Sweet Linda came home today.

Oldskoolfan

Oldskoolfan

Journeyman
Joined
Aug 27, 2022
Messages
192
Location
Ste. Genevieve, MO
Got another Linda today. Beautiful condition. Nary a scratch on the face. Seller said she is blowing fuses. Has 3 service tags on it. Last one dated 10/4/89. I'll have to find out how she comes up on the variac and dbt. Interior and exterior is very clean. Well cared for if nothing else.20250616_144317.jpg20250616_144348.jpg20250616_144432.jpg20250616_144447.jpg20250616_144507.jpg20250616_144821.jpg20250616_144848.jpg20250616_144905.jpg20250616_144927.jpg20250616_145458.jpg
 
Well, I fired it up on my DBT just to see.... didn't blow any fuses, DBT worked like it should. No magic smoke. All seemed well so I checked rail voltages.
Positive rail was 60.2 vdc. Negative rail was -60.2 vdc.
AC input was 50vac both sides of secondary to center tap.
No dc on the output terminals.

I knew these voltages were low so I took out the rail fuses and read voltages again.
Rail voltages were 75.9 and -75.9vdc
AC voltages were 57.2vac both sides to center tap.

Something is loading it down pretty good. This should be educational.
Feel free to offer suggestions. I think I need to start pulling power transistors so here is my first question.
Do I start with the driver row or start with the top row?
 
Test the condition of the main filter caps first. I presume they are original issue? Leaning that way since it doesn't blow coming up slow. In-rush is another story...

Those being bad would drag it down pretty hard, but sketchy output devices would vaporize fuses at any reasonable volume.
 
Looks like you have a few different outputs. They should all be same if possible. Also what is the bias voltage. Also the drivers are different. Looks like who ever repaired it was not familiar with Phase Linear specs
 
Skratch said:
Looks like you have a few different outputs. They should all be same if possible. Also what is the bias voltage. Also the drivers are different. Looks like who ever repaired it was not familiar with Phase Linear specs
Click to expand...
I've got plenty of like drivers if that might be dragging down the rail voltage. Will the bias voltage be valid at this point? With the rail voltage down 20%?
 
If the DBT is still in circuit, those measurements are not that bad...
 
laatsch55 said:
If the DBT is still in circuit, those measurements are not that bad...
Click to expand...

Took the DBT out of the circuit and brought it up on the variac.
Rail volts are 75.4vdc and -75.4vdc

Left channel bias at .475vdc. Reset to .350vdc - stable

Right channel bias at .438vdc. unable to set below .384vdc - stable

Right channel output transistors are all RCA D3937-6 with No name C5869 pre-drivers

Left channel PL XPL910 with Motorola SJ2741

I have sufficient old/used transistors to match right to left. Think that might help?
 
Took a chance and hooked up to my cell phone for input and a pair of cheap test speakers - All seemed well for about 15 minutes then I was getting intermittent popcorn noise in the right channel. Enough to peg the meter so I shut it down. I think I'm gonna make sure I got good transistors and replace the right side so it matches the left and test again.
 
Likely those right channel outputs do not have the required Vce voltage rating
 
Many folks think that since the B+ and B- voltages are +/- 80V that they only need outputs that can handle 80V Vce. Wrong!

Each has to do 2x that or 160V plus a bunch of extra volts of margin to support current carrying capability when at the 160V extreme (driving capacitive electrostatics for example). Not easy to comprehend and that is why many repair techs go astray...
 
Ok, so here is where it stands; Sorted thru 3 full sets of output transistors from previous rebuilds. Came up with 8 FPL 909's, 2 RCA410's, 4 RCA 66546's that were indicated good on my transistor tester. Installed 6 FPL 909's and the RCA 410's in columns 3 & 4. Been running now for 2 hours without a single click or pop. Only outstanding issue is bias won't go lower than 366 millivolts and exhibits some minor fluctuation (+/- .2millivolts). Could this be something with Q6?

14A control board BTW.
 
Guess I spoke too soon. 20 minutes after I posted above, it popped. Put a voltmeter on the right channel output and I'm getting a large DC spike. This is going to be a bear to run down as it's so intermittent.
 
That's a different matter.

But, do be careful of creating collateral damage during the learning process. I know I usually do...
 
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