Technics SUV4A amplifier - voltage and current adjusting pots
I'm having a problem fixing an old SUV4A amp. I replaced the two main ICs (STK8050s) and also IC303 after finding that these were shot.
I then ran it for about 10 mins on a very low volume hooked up to my speakers and tuner - and then found that the volume control was very dirty giving rise to loud audible scratching noises in the speakers. The volume also seemed to be excessive even with the volume control knob at number 3.
However, I neglected to wait 10 mins to warm up and probably had clamp voltage pots (R337, R338) pots set too high. Both Icq pots (R368 and R367) were set prior to powering up - at the fully anticlockwise position as per the service manual.
20 minutes later, there was a click and I lost all sound. I did some checks on the STK8050 ICs and both were blown much to my disappointment!
All diodes, transistors and resistors etc were checked and all were found to be giving correct values and no sign of any burnt out/visibly damaged components anywhere on the main board.
Capacitors have slightly raised ESRs due to age, but nothing that is excessive.
I have read that most replacement STK8050 ICs are slightly different to the original factory fitted STK8050 ICs, therefore needing a different setting on the adjusting pots to prevent the ICs from blowing. Presumably this would be the setting in which the pots are at maximum resistance or at least close to this.
However - I have found that by using my multimeter - two of the pots - R367 and R368 only give the highest resistance if you turn them clockwise. These are the Icq pots (bias adjustment).
R337 and R338 are the clamp voltage pots. When these are turned fully anticlockwise, this results in the highest resistance on the multimeter.
This seems to go against what it says in the service manual - as it says to turn R367 and R368 both fully anticlockwise - presumably to start at the lowest setting and work upwards, thus reducing the risk of blowing the ICs? Why is this the case if the "minimum" setting on R367 and R368 gives the least resistance on the multimeter?
If anyone can offer some pointers I would be grateful. The amp sounded really good whilst it was alive for 30 minutes!
My guess here is that I probably had R337 and R338 initially set too high before letting the amp warm up for 10 minutes, along with a very dirty volume control which I don't think helped much at all.
So - once new STK8050s are soldered in and the heatsinks added with thermal paste etc - should I therefore follow the manual to the letter and:
1. Set R367 and R368 to fully anticlockwise (stated as "minimum setting" in the manual and gives the lowest resistance on the multimeter)
2. Set R337 and R338 to fully anticlockwise (this adjustment position gives the highest resistance on the multimeter)
3. Ensure amp volume is set to zero
4. Ensure speaker switch Main is enabled.
5. Power on amplifier. Verify relay latching.
6. Leave amp on at volume set to zero for 10 minutes.
Listen to music on low volume (At level 2 on volume) for 10 minutes.
Slowly turn up volume (keeping below 12 o'clock position) after 30 mins of operation, hoping that the ICs won't blow again!!
Any pointers on why it says R367 and R368 to be set to minimum and yet "minimum" gives the least resistance on the pot would be helpful - I can't figure out why this is so! It goes against my thinking!
I really DON'T want to blow another set of ICs and delay the job any further for the customer, so any advice would be massively appreciated.
I think these STK8050 replacements will certainly need to be on the lowest possible settings on the pots to prevent another failure. It looks like they are known to be fussy types.
Many thanks everyone,