Should I power this thing up?(I HAVE NO VARIAC)

**ReeceyBurger123** · 08-01-2016, 05:37 AM

Re: Should I power this thing up?(I HAVE NO VARIAC)

Use diode mode on your meter.

**TechGeek** · 08-01-2016, 05:38 AM

Re: Should I power this thing up?(I HAVE NO VARIAC)

uhhh... I don't have a diode checker on my DMM, only resistance, VAC, VDC, and that's about it. Need a photo?

**ReeceyBurger123** · 08-01-2016, 05:47 AM

Re: Should I power this thing up?(I HAVE NO VARIAC)

Ahh right ok, its a little harder with resistance mode as it wont be able to find a voltage drop just resistance across the part/circuit. Maybe pick a cheap DMM up ? You can get them off ebay for £3 with probes with diode test. Still with resistance as its high as you said, chances of it being shorted is nothing.

**jiroy** · 08-01-2016, 05:47 AM

Re: Should I power this thing up?(I HAVE NO VARIAC)

Originally posted by TechGeek

uhhh... I don't have a diode checker on my DMM, only resistance, VAC, VDC, and that's about it. Need a photo?

It's okay , you can always test at 20 k-ohms .

**TechGeek** · 08-01-2016, 06:09 AM

Re: Should I power this thing up?(I HAVE NO VARIAC)

mine can go all the way up to 2000k. So plug it in and give it a go?

**jiroy** · 08-01-2016, 06:15 AM

Re: Should I power this thing up?(I HAVE NO VARIAC)

Originally posted by TechGeek

mine can go all the way up to 2000k. So plug it in and give it a go?

Put it at 20 Kohms (assuming it is digital and not Analog ) , and do tests on diodes and rectifiers without turning on the Fisher unit ..

**TechGeek** · 08-01-2016, 06:19 AM

Re: Should I power this thing up?(I HAVE NO VARIAC)

how do I check rectifiers?(i'm only quite knowledgeable in computers)
And check all diodes?

**TechGeek** · 08-01-2016, 07:57 AM

Re: Should I power this thing up?(I HAVE NO VARIAC)

Hello?

**ReeceyBurger123** · 08-01-2016, 08:25 AM

Re: Should I power this thing up?(I HAVE NO VARIAC)

Check on youtube on how to test a bridge diode easier to do that writing and possibly causing confusion, basically a rectifier has 4 diodes in side of it, normally the pins are labelled on it where the diodes are.

**rhomanski** · 08-01-2016, 08:25 AM

Re: Should I power this thing up?(I HAVE NO VARIAC)

Diodes are a single PN junction. They conduct in one direction only. On semiconductors the arrow always points at negative. Therefore put the negative lead on the band of the diode and it should conduct. It will still show resistance because it's a "semi" conductor. That's why a diode mode is nice. It shows the voltage drop. One way is infinite the other is around a half a volt. A rectifier is merely a diode bridge. It kinda makes a rough DC out of AC. So, figure out which leg is positive and negative and check it. Best done out of circuit. If you can find the paperwork on the rectifier it will show you how it's setup. I would suspect your unit is somewhere from 1976 to 1980 manufacture date. Before then they usually had four simple diodes for a rectifier after that they went to black plastic faces.

**jiroy** · 08-01-2016, 08:30 AM

Re: Should I power this thing up?(I HAVE NO VARIAC)

Originally posted by TechGeek

Hello?

Sorry Techgeek , i had some clients . Well , you treat a rectifier just like it is 4 diodes bridged together , the first one goes from ~ sign to the minus sign , the second goes from ~sign to the positive sign , the third goes from the plus sign to the negative sign and the fourth goes from the plus sign to the negative sign ...

**TechGeek** · 08-01-2016, 04:14 PM

Re: Should I power this thing up?(I HAVE NO VARIAC)

I powered it up, full line voltage... annnnnd it worked. Can't attach photos, Winblows 7 won't let me.

**kaboom** · 08-01-2016, 06:11 PM

Re: Should I power this thing up?(I HAVE NO VARIAC)

Originally posted by jiroy

Rectifiers are primarily diodes , like one rectifier is composed from 4 diodes typically , so you measure rectifiers like a 4 diodes measuring ..

Rectifiers are power diodes. To differentiate them from signal, switching, and varactor diodes.

Originally posted by TechGeek

I powered it up, full line voltage... annnnnd it worked.

Probably had power relatively recently, the caps are mostly good. But recap the driver boards at least.

Originally posted by TechGeek

Can't attach photos, Winblows 7 won't let me.

What do you mean windows won't let you?? You're using IE? Can't think of a better way for windows to "interfere."

**TechGeek** · 08-01-2016, 08:21 PM

Re: Should I power this thing up?(I HAVE NO VARIAC)

Originally posted by kaboom

SNIP

Probably had power relatively recently, the caps are mostly good. But recap the driver boards at least.

SNIP

How come?

**kaboom** · 08-01-2016, 08:37 PM

Re: Should I power this thing up?(I HAVE NO VARIAC)

If DC comes into the amp driver boards from other questionable caps further back, like in the tone controls and preamp, the power amp will amplify this DC if the input coupling caps (on driver boards) are leaky.

You do NOT want to amplify DC and send it to the speakers; yes the prot ckt should pick this up, but you see those non polar lytics on the board with the relay? IF those non polar caps in the protection ckt are also leaky, the prot ckt won't pick up til the DC at the spkr terminals is higher than it would otherwise be.

