500B recap attempt reveals problems...

Winters

Active Member
As I mentioned in my recent thread about choosing the best location for the thermistor protection, I had decided it's high time to dig deeper in my 500B I purchased seven years ago, do a proper recap, add the IBAM and do several minor modifications, for instance modify the 'Tape Head' input to act a second 'Phono' input. I have never had a problem with the unit other than one 7591 tube that gave up the ghost several years ago. I did some basic maintenance on my 500B when I purchased it - silicon rectifier, .1uF coupling caps, screen resistors, cathode to ground resistors etc. It always sounded rather fine, but lately it seemed a bit under the dog - lost some power, some very faint squealing appeared etc.

I decided to tuck the new replacement caps underneath and I purchased Nichicon UPW low ESR electrolytics which are supposedly good for the job.

I started with some minor work - replaced the "death cap", started with the 'tape head' input conversion etc. If the latter works out fine, I'll make a "how to" post.

So I started to do some planning on the recap and noticed that there is something wrong in the power supply area...

First thing that caught my attention is that the R125 (22K) resistor looks overheated.
And then I discovered that someone had been there before me and replaced R131 (1.2K 1W) with a 12 ohm resistor!
Also, R133 seems to be missing!

In this photo you see the R131 resistor which looks way newer than the other components and is a 12 ohm resistor (1.2k on the circuit diagram). It connects C84A to C78C. I have deliberately cut one end off to measure it with an ohmmeter.

DSCF0591.JPG

And where is R133? It's supposed to go between C88 and C84 whereas there are only two things connected to the positive side of C88: CR3 and an orange wire that goes to the B point on the schematic (395V)

DSCF0594.JPG

fisher supply.JPG

I think I'm going to bring it back to original (as shown on the schematic). But why would someone introduce these changes? Is this indicative of some problem?
 
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Winters -- More than likely, you will find R133 over in the output stage area, which was a common place for Fisher to locate the screen grid dropping resistor. It must be in the set, or the set would not work at all. As for R131 being a 12Ω resistor rather than a 1.2K -- somebody either didn't look closely enough, or is color challenged. Either way, it's the wrong part, and needs to be corrected. With the wrong part installed, it's doubtful any serious damage was done, but 120 Hz hum may have been elevated somewhat with the volume at low levels and no signal present. Still, it would be a good idea to check the values of R123, R125, and R126 to make sure the extra current load they were carrying didn't do any permanent damage.

Good luck with it!

Dave
 
Winters.......... you probably realize the need to check the resistor string since you've found some problems already.
I recommend you start at CR3 diode and verify each and every resistor and capacitor.
Sketch it out on paper as you look at each solder joint and what connects to it.
Then go in with a DMM and note the DC voltages on your sketch.
Chances are they are high compared to the schematic, depending on your house line voltage.
Voltages should be within 10%, if they aren't, you may have to play with resistor values.

Jef
 
Thanks for the replies. Of course I didn't bother to check where the orange wire from C88 goes... Hard not to notice R133 which is perhaps the biggest resistor in the unit, a whopping 7 watter.

DSCF0596.JPG

I will check the other resistors in the power supply section. As I mentioned earlier, R125 looks like it's seen some heat as the enamel is flaking off.
The house line voltage problem is nonexistent - we have 230V line voltage in Poland and the Fisher is running off of a simple 2:1 step down transformer so it gets 115V or at times even less than that.
 
In the meantime I have built the IBAM board. It was quite tedious as I chose to build it as compact as possible. The board measures 4cm x 4cm. I should have gone with radial caps as the axials I chose are quite bulky and occupy valuable space on the board. I still have to drill a 3mm hole for the mounting screw in the middle, between the resistors.

One simple question - I can't find any schematic that would show what should go where. I guess that the input (blue wire on my board, it connects one side of all the trimmer pots) should go to the "V" point on the schematic (negative voltage source), the white wire (connects all negative ends of the capacitors and the resistors) should be grounded. The red wires are supposed to go to the 220K (originally 330K) resistors,right?

DSCF0599.JPG
 
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Thank you for the kind words.

After completing the tape head input conversion to act as a second phono input, replacing R125 and R131, replacing the death caps and adding the thermistor protection, I decided to fire it up and check the voltages before I start with the recap and the IBAM. I connected two 6.8 ohms resistors to the 8 ohm speaker outputs and fired it up.

With 115VAC, I get 393V plate voltage, between -14.5 and -15V on grid 1 and 360V on grid 3 of each output tube.
The tubes are a mismatched quad of EH 7591s. And it looks like I was running it on THREE output tubes for who knows how long... No wonder I felt there was something wrong with it!
When I tried to measure the current on one of the tubes, I got a crazy reading of 13V DC. The others read: 270mV, 390mV, 320mV which corresponds to 27mA, 39mA and 32mA. Obviously mismatched.
The 10 ohm cathode resistor on the problematic tube is OPEN. It doesn't look burned but there is some enamel missing.

A new matched quad of Tung Sol 7591s is on the way, should be here tomorrow.

What do I do now? Obviously that open resistor should be replaced but should I presume that there is something wrong with the tube? It doesn't redplate, it holds vacuum and has plenty of getter.

