Fisher 400 My next project with a lot to learn

[curtsad17]

I'm Back 4:17 PM. I think I found it, Rookie mistake. I have the collector and base switched on the 2- KSA1013. I drew a drawing and had them interchanged, I do not know where I got that info. Do you think they are still good? I will check the rest of the components the best I can, and figure out how to rearrange the circuit. These KSA1013 are ECB grindfix you called it.

 

[grindfix]

If resistor smoked they could have been damaged. De-solder at least 2 legs of both transistors and check them with mult-meter's "diode check" function. Sensitive parts like that I often order double quantities especially when they are this cheap.

 

[curtsad17]

Down and dirty,, I switched the legs on the transistors and hooked up the board before mounting it. I read your suggestion about checking the transistors after I was done.
I think I bought 10 transistors.
With line voltage @122vac via variac I get the voltages on Dave G's schematic 435v and 313v at the grids. I can adjust the bias output to -15v . Looks like I dodged a bullet. The 15ohm 5 watt that was smoking hot is resting nicely @ 93* F

Thanks grindfix for mentioning earlier about the leg destination on some transistors. I made me check the data sheet and catch my mistake. Thanks much.

 

[grindfix]

I'm glad you got it under control. I use the forum's help so much that I'm glad I was finally able to contribute.
Good luck with the rest of your mods to 400! Lots of fun and it's worth it!

 

[curtsad17]

One problem fixed another arises. I now have very low output from the right channel. I swapped tubes 8 and 9 and also 10 and 11. Swapped output tube pairs, checked voltages across heaters in V 8,9,10,11 all about 14 volts. Looked around a lot for anything I may have nudged while connecting wires for the EFB board. Could the mistake with the transistors and R 138 getting hot have caused this, If so where should I start to look?
..

 

[grindfix]

Check your cathode resistors. One could have had it with almost no negative bias voltage. Your tubes were conducting way above maximum ratings. First place I'd check.

 

[curtsad17]

Ok will do, OTs were not in place.
Grid resistors are good.

 

[curtsad17]

I checks most components downstream of the volume control. The resistance between the lugs of the volume control varies greatly between channels???

 

[gadget73]

Not uncommon, the pots tend to age kind of badly and it leads to tracking problems. If you've already cleaned it, short of a new volume pot there isn't a lot you can do about it. If the imbalance is bad enough to notice, the balance control will sort you out. Its not a Fisher-specific issue, everything from this era used the same pots and have the same issues.

 

[curtsad17]

I think it is more than that but I am open to suggestions . Leads are connected , at 12 o'clock setting of volume control right channel 1.2K left channel (the working channel) 127 K

 

[curtsad17]

Poked around some more, gave some areas a shot of Deoxit. A few hours later I powered it up and the right channel is back but balance control needs to be way off center. The right channel dropped out again while playing after about 15 minutes. I have an intermittent problem I guess.

 

[grindfix]

That could be either of the switches on the front panel as well, not just volume and balance pots.

 

[curtsad17]

I found the culprit. It is the shielded lead from the volume control to V13 network. I hooked my meter up to the volume pot and started to poke around the area of V13. It took a while but it was obvious once I found it. Powered the unit on and I could control the right channel volume by flexing the lead.
I'll replace that lead, then on to the next mod. I am thinking IBAM should be easier than EFB. I am glad I did not do any damage with my transistor mistake. My confidence is rising.

 

[curtsad17]

Well it took me a while but I have finished up all of Dave Gillespie's mods as outlined in " Improving the Fisher 400" The unit is sounding very good. I have a question about adjusting the AC balance. I have the PI modification in place and I tried to use this quick method Posted by Larry :

NEW AND IMPROVED way, and the way I did it. This involves no desoldering, soldering, or math. Just a DMM and a Jumper wire with 2 alligator clips. AGAIN! TURNED OFF AND UNPLUGGED!!!!

NOTE: PART #'s change due to SERIAL #'s.
For those having units with serials of 29999 and below, the relevent part #'s the B+ to Ground is C75C.
For those having units with Serials 48001 and above, the relevent part # of the B+ to Ground is C52C.
It's the SAME PART Just uses different part # on the schematics.

