Heathkit IP-27 power supply

[andrewn]

I have a Heathkit IP-27 which I am repairing. I got it second hand many years ago so I didn't build it myself. The current limiter has never worked so I thought I would have a go a repairing it. The DTG-600 transister was s/c. I thought I would try to fit a silicon transistor in it's place. The only PNP TO3 type I had was a BDX62 but the PSU won't give much output current with it installed. I later found a germanium AD149 and the PSU works correctly but I feel this transistor is under-rated with regard to maximum voltage. Any suggestions on a suitable transistor?

Several years ago I had C4 (250u) explode when I was using the PSU on maximum voltage. The cap had 70V across it but was only rated at 50V and is in the same circuit area. The parts list says this is a 25V cap I fitted a 100V cap and carried on using it (with the defective current limiter).

Any other reliability/safety mods I should concider doing?

Many thanks
Andrew.

[Chaparal]

Andrew,
Sony used 50V caps in there power amps with a 48V supply voltage.....?
Ok, it worked, but i find it very strange.
Do you have the full manual?

Here an service add
Peter

[andrewn]

Yes I've got the service manual but the only thing I havent got is the wiring diagram but it's fairly easy to compare the wiring against the circuit diagram. I see my unit doesn't have the mod shown so I will do that. There is a 50v 500uF cap accross the o/p and the psu is capable of a 50v o/p. If it was out of calibration then the cap is likely to suffer damage. I suppose it was physical size limitations that prevented designers over-rating components in those days. A modern 50v cap is a fraction of the size now.

[HamishBoxer]

Absolute min 63V working if not higher of those caps.
David gm8jet

[wd40addict]

C4 only has 8V *across* it so 25v rating is more than adequate. It will, however be floating 60-70v wrt negative terminal.

I have the IP-20u version of this supply and when I got it all transistors bar the DTG-600 were blown, or had been replaced by inappropriate types. What I think had blown them in the first place was that the heatsink isolating washers had not been fitted correctly, by the original builder, causing intermittant shorts to deck.

I replaced all the dead transistors with Silicon devices, paying particular attention to the fact that Q2 needs to have VCE of 50 volt.

If you want to change Q1 to a silicon device you'll also have to change D7 to something silicon as well - note I've not tried this on my unit.

Good luck,

Paul

[andrewn]

Hi Paul, I've now got the PSU working with a silicon transistor and diode. The current limiting now works as it should however there is a still a problem. R4, 200 Ohms begins to smoke if I advance the course voltage switch beyond 20V. The resistor is supposed to be 0.5W but I have fitted a 1W resistor but it's still overloaded. The original owner had fitted a 1K medal clad type. On measuring the voltage accross the resistor it just goes higher and higher as the voltage switch is rotated. The same goes for the voltage accross C4. I'm not sure if there is a wiring fault or if the resistor value is wrong. Any ideas?

[wd40addict]

This does sound like a wiring error. Is this still true with no load on the supply output?

If you completely disconnect Q1 and bridge the Emitter and collector wires together (leave base wire floating) does C4 voltage behave sensibly (sit at ~9v) or does it still climb with the main output?

Regards,

Paul

[andrewn]

Hi Paul, I tired isolating the transistor as you suggested and I still found the voltage climbed with the output. On further investigation I found there was a wiring fault. The yellow wire from the transformer that forms part of that circuit had been connected to the AC side of the bridge rectifier not to the +DC side as shown on the circuit diagram. On connecting it to the correct terminal it now works properly Many thanks for your help. Regards Andrew.

[wd40addict]

Well done!!

One of the problems with Heathkits is that you are at the mercy of the original builder.

Do note that now you've 'siliconised' the current limiter there will be more power dissipated in the series resistors so I wouldn't recommend using it in current limit for extended periods.

Have fun!

Paul

[lehrer_pt]

Originally Posted by andrewn

Hi Paul, I've now got the PSU working with a silicon transistor and diode.

Hi
I'm new to this forum and would very much like to know what type of silicon transistors and diodes you used to replace the ones in the IP-27 power supply - which I've recently bought 2nd hand and is not functioning properly. The germanium types I found online are very expensive, probably because they are no longer manufactured. They cost more than the entire power supply.

[andrewn]

Sorry for the long delay in replying. Another member contacted me asking for details which prompted me to look at the thread again where I saw your question.
The transistor I used was a TIP42B and I used a 1N4004 diode for D7. These were components I had in stock so I didn't buy them specially. The TIP42B is rated at 80V Collector-Emitter, 6A max. I would think any general purpose PNP transistor with a similar rating will be fine. I installed it inside the casing using an insulator as shown in the photo.
Regards Andrew.

[lehrer_pt]

Thank you very much Andrew. I had put the IP-27 aside for a deeper analysis and repair but with your info I'm going to try and repair it now.
Regards
Rolando