Tone control problem in X-101-B in PHONO ONLY

[larryderouin]

Today I was playing some records and noticed a difference in tone from the left speaker (KLH 17) when running the X-101-B. The Left side sounded muffled, almost like the Left Tweeter wasn't working. Right side sounded normal with both drivers working.

CONTROL POSITIONS
Mode = MAG 1
Selector = STEREO
Volume = 2
Balance = Centered
Dimension = MAX
BASS & TREBLE = Centered
Loudness = OFF
Hi/Low Filters = OFF
Tape / LP EQ = TAPE and LP. LP is definately quieter than tape, a little thinner sounding, and with less bass. But no difference in sound left to right. Left is STILL MUFFLED.


So I did some switch work which discounted a speaker malfunction.
Stereo/Mono Selector Set to REV = Muffled sound swapped sides.
Function Set to AUX, FM, and TAPE MONITOR (Aux has cassette, FM is KM60, Tape Mon is TEAC 4010GSL). Muffled sound STOPPED and normal sound resumed.

So muffled sound is only on PHONO So Far.

Tubes: All tubes are NEW Russian tubes from Jim McShane with the exception of the Rectifier. Rectifier is a Mullard GZ34 from my TA-600, branded IEC. I had gotten a Sovtek GZ34/5AR4 for it and had been used for a month, but now a spare. The 12ax7's are 2-12ax7LPS in Phono, 4 Tung-SOL 12ax7 Re-ISSUES in Tone Control and Phase Inverter positions. Output's are Tung-Sol 7591 ReIssues. 12ax7's were Matched in pairs by Jim, and the output's were a matched quad. They seem to like the bias on the 101 # 43v to 44v @ pin 5.

Testing. I tested all tubes on my Philco 9100 (rebranded Hickok 533b) for shorts and gassy tubes, gm and emissions. All Normal and all within 1% of new readings when I got the tubes.

1.) Phono Tubes(V6 & V1) were swapped L to R and R to L. Phono tubes are 12ax7LPS. NO CHANGE on
PHONO

2.) Phono (V6 & V1) were swapped with both the Tone Amps (V7 & V2) and then the Phase inverter tubes (V8 & V3) V2,V3,V7, & V8 are all Tung Sol Re-Issue 12ax7's. These were all matched by Jim McShane.

3.) OUTPUT TUBES(LEFT SIDE V4 & V5) were swapped with (RIGHT SIDE V9 & V10) and put back in original position after testing revealed NO CHANGE!

I then removed and moved the RCA's from MAG 1 to MAG 2/cer. NO CHANGE. This unit had a full recap done to it back in Aug/Sept rightbefore I had the back surgery. This is a new development I noticed today. I'll go thru the Phono stage and check for parts that are touching others, dry joints, and check resistors for bad or out of spec readings. I've got plenty of caps so I can change the caps if needed. I can rebuild the PEC's for PHONO if needed. but with the way it sounds I think it's ok.

If anyone had other ideas, I'd appreciate them.

Thanks

Larry

 

[gadget73]

sounds like an issue specifically in the phono stage. If all the resistors and whatnot check out OK, maybe try swapping the two phono PECs to see if it moves to the other channel.

 

[dcgillespie]

Larry -- What happens if you swap the phono interconnects between the two channels (L to R, R to L). Does the muffled sound still stay in the left channel?

Also, if you switch the unit to mono operation (still in phono), do both channels sound the same then?

Dave

 

[larryderouin]

Swapping RCA's makes no difference. Turntable being used is a Technics SL-1200 (Servo speed control) MK-1. Cartridge is a Shure V-15-III, Jico HE stylus running @ 1.25grams. Original RCA Cables, I ran continuity check on them and got a resistance of 6 ohms for 4 ft cables, or 1.25 ohms per foot. I just went thru all the connections underneath last week before putting it back in service, reflowed all suspect solder joints on the boards. Cleaned and lubed. Speed on strobe is accurate to 1/2 mark per minute @ 33-1/3rpm. That's the closest I could get it to perfectly still. Not too shabby for a servo unit I would think.

Mode Selector is "Mono Phono, Reverse, Stereo, A(LEFT), B(Right)
I used Tijuana Brass "Going Places" as test Record for this. Lots of high brass, and hellacious separation. All controls centered except for Mode Selector as follows. Volume at 10:00. Scared the wife on 3rd Man Theme, after I told her it was gonna be loud! Cranked it up to 1:00!!!!

Tijuana Taxi, 3rd Man Theme, Zorba the Greek were used for test.

