Paramount V1.1, 300B static/distortion

[Frank Breech]

Hello all,
It's been a while since posting here. I'm the happy owner of a Seduction-FPIII-Paramount 300B (& Crack), all stock. Put all these together over the course of a year or so, close to 10 years ago now. Been enjoying them trouble free ever since. Just recently one of my Paramounts started producing a staticy/white noisey/crackily distortion (out of the speaker) intermittently. It was VERY intermittent (i.e. "what was that?! did I just hear a crackle?!") and then suddenly became worse (getting fairly crackly up to 20% of the time it is up and running last weekend).

So, I'm just a "follow the instructions" kit builder and these Paramounts v1.1 are totally stock. Thanks in advance for any tips you have to help me get back up and running. Here's what I have so far ...

-Swapped tubes between good Paramount and noisy one. The problem does NOT follow the tubes.

-It produces the noise to the speakers even when no audio input is attached, so it is nothing upstream of the Paramount.

-I took the unit in question off the gravity mount and broke out the old multimeter (actually, I had to buy a new one) and resistance tests were normal with the exception of the following outliers:
-Terminal 9: should be 124Kohms, actual - 300k
-Terminal 11: should be 3Mohms, actual 20Mohms (this may not be that bad but figured it was worth mentioning)
-Terminal 19: should be 128Kohms, actual - 300k (coincidence?)
-B4 (nine pin socket): should be 130Kohms, actual - 300k (another coincidence?)
-B6 (nine pin socket): should be 177Kohms, actual - 4.7Mohms

I know it doesn't smoke, so I powered it up and did the voltage readings from the manual. Soft start works according to spec. All other readings were pretty much spot on, with the following exception:
-Terminal 9: should be 200vDC, actual - ZERO (yikes!)

Seems like Terminal 9 is a troublemaker. I should have time over the next day or so to resolder that point. (gotta find that iron). Aside from a bad solder at terminal 9, what should I be looking at?

FYI - the 294 Kohm resistor between terms 7 & 10 tests as a an almost perfect 293Kohms. I'm not sure how to test is that .1uF cap at terminal 9 is good or not ...

Again: thanks in advance.

 

 

[Paul Birkeland]

Is this a 12AT7 or 5670 Paramount?

If 19 is 0V, you wouldn't get sound out of the amp.  If you're getting sound out of the amp, 19 isn't 0V.

[Frank Breech]

Thanks for the reply.

So sorry - of Course I meant to say these are the 5670 Paramounts.

I had double checked that I had a good clip on Terminal 9 for sure when I got that 0v reading. But I just fired up after reading your message and am indeed getting music (& distortion) out of the amp so I will go back in there and reflow that joint and retest and report back.

I should be able to get to that this evening (I'm in Pacific time, too). If you see this in time, and there's anything else you think will be valuable to check while I'm in there, now that you know this is the 5670, let me know.

thanks again

[Paul Birkeland]

It would help to post:

IA, OA, Breg, and Kreg voltages on the driver board.

On a kit that's this old, it's far easier to understand what's going on if we get those voltages.  Otherwise we have to delve into old manuals to figure out what terminal 19 goes to (looks like it goes to OB).

[Frank Breech]

Hello,
Here are those readings;
IA - 468vDC
OA - 296vDC
OB - 174vDC
KregA - 4vDC
KregB - 5.5vDC

Yes, it does look like Terminal 19 goes to OB. It makes a stop at pin 4 of the 5670.

I retested terminal 9. It seems like when I touch the solder I get no voltage. That must be how I was clipped on to it yesterday. I used the probe to touch the terminal directly and got 174vDC.


Thank you.

[Paul Birkeland]

Those voltages are fine.

I would bet you have a loose joint somewhere causing the distortion, but not in a place that will manifest a voltage issue.  Something silly like a disconnected cathode bypass cap on the output stage would allow for perfect DC voltages but not such great sound coming out of the amp.

[johnsonad]

Does it happen or get worse when you turn the hum pot?

[Frank Breech]

Johnsonad,
No, the hum pot does not seem to affect it. Thanks for the suggestion, though. I hadn't tried that.

Paul,
RE Something silly like a disconnected cathode bypass cap: In this case, would that be the 47uF 450v cap between terminals 5 & 10? (see attached) I can only "kind of" read the schematic. Any guidance on which cap to target is helpful.

thanks Again fellas

[Paul Birkeland]

I'm not saying it's the cathode bypass cap, I'm just giving an example of something that could be disconnected and would cause substantial sonic problems, but would otherwise leave the DC voltages alone.

[Frank Breech]

Understood. It can’t hurt to reflow solder at the cap you mention and I don’t have much else to go on. I’m just trying to understand which one it might be. I don’t have much experience reading the schematics so was just looking for a confirmation if I ID’d the right one.
Thanks

[Paul Birkeland]

Yes, it's the electrolytic cap mounted by the 4 pin socket.  It's not just the cap itself, but any wiring going to and from the output transformer would have the same effect.  A loose coupling cap could do the same thing.

[Frank Breech]

Hi Paul,
Just a quick thanks for jumping in quickly to provide help and get me going to repair this. That's it. The customer service is appreciated.  No need to read on, but here's what I did, at least for my own future reference.
thanks again!

 I have this week off so my top priority was getting this amp back in action. Resoldered points along the path you mentioned to the best of my ability to figure out what that path was (!). When I was reflowing a point on the OT the conductor/tab appear that it had a pretty tenuous connection to the transformer. I made sure that was jammed back in the OT nicely. I'm thinking that may have been the culprit all along.

I was also worried about the 4.7MegaOhms I was reading on pin 6 of the 5670. Resoldered that pin and all the points back to it that I had that high reading (which was basically just up to the transistor on the driver board). Retook the measurements and got 300k ohms at pin #6, which is coming up on twice as much as the spec, but matches the other Paramount perfectly. If you're still reading: is there any reason to chase down the issue that is causing this 300k ohm reading where the spec calls for 177k ohm?

Have had it powered up for the last 10 hours. Not a hint of the distortion/crackle. Very relieved to have it up and running for the holiday.

[Paul Birkeland]

300K at pin 6 would be correct.