Restoring a Harman Kardon A402

[bruno91]

After reading the post of jheu02 on the restoration of an A402 I decided to undertake a restoration of my A402.
This one was bought out of stock, there was almost no sound, the switches no longer worked and modifications had been made to work around the problem.
A complete disassembly is therefore necessary.
With change of capacitors, transistors, diodes and zeners.
My knowledge of electronics being more than modest, I count on the experts of this forum to intervene if necessary, all advice will be welcome as well as your suggestions.






Main Amp P.C. Board





Fonction Selector Switch











note here the modifications made to overcome the malfunction of the switches.

 

[bruno91]

the rest of the disassembly













sorting transistors

I chose to replace the capacitors with Panasonic FC et Nichicon ES.



2SC1400 with KSC1845

2SB549 - MJE253

2SD415 – MJE243

2SC853 – BD139

2SA872 – KSA992

2SC1345 – KSC1845



Les 1S1211 with 1N4148

VD1212 – 1N4148

1S1211 - 1N4148

1S2076 – 1N4148

 

[bruno91]

The change of components ending, it is time to move on to reassembly.














Well here is the reassembly is finished phew...!

 

[bruno91]

Before start-up, check the supply voltages.

+B1 ………. SM +37,72V mesure : +35,2V
+B2 ………. SM +37,72V mesure : +35,2V
+B3 ………. SM +5,75V mesure : +4,62V

-B1 ………. SM -37,72V mesure : -35,2V
-B2 ………. SM -37,72V mesure : -35,2V
-B3 ………. SM 0V mesure : 0V

+B4 ………. SM +20,44V mesure : +19,9V
+B5 ………. SM +18,55V mesure : +18V
+B6 ………. SM +11,69V mesure : +12V

-B4 ………. SM -20,44V mesure : -19,9V
-B5 ………. SM -18,55V mesure : -18V
-B6 ………. SM -11,69V mesure : -12,3V

That doesn't seem too bad to me.

On the other hand, trimpot VR401, VR402 have no action on the voltage measured at the terminals of R435, R436, nor on the collectors of transistors Q418 and Q419, Q417 and Q419.
4mV across R436 terminals.

Do you have any idea what to check?

 

[llwhtt]

It might help to know what brand you are working on.

 

[ryuuoh]

Harman Kardon A-402

 

[bruno91]

yes Harman Kardon A-402

 

[bruno91]

On this card all capacitors have been changed as well as transistors and diodes.

Here is a reading of the voltages across the transistors.



I think I'm going to have to redraw and test all the resistors.

Here is the trimpot change, I got the size wrong, doesn't seem like a problem to me.
Is it possible to make a mistake on the assembly of a trimpot????



https://www.mouser.fr/ProductDetail/Bourns/3362M-1-202LF?qs=cvHLLyFtoE3NRudNfuuoKA==

 

[bruno91]

 

[dlucy]

Excellent beginning to a thread!

 

[bruno91]

C'est parti pour un contrôle de toutes les résistances.

 

[bruno91]

well here are all the resistances tested, all are good, at this stage I do not see what to do.
Do you have an idea?.

 

[bruno91]

Can anyone tell me what to replace these green, small and large, tantalum, ceramic capacitors with?, I'm thinking of replacing them at this stage as I don't have the equipment to test low capacitor.


values.

 

[jheu02]

The small green are ceramic. If you desire to replace, C0G/NPO are great replacements vs standard ceramic. The larger green “chicklet” capacitors are Mylar film. You could replace with same or Polypropylene versions. Keep the capacitance the same, but you can increase the voltage if that’s what it takes to fine replacements. 50V or higher should work everywhere as the rails are only 37-40V depending on what’s on the line at the time.

 

[bruno91]

thank you, I lent myself a fulke 87 so I was able to test the 3 large green capacitors and they are good, I found a cut track, I go up the resistors since they are good and I test all the tracks.

 

[bruno91]

Well here are all the resistors are reassembled, the card reassembled, and the problem persists, it is identical on both channels.
Where is the error, a badly mounted or inverted component, if according to the different voltages someone is able to give me a direction of research that would be nice.

 

[bruno91]

I did a test with the old transistors, the result and identical.

 

[bruno91]

Also checked 4 or 5 times the diodes, I don't see any problem.

D404 - 1s1211 remplacement 1N4148
D408 - 1s2076 remplacement 1N4148
D406 - 1s2076 remplacement 1N4148
D402 - RD5.6c remplacement BZX85B 5.6

 

[bruno91]

Small question for connoisseurs, is it possible that a problem on the preamp board can generate this type of problem, short circuit, bad impedance, I did a test by uncoupling the preamp using the jumpers on the back of the amp, which does not change anything.

 

[bruno91]

the white resistors are good, also check the transistors, test plus pinout, same for the powers hfe=130 except 1 which peaks at 35.
I confirm 0.004v between the collectors of the power transistors. 0.075v on collector. measurements taken between collector and ground.