NOTE: I ran the below test twice. Once with the RCA's Red Right, White Left, and then RCA's Red LEFT, White Right. No Change from the speakers. The instruments DID swap places which is normal. For test 2 swap Reverse and Stereo positions for Trumpet. I listened to the Trumpet and tried for the trombone but with it on the opposite side of the trumpet it was kind of hard to pick up as it was being overpowered by the trumpet most of the time.

MONO PHONO: both speakers diminished slightly compared to stereo. It sounds like the The Right side is over powering the left side.

REVERSE: 1st Trumpet on Left and Very DULL, almost like he's using a plunger for a Mute. All music on Left side is dull and muted.

STEREO: 1st Trumpet on Right Side and lifelike. No dullness.

A (LEFT): Dull and boomy in both speakers. It's as if I turned the treble all the way down and the bass all the way up, loudness on and cranked it up. All controls as noted above. Had to turn down the Volume as it made my V15III start jumping on the 1200. Set to 1.25gr. Speaker 4 ft sideways and 1 ft in front of turntable. No other position did this!!!!

B (Right): Sounding Normal thru both speakers.

 

[gadget73]

still kinda thinking it sounds like an issue in the phono stage. Big bass and lousy treble sounds like the RIAA curve is over-correcting. It should bump the bass and cut the treble but not like that. Looking at the PEC internal schematic, it seems to me that if the cap inside the pec shorted it would do that. You'd be able to measure that from pin 1 to pin 3 easy enough. Should be reading 2.27 meg.

Agree on that being a pretty decent test record. I have a copy and have used it myself. Also I like it, which is helpful.

 

[dcgillespie]

Agreed. The left channel phono preamp sounds like it is ground zero for the problem.

Dave

 

[larryderouin]

Gotta run to Delaware to see parental units. So it'll be a couple days before I do anything with it now.

 

[gadget73]

I tend to feel a bit shell-shocked whenever I have to go to Delaware too. The traffic over there, ugh.

 

[larryderouin]

Jeez, and I thought Northern N.J traffic was bad!!! They make a sport out of seeing how close to a red light they can run. A couple of times I thought I'd end up taking a shot of sub lingual Nitro, but wasn't needed. Offenders got past by the skin of what few teeth they had!!And I've got to go back on Saturday. Whoopee!!

Larry

 

[larryderouin]

STUPID ROOKIE SCREWUP's will get you every time!!

I started on this this morning. All of the values on the resistors are within spec, as is Pin 1 to 3 on both sides with readings of 2.65m and 2.71m (L & R). So I started working backwards from the 101 thru the connections all the way to the friggin needle. Remember when I had that squeeealing in the phono a month or so ago? Well it turns out the same cartridge was partially at fault, and a ground wire connection internally was also at fault. Cleaned up the ground connection with a new solder application and took each cartridge to headshell connection (both sides of the wire), and found both the LEFT Hot (+) and LEFT Ground (-) were both loose on the cartridge but on initial glance appeared fine as they were all the way pushed in. cleaned with Deoxit and a dental brush, Tightened and re-installed the cart on the turntable I'm currently using. BINGO! No more muffled tone in left channel. Swapped the RCA's just to make sure it wasn't the speakers and both are loud and clear!

I swapped in the PLA-45D with the same cart/headshell (V-15-III on a Stanton Headshell) and with the X-101-D turned on I engaged the PLA-45D. Sound was loud and clear in all ranges, and upon cranking up tone controls, then bringing up the volume to 1:00 had no squeeal. NONE, NADA, ZILCH! Lifted the cart off the LP and it was DEAD QUIET! No HUM either!

SO CALL THIS ONE FIXED! I'm gonna go out behind the wood shed and apply a nicely cut branch!!!

 

[dcgillespie]

Some issues you just shake your head at. Who would have thought? Bad connections can cause so many different problems..... Glad you found it!!

Dave

 

[larryderouin]

I was shaking fists at the headshells after that. Pulled every headshell with a cart on it and tightened up every connection on all of them. Thos mini needle nose got a workout too.

 

[gadget73]

I've done stuff like that. I was halfway through planning an extensive re-engineering effort on an amp to fix a serious high frequency roll-off problem when I found that at some point one of the terminals on a front panel pot had gotten bent and shorted to ground. If I'd measured both channels I would have seen the problem quicker but it just so happens that the one I decided to use for performance testing was the one that was screwed up. I didn't notice it sounding funny because the tweeter on the "good" channel was messed up, making it sound like both channels were extremely dull.

 

[7chord]

I recently fixed a guitar amp where I got myself screwed up and swore I had checked all the tubes. After replacing some burnt screen resistors I still had issues. All the voltages were right, but one tube had twice the bias current of the other 3 output tubes. Felt pretty stupid when I realized I hadn't done what I thought I did...

I think I confused that amp with the one I worked on just before it. Anyway it happens...