A-700 Restore

[Scottfer55]

Are you doing a "pull the plug and see what happens" power test at the hospital? That's always stressful.

Well that’s a by product but we have a scheduled outage to install a 600amp feeder to a new emergency department. The breaker has to be installed and wire pulled into the existing “hot” panel. Once we turn it off we wait until the hospital personnel confirm nothing unexpected shuts down before we start. Fun stuff

 

[zaibatsu]

All look good

I think the amp hasn't suffered any major failure.
Must be something minor, you're in good hands with Hamish and avionic.

Did you definitely check the DC V on the relay contact pairs that face each other? It would have been 36V on one pair and ~0V on the side they would contact when engaged, from what you said?

Need to confirm TR151 / TR152 voltages, but worth double checking the relay contact voltages too.

 

[Scottfer55]

A picture of the transistor orientation is worth a thousand words.

The bias transistor old vs new, on tester. And as installed. The writing faces the heat sink on the new.

 

[Scottfer55]

Please verify your voltage measurements/

As you suspected and I apologize guys it was late, I’ll do better! These are the correct measurements

TR 152
E. 0
C. +57.8
B. 0

TR 151
E. 0
C. +57.3
B. -4.0

 

[Scottfer55]

With your meter's black/negative lead welded (not clipped) to the 0V reference point, please measure (at both individual component leads) the voltage and polarity at C205, C210, D149 & D150.

C205- 0
C205+ -2.4

C210- -57.5
C210+ 0

D149
Anode +17.1
Cathode 0

D150
Anode 0
Cathode -17.3

 

[Scottfer55]

DO NOT use/select the PHONO input until the current issue is resolved.
DO NOT engage CLASS A until the current issue is resolved.

DO Engage the SUBSONIC filter - it will block any DC voltages coming from the pre-amp stages.

Have not engaged phono or class A since day one of rebuild.

Engaged subsonic just now after ^^^ those last measurements

 

[Scottfer55]

Did you definitely check the DC V on the relay contact pairs that face each other? It would have been 36V on one pair and ~0V on the side they would contact when engaged, from what you said?

The best way I can describe the contacts are front (closest to front of amp) and rear
I think front is right channel
Rear is left channel ?
How do I identify circuitry related to left or right for future reference please.

DCV on relay measured now
Front hot side fluctuates -34.3 to -38.4
Dry side 0

Rear hot side Fluctuates -36.2 to -38.5
Dry side 0

 

[Hamish119]

C205- 0
C205+ -2.4 (it should be +57.5V)

Excellent work! And it potentially points to the problem.

With the amp powered off, measure the resistance of R335 (22R and mounted vertically) from adjacent connected solder pads.
Re-solder, replace R335 or fix the broken track.



VERIFY!

Please post your diagnosis.

If your diagnosis is to replace R335, use 0.5W (or greater, but don't go nuts - 1W max) Metal Film resistors. You may also choose to replace R358 with the same...

Is the following issue resolved?

...The amp has been sitting untouched since 2003, it was working but had sound issues, dim and some not working lights...

Good Luck!

EDIT: Corrected resistor number - R335. Apologies for any confusion...

 

[Scottfer55]

R335 is one of the resistors I replaced when cleaning up the corrosion. It did test bad. I’m including a picture of the mouser part # and data. Can you confirm this is the correct replacement.

 

[Hamish119]

R335 is one of the resistors I replaced when cleaning up the corrosion. It did test bad. I’m including a picture of the mouser part # and data. Can you confirm this is the correct replacement.

Does the failed R335 appear to be burnt?

I would use something much better. But as you have stock available, go ahead and use what you have. It may fail again.

Good Luck!

 

[Scottfer55]

Does the failed R335 appear to be burnt?

No,it is the one in the picture with cut leads laying on the bag
R358 checks good in circuit

 

[Hamish119]

...How do I identify circuitry related to left or right for future reference please.

If you look at your supplied picture, you can see L & LE and R & RE to the right of the yellow output leads.

N.B. You can measure the DC offset for each channel on the 10R grey wirewound resistors adjacent to the yellow output leads.

Good Luck!

 

[Hamish119]

No,it is the one in the picture with cut leads laying on the bag
R358 checks good in circuit

That's good news.

Components fail!

Go ahead and replace R335 with what you have available. If it fails again, you may be looking for another issue.

After replacement of R335, please verify and post the voltages at C205 & C210 and the voltages on the 22000uF capacitors.

Good Luck!

 

[Scottfer55]

Is the following issue resolved?

YES!!!
Adjusted both channels idle adjustment to 15.5mvdc!
While on the tester.
Is it safe to take off tester and readjust idle adjustment?
EDIT! I just saw your last post voltages to follow

 

[Scottfer55]

After replacement of R335, please verify and post the voltages at C205 & C210 and the voltages on the 22000uF capacitors.

C205+ +45.3
C205- 0

C210+ 0
C210- -45.5

22000uf
measured from red wire to black +46.2
Blue wire to black -46.2

 

[OMGCat!]

I would adjust idle low while on the tester as it may get a significant bump when going to full power.

 

[Scottfer55]

N.B. You can measure the DC offset for each channel on the 10R grey wirewound resistors adjacent to the yellow output leads

Is this DC offset the same as idle adjustment?
I have 0vdc on either side of that resistor.
The service manual only has idling adjustment and adjustment for switching voltage on class A.

 

[Hamish119]

C205+ +45.3
C205- 0

C210+ 0
C210- -45.5

22000uf
measured from red wire to black +46.2
Blue wire to black -46.2

These numbers look good.

Leave the amp powered up on the bulb limiter and DO NOT adjust the BIAS potentiometers.

Have a coffee (or something stronger) and watch this video.

If you don't understand what the bias potentiometers are doing, watch the video again.

After watching the video, measure the capacitor voltages again. If they're still the same as previously measured, measure the BIAS voltage across the resistors as before. Note the difference, but do not adjust.

Power off and remove the bulb limiter.

Power on and measure the capacitor voltages and the BIAS voltages as before, and post the results.

The voltages should now be at their normal operating values.
The BIAS should be very similar to your previous measurements. If it's wildly different, correct it, but be extremely careful - there may be other issues.

Let it idle for 30 minutes and check the voltages and BIAS again.

If it's ALL good, use the amp.

Good Luck!

 

[OMGCat!]

DC offset is basically what you were doing before, checking voltage at the pins of the relay relative to ground. If it's low, as in mV you are looking good.

 

[avionic]

Is this DC offset the same as idle adjustment?

No.