Kenwood KA-5500 issues

[eizner23]

Pulled the KA-5500 onto the bench, problem was distorted output, protection engages when pushing the unit. Bulb tester was dim. The relay engages ( but slowly) and powers up.

Bias was a little high, left channel 110mv right 72mv. Checking differentials, I see they are underpowered.

Looking at schematic and checking test points I find B+ rail in power amp is underpowered, reading -30v, should be -44v

Tracing upstream to power supply, I am only getting 30v to pin 8, should be 44v
Pin 5 is -31v, should be -44v. Pin 11 reads - 19v should be -24v. Pin 9 reads 20v should be 24v
Between dk2 and rk1 I only get 22v. Between pin 4&6 I read 26vac, should be 32vac.

Are these symptoms of a defective rectifier? Would that cause the bulb to shine dimly? Thanks for any input.

 

[EchoWars]

Your DBT is dropping too much voltage. You already know that the outputs are OK and that your problem is somewhere else, so either increase the DBT size (100W or 150~200W) or take it off and run straight from the AC line.

The tone defeat switch on the 5500 is a real PITA and causes lots of distortion problems. Make sure it is clean (DeOxit D5). It's also a PITA to access.

 

[Hyperion]

Would that cause the bulb to shine dimly?

Bulb shining dimly is not a fault, you definitely don't want the bulb to shine brightly, except briefly after power on.

 

[eizner23]

Thanks guys, It turns out the DBT was causing my readings to be off. Everything checks out taking it out of circuit and after bias adjustment the unit sounds great.

Lesson learned on the DBT.

 

[Hyperion]

Well Done! - everyone likes happy endings

 

[eizner23]

The unit worked for a few months then started acting up again. Left channel was dead. Cleaning the tone defeat switch brought it back, but only temporarily. After cleaning the switch at least 5 times i checked the switch for continuity, checked out fine.

Learned to use my oscilloscope enough to trace the signal down in left channel of amp section to base of q19. Nothing going to pin 8 base of q23 power transistor from q19 emitter. Tested q19 with chinese tester, says its working. Freeze sprayed and tested when hot no problem. Schem says 2sd415 , installed is 2sc1565-R. Solder work shows q19 q20 have been previously replaced by another person. I wonder why? I measured voltages at q19 c=45v b=1.2v e=0.5v

Other channel traced signal all the way through. Would q19 be faulty in this case?

 

[eizner23]

Exactly the same as q19

 

[eizner23]

Thanks for the help. I ran a continuity check between the emitter of Q19 and the base of Q23 and it checked out. Unit was cold, I Feed a signal through and I wasnt seeing anything at Q19 base or R27 or Collector at Q9 nothing. Then after some head scratching for whatever reason I checked pin 9 emitter of Q23 and i saw the signal. Now everything suddenly started checking out. Hit everything around that circuit with freeze spray, still works. I Inspected solder joints and traces, didnt see any faults but reflowed everything around that circuit anyways. Must have had a cold solder joint.

I will see how it acts after another cold start tomorrow and maybe try hitting solder joints with freeze spray if it does.

 

[eizner23]

Acted up again on a -20 F day. I narrowed it down to Qe1. I freeze sprayed it and signal went in and out numerous times. 2sa841 is installed as differential inputs. Will ksa992fbu work as sub? Only issue i see is original is 70 mhz while the 992 is little slower at 50mhz. Plan on replacing all 4 diff input devices q1-q4 w. matched gain.

 

[eizner23]

992 did work fine. Unit is operational again. Thanks for the help.