, I do not know whether the original LM431 had failed as I had a bunch of other issues when I installed the new one which I managed to eliminate.  The soldering/desoldering was tedious because of the lack of space caused by the chokes, the large bypass caps and power film caps.  With respect to the last, I still have to somehow glue them somehow to make sure they are stationary.

But I still have another potential problem - on checking the resistance, I got 4.9K instead of 1.4K on T11 which I consequently resoldered.  I checked this anomaly many times and the funny thing is I got 1.4K a few times too.  Since one leg of the parafeed cap is connected to T11, I checked the capacitance and it was very close to 3.3 uf.  In addition, I am getting 4.7K instead of 1.2K on C1 and C4 pins.   The corresponding resistances on the A side terminal and pins were all normal.  The voltages between C1 and C4 were 2.56V, same as between A1 and A4.  However, the voltages on both C1 and C4 were 77 volts whereas the A1 and A4 were 64volts, so I do not know whether the higher voltages on the C side has any correlation to the abnormally high resistance values above.  My concern is whether this higher resistance and voltage will have any impact on the power tube or output transformer.  Sonically, the system is OK.  It has been so long that I cannot remember how the stock circuit compares with it. unless I revert back to the stock circuit.    I would appreciate any comments or suggestions on the high resistance values  on T11 and C1 and C4 described above.  Thanks in advance!!

[rusty-jade]

I just also noticed there is a popping noise in the left channel, the same side as the resistance deviations I described for T11, C1 and C4. Perhaps they are related. The popping noise is not tube related as I switched all the tubes and it still is there.

[Paul Birkeland]

I hate to be the one to say this, but I would remove the parts changes, one by one, until the problems resolve themselves.

[rusty-jade]

Thanks.  I did several things subsequent to your post, and I think most of the problems have been solved. To recap, I had a popping noise on the left channel, the slight white noise from both channels, fluctuation and inability to adjust the left hum pot .  Taking your suggestion into consideration, I replaced the 33uf power film cap with the stock power 100 uf electrolytic.  The problems still remained. Next I the lengthened the leads of the Triad CX-7s so that they could be looped over the  circuit boards and I swapped the positions of both chokes. Lo and behold, the popping noise on the left channel disappeared, and the white noise seemed to be less but the hum pot fluctuations still remained. So I then unsoldered the two Mundorf film bypass caps, resoldered the hook up wires on them ( their leads were not long enough) and switched their positions.  All resistance and voltage checks readings became normal. The hum pot adjustments now is 1.7 on the right and 2.0 -2.1 on the left. So essentially, I am left with the slight white noise, audible only when no music is playing. I believe this is caused by the CX-7s's induction as the noise has lessened when I lengthened the wires and there is not much more to be done with it because of lack of space. Eventually, I may remove the chokes and reinstall the 360 and 1.2K resistors ( with better quality ones) and replace the electrolytic cap I reinstalled with the 33uf 630v film cap I took out as mentioned above.