If the non polar caps on the prot board are shorted, it won't drop the spkr relay even with full DC on the output(s)!

Also, for driving the "high side" of the power amp, there's either a discrete constant-current source or a "boostrap" network with an electrolytic (100-470u, 35-63V typically) and two resistors. If the cap opens up, the high side of the amp is poorly AC driven- you get distortion. If the cap shorts and depending on the bias chain, the output transistors can be over- or asymmetrically biased.

If it were mine, I'd change the caps indicated in the power amp. I've shown one channel; you've got twice as many to change as the attached image shows. Also replace all the lytics on the prot relay board. Clean the bias pots (VR01 and compliment on the other channel)- if dirty or open, full bias is applied to the output transistors. This unit was from before the pots were wired such that an open was "failsafed" against. Bias current is obtained from a discrete CCS; no bootstrap ckt or cap in this one.

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**TechGeek** · 08-01-2016, 08:53 PM

Re: Should I power this thing up?(I HAVE NO VARIAC)

I checked for DC on the outputs of A, B, and C, and I saw as high as 6VDC at full volume, one connection only. The protection circuit cut the outputs for a 1/4 second, brought the outputs back up and didn't bother to cut them out again. So it needs recapping?

**kaboom** · 08-01-2016, 09:02 PM

Re: Should I power this thing up?(I HAVE NO VARIAC)

Originally posted by TechGeek

I checked for DC on the outputs of A, B, and C, and I saw as high as 6VDC at full volume, one connection only. The protection circuit cut the outputs for a 1/4 second, brought the outputs back up and didn't bother to cut them out again. So it needs recapping?

Three things need recapping.

The DC protect relay board (not dropping out, even with DC on the outputs)

The amp, because there are small caps which can upset the center points of both long-tailed pairs. Each dual-transistor, near the input, is an LTP, each with input and feedback nodes.

The "low level" preamp-tone ctl board, which is putting DC into the power amp. There's leakage from one or more caps, leading to DC at the top of the volume control, which explains why it varies when you turn that pot. I have a Sansui 881 which did the very same thing, before I recapped it. You'd turn the vol pot UP, spkr cones moved out. Turn the vol pot down, from mid-range, the cones sucked IN. Don't worry, I used junk speakers.

To verify the DC is getting into the amp via the preamp, remove the two "Control Out->Power Amp In" jumpers in the back of the unit. That ~6V should vanish. You noticed one channel gave 6V, but the other did not; the one that's dumping DC corresponds to that preamp channel with the offset.

The output channels come into the prot board via R01 and R02 (10k). The audio is shunted by two 100u nonpolar caps (C01, C02); any remaining DC is supposed charge these up and trigger the Darlington-connected transistor pair (Q01-Q02); anytime DC is present on the spkr outputs, this shunts drive to the relay drivers Q03-Q04 dropping the speakers.

C03 is part of a time delay that causes the relay to close some time after AC power is switched on. If leaky, the relay never closes (also happened on my Sansui); if open, you lose the delay-close.

Looking at the R-C ratios in the triple tone control, they're pretty typical for better sounding Japanese tone ctls. Quite similar to my Sansui (again), you'll like the way it sounds, even if you're a "basshead." You've also got the loudness control... Judging by the B+/- voltages and size of the transformer, it's an easy 60-65WPC.

This is a nice "hands on" project that you'll enjoy for years to come, unlike any win10 garbage. After fixing this unit, you're practically guaranteed to become a speaker builder.

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**TechGeek** · 08-01-2016, 09:29 PM

Re: Should I power this thing up?(I HAVE NO VARIAC)

So pretty much everything except the PSU needs recapped?

**kaboom** · 08-01-2016, 09:47 PM

Re: Should I power this thing up?(I HAVE NO VARIAC)

Originally posted by TechGeek

So pretty much everything except the PSU needs recapped?

If you want. But I wasn't counting the tuner/IF, only the power amp, pre/tone amp, and prot relay board.

FWIW, the following boards do the following functions in that unit:

Bottom of chassis:

Prot relay board- centrally located, near back
Phono EQ amp- near rotary switch, in back near input jacks
Volume, loudness, tone controls- two boards in front, behind control panel

Top of chassis:

Driver board- centrally located
AM RF, IF, MPX- right side
Minor-rail power supply & regulators- left side in front of transformer

The two main-rail caps (10,000u 50V) can stay if not leaking. Yours look OK in the pix- check where the terminals pass thru the seal. The rectifier is bolted to the chassis- excellent. One thing the old "Fisher" Fishers had were small rectifiers, which were/are a common thing to upsize.

This unit resembles some mid-70's Sansuis in how it assembled. You've got a mix of point-to-point and also boards, but the boards come out fairly easily. Nothing like the late 60's Fishers, which were rather "fun" and involved... been there, done that.

For example. all you have to do to remove the tone ctl/volume/preamp boards is remove the nuts on the pots and the screws from the switches. They should pull out from behind the panel. Easy. You might have to desolder a few leads near the front of the driver board- don't know, can't see it. Take lots of pix and make diagrams if unsure. But once you recap the driver board, you won't be doing it again. Use Nichicon PW/HE or Panasonic FC/FR here.

**TechGeek** · 08-02-2016, 06:21 AM

Re: Should I power this thing up?(I HAVE NO VARIAC)

why'd they have the epic ides to use WHOPPING 10000mfd CAPS!?

Should I power this thing up?(I HAVE NO VARIAC)

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