1.) Using an alligator clip lead, short the B+ supply point to the phase inverter tubes to ground. This would be the 320V source that appears across C52C. This is the 4 section CAN CAP.

2.) Ground the common lead of your ohm meter.

3.) Measure and record the resistance value that appears at pin #1 of the left and right phase inverter tubes.

4.) Connect your ohm meter to pin #3 of the left and right phase inverter tubes, and adjust their respective phase inverter control for the respective value noted in Step #3 above.

5.) Remove the alligator clip lead.

I cannot adjust the 5k pot enough to match the pin 1 reading. I can get within 1.5k. I remember reading with the PI mod, Dave had mentioned some imbalance may be desired due to the construction of the OPTs . I need some help with this one. Attached is a photo of the underside, and the area of the PI mod, of the finished 400. If anyone sees any issues , please feel free to comment.

 

[grindfix]

Did you do this mod?
Did you check resistor values before you installed them? I always do. I actually caught Mouser's packaging mistakes twice within last 5 or 6 orders!!! Other then that, recheck your work. Do you have same problem with both channels?

 

[fred soop]

Always check the parts before using. Todays resistor colors are not as clear, as large, or as uniform as vintage Ohmite. Capacitors are now very small and some of those markings are also difficult to read.

 

[curtsad17]

Yes I check the resistor values when I remember . Both channels are close. Pin 1 resistance measures 48.97 K and 49.03 K Pin 3 fully adjusted measure 47.93 47.66. Another member had a similar problem after installing this mod on a Fisher 500. There is pair of parallel resistors just before the 5k pot, which goes to ground, that have an equivalent Resistance of 42.7K I figure I can replace those two resistors with a 47K and that should allow the 5k pot to adjust the balance and be near the center of it's range. Am I thinking correctly?

 

[grindfix]

Have you checked voltages on PI tubes? Do they match Dave's specs? I believe this mod includes moving plate supply to a lower voltage source. Did you do it?
Another thing, make sure your capacitors are drained (to 0.00v) before trying to adjust AC balance.

 

[curtsad17]

Yes I moved the plate supply voltage of the AF stage to 306V ( Dave's Schematic 310v) My AF stage plate voltage 98V vs Daves 90V and My PI stage Cathode voltage 99V vs Daves 91V

 

[dcgillespie]

Curt -- Congrats on all your progress!!

Regarding the AC Balance control setting with the modified power amplifier circuit; the efforts to adjust this control as per my general instructions for setting such controls in Fisher gear (that Larry kindly posted) won't work in this case, because the lowest distortion setting of the control is NOT produced when the plate and cathode legs of the phase inverter stage are made equal -- as is customarily the case. As a result, the control is intentionally designed so as to be in a centered position when the lowest distortion setting is typically produced.

Even when highly matched components and tubes are used on each side of the push-pull connection, an AC Balance control will allow any remaining minor differences on each side of that connection to be precisely balanced out to achieve production of absolute minimum distortion levels. However -- this all assumes (and it's an assumption regularly made), that the OPT itself is perfectly balanced, with the AC Balance control then only accounting for minor variations in the tubes, capacitors, and resistors. In the course of developing the modified power amplifier circuit however, it was determined that this was one of those cases where the OPT is in fact NOT perfectly balanced, so that even when precisely matched signals are presented to each side of the OPT's primary winding, minimum distortion is not produced. Ultimately, the amount of imbalance was determined, and then baked into the design of the AC Balance control circuit, so that with the circuit otherwise well balanced, minimum distortion is then produced with the control set at a centered position.

This approach was adopted, because it was felt that many (if not most) AKers would not have the appropriate equipment to measure and set the control for a minimum distortion setting. By designing the control's action the way it is however, it allows for those who do have the equipment to achieve the lowest possible distortion setting, and those who don't, to account for the built-in imbalance of the OPT, and therefore still achieve a very low distortion outcome if good attention is otherwise paid to the remaining components comprising the push-pull connection.

I hope this helps!

Dave