The open cathode resistor:
DSCF0603.JPG


How I mounted the thermistor - I replaced one of the self-tappers that hold the tuning flywheel bracket with a machine screw and used it to mount a terminal strip. The thermistor may look close to the plastic cap that ends the metal jacket but there is an inch of space between them.

DSCF0605.JPG


Best regards,
Pete
 
If you haven't replaced the output coupling caps, do so now. The 13V on the one tube cathode is indicative of a bad cap. Even new ones can fail abruptly. (I just went thru this with a Scott LK-72.). Same scenario.
 
I replaced the output coupling caps in 2014 using NOS Russian PIO K42Y-2 0.1uF 630V caps. They are supposed to be very close to K40Y-9 everyone raves about.
Is there any way to test them further or is it better to replace all four of them?

DSCF0608.JPG

If I should replace them, what to choose? As I'd rather avoid Russian PIOs, the options are:

- TAD "mustard" (supposedly replicas of the Philips/Mullard mustard caps from the 1960s)
- Mallory 150s
- Orange Drops
- SoZo Mustard NextGen (expensive)
- TAD "vintage oil" (crazily expensive)

Best regards,
Pete
 
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Take the "bad" tube and swap it into another socket and check the voltages again. If the 13V stays with the tube, it's the tube, if it stays at the same socket, most likely it's a cap.

If it's a Cap, either the Mallory 150's or the O.D.'s are very close to sounding what the Original ERO-FOL's. Nothing really wrong with the Russian K-42's other than the metal casings and close proximity to other components. A K-73 Russian cap would be another one to look at. Similar in construction to the 150's.
 
Well, actually none of the above.

I replaced the open 10 ohm resistor and all seems fine. The tube that previously gave me that 13VDC reading now gives 240mV which translates to 24mA. Could it have simply been a bad resistor?

In the meantime I redid the 100 ohm screen stabilization resistors using the unused pin 1.

DSCF0610.JPG

Best,
Pete
 
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Yeah the 10 ohm bad resistor would do that. I would definately install an IBAM or better yet the IBBA circuit. Tung-Sol 7591A (Russian's) tend to run "HOT" (ie: they want more negative voltage to work at a set ma than a EH. If the EH is running at say -18.5V and the ma is 34, then the Tung-Sol will need approx -22v to -23V. Concentrate on theCathode MA (LOADS) being equal rather than getting the grid votage the same (at that point the loads will all be different.). The IBBA is a better setup as you balance each pair to a 0.0v differential, then bias the pair to the same MA.
 
I've already built the IBAM board (there's a photo of it several posts earlier). I haven't installed it yet.
I heard about the fact that the Tung-Sols tend to run hot. Will the IBAM board let me dial in a negative bias voltage greater than what it is now (circa -15V)?
 
You might have to change the resistors to a larger value. Put it in and test it with the pots set for ZERO OHMS across them (max voltage reading). What's the voltage coming off the bias caps. Should be around -22v or so. Remove R129 and R135. This should bring the bias voltage up some and they get removed with installation of the IBAM or IBBA anyway.
 
Got the new Tung-Sols today. Without IBAM and -15VDC bias voltage they certainly run hot - I got 45-46mA on all four of them which translates to 18W plate dissipation when multiplied by my plate voltage (390V). The maximum plate dissipation for 7591A is 19W.

I get -24V on the second 1000uF cap.

I will certainly install IBAM in the closest future. Can I remove R129 and R135 before I do that and run the receiver without them to bring the negative bias voltage up some more? FWIW, the tubes I bought seem to be closely matched, so getting some more negative volts on them should do the job temporarily.
 
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Winters... I have the Tung-Sol 7591A in my 500-C and run them at 34mA which is about 70% of max. That's supposed to be the sweet spot for tube life and great sound. Running at 18W for a long time won't be too good for your tubes. Thorne
 
Winters... I have the Tung-Sol 7591A in my 500-C and run them at 34mA which is about 70% of max. That's supposed to be the sweet spot for tube life and great sound. Running at 18W for a long time won't be too good for your tubes. Thorne

Sure. I certainly don't want to do that!
 
Can I remove R129 and R135 before I do that and run the receiver without them to bring the negative bias voltage up some more? FWIW, the tubes I bought seem to be closely matched, so getting some more negative volts on them should do the job temporarily.

You need to be careful as the heaters for the phono preamp tubes (V7 & V8) and tone control tubes (V9 & V10) are part of that circuit. You could try a 10k pot to see how that works but I'd try to keep that filament string to those preamp tubes around -42vdc.

When you install your IBAM get rid of R129, R135 and C85. If the Tung Sols are still too hot decrease those resistors on your IBAM board to around 10k but don't go lower than 8k.
 
You need to be careful as the heaters for the phono preamp tubes (V7 & V8) and tone control tubes (V9 & V10) are part of that circuit. You could try a 10k pot to see how that works but I'd try to keep that filament string to those preamp tubes around -42vdc.

When you install your IBAM get rid of R129, R135 and C85. If the Tung Sols are still too hot decrease those resistors on your IBAM board to around 10k but don't go lower than 8k.

Thanks! I think I've got it now. I connect the IBAM "input" directly to the C79 negative end (can), am I right?


DSCF0612.JPG

If it's so, I'm going to install my IBAM tomorrow. I will have to split my 220K resistors as they are connected in pairs.

Best regards,
Pete
 
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